section 1 - the physics of sound

7/23/2019 Section 1 - The Physics of Sound

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The Physics of Sound

Sound is simply the disturbance of molecules. When an event occurs through amedium (anything that carries sound – e.g. water, air etc.), their restful state

forces them to vibrate, causing a disturbance known as sound. When thedisturbance is in process, molecules are compressed together and shoot o inevery direction, making sound omnidirectional. hese disturbed molecules willthen bump into one another and spread out in compressed waves.

Sound Waves

! sound wave is the completion of a compression and a rarefaction cycle."ompression happens when molecules are forced or pressed together. #owever,

rarefaction is the opposite, as it occurs when moleculesare given e$tra space and

allowed to e$pand. !s the

molecules are pressed against oneanother, they pass the

kinetic energy to each other –giving reasoning for why sound energytravels outward from the sound source.

%t is important to remember, that when a molecule to the right of the diaphragmis e$periencing compression, a molecule to the diaphragm&s left is e$periencingrarefaction. Sound waves, like water waves, repeat in succession with the laterwaves getting weaker and weaker as time progresses.

Waveforms

! waveform is the shape of a signal moving in a physical medium, in order to becarried by sound. %n many cases, the medium in which the wave is beingbroadcast does not permit a visual view of the form, referring to the shape of agraph of the varying 'uantity against time and distance. !n instrument called anoscilloscope can be used to represent a waveforms repeating image onscreen.

any programs show waveforms to give the user a visual aid of what hasbeen recorded by imagery. %f the waveform is low, the recording was most

likely soft. *n the other hand, if the waveform is large,the recording may have been recorded with high

levels. Waveforms can vary, asthey may be small when a vocalist is

performing, but may becomemuch larger when the drumsand guitar starts to come in.

+re'uency, !mplitude, Speedof sound and nvelope

+re'uency is the number of wave cycles that occur within aperiod of a second. *urhearing mechanisms are onlycapable of perceiving afre'uency range of - cycles

per second up to -, cycles per second. his de/nes the lowest bassfre'uencies and the highest treble fre'uencies we can perceive. +re'uency is



also measured in #ert0. #owever, it is common to use kilohert0 (k#0), megahert0(#1), and gigahert0 (2#0) to be used with high fre'uencies. %n reality, mostpeople do not have the capability to perceive sound at this full range, and nor domost people need to. he human voice occupies a range that is well within thoselimits – these are the fre'uencies that we are most sensitive to, as they de/neour ability to communicate though speech. 3eople who work with music andrecording and really train their ears are able to keep this e$tended fre'uencyresponse, as long as they don&t abuse their hearing mechanism. %f sub4ected toloud or hurtful volumes on a consistent basis, those capabilities can bepermanently lost.

Wavelength and the speed of sound are the basics audio termsthat are dependent on each other. he length of a givenfre'uency wave is dependent on the speed atwhich the sound wave travels. he speed at whichsound travels is dependent on thetemperature where the sound wave is occurring.

5ower fre'uencies have a longer wavelength, whereashigher fre'uencies have a shorter wavelength. his isdetermined by some simple math. easure how far soundtravels in 6 second and divide that distance by the number of cycles that happenin that same e$act second.

Sound travels at a rate of 667 feet per second at 8 degrees +ahrenheit (whichis also 797 metres per second). %f you want to know the length of a 6 hert0sound wave, you must divide 6 into 667 and you will get 66.7 feet. his

shows the distance that it takesfor a 6 hert0 waveform tocomplete one compression and

rarefaction cycle. :ou can also usethe same math to /nd out whatfre'uency is 6 feet long bydividing 6 feet into the speed of sound. 667 divided by 6 e'ualsis 667 #ert0. !ll this math is basicand very important for thephysical design of recording

studios and the acoustic materials that are used to control those fre'uencies in arecording space.

!n envelope is a smooth curve outlining its e$tremes. he envelope generali0es

the concept of a constant amplitude.

3hase

! phase has sinusoidal functions – this being two dierent but closely relatedmeanings. *ne of the initial angles of a sinusoidal function is called phase osetor phase dierence. !nother usage is the fraction of the wave cycle that hasintervened relative to the origin. 3hase oset modulation works by overlayingtwo instances of a periodic waveform on top of each other. his e$presses thedierence in degrees or time, between two waves having the same fre'uencywhen referred to at the same point in time. wo oscillators that have the samefre'uency and no phase dierence are said to be in phase. wo oscillators that

have the same fre'uency and dierent phases have a phase dierence, and theoscillators are conveyed as out of phase with one another.



#armonics

! harmonic is a signal or wave whosefre'uency is an integral multiple of the fre'uency of some

reference signal or wave. heterm can also refer to the ratio of the fre'uencyof such a signal or wave to the fre'uency of thereference signal or wave. his term is employedin various disciplines and is typically appliedwhen considering the fre'uencies of repeatingsignals that happen to relate as wholenumbered multiples. he harmonics have theproperty of being all periodic at the fundamentalfre'uency, therefore the sum of harmonics isalso periodic at that fre'uency. !s multiples of the fundamental fre'uency, successiveharmonics can be found by repeatedly adding the fundamental fre'uency. anyacoustic oscillators produce comple$ tones that are more or less periodic andthus are composed of partials that are near matches to integer multiples of thefundamental fre'uency and therefore resemble the ideal harmonics and arecalled ;harmonic partials< or simply ;harmonics< for convenience. *scillatorsthat produce harmonic partials behave like onedimensional resonators, and areoften long and thin.

he table below displays the stop points on a stringed instrument, at whichgentle touching of a note will force it into a harmonic mode when vibrated. %t isunusual to encounter natural harmonics higher than the /fth partial on any

stringed instrument e$cept the double bass on account of its much longerstrings.

=ecibels

he decibel (d>) is a logarithmic unit used to e$press the ratio of two values of aphysical 'uantity, often power or intensity. *ne of these values is often astandard reference value, in which case the decibel is used to e$press the levelof the other value relative to this reference. he number of decibels is ten timesthe logarithm to base 6 of the ratio of two power 'uantities, or of the ratio of the s'uares of two /eld amplitude 'uantities. *ne decibel is one tenth of one bel,named in honor of !le$ander 2raham >ell? however, the bel is seldom used.

oday, the unit of measurement is most prominently in acoustics, electronics,and control theory. %n electronics, the gains of ampli/ers and signaltonoiseratios are often e$pressed in decibels. ! change in power by a factor of 6corresponds to a 6 d> change in level.



>y !kai.

section 1 - the physics of sound

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