--- In wrote:
> Dear Experts,
>
> I seem to remember form my class at Cornell many years ago that
secondary
> harmonics as shown on a spectrogram are really a result of the
recording
> equipment and filtering these out of a cut will not degrade the
final audio product.
> Actually, what I am trying to determine is if I filter a cut to
reduce insect
> noise and this noise is also in the band where the secondary
harmonic is, will
> I be significantly degrading the final audio product?
Any non-linear element in the signal path of the recording equipment
may produce spurious (secondary) harmonics. The parameter
quantifying these effects is the distortion (THD) specified in
percent. This distortion can take place within the microphone
(especially when we are close to the maximum sound level specified
by the manufacturer), in the pre-amplifier (at higher gain settings)
and of course, any clipping would be an extreme source of distortion
when the recording level was not adjusted carefully. A limiter would
also add false harmonics. Analog tape recorders may introduce
additional distortions caused by the properties of the magnetic
tape. However, when using high-quality microphones with flat
frequency responses and digital recording gear, the distortion added
by the equipment should be very low and usually not visible on a
spectrogram. Things might get more complicated when using a
microphone that lacks a flat frequency response. A parabola may add
more gain to the harmonics than to the fundamental. In that case,
some kind of low-pass filtering would be appropriate in order to get
a flatter overall frequency response.
Most animal sounds, even those referred to as pure-tone whistles,
actually have some weak primary harmonics. Only a perfect sine
generator (or a less perfect generator in conjunction with a steep
low-pass filter) were able to produce whistles without any
harmonics. But such ideal sine generators are difficult to find in
animal vocalizations...
Generally, all harmonics will influence the "color" of a sound (we
can hear that when listening to the same note played on various
musical instruments). It depends on the intensity of these harmonics
and their pitch whether they are important to our perception or not.
Listening before and after filtering would reveal their importance.
In many whistle-like bird songs, the harmonics are not very
important and filtering them out would not significantly degrade the
final audio product. However, applying a low-pass filter to a
recording is similar to reducing the sample rate and may therefore
lead to some degradation. It depends on the personal preferences,
whether to remove insect noise at the expense of loosing some other
high-frequency signals.
Regards,
Raimund Specht
Avisoft Bioacoustics
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