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POINT BLANK THE ART OF MIXING

Point Blank 2007 1

Week 1:

The Environment

Contents

1 Introduction Page 22 About the course Page 33 Choosing & installing monitors Page 44 Some basics about Acoustics Page 115 Absorbers to the rescue Page 156 Further reading Page 21

Art Of Mixing


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Introduction

Firstly I would like to take this opportunity to welcome you to Point Blank. You haveshown a great deal of enthusiasm, interest and dedication to this subject by enrollingon one of our courses. I hope you will keep up this enthusiasm while attending thecourse and most of all I hope you enjoy your time, your fellow peers, the facilities andTutors while you are studying at POINT BLANK.

Over the past twenty years there have been great technological advancements in howpeople can write, record and produce music. I have personally seen the introductionof MIDI, the introduction of computers, digital synthesis, drum machines, and reliablesoftware sequencers (thats right Im a Rave Granddad).

Back in the early 1990s if you had any aspirations of making music you where faced

with two main options:

1 - Spending a small fortune on enough studio equipment to produce it at home, aswell as taking over your old mans garage to set up your studio.

OR

2 - Spending a slightly smaller fortune paying for a studio, an engineer, andprogrammer and hoping that you could finish off the track quickly before the costsstarted spiralling out of control.

However, the past five years have seen a revolution occur in Music Production. Theprice of computers has dropped dramatically their power and processing speed greatlyincreased and this has all had a knock on effect in major advancements in softwareprograms now available. For the first time in the history of music computers arepowerful enough to do the whole kit-and-caboodle all by themselves. They can act asa sequencer, sampler, synthesizer, a mixing console as well as a multi-track recorder.And then once you have finished your tune you can master it, design the artwork andburn a CD of your bang smacking tune. OK so its not quite as simple as that, but noone denies that it has never been easier to bang out a TUNE in your bedroom.

Jc ConcatoHead of College.


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About the course

Mixing is a very subjective process in which many parameters come into the equation,such as style of music, equipment, environment and maybe more importantly YOUREARS.

There are a lot of different techniques and approaches to mixing, depending on thestyle of music. You may also find that even within the same genre, different mixengineers work in different ways (use of eq, compressor, Fxs, etc) to obtain what wemight sometimes perceive as a fairly similar result. However, when it comes toModern music production, whether you use a desk (analogue / digital) or mix withinyour DAW, they are some common rules that will help you in your quest to perfectmixes.

Trying to get this perfect mix in a professionally designed mixing room is achallenge in itself. The recent changes in Music technology & music Business meanthat more & more music is produced from Home/bedroom studios, which can make

the challenge even trickier. In this module we will take you through each step toimprove your mixes, from how to create a better listening environment to masteringyour tunes.

And remember, dont get too absorbed in the world of room modes, frequencies & ratio, its all about the music, there are a lot of very successful records out therewhich technically dont sound that great but the tracks just work, and people lovethem...

The bottom line is that if your track sounds exciting wherever you play it, youveprobably got a winner.

JC [djesi] Concato


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Choosing and Installing MonitorsChoosing monitors

Before looking at acoustic treatment, I would first like to discuss loudspeaker choiceand mounting. The positioning, orientation and mounting of loudspeakers can have ahuge influence on the final sound of a room. So dont go out & buy massive speakers

& sub woofer if you have a tiny room & dont intend to apply a lot of acoustictreatment.

What makes a good studio monitor? In two words, we should be looking for the 'neutral average', neither too bright nortoo dull. If the aspirations we have for our work are that it should sound tonallyacceptable on the widest range of systems out there, from top of the range hi-fisystems, portable radio to MP3 players; then the perceived tonal balance of ourmonitors should be as close to the 'population average' as possible. The mostcommon monitoring systems found in home studios are either near field monitors,usually used up close and probably with its back to the wall, or a pair of hi-fi speakers(wall mounted or on stands).

Neural average? The perceived tonal balance of a speaker is the combination of thedirect sound from the drivers and reflected sound from nearby surfaces. A neutraltonal balance, however, is not the same thing as a flat axial frequency response.

There is a wide range of speakers out there that will be suitable for the job you needto do. Remember that you are not necessarily looking for the best sounding speakers& most pleasing experience for your ears but most importantly for a tool that will helpyou to make the correct decisions.

A typical monitoring set-up found in professional recording studios will include:

A: Main monitorsmounted in walls

B: One or more pair of nearfield monitorsmounted on the deskmeterbridge or stands.The Nearfield monitorsare used to recreatethe experience of alistener @ home.

The idea is that byusing different types of monitors, you will beable to identify possibleproblems in differentarea of the mix.


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Nearfield monitors

The most common use of monitor speakers in the budget studio is near-fieldmonitoring. This simply means you have the speakers mounted only a few feet awayfrom your ear so that you get mostly direct sound and very little reflected sound; theminimizes the extent to which room acoustics affect the sound. The term 'nearfield

monitor' was an invention of the early '80s. It just about predates the explosive riseof the home and project studio and was originally the term applied to auxiliarymonitors that sat on the meterbridge in large commercial studios, and were supposedto reflect the sound of typical home audio or TV speakers. One speaker originallydefined the breed: the Auratone 5C. The Auratone was, and is, little more than a five-inch 'full-range' driver screwed into a small cube-shaped enclosure. It had littlepretence to audio accuracy or wide bandwidth, and was simply intended to provide areference for the likely sound of recordings when reproduced on an AM radio, or via aTV. So the Auratone was not really a 'nearfield' in the sense that we understand theterm now, but it did set a precedent for auxiliary monitors, and prepared the groundfor the second nearfield icon the Yamaha NS10M. We now live in different times.The huge studios, if not quite heading the way of the dinosaurs, have long been underthreat from small-scale recording spaces and control rooms. And being very muchsmaller nowadays, the typical control room now has little space for vast mainmonitors. These days, the nearfield has had a promotion. More often than not, it'snow out on its own, the top dog. And for the same reasons (well space at least) mosthome studio owners use nearfield monitors. Even when working in top mixing rooms,which offer a vast range of monitoring, most professional engineers will still usenearfield monitors as their main reference, typically of two different designs, as wewill discuss in the next chapter.

Ported vs un-ported?Choosing a monitoring system can be a difficult and confusing task, not least because

of the enormous number of models and designs on offer. In this lesson we will focuson the two main designs for nearfield monitors: infinite baffle (sealed box) & reflex(ported).

The major role of the enclosure is to prevent the out-of-phase sound waves from therear of the speaker combining with the positive phase sound waves from the front of the speaker, which would result in interference patterns and cancellation causing theefficiency of the speaker to be compromised, particularly in the low frequencies wherethe wavelengths are large enough that interference will affect the entire listeningarea.

The ideal mount for a loudspeaker would be a flat board of infinite size with infinite

space behind it. The infinitely large baffle divides the sound coming off the front of theloudspeaker driver from sound coming off the rear, consequently the rear soundwavescannot cancel the front soundwaves, but both sides of the loudspeaker cone areworking into the same infinitely large volume of air and thus are loaded identically.

IN PHASE SOUNDWAVES OUT OF PHASE SOUNDWAVES

EQUAL VOLUME OF AIR ON EACH SIDE


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Infinite-Baffle or sealed box : Of course, the above concept is not workable inpractice, and the closest we can come to the ideal is to build a large sealed box andplace the loudspeaker in the front baffle, hoping that the sound coming off the rear of the speaker cone will be absorbed within the box. The box is usually filled loosely withfoam, pillow stuffing, fibreglass, or other wadding.

The general rule is that the bass response of this kind of cabinet is relatively limitedcompared to that of other arrangements (i.e. ported), for a given cabinet size; thelow-frequency roll-off starting at a relatively high frequency.

On the plus side sealed enclosures, properly designed, will have better transientresponse at lower frequencies, leading to what people generally refer to as "tighter"sound. As well the phase response can be very smooth, with relatively little phaseshift, and the slope of the roll-off is also quite shallow, averaging 6dB/octave. Indeed,because of the shallow slope, even small infinite-baffle speakers can produce audiblebass at surprisingly low frequencies, though you wont have that sub bass filling inyour room.

For many, the infinite-baffle design is the most highly regarded and leastcompromised solution to loudspeaker monitoring. It is also interesting to note that themost widely used mixing reference; the Auratone and the Yamaha NS10 are bothinfinite-baffle designs. Ironically, this is not the most common design.

Ported or Bass reflex: The most common cabinet design is the reflex or portedcabinet, which makes deliberate use of the resonance of the cabinet to takeadvantage of the sound coming off the rear of the loudspeaker cone. The idea of theport is to make the cabinet resonate at a carefully chosen low frequency, not unlikethe effect of blowing across the top of an empty bottle. Instead of being completelysealed, the cabinet has a hole in it through which the internal sound can escape and

contribute to the overall sound in the listening environment. The vent may be locatedon the front baffle, it may be on the rear, and it may take the form of one or moreround holes or slots.

The advantage of this approach is that it allows a much greater acoustic output atlower frequencies than the infinite-baffle design. You get a far more impressive bassresponse and overall volume level for the size of the box.

However, there are a few disadvantages, one being that any resonant system smearstransient signals over time. In monitoring terms, this inherent time-smearing andresonant behaviour can obscure small dynamic changes in the signal being auditioned,and may also reduce the transparency of the mid-range. In practical terms, a poorlydesigned reflex system can make it extremely hard to judge the relative levels of bassinstruments properly.

Another issue is the frequency and phase response characteristics of the portresonance. While the low-frequency roll-off point can be extended to a significantlylower frequency using a reflex design than with an equivalently sized infinite-bafflecabinet, the slope is far steeper, and the phase shifts far greater. Thus the level of bass output is greater down to the roll-off point, but then falls away much quicker,and a reflex cabinet is likely to reproduce very low frequencies at a far lower levelthan an infinite-baffle speaker. The inherently large phase shifts of this design alsoreduce (or at least affect) the naturalness of the bass end.


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If in the upper frequencies, the sealed enclosure is more critical, why is thesealed speaker design not more dominant? Because the ported box tuningfrequency is generally tuned to the lowest fundamentals that the woofer in the boxcan produce, and only comes into play at and near those frequencies. Significantlyabove that, there's not much difference in phase shifts or transient response (in well

designed ported speakers).Here's the kicker though; human ears are far less sensitive to things like transientsand phase shifts at really low frequencies, so it becomes a matter of picking nits, if you get my drift. This is why there are many, many successful examples of both onthe marketplace. Best thing to do is let your ears decide which box you like best forthe job at hand... ;)

If accuracy is what were looking for, why have some Lower quality orlimited range monitors have become standard?It all started with the little Auratone 5C cubes. The idea was that if the mix soundedwell balanced on this terrible little speaker, it would sound okay on anything, a verysimple, but surprisingly reliable rule. After the Auratones came the infamous YamahaNS10Ms which, although a two-way design, shared many of the same characteristicsand served the same purpose.

The reason why the Auratone and the NS10 have survived the test of time, andbecome indispensable tools to the majority of accomplished mixing engineers, is all todo with their skewed frequency response, lack of LF resonances, very low distortionand remarkably accurate transient response behaviour, all of which are inherentqualities of infinite baffle designs using small, responsive drivers.

The two most critical aspects of a rock or pop music mix are the ratio between bassguitar and kick drum, and the balance of mid-range instruments, such as the snaredrum, guitars, keyboards and voices. Speakers like the Auratone and the NS10 areparticularly good at exposing mid-range balance errors. The middle of the audiospectrum is emphasized in such a way that small changes of balance become veryobvious. Hence, if the mix sounds right, it is right and will translate far moreconsistently to other systems, whether quality hi-fi, in-car, or 'cheap and cheerful'portable radio.

The problem in balancing the bass guitar and kick drum is often to do with 'overhang',a low-frequency resonance largely inherent in ported (reflex) speakers, which masksthe true envelope of low-frequency signals. With no port to resonate, infinite-bafflespeakers tend to have very accurate transient behaviour, and this makes it far easierto hear what is really happening with those low-frequency instruments. These

speakers can be extremely revealing of less than ideal balances, such as betweenvocals and backing instruments, for example. They can also highlight mistakes in bassinstrument equalization very well, such as poor EQ allowing the kick drum todisappear entirely on anything smaller than a pair of full-range monitors - oops!

The traditional grot boxis an essential tool formix engineers, but notall of them are bornequal. The Auratone andNS10 are pretty muchunique in this role, and

while few would choose to listen to them forpleasure, they are well suited to the task of fine-tuning a mix.


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Buying monitors : 95% of people listen to music in their cars or on a cheap homestereo. If your mix doesn't sound good on a pair of small speakers, what's the point?A pair of 5,000 powered monitors may sound pristine, but no one else has them, soyou're more likely to have a translation problem. Ideally, you would like a pair of really great sounding speakers, and a pair of real sounding speakers. Typically a

sealed box type NS10 Auratone (check the Triple pyramide 250), and a porteddesign.

Ports are used in the majority of project studio monitors , primarily becausethey help boost the output level at low frequencies.

Lower price range : In lower-cost monitors (such as the M Audio BX5, SamsonResolv 80A, Event TR5, and Behringer B2030A Truth), the main side-effect of thisdesign is a smearing of low-end transients which makes it difficult to judge thebalance of bass instruments. However, the problems of ported cabinet design can beovercome, and more expensive models are able to achieve professional performance.

Higher price range : There are many excellent reflex designs around, including the

larger ATC monitors, all the Genelec models, various Dynaudios, Mackies, andTannoys, and many others. The Mackie monitors are an interesting alternative of reflex design. This is a more complex arrangement, sharing some characteristics withboth infinite baffle and reflex designs, although it falls most comfortably into the lattercamp.

Note about lower quality speakers : We now understand why the use of lowquality speakers is a very revealing tool in the mix process. Why spend around 250on low quality speakers when you can use some old cheap hi-fi?

Low quality hi-fi speakers usually have all the inherent problems of the Bass reflexdesign discussed previously. Most cheap domestic speakers have an overly bright highend (to make them sound exciting), with a slightly reduced mid-range (to make themsound more pleasant and larger than they really are), and a lumpy, resonant bassend. The other thing to mention is that many cheap hi-fi speakers might not be ableto cope with the often continuous high levels required in a professional mixingsituation.I agree that paying 250 for what are clearly very simple speakers that don't evensound particularly pleasant seems a foolish enterprise. However, these things aredesigned to serve as an accurate and reliable mixing tool, and a lot of engineers havefound Auratones and NS10s indispensable in helping them to craft the mixes thatkeep them in employment. That said, high-end monitoring has improved considerablyin the last twenty years, so the role of monitors like the Auratones, NS10s and othersis arguably not nearly as crucial now as it was then.

Genelec 8030A585

Tannoy REVEAL 6D385

Mackie HR824999

Alesis monitor1 MK2220


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When buying speakers for your studio, try to audition them with a placement similarto how you will use them. If they will be used against a wall, try them this way at thedealer. Conversely, if they will be used well away from walls, don't be tempted to

judge speakers which are grouped together with many others in the showroom. Insistthat they are moved into a sensible room position -- any good dealer should expect todo this as a matter of course

Where should I put my Monitors?

You ideally want yourself and your two speakers to sit at the corners of an equilateraltriangle, with your head the same distance from each speaker as the distance theyare spaced apart (see diagram) something between 1 and 2.5 meters (3-8 feet)should be about right, depending on the size of the room and its acoustics. Thespeakers should ideally be mounted symmetrically in the room, and on rigid standstall enough to place the tweeters roughly level with your ears. If your room doesntallow this, it is better to have the speakers too close together than too far apart. If the speakers are spaced too widely, you will get a 'hole' in the middle of the image,making accurate panning and acoustic placement impossible.

Most speakers are designed to be used well away from both side and rear walls, but if you have a choice, it's usually better to put the speakers closer to a back wall, ratherthan sidewalls, and avoid corners at all cost. If you want to mount the speakersagainst the back wall, it is vital that they are not rear ported, as the proximity of thewall to the rear of the speakers will stop the ports working correctly and will result inuneven bass. Also, when positioned in this way, the apparent bass output of anyspeaker will increase. Therefore it is important that you don't choose bass-heavyspeakers that were not intended for this usage. Having made your choice, the nextstep is to devise a secure mounting arrangement. The sound of many speakers will bedisappointing and unpredictable if they are not mounted securely; spiked loudspeakerstands have become the norm. Whether you are mounting your speakers on a shelf ora stand, placing a small blob of Blu-Tac under each speaker corner will stop themmoving. If they are floor-standing types, use a dedicated spike kit, especially if thestands are placed on carpet.


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Most monitors are designed to be used in a particular orientation (usually with thetweeter above the woofer), so make sure you place them the right way up. Turningspeakers upside-down or on their sides can have disastrous effects on the stereoimaging and frequency response!

You also need to think about where the speakers are pointing. Although a lot of designs (Genelec monitors, for example) are intended to be aimed directly at thelistener, often referred to as a degree of 'toe-in'; many others are designed to facedirectly forwards into the room so that the listener is effectively placed slightly off-axis to each speaker. Monitors that are designed to be used this way, if pointeddirectly at the listener, will sound slightly brighter than intended.

The degree of toe-in or toe-out can also have a significant effect on the accuracy of the stereo imaging and the stability of the central image, so it's worth experimentingwith small changes of angle to try to optimize the precision of the imaging and thewidth and stability of the listening 'sweet spot'

Shape of the RoomThe diagrams of figure 8 show the three most common room shapes: square,

rectangular, and L-shaped along with generally favourable speaker location areas.A perfectly square room is probably the worst room shape for most loudspeakerapplications. In a square room, standing wave production is most efficient, and henceobjectionable.

Long, narrow, rectangular rooms also pose their own special problems, creatingseveral standing wave modes along their length. Although maximum low frequencyresponse would probably be achieved by placing the loudspeakers along the narrowwall of the rectangular room, (figure 8A) optimal stereo imaging and midrange claritywould be achieved by placing the loudspeakers along the long wall (figure 8B).

L-shaped rooms would seem to present the same dilemmas as a long, rectangularroom; however, the asymmetry of the acoustical space tends to break up standingwaves and can provide a better listening environment (figures 8A & B).


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Some Basics About Acoustics

One of the most important factors in my opinion is the environment you are mixing in.When you mix music (or anything else for that matter) in a room with acousticsproblems, the material may sound fine in that location, but when the mix is playedback somewhere else; it may sound quite different; and often not good at all!

If the mix environment has minor problems, the effects may be quite small, such assmall changes in bass or treble content but in extreme cases, the balance may fallapart, or may contain elements that weren't heard during the mix.Imagine, for a moment, that your room is bass light. While you mix, to get whatsounds like a good result, you will instinctively add more bass on bass instrumentsand kick drums. When played back in a 'normal' listening environment, the mix willsound very bass-heavy and uncontrolled, which is why the environment youre mixingin is so important. Ultimately, the only real imperative is that the room should workfor creating mixes that sound 'right' when played on other systems outside the studio.

Even though mixing rooms in top recording studios have been very carefully designed

acoustically, you would be surprised at how many professional mixing rooms stillsound a bit weird. That shows that there isnt such thing as a magical formula.

First lets clarify a point, there is a lot of confusion between soundproofing andacoustic treatment, Soundproofing is simply concerned with reducing the amount of sound getting into or out of a room, but in no way defines how the room behaves as aspace for listening to music, which is why Acoustic treatment is often needed in yourstudio.There is no point spending all your savings on very expensive equipment, if theenvironment you are working in is not adequate to listen to music. Buying the bestspeakers in the world (if there is such thing) is not going to guarantee that you willhave a reliable monitoring environment; the Room is part of the equation. The Roomhas a Sound.The first thing to consider is why a room should have 'a sound' in the first place; afterall, it's the speakers that we're listening to. Sound bounces or reflects off all solidsurfaces, so that when the sound source such as a loudspeaker stops producingsound, the reflections continue for a period of time until the energy is absorbed.

There is a very close relation between the speakers, their position and the Room(size, shape, material, etc). Try setting up your speakers in your garden or roof terrace and you will notice the huge difference, the bottom end will be clean and tight,the top end imaging will be much clearer and the centre will be really tight anddefined. This is because you listen to the sound from the speakers only, the sound

leaves the speakers, reaches you and doesnt come back, this is known as ananechoic environment (i.e. no reflections or reverberation). Unfortunately neither ourneighbours nor the weather (in UK) allow us to do that.

One of the favourite locations still to listen to music (except on the tube with an iPod)is the car. I know a lot of engineers who check their final mixes in the car, so why thecar can prove a reliable place to check mixes?There are a number of factors and it is these factors that go into making a goodlistening environment.


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The Shape: There are no parallel walls in a car and the walls are thin andcurved.

The Speakers . They are almost always mounted into a flat surface like belowthe rear window or in the side door panels, as a result there are no out of phase signals coming from the rear of the speaker.

High Frequencies : In the car the windows are the main high frequency

reflectors but they are all at angles and are usually curved as well. The highsalso get diffused evenly throughout the cabin by the dash board. Also theceiling, sides and floor are covered in high frequency absorption.

Mid Frequencies : The seats, door panels and passengers all act as low mid tohigh mid absorbers. Most of the car's acoustic treatment for cutting downengine and road noise is also on the inside and acts as acoustic treatment forthe car stereo.

Low Frequencies: What I love the most about the sound in cars is the bottomend response. With a couple of hundred watts a side, a sub-woofer under theseat and the loudness switch on the bottom end thumps away and soundsgreat. Actually most of the low end goes straight through the walls anddisappears; consequently it doesn't hum around the internal body causingphase problems. Any vibration is dampened by the foam lining and carpet andas far as the low end is concerned the car is equal to open air. This is whatyou hear when a car passes you by, playing loud music with a sub woofer, thebass goes straight out.

So how do you create the effect of your car or the open air in your studio? - By usingAcoustics!! Treating the walls in your control room and studios so as to control thesound and thus improve the quality of the sound that you hear, record & mix. I reallybelieve that good acoustics can really make a huge difference to your final recording & mixing, more than the latest fancy FX or plug in.

So before looking at the practical part, lets first consider how the room interfere withthe music played from the speakers.

RT 60?

Because of the rooms reflections, we don't just hear the direct sound from ourloudspeakers; we also hear an appreciable amount of reverberation as the soundbounces around the room. In a good listening room, the reverb time will be too shortto be perceptible under normal circumstances; although you'd notice a hugedifference if it were removed altogether. However, different materials and structuresreflect different parts of the audio spectrum more efficiently than others, and thedimensions of the room cause resonances or modes to be set up (resulting in standing

waves), so the reverb we hear is 'coloured', i.e. it doesn't have a flat frequencyresponse.

The ideal listening room needs a touch of reverb to help increase the perceivedloudness of the monitors and also to prevent the room sounding unnaturally dead. Butthe reverb time also needs to be roughly equal at all frequencies across the audiospectrum if coloration is to be avoided. Reverb times of between 0.3 and 0.5 secondsare ideal for control rooms. This is the amount of time it takes a loud short sound todie away. "Dying away" can be defined more scientifically as a drop in loudness of 60dB, so acousticians call reverberation time RT60.

Reverberation time is determined by the volume of the room. It can be reduced byreplacing some of the hard, reflective parts of the walls with soft, absorptive sections,as we will see in the next chapter.


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Standing waves

We mentioned earlier room modes or resonances, which cause the spectrum of thereflected sound to vary at different points in the room. Assuming you have solid walls,room modes are directly related to room dimensions. Standing waves are createdwhen you have two parallel facing walls. There will be a particular set of frequenciesthat are reinforced by the distance between the walls.

How does it work?If a sound wave is generated that has exactly the same wavelength as the longestdimension of a room, it will be reflected back and forth from the facing walls in phasewith the original, thus reinforcing it.As you move around a room with standing waves you can hear as you walk in and outof a standing wave, in one spot the bass is booming yet in another there is hardly anybass. Makes it hard to figure out how much bass you have?

What happens is that if you stand at the high point of an in phase standing wave youhear double the volume of the frequency but when you stand at the same point in andout of phase standing wave the waves cancel each other and you hear nothing. It's

pretty hard to figure out your sound frequency balance when this happens throughoutyour control room.

It also happens at all the octaves of the frequencies as well so if the frequency is440Hz it also happens at 880Hz, 1760Hz, 3520Hz, etc. This is what creates colorationin the room. As you move around the room the frequency response keeps changingcausing room coloration.

One solution is to have angled wall (non parallel walls). A wall should be at least 12degrees off parallel to stop parallel wall standing wave interference. That's either onewall at 12 degrees or two walls at 6 degrees each.

(Note that having non-parallel walls doesn't entirely stop standing waves, they stillform within a room but along different lines of repetition). Non-parallel walls haveminimal effects at low frequencies: the low frequency modes will develop much asbefore based on the mean distance between walls.The main reason for having angled walls in a control room is because of reflectioncontrol of the high frequencies for true imaging from your speakers.

This effect happens between every parallel facing wall in the room, typicallyfront/back wall sidewalls & floor ceiling. These are known as axial modes.

Room ModesAnd now down with the math!!! You can determine what is the fundamental frequencyof your room with a simple formula: f = V / 2d

f = Fundamental frequency of the standing wave V = Velocity of sound (343m/sec or 1130 ft/sec) d = Room dimension being considered in meter or feet (length, width, or

height)

Other standing waves occur at harmonics of the fundamental frequency - that is 2, 3,and 4 times the fundamental.


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So a room with a 6 meters dimension has standing waves forming at 27.5Hz (the fundamental frequency) 55Hz (the first harmonic) 82.5Hz (the second harmonic) 110Hz (the third harmonic)

And a room dimension of 3 meters gives 55Hz (the fundamental frequency) 110Hz (the first harmonic) 165Hz (the second harmonic) 220Hz (the third harmonic)

In other words rooms with dimensions that are multiples of each other create similarroom modes (i.e. 6m X 3m).

Unless you make at least one of every opposing pairs of surfaces completelyabsorbent across the entire audio spectrum, modes will exist. In other words, if aroom has dimensions, it has modes, though the absorbency of the walls will influence

the intensity of the modes. The best-sounding rooms tend to have their modes fairlyevenly distributed, so there are no drastic peaks or dips in the room's response. Inpractice, modal problems are most serious at lower frequencies, and unfortunately,smaller rooms tend to be worse affected than larger rooms. Cube shaped rooms arethe worst, as the 3 axial modes (length, width & height) occur at exactly the samefrequencies, reinforcing each other to form very noticeable peaks.

A lot of these potential problems can obviously be improved at the planning stage, if you intend to build a studio from scratch, but choosing adequate dimensions andangled walls. However most of home studio users usually have to do with a spareroom in their home or incorporate the studio in their bedroom, which means parallelwalls made of hard reflective material (i.e. plaster). So how can we attenuate the

sound of a room so it becomes an acceptable listening environment? How can westop all these reflections & side effects?


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1- Echo/reverberation: Sit at your mix position and clap your hands together sharplyonce. Listen carefully to the 'crack' and, more importantly, what happens after the'crack'. In an ideal world, you would hear nothing after the initial crack. What you willprobably hear, however, is a short period of sharp 'flutter' echoes, which combine tomake a 'boing' sound. When the wall is flat and hard, the sound will be reflected. Asingle strong reflection can sometimes be heard as an echo, but in most rooms a lot

of reflections (including reflections of reflections) combine into the reverberation.Slap echo destroys the sound quality of a stereo system primarily in two ways: byadding harshness to the upper mid-range and treble and by destroying the delicatephase relationships which help to establish an accurate stereo image. Reflections off flat walls can sometimes combine to produce undesirable effects. The worst of these isthe standing wave.

2- Standing waves: LF standing waves occur usually around each speaker, but withlarger monitors, the back wall will interfere as well. Remember standing waves notonly happen at lower frequencies but higher ones too, though easier to eliminate.

3- Side reflections interference: If a sound arrives at a single point via two paths at

slightly different times, certain frequencies will be reinforced and others will beweakened. You can easily hear this by putting your ear close to a wall, the quality of sound will change because the reflections off the wall interfere with the direct sound.Side reflections interferences add roughness to the sound, a reduction of harmonicrichness and a smearing in the stereo image.

Absorbers to the RescueThere are different degrees to which you can apply Acoustic treatment; if designing acommercial recording studio you should really consult an acoustic designer specialized

in recording studios. Today we will focus on how to improve your home recordingstudio, using acoustic panels, curtains, blankets, etc The goal is very simple: wewant to get the sound from the speakers to your ears without messing it up. This isreally just a matter of what becomes of the sound after it passes your ears.

Fortunately, most carpeted domestic rooms with just a few items of furniture arealready pretty close to being acceptable listening environments. The purpose of thislesson is to explain some of the basic rules of acoustics so that you know what you'redealing with, and hopefully also to dissuade you from doing anything that might makethe situation worse. The secret is to improve the room by doing as little as possible toit.

There are three things that can happen when sound hits a wall. It can be reflected,absorbed, or diffused .

Different building materials produce differing sound reflections. A flat concrete wallwill produce a distinct echo when you clap your hands and that sound is reflected off of it. However a brick wall, even though it is somewhat reflective, will tend to diffusethe sound reflections and produces a much less distinct echo. This is due to thesurface of the brick, the mortar between the bricks etc Proper diffusion of sound canmake a small room sound larger (great for live recording area but maybe not quitewhat you want for your control room)

Diffusers work in a different way, breaking up the flat surfaces. When sound isreflected off a rounded or complex surface, it is diffused. Diffusion spreads the


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reverberant sound evenly throughout a room, which not only prevents standing wavesbut also eliminates "dead spots" - places where components of the sound are missing.

We can break up flat surfaces by hanging large objects called diffusers. The shapeschosen for diffusers are really a matter of taste and cost. Avoid concave curves, whichfocus sound instead of dispersing it, but otherwise pyramids, lattices, or computerdesigned random surfaces all work well. The depth of a diffuser determines the lowestfrequency that will be affected. A diffuser one foot deep will scatter sound down to160 Hz. Typically diffusers would be applied to the back wall in a control room todisperse the sound waves coming from the speaker.

However, diffusers can cause further problems when the principal activity in a room islistening to loudspeakers. The diffusion can make everything sound airy and openwhere what's on tape might not have that factor. A control room is a workingenvironment, not a listening room. In a control room you want to be able to hearexactly what is on tape and you want to be able to analyze it completely so that youcan add the necessary components such as EQ, reverberation, compression etc. Directsound from the speaker is the principal aim in a control room. I have always preferredcontrol room with little diffusion, I find that you have a tighter bottom end (if properlydesigned) and you have a better perception of the direct sound coming from thespeakers. In the home studio your room sizes are usually small and low and low-midfrequency coloration is your main problem, so the use of deep diffusers wouldprobably increase these problems.

However, room furnishings, such as bookcases, shelves and furniture can act asdiffusers, although pretty ineffective at lower frequencies they can help diffuse highfrequencies, typically from the back wall, thus preventing standing waves to occur athigher frequencies. Something you might want to try.

Absorbers

Walls with absorptive properties help to reduce the reverberation time as well aspreventing standing waves, as the sound isnt reflected as much from the wall, thusfewer sound waves messing each other up in the room. To put it simply Absorptivematerials is going to be your best friends in your home studio.

The idea is that if you have two reflecting wall facing each other, by making at leastone of them absorbent, it will help to prevent the problems previously described. Butunfortunately again, this is not quite simple. To balance a room, you must have a truebalance of the high and low frequencies, with the right amount of deadness toliveliness (reverb time). The coefficient of absorption is the basis of acoustictreatment, this is the amount of sound energy a surface absorbs and reflects and ismeasured at different frequencies. If we say that a surface material has an absorptioncoefficient of 0.25, it means that the surface will absorb 25%, while reflecting back75% at various frequencies. These numbers can be used to compare materials and topredict the results of treatment (look at table below).


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Materials 125 Hz 250 Hz 500 Hz 1 KHz 2 KHz 4 KHz

50mm Acoustic Foam 0.08 0.25 0.6 0.9 0.95 0.9

100mm Acoustic Foam 0.2 0.7 0.99 0.99 0.99 0.99

50mm Mineral Wool (MedDensity)

0.2 0.45 0.7 0.8 0.8 0.8

Plaster on brick 0.013 0.015 0.02 0.03 0.04 0.05

Cotton drapes draped tohalf area. 15oz/sq yd

0.07 0.37 0.49 0.81 0.65 0.54

Foam backed carpet onconcrete

0.05 0.16 0.44 0.7 0.6 0.4

Heavy carpet + heavyfoam underlay on concrete

0.15 0.25 0.5 0.6 0.7 0.8

Coarse concrete 0.36 0.44 0.31 0.29 0.39 0.25

Painted concrete 0.01 0.05 0.06 0.07 0.09 0.08

Wood floor 0.15 0.11 0.10 0.07 0.06 0.07

Window glass 0.35 0.25 0.18 0.12 0.07 0.04

Plate glass 0.18 0.06 0.04 0.03 0.02 0.02

6mm glass 0.1 0.06 0.04 0.03 0.02 0.02

Plaster on brick 0.013 0.015 0.02 0.03 0.04 0.05

9mm Plasterboard over20mm air gap

0.3 0.2 0.15 0.05 0.05 0.05

Brickwork 0.05 0.04 0.02 0.04 0.05 0.05

Vinyl flooring 0.03 0.04 0.05 0.04 0.05 0.05

Breeze block 0.25 0.40 0.6 0.5 0.75 0.5

LF panel absorber 0.28 0.22 0.17 0.09 0.10 0.11

Perforated Helmholzabsorber, 4-inch depth,mineral wool damping,0.79% perforation.

0.4 0.84 0.4 0.16 0.14 0.12

Perforated Helmholz

absorber,8-inch depth,mineral wool damping,0.79% perforation.

0.98 0.88 0.52 0.21 0.16 0.14

Broad-band absorber, 1-inch fibreglass slab atmouth of 7-inch deepcavity

0.67 0.98 0.98 0.93 0.98 0.96

Padded seat (unoccupied) 0.1 0.2 0.25 0.3 0.4 0.3


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The 2 major acoustic problems afflicting home studios that we will be looking at are: 1 - High-frequency problems: too much reverberation in the room, or poor

imaging. 2 LF standing waves. Leading to Low-frequency problems: bass hot and coldspots throughout the room.

Flutter echo is a distinctive ringing sound caused by echoes bouncing back and forthbetween hard, parallel surfaces following a percussive sound such as a hand clap.These High-frequency problems are most common in home and project studios,although, fortunately, they are easy to diagnose and treat using inexpensive methods.If using well positioned small nearfield monitors, that might be the only thing youllneed to do, just reduce the echo/reverberation at higher frequencies.

A very quick, if unscientific fix, is to hold a blanket across one of the hard walls. It

helps to have an assistant whilst playing this game! With the blanket held an inch ortwo from the wall, so that the wave is attenuated both on the way to and away fromthe wall, clap again. If you identified the most troublesome wall, the 'crack' shouldsound much cleaner and shorter. If not, try another wall.

If you are on a tight budget, hanging blankets or heavy curtains on the wall is fine,although, for safety, only use fabric that is fire retardant.

NOTE that the common mistake is to try to completely cover every wall with blanket,curtains, carpet, egg boxes & others Although some of these materials can do adecent job at reducing reflections at high frequencies, they are pretty ineffective atmid & lower frequencies, by applying them to each entire wall; you could make the

room sound very dull. What actually happens is that you are lowering the reflections(Reverberation time) of the Room but only marginally in the high frequency range!The lows are still humming around the room, which creates what we call a MuddySound.

Beyond this, there are a large number of acoustic tiles available for this type of problem.A small number fixed to the wall with heavy double-sided tape or adhesive will workwonders. In many cases, a pair of acoustic foam tiles fixed to the side walls on eitherside of the engineering position, as shown in the diagram here, will be all that'sneeded. Acoustic tiles or Rockwool ceiling tiles will also be of benefit on the ceiling. Amajor benefit of reducing high-frequency standing waves is much better detail andimaging from your monitors. This area is perhaps the most overlooked in homestudios.

You could find out more precisely how much absorptive material you need to apply tothe room, by calculating the Reverb time. There are formulas to calculate reverb timeor T60 such as the one devised by WC Sabine at the turn of the century, though thisformula is more accurate when applied to larger rooms than to small ones. There is amore accurate and rather more complicated formula attributed to Eyring.

Don't panic, though, because in a typical domestic room, using nearfield monitors; theamount of acoustic treatment needed isn't usually that great. Most of us wont feel theneed to go down that road. What's more, using maths, the result is only going to be

an approximation, due to the uncertain absorption coefficients of various materials.Furthermore, the overall effect of the same area of absorbent material will be differentdepending on whether the material is concentrated in one place or distributed around


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the room.

When treating facing surfaces, it's most effective to distribute the absorptive materialbetween them, rather than putting everything on one wall and leaving the other

reflective; in the case of sidewalls, this is essential to maintain a nominallysymmetrical listening environment, otherwise the stereo image could be strangelyunbalanced.

However, it's not always possible to treat opposing surfaces in exactly the same way,the floor/ceiling pair being the most obvious example. If the floor is carpeted, it willabsorb the higher frequencies very efficiently but will hardly affect the bass or lowermid-range at all. One answer might be to mount bass traps (well discussed themnext) in the ceiling to absorb the bass but to reflect back the mid and higherfrequencies absorbed by the carpet.

The easiest way to tell where to place absorption to avoid early reflections is with a

mirror. While you sit in the listening position, have a friend place a mirror flat againstthe side walls and move it around. Any location in which you can see either loudspeaker in the mirror should be covered with absorption. It's a good idea to treata larger area of the wall than you identify with the mirror, so you'll be free to movearound a little without leaving the Reflection Free Zone. Once the side wall locationsare identified do the same on the ceiling. Although it's more difficult to slide a mirroraround on the ceiling, one way is to attach a hand mirror to a broomstick with rubberbands.

High frequencies are the easiest to absorb and it gets harder as the frequency lowers.Most home studio enthusiasts only seem to treat the high frequencies in the room yetit is the mid and low frequencies that cause all the room problems.

Bass Trap

The next problem is bass peaks and troughs. There are two main methods for fixingthis. The best solution is not normally domestically acceptable; in that it involvesmaking sure that no walls in the room are parallel with each other. This is animportant concept and should be applied to any new construction. Making theopposing walls non-parallel stops the waves being trapped and combining (thoughremember it is not that effective with lower frequencies). This can be achieved with anout-of-square wooden frame, double-tacked with plasterboard, on two of the wallsand, ideally, on the ceiling.

A less drastic and often complimentary solution is to add a certain amount of absorption in strategic places. This works by attenuating problem frequencies,therefore reducing their summation. By absorption, I do not mean sound-absorbingwall tiles -- whilst these are great for high-frequency problems, none of them worklow enough for true bass problems. Bass absorbers, or bass traps as they are alsoknown, are available ready-made in a range of domestically friendly styles andcolours, although they tend to be expensive. There is a DIY alternative, however. Witheither of these solutions, the absorbers should be located at some point along aconvenient wall. Room corners are also good, and the key to placing themsuccessfully is, as always, lots of trial and error.

The absorbers will work over a range of frequencies, rather than at one specific

frequency, so it's therefore quite difficult to predict the exact placement and quantityrequired. Start with the absorbers around a third of the way along the wall, thenmove them and listen some more. Add more if required. Remember, we are notnecessarily after a total removal of problems, just enough to stop you reaching for


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that EQ knob!

Some general guidelines about positioning absorbers.

Proper acoustic treatment is both art and science. All rooms present uniquechallenges. Placing too much or the wrong type of material can have disastrouseffects. With acoustic design, less can be more. There is a point where unevenly oroverly treated rooms expose greater acoustic anomalies, this can be worse than notreatment at all.

In some circumstances rooms with carpet and a few pieces of furniture already areclose to sounding good. It is important to vary materials, both in thickness and performance. Applying too

much of one material will slant response to create a coloured and artificial quality.Opposing materials of different types are more efficient as the impedance of materials differs, creating greater resistance.

Often we place materials with small spaces between. There is no loss of absorptionas long as the space does not exceed the combined thickness of the absorbers, 2 thickness plus 2 thickness = 4 space. Be careful not to exceed four inches if parallel flutter problems are a concern.

Symmetry is critical. Do everything possible to make left and right side walls equalin space and quality.

The wall directly behind the speakers needs to be an area of absorption. Generallya minimum 60% covered, starting at the ceiling with the most efficient absorberyou can afford. Do this first, as this will give great benefits.

Bass energy collects in corners. Corner treatments allow for thicker materialswithout giving up valuable space. (Bass traps)

Sidewalls in front and adjacent to the mix position require 50% absorption. Place4 of absorber one foot below the ceiling on an 8 wall. If the room has splayedwalls, added shape, or generally needs to be quieter, use 5 of treatment startingat the ceiling. Continue this treatment at least 6 behind the mix position. At thispoint, you can begin to stagger materials.

Absorbers should be opposite flat walls or diffusers. The sidewalls can have lessabsorption value than front walls to vary the materials. For example, if the frontwall is 4 foam the sides may be 2 foam.

On the back wall do not use diffusion unless it is a minimum of 8 away from themix position. The type and amount of diffusion will vary greatly due to thefollowing factors: the size of speakers, dimensions, distance, aesthetics, andbudget. The back wall is an ideal place for resonators. Again, it is important tohave a different type of absorber than the one used on front or sidewalls. Inhomes, the back wall is a good place for shelves and storage. Keep space betweenitems on the shelves and vary their depth.

As a concept, I prefer hard floors (typically wood flooring) and soft ceilings. If there is carpet, a sparse array of absorption on the ceiling will help.

If the floor is hard, treat 80% of the ceiling with absorbers from the front wall to 4

behind the listener, then begin to stagger the treatment.


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Further reading

www.acoustics101.com

www.theprojectstudiohandbook.com

www.acoustics.salford.ac.uk/acoustics_world/encyclopaedia.htm

www.auralex.com

www.acousticalsolutions.com

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