Hearing

Ear Mechanics  The ear is collects and amplifies sound.  Initial receiver is the eardrum. Sensitive to tiny pressure changesSensitive to tiny pressure changes  Middle ear transmits the sound through bone. Act as levels magnifying force by 40Act as levels magnifying force by 40  Inner ear (cochlea) separates pitches. Breaks up superposed wavesBreaks up superposed waves Accurate to better than 1% (0.1% with training)Accurate to better than 1% (0.1% with training)

Frequency Range  The human ear is sensitive to sounds from 20 Hz – 20 kHz.  Maximum sensitivity is from about 3 kHz to 5 kHz with 120 dB range.  Sensitivity is much worse at low frequencies. Loudness control for audio equipment boosts bass and treble.

Threshold of Pain  At 120 dB most people feel pain. Rock concert at 4 m. Jet aircraft at 150 m.  Eardrums burst at 160 dB.

Speaker Systems  The sound from a source reaches each ear at a separate time. Brain uses the time difference to tell direction.Brain uses the time difference to tell direction. Hard with short wavelength.Hard with short wavelength.  Stereo speakers reconstruct timing differences. Constructive and destructive interferenceConstructive and destructive interference

Beats  When two wavelengths are at nearly the same frequency the superposed waves create beats. The separation of peaks is the period T = 1 / f The difference in frequencies is the beat frequency  f. The beat period T = 1 /  f.

In Tune  A 440 Hz-tuning fork is held near a vibrating piano string. The tuner hears 20 beats in 5 s.  What is the pitch of the piano string?  Will the tuner ever get it tuned perfectly?  The beat frequency is 20 beats / 5 s = 4 Hz.  The piano string could be either at 444 or 436 Hz.  Note: a beat period of 1 min would get a frequency accurate to Hz. next