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:
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.