The Radio Shack Analogue SPL meter is not very linear in the range we subwoofer builders live in. I found this correction table on the Internet, which can make your frequency response measurements more accurate
This
table comes courtesy of Dan Marx, who runs AudioInnovation
,
a site for fellow Audio Enthusiasts and Speaker builders. As well as his
projects, he has some good info on speaker theory and a links section
There
are also some modifications that can be done to the meter to make it more
linear. A good discussion thread with both this table and the mods is
at Audiogon
|
Frequency
|
Correction
|
Frequency
|
Correction
|
|
|
10hz
|
+20dB
|
500hz
|
-0.5dB
|
|
|
12.5
|
+16.5
|
630
|
0
|
|
|
16
|
+11.5
|
800
|
0
|
|
|
20
|
+7.5
|
1.0k
|
0
|
|
|
26
|
+5
|
1.25k
|
0
|
|
|
31.5
|
+3
|
1.6k
|
-0.5
|
|
|
40
|
+2.5
|
2.0k
|
-1.5
|
|
|
50
|
+1.5
|
2.5k
|
-1.5
|
|
|
63
|
+1.5
|
3.15k
|
-1.5
|
|
|
80
|
+1.5
|
4.0k
|
-2
|
|
|
100
|
+2
|
5.0k
|
-2
|
|
|
125
|
+0.5
|
6.3k
|
-2
|
|
|
160
|
-0.5
|
8.0k
|
-2
|
|
|
200
|
-0.5
|
10k
|
-1
|
|
|
250
|
+0.5
|
12.5k
|
+0.5
|
|
|
315
|
-0.5
|
16k
|
0
|
|
|
400
|
0
|
20k
|
+1
|
After
taking your measurements with the meter set to "C"
weighting and "Slow" response,
add the figure shown in this table 
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