Transcript Hearing

Hearing
Ear Mechanics

The ear is collects and amplifies sound.

Initial receiver is the eardrum.
• Sensitive to tiny pressure changes

Middle ear transmits the sound through bone.
• Act as levels magnifying force by 40

Inner ear (cochlea) separates pitches.
• Breaks up superposed waves
• 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.
• Hard with short wavelength.

Stereo speakers reconstruct
timing differences.
• Constructive 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 Df.
The beat period T = 1 / Df.
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 0.016 Hz.
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