HUMAN PERCEPTION OF SOUND

How your ears work

• Sound propagates as a longitudinal wave.– Energy transfer

through collisions of air particles

• Wave hits the ear drum, causing it to vibrate.

Image from http://www.nidcd.nih.gov/health/hearing/innear.asp

How your ears work (continued)

• Vibrating ear drum causes 3 little bones to vibrate (incus, malleus, and stapes).

• They bang into the cochlea, a fluid-filled sack that looks like a snail. Image from

http://www.nidcd.nih.gov/health/hearing/innear.asp

How your ears work (continued)

• Tiny hairs inside the cochlea sway back and forth as the sack vibrates.

• Motion of the hairs creates electrical signals (nerve impulses) that the auditory nerve carries to the brain.

• Brain interprets these signals as sound.

Image from http://www.nidcd.nih.gov/health/hearing/innear.asp

Causes of Hearing Problems

• Inefficient transmission of sound waves– Blockages (ear wax!)– Damage to ear drum– Limited ability for little bones to vibrate

• Abnormal bone growth fuses them together• Arthritis

• Damage to the electrical circuit– Impulses not generated by hairs– Impulses not detected by auditory nerve– Impulses not received by brain

The Physics of the Human Ear

• Frequency Range: 20 Hz - 20 kHz– That’s better than stereo speakers!

• Typical woofers: up to 2 kHz• Typical tweeters: 2 kHz – 20 kHz

• Amplitude range– Volumes are measured in decibels [dB]

– Quietest audible sound, the threshold of hearing, is 0 dB– Loudest tolerable sound, the threshold of pain, is 120 db

– The dB scale is logarithmic, which means it “counts” in powers of 10. 120 db is 1012 times louder than 0 dB!

Perception of simultaneous sounds

• When you hear sounds of slightly mismatched frequency, your brain “averages” them.

– Suppose the sopranos sing a 442 Hz note, and the altos sing a 438 Hz note.

– Your brain tells you that you are hearing a 440 Hz note.

Perception of simultaneous sounds

• Not only do you detect a different pitch, you also hear a volume that varies!

• Why?– The waves interfere constructively at some

points and destructively at other points– Points of CI produce high amplitudes (volumes)– Points of DI produce low amplitudes (volumes)

You saw & heard this in the java applet

http://library.thinkquest.org/19537/java/Beats.html

• This periodic variation in amplitude is called a beat

• Beat frequency is calculated asFbeat = fhigh - flow