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Old 3rd March 2009, 10:04 PM   #1
jrwone0 is offline jrwone0  United States
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Default strange port frequency response

I measured the frequency response of the slotted port in my cheap studio monitors (Samson A5) and it showed it this weird bump in the midrange around 800hz. The port seems to be tuned around 70hz but the bump at 800hz is almost at the same level in the graph (see below). The response measured the same using sweeps and RTA with pink noise. To make the measurements I put a measurement mic inside the port and also at the edge of the port with about the same results.

The slot port is about 7 inches wide and .5 inch tall and extends 6 inches deep into the speaker. The internal dimensions of the speaker box itself is about 6.75 inches wide x 10 inches high x 7.75 inches deep. Overall this monitor sounds pretty clear but a little bit boxy in the mid range and I'm thinking that might be related to what shows on this graph.

I don't know much about acoustics but I would guess the 800hz frequency is too low to be caused by standing waves in any dimension in this speaker box. I would think it would be too high to be a harmonic from what is going on in the port as well, but maybe I am wrong? The box is made out of MDF so I gather its panel resonance frequencies would be much higher than this, but again I don't know enough to be sure. Ideas?

Thanks! -Jason
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Old 3rd March 2009, 10:18 PM   #2
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Speed of sound = 343 meters/Sec, or about 13,504 inches/sec. An 800 Hz wavelength is 16.88 inches long (13,504/800=16.88). Divide by four and you get 4.22 inches.

It's a safe guess you're seeing a quarter-wave resonance coming from the pipe (otherwise known as organ-pipe modes). There should also be a three-quarter-wave resonance at 2400 Hz, or nearby.

No fix except to fill the complete pipe with fiberfill or some other absorbing material and accept the loss in bass.
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