Audio Levels - Toa A-503A Manual

Audio Levels: Voltage, Gain and the Decibel
A basic characteristic of any audio signal is its amplitude, measured electrically in terms of voltage or
acoustically in terms of sound pressure. When assessing the loudness of a signal, the amplitude or
pressure is converted to a decibel value. The decibel scale gives a relative number referenced to a cer-
tain voltage or pressure. For example, 0 dBV is a popular standard reference for audio levels, and rep-
resents one volt. Note that amplitude is expressed as a voltage, while level (or loudness) is expressed
using a dB scale.
When working with audio electronics, levels are com-
monly divided into three ranges: mic level, line level,
and speaker level. Mic level is the smallest signal.
Microphones and other passive transducers (devices
that convert energy from one form, such as sound, to
another, such as electricity) produce signals ranging
from a few microvolts to a few millivolts. A typical nom-
inal operating level for a microphone output would be
–55 dBV. Line level is hundreds of times greater in volt-
age terms — typically ranging from several millivolts up
to around 1 volt, with a nominal level of 0 dBV. Speaker
level is the strongest, ranging from a fraction of a volt
(during quiet periods) to several dozen volts depending
on the output rating of the amplifier. Of course, sound
is very dynamic in nature, so whatever the nominal
operating level of your signal is, if you read it with a
meter during operation, you are likely to see large fluc-
tuations from moment to moment within that range.
An important function of amplifiers is providing the
"gain"needed to raise signals from mic or line level up to
speaker level. Gain is another word for amplification,
and simply means an increase of the voltage or power.
The opposite of gain is attenuation. Both gain and
attenuation are commonly measured in decibels.
The dBV scale is not the only one used for audio levels.
Another popular reference scale is the dBu, where 0 dBu
represents 0.775 volts. The historical predecessor to
these two scales is the original dBm scale, where 0 dBm
represents one milliwatt, or 0.001 watts. Other scales
you might encounter include dBW (referenced to one
watt) and dB V (referenced to one microvolt). These
scales are seen mostly in the radio broadcast industry.
Care should be taken not to confuse one scale with
another, especially the common dBV and dBu scales. To
make things especially aggravating, the term for dBu was previously dBv — with a lower-case "v"; so
if you encounter dBv on an old spec sheet, it means dBu, not dBV.
TOA Electronics Amplifier Guide
What is RMS Power?
An audio signal is defined by its amplitude
(loudness) and frequency (pitch). When the
sound is represented as a waveform, the ampli-
tude is the vertical dimension, while the fre-
quency is the number of up and down cycles of
the wave per second, with seconds running
from left to right.
Amplifier power ratings are based on the
amplitude of the waveform. Since the peak levels
of a complex waveform (one containing many
frequencies) may occur rarely or frequently, an
averaged value is used, based on the "root
mean square" or RMS method. In this method,
the amplitude is squared (so that all values are
positive), then the resulting values are aver-
aged, and the square root of this average is the
RMS value. For simple sine wave test signals,
the RMS voltage will be 0.707 times the peak
voltage. After calculating RMS voltage, the
RMS power is calculated by squaring the volt-
age and dividing by the load resistance.
Peak
RMS
8
Time
Peak
to
Peak