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Page 10
This disc allows the user to play 10Hz through 98Hz in one hertz increments allow-
ing very good resolution over your measurements. Other good examples for test
discs are Avia’s Guide to Home Theater and Digital Video Essentials. When meas-
uring the room’s response use a minimum resolution of 5Hz. Figure 3 on the previ-
ous page shows a theoretical illustration of a room’s response.
Before you plot your results however, it’s important to note the Radio Shack sound
meter is not totally "linear" in its accuracy down low. All similar meters are off by
the similar amount depending on the frequency; so while its sensitivity below 20Hz
is not what it is at 50Hz, there is a simple way to compensate for this deficiency. It's
called a compensation chart.
Simply look to the below frequencies and add the required decibels (dBs) at the fre-
quency in question. So if your system seems to be measuring 70dBs with a 20Hz
tone, then really it's putting out 77.5dBs. This is a VERY substantial difference,
roughly TWICE the acoustical energy, requiring several times the amplifier power to
achieve. You can see then how critical the below chart is if you are using your PEQ
to even out or “flatten” the room response of your bass.
Add the following to your measurements:
10Hz add 20dBs
12Hz add 16.5dB
16Hz add 11.5dB
20Hz add 7.5dB
25Hz add 5dB
31.5Hz add 3dB
40Hz add 2.5dB
50Hz add 1.5dB
63Hz add 1.5dB
80Hz add 1.5dB
100Hz add 2dB
125Hz add .5dB
Explanation of the controls. Now that we understand how to measure the
room’s response the next step is to correct the problem areas with the parametric
equalizer. Before we start turning the controls we need to understand what the ad-
justments are and how they work (the controls of your PC-Ultra may be arranged
different than pictured).
“
Q”. This control determines how wide the correction will be. Each “hash mark” on
the amp’s front plate represents 0.1 octave adjustments. The “HI” mark is equal to
0.1 octave (narrow) and “LO” mark is equal to .9 octave (wide). In F igure 3 the
highest peak is located at 40Hz and becomes flat again at 50Hz (the Highest Fre-
quency of the peak). To determine the Q adjustment the following formula is
needed. More on “Q” in a moment.
Formula 1 (Highest Frequency [X] – Center Point [Y])
Center Point (Y)
SVS PEQ Controls