120 Chapter 5
particularly helpful when budget is a concern since diffu-
sive treatments tend to cost twice to ten times more than
absorptive treatments on a per square foot basis.
5.3.4 Applications of Diffusion
Like absorption, major differences in the size of the
space being treated need to be considered when applying
diffusion principles. Diffusion tends to provide the most
per-dollar-spent benefit in large spaces. Diffusion tends
to happen in the far field. In the near field of a diffuser,
the scattering effects are typically less pronounced and
can actually be less subjectively pleasing than a flat
reflective surface in some applications.
For large spaces, diffusion is often employed on the
ceiling or rear wall of a space. This provides distribution
of the sound energy in the room that envelops a listener.
There will be a noticeable (and measurable) reduction in
RT, but the reduction is not nearly as severe as it would
be with a similar area of absorptive treatment in the
same space. Additionally, a typical listener tends not to
notice that RT has been reduced, but rather that the
decay has been smoothed out, intelligibility has
improved, and the room generally sounds better after
diffusion has been appropriately applied.
For small spaces, the decision to use diffusion is
more challenging. D’Antonio and Cox suggest that the
full benefit of diffusers is realized when the listener is a
minimum working distance of about 3 m (10 ft) away
from the diffusive surface.^34 This tends to be a good
rule-of-thumb from which to start. This rule provides
something of a size threshold that must be exceeded to
get the most value from the application of diffusers.
When small rooms are being treated, it is not uncommon
to be able to get far more value from absorption relative
to the same area of diffusion.
The most common applications of diffusion in small
rooms, such as recording studios and residential theaters
or listening rooms, is (like larger spaces) on the rear wall
and ceiling. The application of diffusion to the rear wall
was particularly popular in the heyday of Live End/Dead
End (LEDE) recording studio design. While LEDE is
still a popular approach, it has been replaced with the
more general reflection-free zone (RFZ) approach to
control room design. Regardless of the design method,
diffusion can be useful for removing reflective artifacts
from a small room without making the room too dead. It
can also be used to great effect, for example, on the
ceiling of a home theater. A diffusive, rather than
absorptive, ceiling can sometimes provide a better feel
to a home theater, regardless of the ceiling height.
5.4 Reflection and Other Forms of Sound
RedirectionIn addition to diffusion, sound can be redirected by
controlled reflection, diffraction, or refraction. Diffrac-
tion takes place when sound bends around an object,
such as when a passing train is audible behind a wall.
The low frequencies from the train rumble have large
wavelengths relative to the height of the wall, allowing
them to bend over the top of it. This can come into play
indoors when sound bends around office partitions,
podiums, or other common obstacles.
Refraction is the only form of acoustic redirection
that does not involve some sort of object. Acoustic
refraction is the bending of a sound wave caused by
changes in the velocity of sound through a medium.
Refraction is often thought of as an optical phenomenon;
however, acoustic refraction occurs when there are
temperature gradients in a room. Because the speed of
sound is dependent on the temperature of the air, when
an acoustic wave passes through a temperature gradient,
it will bend toward the cooler air. This can occur indoors
in large rooms when cooler air from air-conditioning
vents located on the ceiling blows into a room with
warmer air below; the sound will bend upwards until the
temperature reaches equilibrium. Even in recording
studios, a heating vent blowing warm air over one loud-
speaker in a stereo pair can skew the sound towards the
other loudspeaker and wreak havoc on the stereo image!
Finally, everything in the room, including the room
itself, reflects sound in some way, even the absorbers.
One could look at an absorber as an inefficient reflector.
When an item is small with respect to the wavelength of
sound impinging upon it, it will have little effect on that
sound. A foot stool placed in front of a woofer will have
little effect on the 1.7 m wavelength of 200 Hz, the
4.4 m wavelength of 100 Hz, or the 6.9 m wavelength of
50 Hz. The wave will diffract around it and continue on
its merry way. The wavelength of 8 kHz is 4.3 cm. Just
about anything that is placed in front of a tweeter that is
reproducing 8 kHz and up can effectively block or redi-
rect the sound.
Although it may seem odd, reflection is a very useful
form of acoustic treatment, especially in concert halls.
Adding reflections to the direct sound is what makes
concert halls what they are. However, adding reflections
to a monitoring environment can impede critical analysis
by coloring the sound coming from the speakers. It is not
a question of reflections being good or bad; rather, the
designer must decide to include or exclude reflections
based on the use of the facility and the desired outcome.