INTELLIGIBILTY, 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
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 waveforems 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.
Figure 6: Examples of sine and square waves at the output of a 100-watt amplifier.
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.
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.
Now, let’s feed this speech signal to an amplifier with an output capability of 100 watts into an 8 ohm load, as shown in Figure 8. We have labeled the left axis with the actual output voltage produced by the amplifier, and we have indicated the approximate average signal voltage at the right axis.
Figure 8: Speech envelope at the output of a nominal 100-watt amplifier.
It is clear in this figure is that the average signal output is about ±10 volts, while the full voltage output capability of the amplifier is ±40 volts. The difference here is 12 dB, which corresponds to a power difference of 16 to 1.