Small Room Acoustics 141Examples of rooms in this category are recording control
rooms, mastering suites, and audio production rooms.
The state of the art at this writing does not permit us to
design transducers or electronics that are perfect so as to
afford the user 100% confidence that what is heard is
fully equivalent to what has been or is being recorded.
We can, however, design rooms that fully meet this
criterion. An anechoic chamber would indeed be 100%
neutral to the loudspeaker, allowing the user to hear
precisely and only what is coming out of the speaker.
The problem is that anechoic chambers are quite
possibly the most acoustically hostile places we can
imagine. It is difficult to spend a few minutes in an
anechoic chamber let alone try to be creative and make
artistic decisions about music in one. The challenge is to
build a room that will not significantly interact with the
loudspeaker by means of room modes or reflections that
arrive at the listening position and still be a place that is
subjectively acceptable to the user. There have been a
number of good approaches to this problem over the
years starting with LEDE¥,^19 Reflection Free Zone¥
(RFZ),^20 and the Selectively Anechoic Space¥.^21 Later
came Tom Hidley’s neutral room or nonenvironment
design and more recently, David Moulton has proposed
his wide-dispersion design. These approaches all
endorse attenuating or completely eliminating all the
early reflections, creating a space that is essentially
anechoic when energized by the loudspeakers and
listened to in the prescribed position, but in all other
ways it is an average room. Reflections can be elimi-
nated or reduced at the listening position by changing the
angle of the reflector, by using an absorber, or by using a
diffuser. It should be noted that Angus questioned the
use of diffusion in controlling lateral reflections.^22
On the surface one might wonder why all sound
rooms are not built this way. The reason is that most
people do not listen to music analytically. In precision
rooms, music that is poorly recorded will sound that
way. One can certainly design rooms where the music
sounds better than it does in a precision room. There are
artifacts that one can build into a room that are subjec-
tively very pleasing, but they are part of the room and
not the recording. The recording engineer generally
wants to know what exactly is in the recording. The
engineer generally listens to the product in a number of
different environments before releasing it to insure that
it does hold up even under nonideal conditions.
So-called good sounding artifacts can be observed in
the frequency domain as well as the time domain. For
example, if a room has an audible room mode at 120 Hz
music might sound full and rich in the upper low end
and be quite pleasing, however the fullness is in the
room, not the recording. The recording may in fact be
“thin” or lacking in the low end because the room is
adding to the mix. In the time domain, a reflection that
occurs in the first 10 ms or so and comes from the side
(a lateral reflection) might result in a perception of a
stereo image that is much wider than the physical sepa-
ration of the speakers might allow. This might be
perceived as a very good sound stage, but it is an arti-
fact of the room and not of the recording.^23
Designing such a room is an art and a science. It is
beyond the scope of this book to detail a complete room
design protocol, however, the steps in designing such a
room must include:- Choosing a set of room ratios that yield a modal
distribution that will result in the best possible low
frequency performance. - Choosing a symmetrical room shape so that each
loudspeaker interacts with the room in exactly the
same way. - Choosing and placing acoustical treatment so that
the early reflections (at least the first 18 ms) are
attenuated at and are at least 18 dB below the direct
sound. Care should be taken to insure that the treat-
ment chosen exhibits a flat absorption character-
istic at the frequency range of interest and at the
angles of incidence. The energy time curve should
be measured to insure that the direct sound is not
compromised over the entire listening area. - Placing equipment and furniture in the room in
such a way as to not interfere with the direct sound.
It should be noted that the recording console is
often the most significant acoustical element in the
control room. - Insuring that there are enough live and diffusive
surfaces in the room so that the overall subjective
feel of the room is that of a normal room and not an
anechoic chamber.
6.8.3 Rooms for MicrophonesDesigners are frequently asked to design rooms that are
intended for recording or use with live microphones.
Recording studios, vocal booths, and even conference
rooms could be part of this category. The criteria in
these rooms are almost all subjective. End users want
rooms that sound good and that are comfortable to work
in. The acoustician is well advised to work with a good
interior decorator as a significant part of what makes
someone feel comfortable in a room is the way the room
is decorated and lit. Obviously noise control is a large