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
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
This concept has been greatly expanded upon and now includes;
Rear Surrounds, Overhead speakers and the whole package is known as
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 involves a frequency called a
This carrier frequency is demodulated (eliminated) by your receiver.
When you tune a radio receiver to a station, you are actually tuning in a carrier
frequency to demodulate.
In the case of AM radio, the carrier frequencies used are in the Kilohertz range
(100,000 cycles per second).
Amplitude Modulation simply means that the carrier frequency is modulated by
the amplitude (volume) of the signal being transmitted. Once your radio receiver
strips out the carrier frequency, what’s left is the audio signal being transmitted.
The number 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 (kHz).
Audio, as we know it and are used to hearing it, consists of symmetrical sine
waves 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
that is one tenth of a
, which itself, is a unit of measurement that is seldom
Decibel calculations are made using a logarithmic scale.
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
named for the founder of Bell Telephone, Alexander Graham Bell. It’s worth
noting 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
Volts RMS that is not referenced into a given termination or load.
This signifies a reference level of 1 volt RMS, regardless of the load and
thus, will vary as the load (or resistance) varies.
This is actually a definition as opposed to a reference level.
It refers to the decibel level below ‘0’ VU, full scale. It came about with the
advent of digital recording where the absolute maximum level is 0VU or
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 microphone is placed in
close proximity to the speaker because, in effect, you’ve 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 its input……Feedback!
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 Megahertz (millions of cycles per
second or MHz). The 87.5 to 108 MHz band is common throughout the world
with a couple of exceptions; Japan and Russia, where a wider frequency
spectrum is used.
Frequency Modulation is accomplished by modulating the carrier frequency with
the frequencies of the signal being transmitted. When you tune a radio receiver
to a station, you are actually tuning in a carrier frequency to demodulate.
Once your radio receiver strips out the carrier frequency, what’s left is the audio
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 signal-to-noise ratio 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 or a product of a fundamental
Imagine a guitar string. If you pluck the string exactly in the middle, it vibrates at
its fundamental frequency plus a
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 second harmonic,
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 decibels. Some distortion analyzers do the math
and display the results as a percentage. 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 harmonic distortion because IMD is the
byproduct of non-linear amplification and is not harmonically related to the signal
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.
All types of distortion are cumulative. Each time the signal is processed, the
overall distortion is increased, regardless of the type of signal.
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 with the surnames of Fletcher and
Munson, documented this phenomenon in the 1930’s.
Their findings resulted in what is known as the Fletcher-Munson curve.
The loudness control on your home audio equipment is designed to create what
is known as an
equal loudness contour
that compensates for our level-
Technically speaking, Minus 10 is 10 dB below the reference point of 0 dBu.
So, what is 0 dBu you ask? 0 dBu is 0.775 Volts RMS
referenced into a
given termination or load.
That said, Minus 10 dBu, is actually the ‘0’ reference level for most 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 records 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 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 20Hz 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. The sound of a waterfall (up close) would be defined
, or all of the frequencies combined
not within a given spectrum
but usually 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 QS system, (regular matrix) 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 release prints.
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.
expenditure of 1 Watt-hour represents 3,600 joules
or 3.600 x 103 J.
A Watt-hour is expressed as Wh. A joule is expressed as the symbol J.
To obtain joules when the watt-hours are known, multiply the number of watt-
hours by 3.600 x 103. To obtain watt-hours when joules are known,
multiply the number of joules by 2.778 x 10-4.
Your electrical bill is usually expressed in Watt-hours so, it’s worth
The abbreviation for ‘Volume Unit.’
The VU meter is a reference meter
calibrated in decibels. ‘0’ VU on a VU
meter originally corresponded to 0 dBm,
or 0.775 Volts RMS referenced into 600
In professional audio,
‘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
There are actually several noise colors.