The measurement:
A calibrated loudspeaker has been fed with a sinus wave on each frequency level. The loudspeaker has been placed upon a calibrated pressure sensitive microphone. For each frequency the signal is fed through the loudspeaker and the output level is adjusted to specific level which is referred to as 0 dB.
Then the spike is placed between the loudspeaker cabinet and the pressure sensitive microphone. The same signal with the same output level is used. The level from the microphone is then measured. The direct sound through air from the loudspeaker to the microphone is never measured louder than –30 dB for any of the frequencies. The curves will therefore show the approximate transmission of energy from loudspeaker cabinet through the spike and down to the microphone.
In words the damping of acoustic feedback is:
|
50 Hz
|
-11 dB
|
|
100 Hz
|
-10 dB
|
|
150 Hz
|
-6 dB
|
|
200 Hz
|
-1 dB
|
|
300 Hz
|
-6 dB
|
|
400 Hz
|
-3 dB
|
|
600 Hz
|
-3 dB
|
|
800 Hz
|
-1 dB
|
|
1600 Hz
|
0 dB
|
|
2500 Hz
|
0 dB
|
|
3200 Hz
|
-5 dB
|
|
6400 Hz
|
-2 dB
|
|
9600 Hz
|
-2,5 dB
|
|
12800 Hz
|
-3 dB
|
We do not know of any other that has measured spikes this way before. Until now spikes only have been listening tested as far as we know. This is a way to try to measure the benefits in a way that makes it possible to show the results on paper. There can be inaccuracy because of unknown resonances and other unknown influence, but we are sure these results show how spikes reduce acoustic feedback.
The influence of reducing the feedback at one frequency for another is not measured. Listening tests show that reducing feedback at low frequencies does have large influence on the sound quality at higher frequencies.
The different models, threaded and self-adhesive have been tested. The results are so similar for the different models that it does not influence the curve. At 50Hz (the most important frequency to damp) we had a reduction of 11 dB, which means that appr. 95% of the feedback energy was stopped. | 
