(39) Week 5

• A sound wave is made when force or pressure from a vibrating object sets molecules in air, into motion. They end up moving back and fourth from their normal equilibrium position.

  so sound is just vibrations
• the movement of molecules determines the frequency and amplitude of a sound. this makes the sounds specific pitch and loudness
• frequency- the rate of movement of molecules. measured in cycles per second - AKA HERTZ

  one of the cycles shown below is what is meant by frequency

  A) frequency

  
• frequency decides the pitch of a sound.

  a normal human can hear 20-20000 hz

  amplitude determines the loudness of a sound

  a normal human van hear sounds as low as 0 dbSPL and 120 dbSPL
• the amplitude of a sound is determined by the maximum displacement of the air molecules. The displacement leads to pressure fluctuation in the air - expressed in dB SPL (decibel sound pressure level)

  
• sound in air- 340m/s

  water- (more dense, more closely packed molecules ) 1500m/s
• infrasonic- below 20Hz

  ultrasonic- above 20000Hz

  this used to be called subsonic and supersonic but those words are closer in meaning to speed rather than frequency, so we use these words instead

  when sounds are below 20hz they might be felt rather than heart at a high intensity e.g. bass
• a device which takes sound and represents it as a wave is called an oscilloscope. !
• longer wavelengths means lower frequency and pitch. shorter wavelengths means more frequency and pitch.
• acoustic impedance makes it hard to sound waves to switch from travelling through air and then water. the impedance changes between the 2 medias.

  -The impedance of the air and the fluid filled cochlea is very different. that's why we need ossicles. they change the impedance. the ossicles basically increase the pressure
• a normal conversation with someone thats like 1m away has the pressure of 2mPa (millipascal)

  pressure is higher when ur closer to someone- which makes u perceive it as louder

  explained by the inverse distance law- pressure is inversely proportional to the distance from the source

  
• the pressure value of like- speaking, or breathing or anything is VERY low compared to the pressure from the atomosphere.
• the ambient noise floor is all the surrounding sounds which drown out quiet sounds like e.g. breathing. sounds of lower pressure are drowned out by sounds of higher pressure
• 100 pa at your ear would hurt
• we are able to disturb the atomospheric pressure of over 100k Pa through short bursts of sounds. the largest possible sound is when it meets atomospheric pressure. after that it becomes a shock wave rather than a sound wave. that's how explosives are made
• temperature builds up with inceased pressure/pressure differences between soundwave and atomospheric pressure, but sound waves are adiabatic- theres not enough time for heat transfer to take place. this means heat isn't exchanged with the air from a soundwave.
• db is on a logarithmic scale. this means increases in db arent linear/proportional. like a graph that steeply rises.

  
• so any db above 0 is after the human hearing threshold. any db below is before the hearing threshold. everything is measured against the HEARING THRESHOLD FOR HUMANS.

  the human hearing range is also large so that is another reason its easier to use a log scale
• A sound wave

  
• pure tone vs complex

  a sine wave of a (constant freq and amplitude) is known as a pure tone

  real world has like bg noise and stuff

  
• Fourier analysis

  breaks down repetitive waveforms into a series of sine waves with appropriate amplitudes and phases. so breaks down complex.

  you dont need to know the equation yet
• psychophysics:

  study of perceptual responses to physical stimuli- uses behavioural measures

  --psychoacoustics: responses to acoustic stimuli

  e.g. audiometry, uncomfortable loudness-level measurement, speech discrimination
• absolute threshold- value of absolute stimulus magnitude that creates a response (detection threshold)

  discrimination threshold- smallest diff between 2 stimuli that can be detected
• hearing

  A) 20000

  B) 20

  C) 17

  D) 15

  E) 3