X-Treme Audio HPS - Manual (Page 3)

User’s manual
Compared to the physical size of the sw, these λ values confirm its
very low directivity, already verified on our omni-directional model.
This will come about unless special techniques are used to “force”
the directionality of a sw, making it possible to optimise the direction
of the sound (as with the XTCARDIOID: a cardioid directivity sub-
woofer manufactured by X-Treme Audio).
Another phenomenon related to
λ size is combing. Generally speak-
ing, two sources that emit a specific frequency will create zones of
constructive and destructive combing: the two pressure contribu-
tions are “in phase” for the former, resulting in an increase in acoustic
pressure (the level of sound pressure resulting from two contributions
with equal amplitude and perfectly in phase is the same as that of the
single source + 6 dB). For the latter, the two contributions are “push-
pull”, leading to the cancelling out of the pressure (two contributions
of equal amplitude with perfect push-pull will cancel out one another:
-∞ dB). These two situations depend on the difference of the acous-
tic paths relating to the two sources at the point in question.
Combing does not generally create major problems at the medium-
high end of the spectrum due to the fast spectral and spatial alterna-
tion of the phenomena of construction and cancellation; however its
effects can be significant with sw’s. In this case, the dimensions of
the combing “fringes” are macroscopic and, as such, can be per-
ceived by the listener, who will note clear and undesirable changes in
the level of acoustic pressure when moving from one part of the area
of sound to another.
3. Combing and mutual pairing
Another phenomenon linked directly to the relationship between λ
and the distances involved is pairing of sw’s. Two sources placed at
a comparable or smaller mutual distance than
λ “pair up”, meaning
the total acoustic power delivered is not simply the sum of the two
powers supplied individually: the acoustic power of each one de-
pends on the other one’s performance. This comes from the fact that
the effective acoustic power depends both on the vibratory motion
of the source (which we can basically associate with the movement
of the cone) and on the total acoustic pressure near it: the combing
coming from nearby sources also plays a part in the total pressure
encountered by the movement of the cone. The following examples
show how combing, analysed across the entire acoustic field, easily
explains this phenomenon of power pairing.
a) Two very close sources with respect to
λ in the band considered
(basically: distance between them is less than λ/8): can be consid-
ered coinciding from an acoustic point of view, meaning the acous-
tic paths separating them from all spatial points virtually coincide if
λ is used as a unit of measurement. This leads to phase coherence
throughout the space and so to an increase of 6 dB compared to
the single source. This 6 dB increase in acoustic pressure at all
spatial points leads to a similar increase in sound intensity (this is
true in far field): if the flow of the intensity is calculated on a surface
that encloses the system, the result is an increase of 6 dB in the
acoustic power delivered. So, the acoustic power is fourfold for
actual electro-acoustic sources, even though the consumption of
electrical energy has only doubled. Here are two basic explana-
tions of the phenomenon: the acoustic impedance “seen” by each
source is doubled, as is the efficiency of both of them. We can also
see that the energy balances seem right given the low efficiency of
a sw, namely the very low acoustic energy compared to the energy
of the cone that is largely dissipated out of it.
It should be remembered that for two actual paired elements, an
increase of 5 dB is a more accurate estimate than the nominal 6
dB due to the considerable size of a subwoofer and the frequen-
cies it can reach.
b) Two very distant sources compared to λ (basically distances over
5 λ): the space will typically have alternating zones of constructive and
destructive combing with an average 3 dB increase in pressure. The
power in this case is only doubled: the individual powers are simply
added together, so the sources can be considered un-paired.
c) Two sources placed at a mutual distance comparable with λ (such
as two subwoofers placed a few metres apart): a positive pairing
(more than 3 dB increase in acoustic power) or negative pairing
(less than 3 dB increase in acoustic power) may occur depending
on the distance/
λ relationship.
d) Close sources compared to λ but with push-pull emission: the
pressure is nil (or “very low”) throughout the space, so the acoustic
power is nil (nil impedance, nil efficiency: the cones are unable to
compress the air and simply move it between one and another).
Some of the examples discussed are illustrated in Figure 2.
Single source Two coupled sources
Two uncoupled sources
Two coupled sources
in push-pull
Fig. 2 Combing figures for different source coupling
(instant SPL)
4. Pairing with the environment
The phenomena of interaction with the environment can often be traced
back to the mutual interaction we have just seen, applying the Mirror
Image Source Method (MISM). This says that a flat wall can be con-
sidered almost a twin of the actual source and symmetrical to it with
respect to the wall itself (not unlike a source of light placed in front of
a mirror) provided it has a sufficiently large and reflective surface area.
Take the example of a subwoofer resting on the floor: its image
source is located immediately underneath it and so is well paired
with it. In this case, the efficiency doubles but the electrical power is
not affected because we have not added an actual source. So, com-
pared to the case of two actually paired sources, the acoustic power
is only twofold and not fourfold. However, since it is concentrated in
a single half-space, the increase in intensity and pressure coming
from the floor is again about 5 dB, compared to a hypothetical free
field situation. The same type of pairing will occur if the sw is placed
against a vertical wall (provided this is sufficiently large and solid).
The same wall at a slightly higher distance (so the distance of the
image source is comparable to the average
λ) may result in power
cancellation for certain frequencies and emphasis for others, altering
the timbre and the characteristic “emotion” of the sw used and con-
tributing considerably less than 5 dB in this case.
5. Hence, some pratical ideas...
• You can safeguard optimum acoustic power by installing the sub-
woofers in stacked clusters (groups of subwoofers on top of one
another) and keeping the distance between one and other and
from the floor as small as possible.
3/6