Hi Josh,
Sorry for the very delayed response. I was trying to find a way to post this as a figure but it needs some hosting... and we got carried away by other matters...
We did the measurements a while ago, and it turned out that it's a combination of attenuation of the HiFi module output and the properties of our magnetic speaker. I thought it would be good to report here.
The following is the HiFi module output measured through an oscilloscope, when the amplitude is set to 1.
freq Pk-pk dB re max
1000 5.32 -0.3
2000 5.32 -0.3
4000 5.28 -0.3
8000 5.32 -0.2
10000 5.36 -0.2
15000 5.4 -0.1
16000 5.44 -0.1
20000 5.44 0.0
24000 5.44 0.0
30000 5.2 -0.4
32000 5.08 -0.6
35000 4.84 -1.1
37500 4.56 -1.6
40000 4.24 -2.2
42000 4 -2.8
45000 3.6 -3.7
50000 2.96 -5.4
55000 2.48 -7.0
60000 2.04 -8.7
65000 1.68 -10.5
70000 1.44 -12.1
75000 1.2 -13.6
80000 1.04 -15.1
As you can see, at 60-65 kHz there is around 10dB drop in output.
freq dB SPL effect of speaker (dB)
1000 81.8 2.0
1500 79.4 -0.3
2000 77.5 -2.2
3000 75.6 -4.2
4000 80.8 1.1
6000 74.3 -5.5
8000 79.3 -0.4
10000 80.0 0.2
15000 73.2 -6.7
16000 72.8 -7.1
20000 80.6 0.6
24000 79.3 -0.7
30000 81.0 1.4
32000 80.9 1.5
35000 80.9 1.9
37500 80.6 2.2
40000 77.1 -0.7
42000 77.0 -0.2
45000 77.1 0.7
50000 73.1 -1.5
55000 68.8 -4.2
60000 64.1 -7.2
65000 60.6 -9.0
70000 50.3 -17.6
75000 49.7 -16.7
80000 43.4 -21.5
Our speaker additionally introduced an additional attenuation of around 10 dB at 65 kHz.
Since the second table is measured with the HiFi module amplitude set to 1 (max), this is the maximum intensity we can give.
