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4.3.c Timbre
Timbre refers to the tonal shift or coloration of a signal caused by the
loudspeaker and room interface. This coloration can occur because of the
early reflections, which create a comb filter effect. A comb filter results
whenever two signals are added together after one of them has been
delayed in time. It is sometimes called multi-path. An example of a comb
filter is shown in Figure 4-7 on page 68, where the second signal is delayed
from the first by about 1 ms.
There are two critical aspects of a comb filter for small time delays. The
first is that the notches are spaced wide enough that they fall into alternate
critical bands, thereby being perceivable as frequency effects. The second is
that the small time delays puts the second signal within the ears integration
time, thus fusing the two signals together. The ear is actually receiving the
filtered response as shown in the figure. When the time delay is extended,
the notches become so dense that they are basically not resolvable by the
ear, and, second, they are not as readily fused into the perception as the
shorter delays. The net result is that the ear is highly sensitive to short time
delays, and this sensitivity drops with longer delays.
Timbre is much like localization in that it is strongly affected by the
early refections. In one sense, timbre and localization degradation due to
time delayed signals are the same thing, but they can also be different. A
Timbre
the tonal character of a
sound.
Room Acoustics
68 Premium Home Theater: Design and Construction
vertical reflection causes strong coloration but is not a significant localization
problem. Lateral refections cause both. Timbre problems can also
come from the loudspeaker itself in the form of non-flat power response
(where the direct sound and the reverberant sound do not have the same
timbre) or strong resonances, which don’t tend to affect the localization as
much, but do cause significant coloration problems.
It should be pretty clear by now what we want and what we don’t want
in the way of small room acoustics. Below is a summary:
• First, we want a large amount of sound absorption at low frequencies
to help smooth out frequency and spatial response irregularities.
This absorption helps what is called the modal overlap or the
extent to which the modes interact. Absorption helps to “mix up”
the low frequency modes.
• We want as little absorption at higher frequencies as we can reasonably
get away with. That’s because, in general, we do not see any
positive effect on the frequency response from generic absorption,
and we know that it will degrade the perceived spaciousness of the
room by removing the desirable multiple lateral reflections.
• We want to try and eliminate early reflections or at least minimize
them as much as possible. I have shown how the smaller the reflec-
Figure 4-7.
Comb filter resulting
from the combination
of two signals delayed
from each other.
0 1000 2000 3000
Frequency
-30
-20
-10
0
10
Gain (dB)
Premium Home Theater: Design and Construction 69
Sound Perception in Rooms
tion delay, the more detrimental it tends to be and extending the
reflection delay time while reducing its level is highly desirable. I
have not yet discussed how to achieve this, but there are options.
• We will want to be careful not to go overboard reducing the refections
because that will simply lead to a dead and lifeless room.
• An early reflection arriving at an alternate ear is not as bad as an
early refection arriving at the same ear. The former case has a binaural
advantage in the brain’s signal processing that the later case
does not. This fact makes most reflections in the vertical plane
undesirable, but again we have to balance this requirement against
the desire for little high frequency absorption.
Lets now return to our discussion of the absorption in a small room. I
have shown that it is desirable to have large low frequency absorption with
little high frequency absorption, where there may be a few exceptions used
to control specific early refections. In a practical sense, there is a real problem
with this requirement. Virtually all acoustical treatments for rooms
have large high frequency absorption dropping to almost nothing at low frequencies,
which is exactly the opposite of what we want. Clearly, dealing
with the absorption aspects of a room by the use of standard materials is not
recommended. The use of sound absorption in a small room must be dealt
with extremely carefully. It has been my experience that it is almost impossible
to make a small room too live at high frequencies. Most typical room
construction materials and furniture have significant levels of absorption at
high frequencies. Obtaining the right amount of absorption across the frequency
band requires different construction techniques and room interior
treatments.
I will return to the construction details in a later chapter, however, there
are some specific topics that are more relevant here. How absorption actually
works is an important issue in our current discussion. There are two
principle mechanisms for sound absorption.
The first is to use a porous material such that the sound wave can penetrate
it, and, in doing so, the air moving in and out of this porous medium
dissipates energy through friction. This mechanism is by far the most common,
and there are some specific features to this kind of absorption. First, it
becomes increasingly less effective as the wavelength of sound exceeds the
Room Acoustics
70 Premium Home Theater: Design and Construction
thickness of the material. Thin materials will have no low frequency
absorption. The second is that since the porous material works on the
acoustic particle velocity, the effectiveness of the material is reduced when
it is placed at locations of low particle velocity—places like walls where
the velocity must go to zero. A piece of sound absorbing material placed on
a wall is 1) not very effective and 2) increases in effectiveness as the frequency
increases. This most common of all sound treatments is exactly the
wrong thing to do.
The second major source of sound absorption is through the actual
motion of the room structure—the walls themselves. Of course, if these
walls are perfectly rigid—like poured concrete—then this mode of absorption
is negligible. But, for a common frame and dry wall construction, wall
motion can be quite substantial. Since the wall has mass, its motion will
continue to fall as the frequency goes up—that is, unless it has a resilient
support structure. All walls must be supported in some way. When the support
is resilient (and all supports are to a certain extent, except for maybe a
concrete backing), then there will be a resonance frequency and the motion
of the wall will fall both above and below this resonance. A wall would typically
resonate somewhere below 100Hz—depending on drywall thickness
and the method of mounting. When the wall does move, it dissipates energy
through friction. (All absorption is friction of some sort.) The main difference
with this type of absorption is that it decreases with frequency rather
than increase as the porous material method does. This would seem to be
the ideal mode of absorption for a small room and indeed it is. In fact, if
done properly tremendous absorption can be achieved at low frequencies
with almost no high frequency absorption.
Another concept in sound absorption that comes into play in most HTs
that I have done has to do with sound absorption on opposing walls. In my
book Audio Transducers, I show how, at low frequencies, sound absorption
works the same whether it is on one wall of an opposing pair or it is on both
of them. By this I mean that the sound absorption is the same whether it is
split between two opposing walls or all of it is placed on one wall. This is a
good thing to know because it means that if we need to add low frequency
absorption to a room, we need only do it on one wall of each of the three
opposing pairs. I will show how this is a major advantage when locating a
HT in a home.
