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 Characteristics

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/harmonics 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