Although Radio Shack claims that
under normal use, the meter shouldn't require recalibration,
a hole labeled 'CAL.' is provided which will allow a technician
to gain access to the meter's innards and recalibrate it if
necessary. Finally, the meter is equipped with a phono-type
output jack for connection to other measuring equipment, and
a standard threaded socket for tripod mounting. Mounting the
meter atop a tripod is recommended in order to avoid reflections
off of your body which can alter measurements by more than 2dB.
Using the Meter For Listening
Room Setup
When setting up loudspeakers in
a listening room, the ultimate goal is to choose loudspeaker
and listener positions such that sound pressure levels are fairly
constant over the audible frequency range. While this may sound
relatively simple to accomplish, doing so can be very difficult
in practice as the sound waves reflecting about your listening
room sum and cancel at different spatial locations, resulting
in either increased or decreased sound pressure level at certain
frequencies. If your listening position happens to be located
at a position of increased or decreased sound pressure at a
particular frequency, a boost or suckout at that frequency may
be audible, resulting in a decrease in your system's accuracy
and musicality.
The Radio Shack sound pressure
level meter allows you to measure the sound pressure levels
at your listening position given a particular loudspeaker position,
and can help you avoid those positions that result in large
increases or decreases in sound pressure level. To effectively
use the sound pressure level meter for this purpose, a test
CD containing a pink noise track and tracks containing test
tones over a wide frequency range is required. Such CD's are
available from several sources, but I typically use either Stereophile's
Test CD 2 or Test CD 3 each containing tracks which fit the
bill nicely.
The first step in using the meter
for loudspeaker placement is to pick a starting location for
your loudspeakers and listening seat. Don't worry about the
initial placement too much since you'll most likely be changing
it radically once you start measuring. Pick initial locations
such that your speakers are equidistant from the side walls,
and your listening position is centered between the loudspeakers.
Now set the meter's dial to the 80dB setting, the weighting
to 'C', and the response to 'SLOW', and locate the meter where
your head would be if you were sitting in your listening seat.
If you're not using a tripod, try to stand to the side of the
meter to reduce reflections from your body that might affect
your measurements. Play the pink noise track on your test CD
(track 15 on Stereophile's Test CD 2) and adjust the volume
of your system so that the meter reads 0dB (which means that
the sound pressure level at the meter's location is 80dB since
the meter's dial is set to 80dB). Be sure to leave your system's
volume control at this level so that subsequent measurements
can be meaningfully compared to the pressure at this volume
level. Ensure that the meter is at the same location as when
making the pink noise measurement, and play the test tone tracks
from your test CD (tracks 16-18 on Stereophile's Test CD 2).
For each test tone, jot down the tone's frequency and the meter's
reading at that frequency. When you're done measuring the sound
pressure level for each test tone, you will have a set of measurements
of frequency vs. sound pressure level. In other words, you will
have the data for a frequency response graph of your room, given
the current loudspeaker and listener locations.
Unless you got extremely lucky,
your first set of measurements will indicate fairly gross deviations
from the 0dB measurement obtained with the pink noise track.
In that case, systematically move your loudspeaker and listener
positions in small increments, of say two inches or less, and
repeat the measurements. Try and resist the urge to make large
spatial changes between measurements. Remember, moving a loudspeaker
one or two inches can have a radical effect on measured frequency
response (after each change in loudspeaker and/or listener position,
don't forget to readjust your system's volume so that the pink
noise track measures 0dB on the meter with the dial at the 80dB
setting).
Don't drive yourself crazy trying
to get ruler flat in-room response because in most cases it
simply isn't possible - a variation of a few dB across the range
of measured frequencies is perfectly acceptable. If, on the
other hand, you are measuring gross response increases or decreases
of, say, 10dB or more at certain frequencies, then invest the
time to try to eliminate them as they will not allow your system
to reach its full musical potential.
You Only Get Two...
While simulating the volume level
of a large rock band or full symphony orchestra in your listening
room can be a lot of fun, remember to keep an eye on those SPL's
to ensure that they are within safe levels. The following table
from the U.S. Department of Labor (also included in the excellent
documentation accompanying the Radio Shack sound pressure level
meter) gives data on permissible sound level exposure.
