Specifications

Appendix: Acoustics
Dynaudio Professional AIR reference manual – 2014-08-28 114
Comb ltering
The ltering function that arises when a signal is added
to itself after having been delayed in time is called a
comb lter.
The resulting frequency response resembles a comb,
hence the name.
Fig. 75: Two 500 Hz sinusoidal tones added. The
second tone is delayed 1 millisecond, hence the sum
is zero.
Fig. 76: Two 1 kHz sinusoidal tones added. The
summed level is doubled (+6 dB).
The comb lter function is almost never intentional – but
it is heard all the time in audio productions, where it can
arise both for acoustical and electrical reasons.
Acoustically, comb lter effects typically occur when
part of the sound travels from source to recipient both
on a direct path and via a single reective surface.
This reection must be attenuated at least 10 dB and
preferably 15 dB in order for it not to have an effect on
the sound eld at the recipient’s position. Electrically,
the phenomenon arises when two microphones with a
certain distance between them capture the same sig-
nal and the level from each microphone is of the same
order of magnitude.
Fig. 77: Two typical situations in which comb ltering
occurs, either acoustically or electrically
Generally speaking, all digital signal processing takes
time. This means in practice that comb lter effects can
arise if you send a signal through a signal processor
and combine the processed (and hence delayed) signal
with the original.
Fig. 78: An example of a comb lter effect created by
combining two signals with the same amplitude and
a one millisecond delay between them. Dips occur
due to cancellation at 500 Hz, 1.5 kHz, 2.5 kHz etc.,
and the two signals’ levels double (+6 dB) at low
frequencies with a full wavelength’s delay at 1 kHz,
2 kHz, 3 kHz etc.