GENERAL AUDIO TERMS
5.1 Channel Audio
This is a listening environment that involves the use of six separate speakers.
The placement configuration is: Left, Center, Right, Left Surround, Right
Surround and a Subwoofer. The left, Center and Right speakers are placed in
front of the listener and the Left and Right Surround speakers are placed to each
side or slightly to the rear of the listener. The Subwoofer is generally placed
somewhere to the front of the listener. The exact placement of the Subwoofer is
dependent on its individual design and the acoustics of the listening room. The
term “5.1” refers to the individual channels with the “.1” referring to the subwoofer
since its bandwidth is limited to the lower part of the audio frequency spectrum.
This is an abbreviation that stands for Amplitude Modulation. We all know what
AM Radio is (at least, in the U.S.A.), but understanding what Amplitude
Modulation is will take some explanation:
Radio transmission uses a set frequency called a
. This carrier
frequency is demodulated (eliminated) by your receiver. In the case of AM radio,
the carrier frequencies used are in the
range (100,000 cycles per
Amplitude Modulation simply means that the carrier frequency is modulated by
the amplitude (volume or strength) of the signal being transmitted. Once your
radio receiver strips out the carrier frequency, what’s left is the signal being
transmitted. The number base used to tune the station is representative of the
carrier frequency used by that particular radio station. XYZ 950 would mean that
the AM radio station XYZ is transmitting with a carrier frequency of 950 Kilohertz
Defined simply as a given range of frequencies (audible or not). We humans, for
example, have an audible hearing bandwidth ranging from approximately 20 Hz
(cycles per second) to 20 Kilohertz (kHz).
Audio, as we know it and are used to hearing it, consists of symmetrical
whether it’s a clarinet, a cricket, or a cymbal crash. Whenever the volume
of undistorted audio is increased beyond the limit of a given piece of electronic
equipment or a medium such as digital recording, the nice round tops of those
sine waves are flattened or “clipped off.” The result is very noticeable.
dB is the symbol abbreviation of a
. The decibel is a unit of measurement
expressed as a mathematical logarithm and is one tenth of a bel, which itself, is a
unit of measurement that is seldom used.
For example: A change in power ratio by a factor of ten is a 10 dB change. A
change in power ratio by a factor of two is approximately a
3 dB change.
The bel and the decibel originated at Bell Telephone Laboratories as a unit of
measurement for the signal loss (or gain) in telephone circuits. The bel was so
named for the founder of Bell Telephone, Alexander Graham Bell. It’s worth noting
here that Bell Telephone Laboratories established the ground rules and practices
that have been used by professional audio installations since the early part of the
twentieth century and are still in use today.
There are a few flavors of dB in use. Some examples are:
(The “m” for milliwatt) is a measurement of power where “0” dBm equals
one milliwatt of power. 0 dbm is 0.775 Volts RMS referenced into a 600 ohm
resistor or load.
In this case, the “u” stands for unterminated or unloaded. 0 dbu is 0.775
referenced into a given termination or load.
This signifies a reference level of 1 volt
, regardless of the load and
thus, will vary as the load varies.
This is actually a definition as opposed to a reference level.
It refers to the decibel level below “O” VU, full scale. It came about with the
advent of digital recording where the absolute maximum level is 0VU or
. For example: The professional audio standard most often used for digital
audio recording is -20DBFS which is a reference level of 20dB below full scale
digital or 0VU.
Most commonly associated with that bone jarring, teeth rattling howling and
screeching that occasionally happens at live venues when the volume of a
particular microphone is turned up too high or the microphone itself is placed too
close to one of the PA speakers. This is known as
happens because microphones are very sensitive to small changes in
atmospheric pressure while speakers are designed to effect very large changes
in atmospheric pressure. When a particular speaker is reproducing the amplified
output of a microphone, feedback can happen when the former is placed in close
proximity to the latter because, in effect, you have created a closed loop.
Ironically, feedback at the electronic component level is what makes all this
possible. In the world of electronics, amplifier gain is controlled by routing a small,
carefully controlled, amount of the signal from the output of an amplifier back to
The abbreviation for Frequency Modulation, FM is the household acronym in the
U.S.A. that refers to the commercial radio band which is broadcast using a carrier
frequency between 87.5 and 108
(millions of cycles per second or
MHz). The 87.5 to 108 MHz band is fairly common throughout the world with a
couple of exceptions: Japan and Russia, where a wider frequency spectrum is
Frequency Modulation is accomplished by modulating the carrier frequency with
the frequencies of the signal being transmitted. Once your radio receiver strips
out the carrier frequency, what’s left is the signal being transmitted. An interesting
benefit of FM radio transmission is that the noise power decreases as the signal
power is increased. Thus, a 50,000 watt transmitter will have a better
than a 10,000 watt transmitter.
Harmonic Distortion and Total harmonic distortion (THD)
Harmonic Distortion is a consequence of electrical amplification. The actual
harmonic component measured at the output of an audio amplifier is: Total
Harmonic Distortion plus Noise (THD+N).
First, we need to understand just what a harmonic is:
A harmonic is any frequency that is a multiple of a fundamental frequency. A
harmonic can be
multiple or any number of multiples of the fundamental
Imagine a guitar string. If you pluck the string exactly in the middle, it vibrates at
its fundamental frequency plus a second harmonic frequency resulting from the
string being divided in two when plucked. Each half of the string also vibrates at
the same time as the full length string is vibrating, creating a
which, in this case, is twice that of the fundamental frequency.
Now, as it turns out, all electronic amplifiers create harmonics of the fundamental
frequencies being amplified. The type and amount of harmonics generated are a
consequence of amplifier design and the amount of amplification involved. When
the harmonic distortion of an amplifier is measured, a pure tone is applied to the
input of the amplifier by a piece of equipment known as a
(makes sense). The output of the amplifier being tested is fed back to the
distortion analyzer, which removes the pure tone, measures what's left, and
usually displays the results in
. The displayed results are all of the
harmonics generated by the amplifier plus any noise added by the process of
No, this is not about the car and truck rental company. Hertz, abbreviated Hz,
represents the definition of frequency. Frequency, in this case, is referring to
cycles per second (cps). Hertz is the surname of Heinrich Hertz, a German-born
PhD physicist who, in the late Nineteenth century, was able to conclusively prove
the existence of electromagnetic waves.
The most common multiples of Hertz are:
is one thousand cycles per second (kHz).
is a million cycles per second (MHz).
is one billion cycles per second (GHz)
Intermodulation Distortion (IMD)
This gets a little more complicated than simple harmonic distortion because IMD
is the byproduct of non-linear amplification and is not harmonically related to the
signal being amplified.
As an amplifier creates harmonics of the fundamental signal, some of those
harmonics themselves create sum and difference frequencies. This process is
called….you guessed it!….
. Those frequencies created by the
process of intermodulation are measured as intermodulation distortion. Using a
, IMD is measured by feeding two pure tones into the amplifier
under test, subtracting them at the amplifier’s output, and measuring what’s left.
The switch on your stereo system that compensates for the apparent loss of bass
and treble at lower volume settings.
Our hearing is level or loudness-dependent. As volumes decrease below normal
speaking levels, we tend to loose sensitivity at the lower and higher frequencies,
leaving only the midrange (2-5 kHz) which, curiously, is where most of the energy
of the human voice resides. Two researchers by the names of Fletcher and
Munson documented this phenomenon in the 1930’s. Thus, the loudness control
on your home audio equipment is designed to create what is known as an
that compensates for our level-dependent hearing.
Technically speaking, Minus 10 is 10
below the reference point of 0 dBu.
So, what is 0 dBu you ask? 0 dBu is 0.775 Volts
referenced into a given
termination or load.
That said, Minus-10 dBu is actually the “0” reference level for consumer analog
equipment. The reference voltage (for those still interested) is 316 millivolts (Mv)
or 0.316 Volts.
Defined simply as “single-channel.” Although there was some early
experimentation with stereo recording, nearly all of the recordings produced from
the beginning of recorded sound up through the middle 1950’s were monaural.
Peak Music Power
More of an advertising slogan than an actual reference measurement, Peak
Music Power is measured with an amplifier operating at 100% power. And, this is
assuming 100% efficiency, which few (if any) production model amplifiers can
achieve. In this case, the peak power rating is referring to the maximum output
power that can be achieved for an instant without destroying the amplifier under
test. Manufacturers will often connect the amplifier under test to a very large,
brute-force power supply in order to measure the maximum peak output rating.
Obviously, this is not practical or even sustainable. Manufacturers love this rating
as it typically overrates the product by about 30%.
Yes, some types of noise are defined by color. Pink Noise is all of the frequencies
combined within a given spectrum. And, all of those frequencies within that given
spectrum have the same power. The spectrum for Pink Noise used in the audio
realm is usually 20
to 20kHz. Pink Noise is useful for measuring how flat (or
equal) the frequency response is of a given piece of audio equipment. Pink Noise
was routinely used for calibrating analog recorders. It takes a specially designed
generator to produce Pink Noise, and, in order to view it in real time, a spectrum
analyzer must be used. Pink Noise sounds somewhat like a waterfall.
Incidentally: The sound of a waterfall (up close) would be defined as
or all of the frequencies combined
not within a given spectrum
necessarily of equal power.
This is a discrete 4 channel system that consists of 2 front channels and 2 rear
channels. “Quad,” as it was known, was actually introduced with 4 channel reel-
to-reel audio tapes in the late 1960’s. By the early 70’s Quad formats started
appearing for vinyl record albums. Eventually, there were three competing
Quadraphonic formats for records, and the resulting confusion in the consumer
market may have led to its ultimate commercial failure. One of the formats, the
Sansui SQ system, was resurrected by Dolby Laboratories in the mid 1970’s and
was successfully used as a theatrical sound format for feature films. It’s known as
“Dolby Stereo.” Dolby reassigned the audio channels to: Left, Center, Right and
one Surround channel. This is the analog format that still survives on today’s
feature film releases.
An abbreviation for the term “Root Mean Square.” This mathematical formula is
useful for calculating the statistical average of a given quantity. It is used in
several scientific disciplines such as the physical sciences, statistics, and
electronics. In electronics, the RMS formula is used to measure an audio power
amplifier’s ability to provide continuous power into a given load such as your
speakers. The RMS power rating of a power amplifier is achieved using a single
tone that is fed into the amplifier input at a specified voltage while the amplifier is
terminated into its designed load. The signal at the output is measured using a
meter calibrated to measure RMS voltages. The RMS rating of an amplifier is the
power rating that matters most because it tells you the power that can be safely
and continuously delivered to your speakers. If the manufacturer’s specifications
do not include the RMS power rating, you can closely calculate it by multiplying
the advertised peak power rating by 0.707.
The abbreviation for “Volume Unit.”
The term VU is most commonly associated with a VU meter, which is a reference
meter calibrated in
. “0” VU on a VU meter originally corresponded to 0
dBm, or 0.775 Volts RMS referenced into 600 ohms.
In today’s professional audio world,
“0” VU is often +4 dBu, which is
1.23 volts RMS at 1000 Hz.
The VU meter was introduced in the late
1930's and originally contained two scales:
dB on the top scale and “percentage of
modulation” on the lower scale, calibrated
from 0 to 100. The percentage of
modulation was a useful reference for the
broadcast and feature film industries.
Howard M. Tremane, “Audio Cyclopeia” Second Edition, Howard W. Sams, 1973
Rudolph F. Graf, “Dictionary of Electronics” Howard W. Sams, 1974
Glenn D. White, “The Audio Dictionary” University of Washington Press, 1987
© Corey Bailey Audio Engineering