Page 56 - The Master Handbook Of Acoustics

31
SOUND LEVELS AND THE DECIBEL

Table 2-3. Reference quantities in common use.

Level in decibels Reference quantity
Acoustic

Sound pressure level in 20 micropascal
air (SPL, dB)
Power level (Lp,dB) 1 picowatt(10–12 watt)
Electric

Power level re 1 mW 10–3 watt (1 milliwatt)
Voltage level re 1 V 1 volt
Volume level, VU 10–3 watt

Acoustic Power
Table 2-4. The Greeks had a word for it.

It doesn’t take many watts of acoustic power to Prefix Symbol Multiple
produce some very loud sounds, as anyone who
tera T 1012
lives downstairs from a dedicated audiophile
giga G 109
will testify. We are conditioned by megawatt
mega M 106
electrical generating plants, 350-horsepower
(261 kilowatt) automobile engines, and 1,500- kilo k 103
watt flatirons that eclipse the puny watt or so the milli m 10-3
hi-fi loudspeakers might radiate as acoustic micro 10-6
power. Even though a hundred-watt amplifier nano n 10-9
may be driving the loudspeakers, loudspeaker
pico p 10-12
efficiency (output for a given input) is very low,
perhaps on the order of 10 percent, and head-
room must be reserved for the occasional peaks
of music. Increasing power to achieve greater results is often frustrating.
Doubling power from 1 to 2 watts is a 3-dB increase in power level (10
log 2 = 3.01), a very small increase in loudness; however, the same 3-dB
increase in level is represented by an increase in power from 100 to 200
watts or 1,000 to 2,000 watts.
Table 2-5 lists sound pressure and sound-pressure levels of some
common sounds. In the sound-pressure column, it is a long stretch