Page 60 - The Master Handbook Of Acoustics

35
SOUND LEVELS AND THE DECIBEL

Note that 10 log is used because two powers are being compared.

Example: Microphone Specifications
A Shure Model 578 omnidirectional dynamic microphone open-cir-
cuit voltage is specified as –80 dB for the 150-ohm case. They also
specify that 0 dB = 1 volt per µbar. What would be the open-circuit
voltage,
, be in volts?
–80 dB = 20 log
/1
log
/1 = –80/20

= 0.0001 volt
= 0.1 millivolt

Example: Line Amplifier
A line amplifier (600 ohms in, 600 ohms out) has a gain of 37 dB. With
an input of 0.2 volts, what is the output voltage?

37 dB = 20 log (
/0.2)
log (
/0.2) = 37/20
= 1.85

/0.2 = 10 1.85

= (0.2)(70.79)

= 14.16 volts
Example: General-Purpose Amplifier

This amplifier has a bridging input of 10,000 Ω impedance and an out-
put impedance of 600 Ω. With a 50 m
input, an output of 1.5 V is
observed. What is the gain of the amplifier? The so-called voltage gain
is:
voltage gain = 20 log (1.5/0.05)
= 29.5 dB

It must be emphasized that this is not a power level gain because of the
differences in impedance. However, voltage gain may serve a practical
purpose in certain cases.

Example: Concert Hall

Seat x in a concert hall is 84 feet from the tympani drums. The tympa-
nist strikes a single, mighty note. The sound-pressure level of the
direct sound of the note at seat x is measured to be 55 dB. The first