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 sound is pressure waves. it propagates based on pressure difference

longitudinal wave, amplitude of oscillation is in direction of propagation

human hearing range 20-20khz

female higher frequency , so it reaches longer distance 

sound is perceived when vibration in air is detected by ear. ear converts vibration to electrical impulse

impulses are transmitted to brain by nervous system and brain detects pitch,timber and loudness


sound travel by compression and rarefaction of air molecules


v -= sqrt(T)

T is in kelvin




at 20 degree 344 m/s



Lp=20 log (p/p0)

p0=refrence pressure 2*10^-5

 which is often considered as the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). 


w0=10^-12 W


i0=10^-12 W/m^2



Intensity of sound

unit W/m^2

threshold of hearing intensity = 1 pico W/m^2


threshold of pain = 10 W/m^2 , pressure 63 N/m^2



loudness of sound intensity in db over threshold of hearing at 1000hz is phon


used to determine increase in loudness

loudness sensation produced by 1000hz sound of 40db  is 1 sone


10logL = (p-40) log2 

 p is loudness in phon

 L is loudness in sone


frequency of sound 

in speech and music, pitch depend on intensity , unit is MEL

1 MEL is tone of 1000 hz and 40db intensity

relation between pitch and frequency is non-linear

higher frequency lower loudness, 

human need higher intensity to listen to low frequency and lower intensity to listen higher frequency


tonal characteristic of sound

we distinguish sound based on timbre

our voices also have different timber

timber is defined as per sets of different frequency proportion 

overtones and harmonics

frequencies other than fundamental frequencies

harmonics is integral multiple of fundamental frequency

interval and octaves

ration between two frequencies is interval

interval = f2/f1

octaves = log(f2/f1)/log2

 natural frequency : all sound transducers wit moving parts have frequency based on mass and compliance


f=1/[2*PI* sqrt(LC)]

 f=1/2PI * sqrt(K/M)

M=L , K=1/C, X=Q, V=I, F=V


diaphragm has natural frequency

lower frequency is attenuated due to spring

higher due to mass 


sensitivity : how weak sound signal is detected by microphone

o/p of mic (in mv)  for sound pressure ( 0.1 pa)  at 1000 hz

normal pressure has 0.1 pa 

some manufacturer given sensitivity in dbm i.e db milli volt


represented in terms of sound pressure level(SPL)

ratio of db of output (with SPL of 0.1 pa) to output in absence of sound 

bandwidth of audio within +/- 1db is frequency response

40 hz - 15k hz flat responce winthin +- 1db is hifi audio

80-8k hz for normal mic

carbon,crystal,condenser,moving coil,ribbon microphone


non-linear : distorted harmonics in amplitued

for hq less than 5 , for hifi less than 1


multiple mic with diffrent path of source 



E = actual o/p

E0 = o/p of omnidirectoinal mic


must match output impedance for max power transfer

moving coil mic 

dynamic microphone , based on em induction

Sensitivity 30uv for 0.1 SPL

snr 30db

freq response 60-8k hz for +-1db

distortion less than 5%



robust,heavy,no bias,low cost,25 cm distance


use for PA(public address), broadcast

 poor high freq response due to heavy coil

ribbon microphone

use lightweight aluminum ribbon as diaphragm and coil assembly

corrugated ribbon = having grooves like bump

horse shoe magnet

freq response=  20-12k hz

 reflectivity = bidirectional, no signal at 90 degree

lite ribbon so careful

wind will make noise

excellent transient response

no external bias 

high cost


crystal microphone 

piezoelectric effect

pressure difference on crystal produce potential difference

 quartz,tourmaline,Rochelle salt, ceramic

rochelle salt : high piezo but temp 50degree moist

quartz and tourmaline :  low piezo

ceramic : suitable as high temp and moisture sustain

Sensitivity 50uv for 0.1 SPL

snr 40db

freq response 100-8k hz for +-1db

distortion less than 1%


 op impedance 1 Mhz

can be spoken at close range

high impedance

no need bias

long direct sunlight damage crystal

low cost

capacitor or condenser microphone




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