Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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The discussion above has considered decibels primarily as they measure sound loudness.
Decibels can also be used to measure relative electrical power or voltage. For example, dBV
measures voltage using 1 V as a reference level, dBu measures voltage using 0.775 V as a
reference level, and dBm measures power using 0.001 W as a reference level. These
applications come into play when you’re considering loudspeaker or amplifier power, or wireless
transmission signals. In Section 2, we’ll give you some practical applications and problems
where these different types of decibels come into play.
The reference levels for different types of decibels are listed in Table 4.3. Notice that
decibels are used in reference to the power of loudspeakers or the input voltage to audio devices.
We’ll look at these applications more closely in Section 2. Of course, there are many other
common usages of decibels outside of the realm of sound.
what is being measured abbreviations in
common usage
common reference point equation for conversion
to decibels
Acoustical
sound power
dBPWL or
( *
sound intensity
dBSIL or
threshold of hearing,
( *
sound air pressure amplitude
dBSPL or
threshold of hearing,
( *
sound amplitude dBFS
where n is a given bit depth
x is a sample value,
(
| |
)
Electrical
radio frequency transmission
power dBm 1 mW =
10-3
W
( *
loudspeaker acoustical
power dBW 1 W
( *
input voltage from
microphone; loudspeaker
voltage; consumer level
audio voltage dBV 1 V
( *
professional level audio
voltage dBu 0.775 V
( *
Table 4.3 Usages of the term decibels with different reference points
4.1.5.3 Peak Amplitude vs. RMS Amplitude
Microphones and sound level meters measure the amplitude of sound waves over time. There
are situations in which you may want to know the largest amplitude over a time period. This
“largest” can be measured in one of two ways: as peak amplitude or as RMS amplitude.
Let’s assume that the microphone or sound level meter is measuring sound amplitude.
The sound pressure level of greatest magnitude over a given time period is called the peak
amplitude. For a single-frequency sound representable by a sine wave, this would be the level
at the peak of the sine wave. The sound represented by Figure 4.3 would obviously be perceived
as louder than the same-frequency sound represented by Figure 4.4. However, how would the
loudness of a sine-wave-shaped sound compare to the loudness of a square-wave-shaped sound
with the same peak amplitude (Figure 4.3 vs. Figure 4.5)? The square wave would actually
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