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As we can see in the figure, the two octaves between 1 kHz and
4 kHz are dominant, and this is why, in very noisy listening environments,
sound reinforcement systems are often band-limited to this range.
Ideally, we would like for reproduced or reinforced speech to sound
both natural and intelligible, and this is certainly possible in
reasonably quiet environments.
INTELLIGIBILTY AND AMBIENT NOISE LEVEL
Ideally, the local noise floor should be about 25 dB below average
speech levels for the most natural reinforcement of speech. If the
ambient noise level in a space is only 15 dB below the speech level,
most listeners will have no trouble understanding the message, but
many of them will complain about the noise level. As the speech-to-noise
ratio is further reduced there will be a pronounced loss in intelligibility
for all listeners, prompting sound system operators to increase
the level of the reinforced speech signal. There is a limit to this
procedure however.
When is speech level too loud? Normal face-to-face speech communication
is in the range of 60 to 65 dB SPL; however, most speech reinforcement
systems operate in the range of 70 to 75 dB SPL. If the level of
amplified speech is increased beyond the range of about 85 or 90
dB SPL, there will be little increase in overall intelligibility,
and most listeners will complain of excessive levels. At even higher
levels there will be a diminishing of intelligibility as most listeners
will literally feel oppressed by the too-high levels. The trend
here is shown in Figure 5.
Figure 5: Effect of speech level
on intelligibility. |
There is an optimum operating range for a speech reinforcement system.
For those systems in very quiet surroundings a normal level of 65
to 75 dB SPL is ideal. In progressively noisier environments the
system operating level should be raised so that the signal-to-noise
ratio is at least 15 dB. Typical here would be a transportation
terminal at peak travel times, where noise levels in the 60 to 65
dB(A) range would call for system operation at peak levels of 80
dB SPL for greatest intelligibility.
Sports venues often present high crowd noise levels in the range
of 85 to 95 dB SPL, and under these conditions it is virtually impossible
for a speech reinforcement system to work at all. It is better to
wait until crowd noise subsides before making announcements.
MATCHING SPEECH LEVELS TO CAPABILITIES
We have seen that amplified speech levels must be contained within
a fairly narrow range of about 15 or 20 dB for most effective operation,
and systems should be designed with this requirement in mind. First,
we will show the waveforms for sine and square waves of an amplifier
capable of delivering 100 watts into an eight-ohm load. Note that
full utilization of the amplifier’s voltage drive limits,
the sine wave output is 100 watts, while the output of a square
wave will be 200 watts.
Why then do we rate this amplifier at only 100 watts? All amplifiers
are rated according to their maximum sine wave output capability
into a stated load impedance. The sine wave has a 3-dB crest factor
(peak-to-RMS ratio), while the square wave has a crest factor equal
to unity, as shown in Figure 6. Since music and
speech signals are composed primarily of sine-like waves, the amplifier’s
power nominal rating is stated as 0.707, the actual peak output
voltage rating of the amplifier, or 3 dB lower.
Figure 6: Examples of sine and
square waves at the output of a 100-watt amplifier. |
If we actually record a typical speech signal over a period of
about 20 seconds, the signal envelope will look much like that shown
in Figure 7. You can see that average signal hovers
largely around the baseline, with occasional higher values and only
rarely reaching the full scale of the figure.
Figure 7: Speech envelope over
a 20-second interval. |
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