Amplitude of Sound


The number of molecules displaced by a vibration creates the amplitude of a sound.


The strength or level of sound pressure.


















The number of molecules in the sound wave in b is greater than the number in the sound wave in a, therefore the amplitude of the sound wave in b is greater.



Measuring Amplitude


The transmission unit (TU)

Bell Telephone Laboratory used TU to quantify the reduction of audio level over a one mile (1.6 km) length of standard telephone cable.

TU was renamed to bel (B) in 1923-34 to honor the lab’s founder Alexander Graham Bell.

The decibel (dB)

1 decibel = 0.1 bel

10 dB = 1 B



The decibel (dB) is:


A measure of the ratio between two quantities.

A number used to express relative output sensitivity.

Different dB measurements for different purposes

Acoustics: dBSPL, Electric: dBV, dBu… Radio: dBm, dBW, dBk…..



Used in acoustics, when measuring the “sound pressure level


dBV (electric voltage)

1 dBV reference is deciBels relative to 1 Volt.

dBu (unloaded electric voltage)

1 dBu reference is deciBels relative to 0.775 Volts.



























Equal Loudness Cruves





















Human ears lack sensitivity to low and high frequencies.

Question: A 20 Hz sound would have to be at what level to seem as loud as a 1,000 Hz sound at 20dB?

Anser: 80 dB.




Sound Level vs Distance: the Fall Off

Sound level drops off at 6 dB per doubling of distance.


















Critical Distance: the distance from the source along the line of principal radiation, where the level of the direct sound and reverberant sound is equal.

The less reverberant the room, the longer the critical distance.









a          sound reflected

b          sound absorbed

c          Partially reflected and absorbed











Sound Fields in a Room


a          direct sound

b         1st-order reflected sound

c          higher-order reflected sound

d         reverberant field





Sound Absorption Chart:






























Acoustic LIFE CYCLE of a Sound Wave
The THREE Components


Direct Waves/Direct Sound

Reaches listener without bouncing off any surface.


Early Reflections

Hit at least one surface before reaching the listener in roughly 10 milliseconds after the direct sound.



Also known as later reflections.

When sound reflects from many surfaces, reaching the listener more than 10 milliseconds.