## Hearing — The Ear

• Physics of hearing

• Note that the ear detects frequency directly

• The measured sound amplitude is logarithmic in the sampled sound power: big differences at low power, small differences at high power

• Thus usually use units of dB instead of linear power

P[dB] = 20 log (P[rms] / 10^-12)

• C.f. "type A" and "type B" potentiometers as "volume knobs"

## Hearing — Psychoacoustics

• The brain does things with sound…

• Perceived volume of a sound is a function of

• Background volume level: individual sounds seem less loud in a noisy environment

• Waveshape: in particular, the brain will "fill in" "clipped" waves as though the peaks exist. This is used by broadcasters to make sounds seem louder than their measured power

• Perceived frequency of a sound is

• Relative: "perfect pitch" is rare; pitch is mostly judged by relation to surrounding / background / earlier pitches

• Dominated by high frequency components: dbA frequency weighting is used as an estimate of perceived loudness

• Accomodation over time, etc happens

• Shepard Tone is an interesting audio illusion

## Hearing — Safety

• Easy to damage hearing permanently

• Sound exposure is cumulative: long exposure to a moderately loud sound is more damaging than short exposure to a somewhat louder sound

• NIOSH: More than 85 dBA (heavy city traffic) over 8 hours is hazardous

• Acute trauma over 130 dBA

• Not just general hearing loss: notching, tinnitus

• How to stay safe:

• Wear ear protection around sustained loud noise: concerts, machine rooms, etc. Wear noise-cancelling headphones on airplanes

• Avoid earbuds, as they are prone to hearing damage. If you use them, turn them down to below where they sound best: the ear / brain don't know how to deal with them

• Turn down master volume before powering up any audio equipment. Then raise the volume to a comfortable level

• Put headphones on your neck to start; you should hear silence. Then put them on and turn up the volume

• See above when working with mute buttons, plugs and jacks, etc. So easy to make a mistake

## Hearing — Two Ears

• Normally hear in stereo; thus two-channel audio with separated sources (headphones, left and right speakers)

• Means two audio channels to deal with: stereo is sometimes encoded as sum and difference channels, with the difference channel at lower fidelity

• Localization in space is a function of time difference and level difference between ears

• Angle:

• Phase is used below 1KHz

• Head interference and group delay is used above 1.5KHz

• This is part of what the "ears" (pinna) do

• Distance:

• Softer, lower-frequency sounds seem farther away

• Room effects (reflection) increase perceived distance