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 labÕs 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, dBuÉ Radio: dBm, dBW, dBkÉ..

 

¥dBSPL

¥Used in acoustics, when measuring the Òsound pressure levelÓ

¥ 

¥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:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  source: http://www.acousticalsurfaces.com/acoustic_IOI/101_13.htm

 

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.

 

¥Reverberation

Also known as later reflections.

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