Midbass hump in TL speaker - diyAudio
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Old 28th October 2009, 05:38 PM   #1
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Question Midbass hump in TL speaker

Guys,

I built my Recession Buster kit into a TL, and am struggling with a broad misbass hump between 200-400 Hz and about 6-10 dB in magnitude. Everything is beautiful from 500Hz upwards.

Using nearfield measurement, albeit using an iPhone SPL app, i found that the driver nearfield output is ruler flat, while the line terminus nearfield output is like a gentle hill with the peak centered at 300 Hz.

I believe the 200-400 Hz hump i measured is the resultant of the sum of the flat woofer and humped terminus output.

I have tried stuffing the last section of the terminus which ends at the bottom of the front baffle, but this only diminished bass response below 200Hz, with no effect on the midbass from 200-400Hz.

The gentle curve of the terminus output does not suggest an organ pipe resonance in the last section of the line, which measures 40 cm in length, together with end correction gives a organ pipe mode of just about 300Hz.

Any advice and suggestions to tame this response is much appreciated.

Cheers!
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Old 29th October 2009, 02:22 AM   #2
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Join Date: Jun 2005
Location: Singapore
Maybe the graph of the response will help tickle your collective thoughts.

Note the SPL is relative, it is the shape that i am interested in. The combined response is rather jagged, perhaps due to comb filtering, as i was measuring between the two speakers. But the general shape of the hump centered at 300 Hz is visible, and very audible.

You can see that my efforts at stuffing the terminus only affected the bass at the low range of 200 Hz.

Cheers!
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Old 29th October 2009, 01:34 PM   #3
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Good morning Rhapsodee,

Those measurements are showing 20 dB swings in output, that is quite high. Also, the data-points are far apart it would seem, and I wonder how much of that graph has been extrapolated by the application. You say this was done on an iPhone? Unfortunately, I think the first step may be to verify your measurements with something a little more accurate, unless you can verify the accuracy of the iPhone some how. If it is possible to increase the resolution of the measured data, that would be useful.

I am not able to make a comment on TL parameters as I know nothing about them, but if I had to make an initial guess, your enclosure tuning frequency may be a bit high, likely around 300 Hz. What lowers TL tuning? Added path length, path volume? What are you using for stuffing?

Jim
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Old 29th October 2009, 03:19 PM   #4
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Hi Jim,

Thanks for your response. First of all, i was measuring at 5Hz intervals. Is that too widely spaced? There was no extrapolation of the curves, every single point was measured and plotted. I have a good confidence in the iPhone measurement in that frequency range that i used, partly because of the ruler flat measurement of the nearfield woofer output (with the iPhone 1 inch from the woofer cone). I mean it would be an extreme coincidence that the uneven response of the woofer exactly matched the iPhone to give a flat curve.

The wild swings may be because i was measuring the stereo summation midway between the two speakers. Maybe i should re-measure it with only one speaker, midway between the woofer and vent.

Regardless, going from the nearfield woofer and nearfield vent outputs, it would make sense that they sum with a hump in the response.

The TL is tuned to 50Hz with a 1.6:1 taper. I'm thinking the large size of the terminus (10cm x 10cm) may be allowing a lot of midbass energy to escape. Or the last section of 10cm x 10 cm x 40cm long is having an organ pipe resonance. My efforts at damping has proved fruitless. Perhaps i should try a different type of stuffing material?
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