Specifications

TI 323 (6.0E)
13 - 36
4.4.4. Comb filter effects
With more than one system per side care has to be taken
to minimise the audible influence of the comb filter effect.
An unavoidable problem with multiple sound sources, this
effect creates a very uneven frequency response with
audible peaks and cancellations due to interference
across the coverage area.
What is a comb filter and how does it build up ?
If identical signals from two sources reach a point with a
small offset in time, there is always a frequency for which
this offset difference corresponds to the time for half of
its wavelength. This signal arrives at the listening point
twice, but with opposite phase. The result is a full
cancellation for this frequency.
Two times this frequency is called the comb filter
frequency f
comb
for this point. The result is an in-phase
arrival and, given equal levels, a +6 dB addition.
At 1.5 times f
comb
there will be another cancellation and
so on.
In the example above two sources of equal level and a
time difference of 1ms are added.
1ms corresponds to a full cycle at f
comb
= 1 kHz. 1/2
f
comb
= 500 Hz, corresponding to half a cycle difference
in time, at this frequency we see the first cancellation. In
frequency steps of f
comb
= 1000 Hz starting from 1/2
f
comb
we get more cancellations.
If the levels of the sources is different, the effect stays the
same, only the depth of the cancellations decreases.
To minimise comb filter effects, the mid/high cabinets are
arrayed in such a way that the coverage patterns of the
loudspeakers overlap as little as possible. Arraying C3
and/or C4 cabinets using the angled rear side panels
produces minimal overlap and minimal interference with
maximum uniformity of frequency response over the
horizontal dispersion angle.
If a system has insufficient throw for a certain direction in
a room, multiple cabinets covering the same area can be
stacked vertically. This minimises audible comb filter
effects at audience level.
The cabinets should be placed as close together as
possible, high enough and angled correctly to cover the
far field only. Given that the difference in distance
between the sources is rather small, filter effects are
limited to very high frequencies.
Summary:
Source overlapping is acceptable in the far field because
with greater distances the relative differences in path
length become smaller thus minimising audible comb filter
effects. The benefit is an increased SPL. Try to limit
overlapping to the vertical plane which limits the areas
covered by multiple speakers to the far field. Speakers
covering the same area should be positioned as close
together as possible in order to keep the path length
distances low.
In contrast, horizontal overlapping in an array will also
produce overlapping coverage in the near field. This
causes highly audible comb filter effects starting in the
mid frequency band and gives an increase in SPL where
it is not needed.
4.4.5. Coherent signals and directivity
build up
If identical signals, reproduced by different sources,
arrive at a point without a time difference, these
signals are called coherent.
If there is a small time difference, such that for the
highest frequency of the source's operating band the
time for half a cycle is still much longer than the
difference in arrival time, these two sources are also
summed coherently. This means that a doubling of the
number of sources will result in a +6 dB level
increase.
This effect can be utilized for subwoofers located
close together, these will sum below a certain
frequency, dependent on the total size of the
subwoofer stack.
With a few subwoofers stacked together the
increased level radiates spherically, so in all
directions. Larger arrays will produce directivity
patterns: a vertical column of subwoofers will add
coherently in the horizontal plane, a horizontal line
will add coherently in the vertical plane. As the
listening point moves from this plane, coherency will
decrease until maximum cancellation occurs.
It is only possible to direct low frequency energy by
stacking subwoofers in columns. Horizontal subwoofer
arrays are normally not desirable; they narrow the