slhs 305 : exam two

what is sound ?

sound is produced by an object vibrating

sound need a ___ to travel

medium

• gas

• liquid

• solid

sound travels faster by a ___ than a ____

solid , gas

compressions vs rarefication

compression : peaks of sound waves ( more pressure )

rarefications : opposite

cycle

the repetitive round trip of a sound wave from equilibrium to maximum compressions , then to maximum rarefaction , and back to equilibrium

frequency

the speed of the vibrations ( howe close the sound waves are )

higher frequency produces ___ pitched sounds

higher

amplitude

size and intensity of vibrations

• how tall a sound wave is

wavelength

distance between waves

period

time between waves

• e.g. the shorter the period , the higher the frequency of the sound

damping

the gradual loss of energy in a vibrating system

phase

where a wave is in its cycle at a specific moment

• 0 start , 90 peak , 180 midline , 270 dip , 360 end

phase relationships

• in phase : waves add together

• out of phase : waves cancel each other out

speed of sound depends on …

• temperature : hotter = faster

• elasticity : more elastic = faster

e.g. steel

• density

reflection

how sound waves bounce off a surface

reflection coefficient

• r = 1 : sound is reflected ( tiles , concrete )

• r = 0 : no sound is reflected ( carpet , foam )

absorption

how much sound energy is taken in by material

absorption coefficient

soft materials ( carpets , drapes , etc. ) absorb more sound than hard materials ( tile , rock , etc. )

reflection and absorption tradeoff

the sum of the absorption coefficient + the reflection coefficient always = 1

refraction

bending or change in direction of a sound wave as it passes from one medium to another with a different density or speed of sound

diffraction

the bending and spreading of sound waves when they encounter an obstacle and pass around it

sound intensity

the average rate of sound energy passing through a specific direction

sound pressure

the variation in air pressure cause by a sound wave relative to atmospheric pressure

reference levels

the softest sound the average human can hear

upper limits

the loudest sound the average human can hear that translates as pain

dB SPL reference value

20

dB IL reference value

10 - 12 w / m ^ 2

__ dB HL represents the softest sound a young person with no hearing loss can hear with no hearing loss can hear at any given frequency

0

0 dB HL

indicates normal hearing

positive dB HL values

a person needs the sound to be louder than normal to hear

• indicative of some degree of hearing loss

negative dB HL values

someone may have better - than - average hearing

the decibel scale is logarithmic , meaning …

• if the sound increased by 10 dB , the intensity increased x 10

• if the sound increased by 20 dB , the intensity increased x 100

• if the sound increased by 30 dB , the intensity increased x 1000

… and so forth

simple waves

stereotypical sound wave that comes to mind

complex waves

the combination of two or more simple waves

fundamental frequency

gives a sound pitch and tone

harmonics

higher frequency components in a complex sound

• occur at interfere multiple of the fundamental frequency

harmonics and fundamental frequency …

… all happen at once

periodic waves

waves are harmonically related

aperiodic

waves are not harmonically related

to find the fundamental frequency when given harmonics …

divide the harmonic frequency by its harmonic number

to find the frequency of a harmonic …

multiply the fundamental frequency by the harmonic number

time domain

how air pressure ( amplitude ) changes over time —> waveform

frequency domain

how much energy is present at each frequency —> spectrum

signal to noise ratio

a dB measurement of how the sound ( signal ) that you want to hear stand out from background noise

snr equation

signal sound - background noise

reverberation time

how quickly sound fades away in a room

reverberation time depends on …

• physical volume of a room

• surface materials of a room

reverberation time is measured by …

RT60

• the time it takes for a sound of 60 dB to die down completely

recommended classroom guidelines

• RT60 should be < 1 second ( idea is 0.6 seconds )

• SNR’s should be > or = to + 12 dB

reverberation time does these three things

1. improves speech intelligibility

2. reduces background noise

3. supports certain demographics

band - pass filter

passes frequencies that fall only within a relatively narrow range

• e.g. if someone is talking you can only hear that person

band - reject filter

passes all frequencies except those that fall within a relatively narrow range

• e.g. if someone is talking you can hear everything except that person

low - pass filter

allows lower frequencies to pass through the filter

high - pass filters

allows higher frequencies to pass through the filter

uses of high pass filters

• allows high frequencies to pass

• reduces low frequencies

uses of low pass filter

• allows low frequencies to pass

• reduces high frequencies

cutoff frequency

the frequency at which the output signal is reduced

cutoff frequency and slope interpretion

• for a low - pass filter , frequencies below the cutoff are passed , those above are reduced

• for a high - pass filter , frequencies above the cutoff are passed , those below are reduced

slope ( roll off rate )

the rate at which frequencies beyond the cutoff is reduced

slope interpretation

• a steep slope : stronger reduction of unwanted frequencies

• a gentle slope : more gradual change in south character

inverse square law

when it doubles ( e.g 3 to 6 ) always subtract 6

if shown the threshold of audibility graph

the lower the points , the higher the hearing loss

the higher the dB SNR …

the better it is

low - frequency sounds have ___ wave lengths

longer

• better at diffracting around obstacles

constructive interference

• strengthens sound

• hightens amplitudes