noise & dynamic range

8/19/2019 Noise & Dynamic Range

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INTRO: Noise is generally more obvious with headphones than

speakers and a relatively common complaint among headphone

aficionados. There’s a lot of confusion about sources of noise,

specifications, and how to make valid comparisons.

NOISE DEFINED: Technically noise is anything present that’s not

related to the desired audio signal. We usually only care about noise

within the audible range of 20 h to 20 !h. "nd within that range, the

ear is more sensitive to noise at some fre#uencies than others. The most

common audible noise is relatively random in nature and heard as a

broadband $hiss%. &ow fre#uency hum at power line fre#uencies is also

sometimes audible. "nd digital devices, especially computers and

mobile phones, can generate noises at specific fre#uencies that are heard

as whines, chirps, clicks, bues, etc.

SOURCES OF NOISE: Noise can, and often does, invade the signal

chain in audible ways starting at the microphones used during recording.

'ere are some common sources(

• Recordings ) *icrophone preamps and other gear used during

recording often have audible noise. +ut lots of techni#ues are usedto reduce the audibility of such noise. Noise gating, for eample, is

used to cut noise when there’s no sound from a given microphone

or instrument. Nearly all recordings before the early -0’s were

mastered on analog tape which has significant amounts of tape

hiss. "nd even digital recordings can have noise from all the

electronics in the signal path. "nd, of course, vinyl has lots of

noise.

• DAC ) n theory a perfect / bit 1" has a 3 d+ signal4to4noise

ratio but some fall well short of full / bit performance. 25 bit

1"s often only manage approimately / bit performance and

the very best reach 2/ bit 67N8+9 performance. This is especially

true of 1"s inside a computer. :ome 1"s also produce

significant amounts of their own noise such as as modulation and

http://en.wikipedia.org/wiki/Noise_(electronics)http://en.wikipedia.org/wiki/Effective_number_of_bitshttp://en.wikipedia.org/wiki/Effective_number_of_bitshttp://en.wikipedia.org/wiki/Noise_(electronics)


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#uantiation noise 6although these can also be considered forms of

distortion as they are only present with a signal9.

• Headphone Amp ) 7ven a laptop or portable player has a

headphone amp in it although it might be built into the same chipas the 1". "ny amplifier adds noise, it’s ;ust a #uestion of if it’s

audible or not. 7ven some fairly epensive stand4alone headphone

amps can have significant amounts of noise. They can also further

amplify whatever noise is $upstream%.

• Noise is Cumulatie ) While sometimes there’s an obviously

dominant source of noise it can ;ust as easily be a little from here

and little from there. Noise adds up.

NOISE !EASURE!ENTS: There are two basic kinds of noise

measurements. 8ne is a an absolute measurement of ;ust the noise and

the other is a measurement of the noise relative to some known signal

level. The decibel 6d+9 was partly developed as it more closely follows

sub;ective human hearing. " one d+ change in level is about the smallest

change most people can detect. " /0 d+ change is perceived as being

roughly twice as loud 6or soft9. f


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+oth are reasonable listening levels for many full sie headphones

such as the :ennheiser '100.

• Signal to Noise Ratio #SNR or S$N% ) This is a more open ended

method where both the noise figure and the reference signal levelmust be provided for it to be a meaningful number. The correct unit

is d+r where the $r% means $relative% but it’s often ;ust given in

d+. @nfortunately, many manufactures don’t specify the reference

level. When ;ust :N> is specified with no reference you should

assume it’s referenced to whatever the absolute maimum output

level is for the device44the same as a 1ynamic >ange

measurement. :adly, that’s often not specified either 6see( *ore

Aower 9.

• &olts s d' s d'u s d'r ) *easuring noise in volts only

works for absolute noise measurements. *easurements in d+v are

referenced to / volt which makes the math much easier and they’re

commonly used in professional audio. 0 d+v B / volt. n consumer

e#uipment d+u is more common and referenced to 0.==? volts

making the math more awkward. *easurements in d+r can be

referenced to anything including each other.

D(NA!IC RAN)E: "s eplained above, 1ynamic >ange is really the

same as the :ignal4to4Noise >atio 6:N>9 using the maimum possible

signal. t’s the ratio between the loudest undistorted output of the device

and what’s left over when nothing is playing and is usually a positive

number instead of a negative one. The theoretical dynamic range of /

bit digital audio is 3 d+ so that’s often used a benchmark for dynamic

rangeCideally you don’t want the playback hardware to be worse than

the recording format. With higher output gear it’s not uncommon to seedynamic range measurements well above that value so it’s not an

unrealistic target. :tudies, such as the one conducted by *eyer and

*oran, have shown 3D d+ of dynamic range is transparent for any

normal listening conditions. The only way to epose the noise floor is to

crank up the volume to unrealistic levels. @sing a digital 6software9

http://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://nwavguy.blogspot.com/2011/09/more-power.htmlhttp://nwavguy.blogspot.com/2011/09/more-power.htmlhttp://en.wikipedia.org/wiki/Dynamic_range#Audiohttp://en.wikipedia.org/wiki/Audio_bit_depthhttp://en.wikipedia.org/wiki/Audio_bit_depthhttps://secure.aes.org/forum/pubs/journal/?ID=2https://secure.aes.org/forum/pubs/journal/?ID=2http://en.wikipedia.org/wiki/Signal-to-noise_ratiohttp://nwavguy.blogspot.com/2011/09/more-power.htmlhttp://nwavguy.blogspot.com/2011/09/more-power.htmlhttp://en.wikipedia.org/wiki/Dynamic_range#Audiohttp://en.wikipedia.org/wiki/Audio_bit_depthhttp://en.wikipedia.org/wiki/Audio_bit_depthhttps://secure.aes.org/forum/pubs/journal/?ID=2https://secure.aes.org/forum/pubs/journal/?ID=2


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volume ahead of a / bit 1" and leaving the volume after the 1"

cranked way up may epose the / bit noise floor. n these applications

//0 d+ of dynamic range should be sufficient to keep the noise below

ambient levels.

&O*U!E SETTIN): There are some interesting twists with volume

settings some of which are not intuitive(

• Upstream Noise 4 "ny noise that’s $upstream% of the volume

control will be more audible as you turn the volume up assuming

the music doesn’t mask it. The absolute noise is worse at higher

volume settings but the :N> stays about the same because you’re

also increasing the signal by the same amount as you turn up the

volume.

• Ampli+ier Noise ) 1epending on where the volume control is

located within the gear it may or may not significantly alter the

noise. " digital volume control, for eample, will only affect the

noise in the recording itself 6and not change :N> at all9.

nterestingly some devices with analog volume controls have the

most noise at half volume— such as the Eii8 73. This is usually

because you’re hearing the Fohnson Noise of the volume controlitself where half volume is the worst case situation. This is typical

when the volume control is before the gain stage. When the

volume is after the gain stage, most everything becomes @pstream

Noise 6see above9 and is reduced at lower volume settings.

• Fi,ed Noise ) "mps have a certain amount of noise that’s present

at any volume setting. This is usually noise that’s from the circuitry

after the volume control and, in a properly designed amp, it’sentirely possible to have it always be inaudible.

-ORST CASE NOISE AUDI'I*IT(: :ome define audible noise as

anything you can hear under worst case conditionsCi.e. nothing

playing, the worst case volume and gain settings, a very #uiet room, and

http://nwavguy.blogspot.com/2011/05/fiio-e9-headphone-amp.htmlhttp://en.wikipedia.org/wiki/Johnson_noisehttp://nwavguy.blogspot.com/2011/05/fiio-e9-headphone-amp.htmlhttp://en.wikipedia.org/wiki/Johnson_noise


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using etremely sensitive headphones. "n easily accepted guideline is )

3 d+ un4weighted referenced to a realistic maimum listening level

6see 1ynamic >ange above9 as that’s the maimum dynamic range of /

bit digital audio. :o whatever level produces around //0 d+ peak :A&

6see my Aower article9 should be the reference value and as long as thenoise is about 3 d+ below that it will be entirely inaudible. That’s

achievable with less sensitive headphones but difficult with ultra

sensitive 7*s. :ome eamples

• HD.// ) 2.G H for //0 d+ gives G uH or )-- d+v of noise

• )RADO SR0/ ) 0.= H for //0 d+ gives // uH or )33 d+v of

noise

• U1E1 TripleFi 2/ ) 0./ H for //0 d+ gives /. uH or )// d+v of

noise

3RACTICA* NOISE AUDI'I*IT(: n reality, testing shows that -?

d+ below //0 d+ :A& is sufficiently #uiet for most people 6noise of 2?

d+ :A&9. That puts the limit at )/0? d+v, 64/02.- d+u9 or ?. uH for

sensitive 7*s. With the most sensitive 7*s in a really #uiet room

someone might still hear some noise at that level, but being realistic, it’slikely $good enough%. f you want to be assured of silence with even the

most sensitive 7*s, aim for )//0 d+v 64/0=.- d+u9.

NOISE 4 )AIN: 'eadphone amps have varying amounts of gain44the

maimum amount they can amplify the input signal. :ome have multiple

gain settings. The higher the gain the more they will amplify upstream

noise. "nd, typically, the higher the gain the higher their own noise. This

is one reason you ideally want to use the lowest amount of gain re#uired.

:ee( "ll "bout


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• Noise d' &olume 2//6 ) The 82 measures )//2 d+v un4

weighted and )//? d+v "4Weighted. This is well below the )/0?

d+v guideline and means the 82 will be silent in use.

• SNR Re+erenced to Full Output ) The 82 referenced to = volt>*: 6full output9 measures )/G0 d+r unweighted and )/GG d+r

"4Weighted. These numbers are etremely impressive but also

unrealistic for most users who will never need even close to =

Hrms of output.

HEAD3HONE SENSITI&IT(: 'eadphones vary widely in their

sensitivity. *any assume a headphone that’s /0 d+ more sensitive will

make the :N> /0 d+ worse but that’s often not true. "s headphones become more sensitive, you need less gain, andIor use lower volume

settings. +oth of those typically lower noise :o the ratio of the signal to

the upstream noise, and hence the :N>, stays about the same. 8nly

fied noise 6see above9 is directly related to the headphone sensitivity.

Fohnson Noise from the volume control can complicate this a bit but as

headphones become more sensitive the fied noise becomes much more

important. :ee Noise "udibility Worst ase above for eamples of three

different headphones.

NOISE S3ECTRU!S: :ometimes you will see a spectrum graph for

noise measurements. The approimate $noise floor% in these graphs is

much lower than the actual noise specification. n the graph to the right

the overall noise is about )//2 d+v but the noise floor is down around )

/?0 d+v in the graph. This huge difference is because the )//2 d+

number is the sum of all the noise from 20h to 20 !h. Think of

spreading a cup of sugar out across the floor. t would barely change the

height of the floor. +ut if you gather all the sugar up in a measuring cup,you can know how much total sugar there isCmuch like the noise

measurements shown in the boes in the graph. lick the graph for a

larger version.


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NOISE 'AND-IDTH AND

-EI)HTIN): Typically noise is the sum

of all energy within the audio band. deally

the bandwidth is specified for un4weighted

measurements. "4Weighting is often usedwhich ad;usts the measurement for the

relative sensitivity of the ear at different

fre#uencies and also limits the bandwidth.

"nother weighting standard is T@4> 5-. Eor gear that’s prone to a lot

of out4of4band ultrasonic noise, such as lass41 amplifiers and digital

e#uipment, a wideband noise measurement up to about /00 !h can also

sometimes be useful in addition to weightedIlimited measurements.

CO!3ARIN) NOISE !EASURE!ENTS: Jou can only directly

compare noise measurements given in d+u, d+v, or d+r at the same

reference level. "nd they must use a similar bandwidth and all be either

unweighted or weighted the same way. 8therwise, you can’t compare

the numbers without at least doing some math and sometimes you can’t

compare them at all. 'ere are some eamples(

• R!AA 7 @nfortunately >*"" has no concept of absolute levels.

:o it can’t calculate noise levels referenced to any known value. tattempts to calculate dynamic range against 9 d+E: 6the clipping

level of the 1" itself9 but even that is sub;ect to wide variations

in the device settings 6i.e. volume, $record% level, etc.9, calibration

settings, etc. +asically >*"" noise measurements are nearly

worthless and the noise of the A sound hardware might be worse

than whatever you’re trying to measure anyway. :ome >*""

results are comparatively arbitrary and this is one of them.

• d' to d'r ) f


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• d'u to d' ) These are close. To convert from d+v to d+u the

noise is 2.2 d+ worse. To convert the other way it’s 2.2 d+ better.

• d'r #8// m&% to d' ) updated my own noise measurements

from d+r referenced to 500 mH to d+v 6referenced to / volt9. Toconvert the old 500 mH measurement to d+v the noise improves

by - d+. To convert the other way, it’s worse by - d+.

• )eneric Conersions ) The generic math for the amount to add or

subtract is 20 K &og6 Href/ I Href29. The lower the reference

voltage the worse the noise figure. Noise can also be referenced to

power instead of voltage. n that case it’s /0 K &og 6 Aref/ I

Aref2 9.

o d+v to Holts B antilog6 d+v I 20 9

o 43 d+ in Holts B antilog 6 )3I20 9 B / uH 6 0.0000/ volts9

o Holts to d+v B 20 K log 6 Hnoise 9

• -eighting Comparisons ) t’s impossible to accurately compare

different weighting or weighted vs un4weighted as it depends onthe fre#uency distribution of the noise. "n amp with a lot of hum,

for eample, will have a proportionately lower weighted

measurement than one with only uniform hiss. n general,

however, epect an "4Weighted measurement to be about G to d+

better than an un4weighted measurement.

SOURCE I!3EDANCE: Fohnson Noise is often a dominant source of

noise in headphone amps and preamps. "nd it’s proportional to the

impedance of the input circuitry which includes the source. The higher

the source impedance, the higher the noise. :o, for eample, a given

headphone amp might be dead silent when driven from a source with a

/00 ohm impedance, but using a source with a /0! impedance could

easily produce audible noise. In this case the noise you’re hearing is

really coming from the upstream source not the amp.

http://en.wikipedia.org/wiki/Johnson_noisehttp://en.wikipedia.org/wiki/Johnson_noise


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!EASURIN) NOISE: +ecause noise measurements are a sum of the

noise across the audio band, and etremely low in value, they’re tricky

to measure accurately. The best high4end 25 bit A sound hardware may

have a low enough noise floor, but often cannot accept the full output of

the device being tested. "nd more significant, A sound hardware has noway to set or measure absolute levelsCi.e. measurements in volts, d+v,

etc. Hery few 1igital *ulti *eters 61**s9 have the resolution and low

enough internal noise to measure accurately down to a few microvolts of

" from 20 h to 20 !h. t is, in theory, possible to temporarily

calibrate a 25 bit soundcard using a known accurate meter and suitable

test tones. +ut it’s tricky to do accurately and apply to whatever software

is being used. The source impedance is also an issue. *anufactures tend

to short circuit the inputs for the best noise number, a more realistic testis to use a shunt resistance e#ual to the output impedance of a typical

source. f you try to use a real source, its noise will also be included in

the measurement 6as with >*""9. "lso, when measuring a source with

a 1" it’s necessary to use a very low level test signal as 1"s shut off

completely giving an unrealistic noise value if there is nothing to play. "

proper audio analyer can remove the low level signal from the

measurement leaving ;ust the noise.

R!AA !EASURE!ENTS( 7ven if you somehow calibrate the levels,

you still don’t know what >*"" is doing internally. t’s a magic $black

bo% with no credible documentation about how it arrives at its final

numbers. What bandwidth is being usedL s the result weighted or un4

weightedL Alus the unknown output noise of the >*"" sound

hardware is included in the measurement by design. @ltimately, the best

way to make noise measurements is with an audio analyer such as those

from "udio Arecision and Arism :ound.

'OTTO! *INE: Noise of around )/0? d+v 6referenced to / volt9 will

nearly always be inaudible. Noise around 43? d+v is probably $good

enough% for many. Noise referenced to other values must be converted

to d+v or another consistent reference before it can be fairly compared.

>*"" values are nearly useless because >*"" has no concept of

http://nwavguy.blogspot.com/2011/02/rightmark-audio-analyzer-rmaa.htmlhttp://nwavguy.blogspot.com/2011/02/rightmark-audio-analyzer-rmaa.html


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absolute levels. t can only provide dynamic range and it often gets even

that wrong because it’s difficult to set the levels properly without proper

instrumentation.

noise & dynamic range

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