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Bass Boom - change port length?

3154 Views 5 Replies 3 Participants Last post by  jtalden
I have just built some bass reflex cabinets for a pair of Tannoy Monitor Gold HPD 315s. They are loosely based on designs discussed on the Hilberlink Tannoy, and are roughly 150 litres in volume with 2 100mm diameter, 100mm long ports. (As apparently recommended by Alex Garner of Tannoy)

I am very happy that the sound is much better than my previous speakers, but I did notice that on certain tracks, the kick drum can get a bit tiring. I have set up REW, and calibrated the Behringer ECM 8000 mic and Behringer UCA202 sound card. I have been experimenting with blocking one or both of the ports and analysing the results. The least lumpy curve is obtained if I block the lower port, but looking at the SPL chart, there is still a sharp kick up from about 18Hz. (See image) Is this likely to be causing my "kick drum fatigue"?

This is my first bass reflex design, so I'm not sure if this problem can be alleviated by changing the port length; any advice would be appreciated.

On a side issue; I have used REW on Windows 8 and on Ubuntu. With the setup as close to the same as possible, (calbration, room setup, mike position) I get wildly different-looking phase curves. The Windows ones jump all over the place, whereas the Ubuntu ones (seen in the attachment) show a relatively flat line to 600 and then a reasonably regular curve up from there. Could anyone tell me why they are so different?


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Both the SPL and Phase curves are not believable as real speaker measurements so I suspect there is a problem someplace.

My first thought would be that a speaker measurement was mistakenly saved as a calibration curve. That would explain this type of measurement result. Click the boxes for "Soundcard Cal" and for Mic/Meter Cal" and see if they both look normal.
Here is an image with the calibration curves turned on. The mic calibration curve looks similar to others I have run, as does the sound card, but I don't really know what a typical sound card curve looks like ...


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Yep, I think sound card calibration was incorrect.

So back to the original concern. The image now shows 3 traces; gold is both reflex ports open, green is one port blocked, and blue is both ports blocked. I'm guessing I have a few problems here; the humps at 55Hz and 160Hz, and the dip at 2.1kHz don't seem right. The (Tannoy factory) crossover is specified at 1kHz, so I wouldn't have thought it would have caused the dip.

Could anyone please comment on what my options are?


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Pretty decent curve overall. :T The quick and easy solution is parametric equalization. If you’re averse to that, then a slew of bass traps could reduce the severity of the 50 Hz peak, but probably not totally eliminate it.

Now these measurements look normal. The SC calibration should now look like a flat line flat in the middle of the chart.

The impact of the port configurations looks as expected with dual ports showing a lower cutoff freq but steeper cutoff rate and blocked ports showing a higher cutoff freq but slower cutoff rate.

The peaks and dips <200 Hz are typical of the effect of the room response (room modes). There is lots of info on what it is and how to address it.

The recommended categories are:
1) Find favorable speaker and LP positions in the room.
2) Improving the room acoustics with treatments
3) Tame the modes near the LP by using EQ.

Its not possible to advise specifics without much more info about the situation and what has already been done. The general advice is to do as much as you are willing to do in the first 2 categories and then use EQ as needed to finish up.

To answer your original question: No, a port length change will not be a benefit.
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