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I'm trying to determine the reason for a dip in the frequency response curve of my loudspeaker. I have a loudspeaker mounted on the inside of an enclosure, and a 100mm ID tube mounted in-front of the speaker on the outside of the enclosure.

The measured frequency response is: enter image description here

At roughly 4100 Hz, a large dip in the response is noted. This dip corresponds to the cut-off frequency of the (0, 1) cross mode. This leads me to believe that it has something to do with the higher-order mode propagation, but I don't understand what.

Could someone help me understand this problem please.

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That dip looks suspiciously sharp. Are you sure it's not an artefact of whatever kit you're using to measure the frequency response. – John Rennie May 27 '12 at 17:12
I'm using a kit I put I'm possibly missing something fundamental. The measurement chain is roughly: generate MLS signal->output to soundcard->amplify->playback with loudspeaker->record response with mic->extract signal from mic power supply->convert to digital signal with sound card->circular convolution->system response. I also account for the sound cards response. Adding a LPF to the signal reduces the dip, so does adding fibrous material infront of the loudspeaker to reduce the higher order modes. I will look at trying a different excitation signal. – Busk May 27 '12 at 20:12
Looks like cancellation from reflection? – endolith Jan 4 at 21:16

You should first use a PC's signal generator to validate the response curve with your ears. There's no doubt you'd hear (or not hear as the case may be) a 50 dB drop. :) I've never seen a real-life response with a dip like that.

Try either to generate a sweep from 4.0k to 4.5k or use Visual Analyzer's signal generator.

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