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EXTRONEWS JAN/FEB 1999

Audio Specs Spelled Out

Audio specifications, exactly what do they

mean? What is actually being measured?

Most people in the A/V industry probably

have a general idea of what common audio

specifications mean. However, it is important

to understand how each specification is

measured and what the measurement specifi-

cally tells the reader about the performance

of the equipment. In the following section a

number of the more common audio specifica-

tions will be discussed.

Frequency Response:

Frequency response is the measurement of

signal amplitude over a specified frequency

range. Typically, this is a concern with

buffering and routing equipment (audio DAs,

audio switchers, etc), as opposed to audio

processing equipment (such as audio equal-

izers). Frequency response is usually

measured over the range of 20 Hz to 20 kHz,

which is essentially the range of human

hearing. There should also be a dB value

listed with this range. The dB value indicates

the amplitude variation over the frequency

range, or the flatness of the frequency

response. The less variation of amplitude

over this range, the better.

Frequency response is determined by

applying a series of specific frequencies to an

amplifier and then measuring the output level

for each frequency. A plot can then be made

of the amplifier's frequency response. Figure

1 shows a graphical representation of a

frequency response measurement.

As a real world example, the frequency

response spec for the audio section of

Extron's CrossPoint matrix switcher is: +/-

.05 dB from 20 Hz to 20 kHz (.05 dB is

considered quite low). This tells the reader

the audio amplitude won't vary by more than

+/- .05 dB over the 20 Hz to 20 kHz range.

Occasionally a manufacturer may list

frequency response as simply “20 Hz to 20

kHz,” with no dB value listed. This may tell

the user the range over which the amplifier

will work, but this doesn't tell the user

anything about the flatness of the response.

Further inquiry should be made in such cases.

Signal to Noise Ratio (S/N)

This specification is a measurement of how

noisy the audio equipment being measured is

in reference to the signal level. The actual

measurement is the ratio of the signal level to

the noise level, and is expressed in dB. The

signal measurement is made by applying an

audio signal to the amplifier to be tested

(usually in the 1 kHz range), after the

amplifier has been set to either the maximum

output level, or the normal output level. The

signal output by the amplifier is then

measured.

The noise level is determined by measuring

the output of the equipment with no signal

applied. The noise level is measured across a

specified bandwidth, or through a weighing

filter. Once the signal level and the noise

level have been measured, the ratio of the

signal level to the noise level is computed.

This ratio is then expressed in dB. Typically,

this can be considered the effective dynamic

range of most analog devices.

Note: To measure the noise level of digital

audio equipment it is necessary to supply a

signal to the unit and then use filters to

separate out the noise. The noise level of

digital audio equipment cannot be deter-

mined by simply measuring the output

without an input signal (as is done with

analog audio equipment). When no signal is

supplied, the digital conversion circuits will

likely not work normally, and an actual noise

level cannot be produced.

Crosstalk

Crosstalk is a measure of signal leakage from

one channel to another. The crosstalk

measurement should not be confused with the

measurement of stereo separation-– although

these two terms often get used interchange-

ably. The measurement technique is the same

for both of these specifications, but the

crosstalk measurement deals with channels

that carry independent signals (e.g. input 1

and input 2). On the other hand, stereo sepa-

ration refers to the signal leakage between

two channels (left and right) which carry a

stereo signal.

100

1000

10k

75k

100k

Output

Voltage

Frequency (Hz)

Rolloff Uneven Frequency

Response

Rolloff

FIGURE 1: THE FREQUENCY RESPONSE MEASUREMENT IS MADE BY PLOTTING THE SIGNAL

AMPLITUDE ACROSS A FREQUENCY RANGE. IN THE CASE OF AUDIO BUFFERING AND ROUTING

EQUIPMENT, ITS USUALLY BEST TO HAVE FLAT RESPONSE OVER THE FREQUENCY RANGE.

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