JBL Room Mode Correction User Manual page 9

3. Now advance to Track 3. This track contains a set of warble tones beginning at 126 Hz,
moving downward to the lowest frequencies. As this track is played, keep your eye on
the meter. If none of the tones on Track 3 causes an over-reading, draw a horizontal
line on the chart paper at a level of -7 dB. (See Figure 3) This will be your baseline
reference level.
4. If any of these tones causes an over-load reading on the meter, stop the CD player and
adjust the CD source volume downward slightly. Begin the test again, making sure that
none of the segments in Track 3 will cause an overload. Once you have set a playback
level that causes no overloads, return again to Track 2 and note the new level as
observed on the sound level meter; draw a horizontal line on the chart at the indicated
level. For example, let's assume that your new level setting from the CD source results
in a reading of, say, -9 on the meter; then you would mark the chart paper with a line
at -9, which would become your new baseline reference level. (See Figure 8 below)
-5dB
-6dB
-7dB
-8dB
-9dB
-10dB
-11dB
-12dB
-14dB
-16dB
20Hz 22 24 25 26
5. For now, let's assume that your initial setting of -7 dB was OK and that there were no
overloads when you played Track 3.
6. Beginning with Track 4 and proceeding on to Track 26, make entries on the chart paper
for each warble tone. Before each tone plays, an announcer names the frequency of the
tone. You will note that the warble tone sequence begins at the highest frequency of
126 Hz, progressing downward to the lowest frequency of 20 Hz. Make your entries by
putting a dot on the intersection of the announced frequency and the level as you read
it on the sound level meter. This process is shown in Figure 9. Note that the test tracks
are about 35 seconds long -
probably longer than you will
actually need to make an
entry. You may speed up the
measurement process by
pressing the skip button on
the CD player once you have
made your entry on the chart.
During the course of making
your entries, you may observe
that the meter reading shows
a uniform flickering between two adjacent values. If this occurs consistently, it indi-
cates a value halfway between the two LED's, and you may make your entry according
ly at a half-dB step.
7. After you have finished plotting the sequence of readings, simply connect the dots. You
are now ready to interpret the plotted data with the help of the following sample plots.
31 35 41 45 47 50 53
28
0dB
-1 dB
-2dB
-3dB
-4dB
-5dB
-6dB
-7dB
-8dB
-9dB
-10dB
-11dB
-12dB
-14dB
-16dB
20Hz 22 24 25 26
Frequency
57 62 68 76 85 90 95
45 47 50 53 57 62
28 31 35 41
Hz Level dB
Figure 8
115
105 126
Figure 9
68
76 85 90 95 105 115 126
Width %
9