Acoustics 3 What is Music?

What is a musical note?

A musical note is determined by having repeating cycles of frequency (a). Noise on the other hand is a complex amalgam of the ripples caused by the

disruption of air molecules (b).

Movement of Guitar Strings ! Before a string is plucked it is in a stable state.

! During plucking the string is stretched before it is let go, after which it tries to get back to its previous state. However it is moving too fast to stop so stretches itself in the other direction.

! It tries again to get back to a straight line but again overshoots, and this continues until it runs out of energy.

! Energy is lost because it is pushing air out of the way. It is also passing vibrating energy onto the body of the guitar.

! Simple enough But dont expect the strings to move to and fro in a perfect curve

Get Plucky ! If a string vibrated to a fro from a perfect curve we would hear its

fundamental frequency.

! For this to happen it would have to start from a curve. When we pluck a string, however, we are creating a kink in the string where two straight lines meet.

! So all parts of the string will race off with energy at the same time, but not from the same starting place.

! The string now starts to vibrate in several ways at the same time: imagine a childs swing in action and the child wiggling the part of the chain he or she is holding. The chain is now doing two things a) swinging slowly to and fro and b) wiggling up and down to a fro.

! A guitar string obeys multiple orders simultaneously, so long as each end is attached to a guitar and cannot move.

A complex waveform is created when there is a combination of string patterns happening at the same.

No half measures

! With the ends of the string stationary the movement pattern of the string must divide into one part or two parts, or three in fact any whole number of parts.

! There would not be four and a half parts, for example, as this would require one of the ends of the strings to waggle about which it cant. Its attached to the guitar.

! So the string does not move two and fro as one long string, it involves complicated wiggling. The whole string movement of the guitar is accompanied by some half string, third string, and quarter string vibrations (and others).

Different harmonics depending on where the guitar is plucked

The Fundamental frequency is also known as the first harmonic. The next one half its size is the second harmonic and is twice its

frequency.

strings vibrate in a complex dance

! Short strings vibrate at higher frequencies than long strings (if they are the same type of string under the same amount of tension).

! If you halve a string it will double the frequency.

! When a string is plucked we effectively hear shorter strings. The halves, thirds and quarters etc will all vibrate in their own ways but will always return to base at various intervals.

! These overall pattern of these movements will repeat at the same rate as the lowest frequency involved. This is the note we hear: the fundamental (all the other frequencies we hear are kind of like backing

singers supporting this note, giving it richness and uniqueness).

! Drums and cymbals provide a rhythmic drive to much of the music made today. Why is it vital that these instruments provide a noise and not a note?

drummers are noisy

! If a drummer banged along with two notes all the time, these notes would dominate the tune. (Unless you want them to, like orchestral tympani).

! A thud or a tschhh will provide rhythmic information without hijacking the music.

! Drums are columns so the only way to prevent musical notes is to tune its two skins differently. This will mean the skins of say a kick drum will not be mutually supportive of the pattern of air pressure produced when struck. This lack of mutual support between the skins also means the sound dies away more rapidly if they vibrated in unison they would ring on for longer.

Nearly everything we hear is a complex combination of multiple frequencies.

Fletcher Munson Curves

Loudness, being a psychological quantity, is difficult to measure, so Fletcher Munson averaged their results over many test subjects to derive reasonable averages.

The lowest equal-loudness contour represents the quietest audible tone and is also known as the absolute threshold of hearing. The highest contour is the threshold of pain.

Equal-loudness contours were first measured by Fletcher and Munson using headphones (1933).

In their study, listeners were presented with pure tones at various frequencies and over 10 dB increments in stimulus intensity.

For each frequency and intensity, the listener was also presented with a reference tone at 1000 Hz. The reference tone was adjusted until it was perceived to be of the same loudness as the test tone.

Equalisation (EQ) What is an equaliser?

A device, circuit or piece of software that lets us control the relative amplitude of various frequencies within the audible bandwidth.

What does it allow us to do? - Correct specific problems in a recorded sound (possibly to restore a sound to its natural tone). - Overcome deficiencies in the frequency response of a mic or in the sound of an instrument. - Allow contrasting sounds from several instruments or recorded tracks to better blend together in a mix. - Alter a sound purely for musical or creative reasons.

There are two main types of EQ: Bell (or Peak) EQ and Shelving EQ

Shelving EQ refers to a rise or drop in a frequency response at a selected Frequency which tapers off to a preset level and continues at that level until the end of the audio spectrum.

The Bell (Peak) EQ is the most common EQ. It is created by a peaking filter and as its name implies a peak-shaped bell curve can either be boosted or attenuated around a selected centre frequency.

A Semi Parametric EQ is a Bell (Peak) EQ without the Q (quality factor). Therefore the bandwidth of the bell curve will remain constant.

A Parametric EQ does have the ability to widen or narrow the selected bandwidth.

Using the Q to widen the quality factor.

Using the Q to narrow the bandwidth.