Chapter 3 ÷ Lecture Outline

1) Sound propagation:
 

waves of condensation and rarefaction in a medium

note how weak the sound pressure vs. random pressures.

before we talked about amplitude, frequency, and phase.

now the wavelength: __ = c/f. in air c = about 340 m/sec.

Why is this important? resonance and spatial location.


2) pressure and intensity
 

pressure is usual measure of amplitude, power is used in acoustics

pressure = F/area. Why is this important?

The machinery of the middle ear.

intensity = power = pressure squared/density * speed of sound

(density * speed of sound = characteristic impedance)

density of water = 998 kg/m^3. c = 1481 m/sec, BUT

density of air = 1.21 kg/m^3. c = 343 m/sec. SO?

Why is this important? The problem of the inner ear.
3) The dB scale Measures of amplitude ÷ pressure as RMS, peak, peak to peak

units of pressure are dynes /cm^Squared - But...the problem of scale: threshold = 1, upper limit = 10^14.

Solution? use logs:

dB = 10 log (power A /power reference)
dB = 20 log (Pressure A/ Pressure reference)
What is the reference pressure? Population threshold at 1kHz? SPL
Population thresholds for all frequencies? HL
Lab measures = SPL; Clinical measures = HL.
4) Interference
Impedance matching and mismatching:

matched? == conduction

mismatched = absorption or reflection.

"Forced Movement" Resistance and Reactance -

mass reactance (depends on freq) and spring reactance( 1/freq)

Importance? in inner ear filters. interference by objects and relation to wave length: large object ÷ reflect or absorb;

medium object ÷ shadow;

small object ÷ invisible.
5) Resonance: standing waves
closed tube: node for pressure at end of tube if wave length = 4L
so frequency is c/4L:
importance? ear canal.