QuickEnhance® VST User's Manual - Digital Audio Corporation

Figure 10: Typical Averaged Voice Spectrum
The figure above shows that there is voice information out to 10,000 Hz
and beyond. However, when we expand the upper frequency limit, we
find that the signal usually contains substantially more noise than voice
information. We use the term signal-to-noise ratio, S/N or SNR, to
indicate the relative amount of energy in the desired signal compared
with the interfering noise. The signal-to-noise ratio is defined as:
S / N =
signal energy
noise energy
If we expand the bandwidth, it will only marginally increase the signal
(speech) energy but will substantially increase the noise level. The S/N
ratio decreases as a result, and it is harder to understand the voices. We
want the highest signal-to-noise ratio possible when we are trying to
understand what is being said. We have found that voice quality and
intelligibility are acceptable for a voice frequency band of 200 to 5000 Hz.
Expanding beyond these limits has marginal value.
18