The sound - MuleSlow Services

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Richard C. Heyser
ONE TYPE of distortion a loudspeaker system can produce
is the generation of extraneous harmonics
when the system is driven by a single sustained tone.
Thus, if a speaker drive signal consisting of a perfectly pure
tone with no musical partials is reproduted with tonal harmonics
in addition to the pure tone, then harmonic distortion
has occurred. Since we are concerned with a sustained
tone, the distortion components will consist of whole
number harmonics of the fundamental pitch, that is the second,
third, etc. harmonics of the fundamental pitch.
It is our belief that readers of Audio can relate more easily
to musically based tones in distortion measurements than to
test frequencies which might have a technical but non-musical
basis. For that reason, we use test frequencies based
upon the musical scale.
The test tones for these measurements were chosen with
some care. First, the number of tones needed to be limited
to keep the data from overwhelming the interpretation of
performance. Second, it was decided that the span of the
tones should encompass not only the usual range of fundamental
notes produced by musical instruments, but also be
placed to show possible speaker problems. Finally, the harmonic
structure to be measured needed to be meaningfully
related to both musical experience and to conventional
speaker problems.
The three tones f:,ally chosen were E1 or 41 .2 Hz, A2 or
110 Hz, and A4 or 440 Hz. E1 was chosen to represent the
lowest fundamental one might generally find in music. The
note is also near or below the low frequency cutoff of even
the largest reproducing systems we might be called upon to
test.
A2, in the octave below middle C, was chosen for its tonal
position at the transition point one might conceive between
bass and middle tones, as well as the fact that even the smallest
systems should be able to reproduce it.
A4, a fundamental note for many instruments and vocalists,
was chosen not only for its musical significance, but also
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because it is in the range where many speaker systems are in
crossover or where the bass driver is running out of steam.
In those instances where the crossover is below 500 Hz, we
-replace A4 with a musical tone approximately a half octave
below the actual crossover frequency.
Another reason for choosing these three frequencies is
that the significant harmonics fill up the frequency range
between the three chosen tones without overlap. In analysis
of this measurement, the harmonics or distortion terms
which are significant are the second harmonic (the same
tone in the next higher octave) and the third harmonic (the
fifth in the next octave). Fourth and higher harmonics are
rarely of any significance when the speaker is driven within
the maker's recommendations. When these harmonics do
occur, they show up as an audible breakup or knocking,
which usually signifies mechanical problems.
Measurement Scales
Harmonic distortion is measured in per cent. The fundamental's
level is defined as 100 per cent at each point of our
measurement, and the distortion component is given as a
percentage of this level.
This percentage measurement of harmonic distortion can
be related to musical experience in the following manner.
In an out-of-doors situation, the sound pressure will halve
each time you double your distance to a source. If, for example,
you were ten feet from a soloist and stepped back to
twenty teet, the direct sound would be reduced by 6 dB.
This would take a 100 per cent value at ten feet and reduce it
to 50 per cent at twenty feet. If you stepped back to forty
feet, the reduction would be another 6 dB to produce a 12
dB pressure drop, or a change to 25 percent of the original
ten foot level.
If you divide the harmonic percentage value into 100 this
gives the distance you must move away to hear that level of
sound. Ten per cent is a distance of 10 times, or 100 feet for
AUDIO. FEBRUARY, 1976