1. HUMAN PERCEPTION OF SOUND

2. 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/he aring/innear.asp http://www.nidcd.nih.gov/health/he aring/innear.asp

3. 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/he aring/innear.asp http://www.nidcd.nih.gov/health/he aring/innear.asp

4. 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/he aring/innear.asp http://www.nidcd.nih.gov/health/he aring/innear.asp

5. 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

6. 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 10 12 times louder than 0 dB!

7. 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.

8. 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)

9. You saw & heard this in the java applet http://library.thinkquest.org/19537/java/Beats.html

10. This periodic variation in amplitude is called a beat Beat frequency is calculated as F beat = f high - f low