Page 54 - Master Handbook of Acoustics

level gain of an amplifier in decibels is 20 log (output voltage/input voltage), which holds true
regardless of the input and output impedances. However, for power-level gain, the impedances must
be considered if they are different. It is important to clearly indicate what type of level is intended, or
else label the gain in level as “relative gain, dB.” The following examples illustrate the use of the

decibel.

Example 1 Sound-Pressure Level
A sound-pressure level is 78 dB. What is the sound pressure?

Note that the reference level in SPL measurements is 20 μPa.

Example 2 Loudspeaker Sound-Pressure Level
A loudspeaker with an input of 1 Ω into 8 produces an output sound-pressure (SPL) of 115 dB on
axis at 1 m (3 ft). What is the SPL at 6.1 m (20 ft)?

The assumption made in the 20 log 6.1 factor is that the loudspeaker is operating in a free field

and that the inverse square law is valid in this case. This is a reasonable assumption for a 20-ft
distance if the loudspeaker is operating away from reflecting surfaces. The free field is described in
more detail in Chap. 3.
This loudspeaker is rated at an SPL of 115 dB at 1 m with 1 W. If the input were decreased from
1 W to 0.22 W, what would be the SPL at 1-m distance?

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

Example 3 Microphone Voltage
An omnidirectional dynamic microphone has an open-circuit voltage specified as −80 dB for the
150-Ω case. It is also specified that 0 dB = 1 V/μbar. What is the open-circuit voltage v in volts?