Port resonance simulation in UniBox @ low tuning - diyAudio
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Old 13th December 2009, 11:02 AM   #1
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Default Port resonance simulation in UniBox @ low tuning

I am trying to understand the port resonance with simulation using Unibox.
Tuning frequency =25 Hz, Damping= Heavy Fill, Leakage = No Leaks and Port Type = Two Flared Ends.

Case 1:
No. of ports = 1 and dia of port = 10 cms
Unibox shows several dips starting with about 180 hz as in the attached image (case1.jpg).


Case 2:
Since multiple ports are supposed to generate less port noise, I used no. of ports = 16
and dia of port = 2.5 cms. This results in the same total port cross section area as case 1.
There is no effect on the dips using several smaller ports as in the attached image (case2.jpg).

1) I was expecting a reduction of the dips in Case 2, but the graphs shows the same result.
Can some one please explain?

2) Also how audible are these port resonance noises at higher frequency if the crossover to the mids is at 300 Hz?

3) Are these dips so severe as shown in the simulation?

Thanks in advance
Goldy
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Old 13th December 2009, 11:32 AM   #2
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Images attached
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File Type: jpg Case1.JPG (196.7 KB, 157 views)
File Type: jpg Case2.JPG (197.1 KB, 144 views)
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Old 13th December 2009, 07:45 PM   #3
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Port resonance frequency depends on the length of the port(s), not the number of ports.

Many small ports with the same total area as one large port will have the same length and thus the same resonances. The air velocity in the vents will also be the same, so the amount of port noise will be the same (in theory).

To reduce port noise you need to make the port(s) larger (more area) and longer so they still tune the enclosure to the right frequency. This makes the port resonances lower in frequency which can be a problem if they are in the same band as the frequencies you want the speaker to reproduce.

For example, if you want the box to work from 20 Hz to 200 Hz, and the first port resonance frequency is 150 Hz, you have a problem. But if the box is crossed over so it only works from 20 Hz 10 100 Hz, then no problem.
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Old 13th December 2009, 08:07 PM   #4
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The trick to try would be to use two ports with a smaller diameter which results in a different area to your single port (either bigger or smaller) but if smaller make sure you don't go less than the minimum port diameter specified by unibox. You can go smaller according to your pics the first shows minimum diameter 7.56cm (for single port)... you could try two at 6cm diameter and see what the difference in port resonance looks like. Make sure you use port diameters that are available though Basically using two ports in general will allow you to get a bigger port area than may be practical with one large port, but it also gives you some flexibility in choosing the overall port area, and hence port resonance. But in your case I suspect you need to reduce the port area to aleviate the problem you are seeing.

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Last edited by wintermute; 13th December 2009 at 08:09 PM.
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Old 13th December 2009, 09:34 PM   #5
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One infinity speaker (Overture) had really long ports like that. A reviewer said they could hear the effect with test tones, but not as much with music. It was especially bad with the speaker on its side (it was ported out the bottom). Only the first resonance was evident in measurements.

You might be better off with a PR.
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