The Physics of Sound

Sound: a series of disturbances of molecules within, and propagated through, an elastic medium or…

Sound: is an alteration in the pressure, particle displacement propagated in an elastic medium

Elasticity: capacity of a medium to return to its original position after being altered

Brownian motion: heat effects particle velocity = + heat = + velocity

Sound Wave

Sine Wave

Compression\ Rarefaction

Degrees of Wave

Wave Length

Velocity

Acceleration = increase in velocity

Deceleration = decrease in velocity

Increase density = increase V. (fastest in solids, then liquids, then gases)

Degrees of Sine wave

Sound Characteristi

cs

Damping Free vibration - when no outside force is added to

vibration Lightly damping - when vibrations of a mass decay over

time Heavy damping - “ “ “ “ “ “ “ “ “

rapidly Critically “ - “ “ “ “ “ “ “ before

1 Hz. Forced vibration - when outside force is added to

vibration. When removed = free vibration

Vibrator Length, Mass and Stiffness

If length decreases = frequency increases

If length increases = frequency decreases

If increase in mass = decrease in frequency

If increase in stiffness = increase in frequency

Vibration

Periodic - a waveform that repeats over time (i.e. speech)

Aperiodic - a waveform that doesn’t repeat over time (noise)

Fourier Analysis

Frequency : Pitch :: Intensity : Loudness

Frequency is the number of complete oscillations of a vibrating

body per unit of time (in audiology we use Hertz Hz) Pitch is the subjective impression of highness or lowness

of a sound. The psychological correlate of frequency.

Intensity is the amount of sound energy per unit of area (dB)

Loudness is the subjective impression of the power of a sound.

It is the psychological correlate of intensity (sones)

Resonance Resonance - the ability of a mass to

vibrate at a particular frequency with a minimum application of external force. Also the ability of a vibrator to put another vibrator into vibration.

Resonant frequency - the frequency at

which a mass vibrates with the least amount of external force; the natural frequency of vibration of a mass

Fundamental frequency/overtones/harmoni

cs

Fundamental frequency - the lowest frequency of vibration in a complex wave

Overtones - any whole number multiple of the fundamental frequency of a complex wave

Harmonic - same as overtones except in numbering system

(i.e. 1st overtone is = to 2nd harmonic)

Formant

Impedance

Opposition to the path of sound. Denser the object the more the impedance

Sound Measurement

Audiometer air conduction bone conduction speech (sound

field, free field and under phones)

Sound level meter A scale B scale C scale

Intensity/Decibel It involves a ratio It involves a logarithm It is therefore non-linear It is expressed in terms of various

reference levels which must be specified It is a relative unit of measure 3 dB increase, if intensity levels are

doubled

dB Levels

Force and Pressure Force - the impetus required to institute or alter the

velocity of a body. A Newton is also a measure of force. Pressure - force over an area of surface (i.e. Pascals .0002

dynes per cm2 = the smallest pressure variation for hearing.

Sound Pressure Level (SPL) .0002 dynes per centimeter squared = 20 micropascals = 0 dB SPL

Thresholds

The softest level an individual can hear a sound 50% of the time.

Threshold of pain is about 140 dB (SPL)

Sensation Level (SL) = dB above threshold

Power

the rate at which work is done. Units of measurement are: watts, or ergs per/sec.

Audiometric Zero

Standards ASA 1951 ISO 1964 ANSI 1996