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resonances at different frequencies. It is essential for the frequency response of both speakers
to be similar if you are to hear the full depth and space potential of your speaker systems.
A loudspeaker system could be adjusted to measure an acoustically flat frequency response
across the entire audio range of 20Hz to 20,000Hz. However, this type of system almost always
sounds too bright. There are several reasons for this effect, which include human hearing,
recording methods, and the acoustics of the living room and concert hall.
How the ear hears, differs from typical measuring techniques in ways that can be
misleading. Most measurements combine the effects of the direct sound from the loudspeaker
with the multiple room reflections or reverberation. Since typical room acoustics accentuate
low notes and attenuate high notes, the response of a good loudspeaker will automatically
take on a downward slope in a typical listening room the ear, however, places more emphasis
on the direct sound that has arrived unmodified from the loudspeaker. Equalizing the overall
sound (both direct and reflected) to flat would give a strongly rising character to the impor-
tant direct sound and an overall bright sound quality.
Additionally, most recordings are usually made with the microphones relatively close to
the instruments. This is done to avoid including too much of the ambience of the recording
studio or concert hall acoustics in the original recording. However, it is common practice
in recording classical music to place additional microphones well back from the instruments
in order to intentionally pick up a certain amount of the concert hall reverberation. The out-
puts from these "Far" microphones is then blended in a precise amount with the output from
the "Near" microphones to add a desired degree of liveness and realism to the recording.
Concert hall acoustics will modify live sound and cause a gradual rolloff in high frequen-
cies. This reduction of high frequencies is due to the absorption and losses in air, and in-
creases as the distance from the instruments increases.
Typical home listening rooms are much smaller than a concert hall and the loudspeakers
are much closer to the listener than the instruments in the concert hall. These smaller room
dimensions result in much less high frequency attenuation from the loudspeakers to the
listeners. The effects of reflected sound are also more pronounced in the smaller listening
room.
When recordings made with mostly "Near" microphone techniques are reproduced in a
home stereo system, they produce a much brighter sound than that heard in the original con-
cert hall. The listening room does not furnish the gradual high frequency rolloff that occurs
in the larger concert hall.
Room equalization therefore involves elimination of the most obvious peaks and dips in
response, as well as a general contouring of the overall frequency response. The curves in
the graph below have been compiled from listening tests, room measurements and concert
hall measurements. These curves are intended only as a guide to establishing the desired
system response in the listening area.
Smoothness in the response curve can be corrected by use of spectrum analyzer
measurements. However, the final curve contour should be carefully considered based on
listening evaluation and the individual listener preferences. The type of music the listener
prefers usually is a major factor in selecting the desired curve contour.
Generally, a person who listens to classical music will prefer a response between curves
B and C. A person who likes contemporary or rock music may prefer a response between
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ROOM ACOUSTICS
EQUALIZING
FOR OPTIMUM
LISTENING