2013 Subwoofer Roundup: 6 Subs For $500 Or Less

[ousooner2]

So what if you extended the port on the PA-150? It's likely tuned too high (and in too small of a box) and that's why it's falling off a cliff right? Why not try to extend the port and see what happens

 

[theJman]

You stand the chance of damaging the driver by doing that.

The manufacturer will tune a bass reflex subwoofer with the port, so the driver will "unload" at a frequency within it's design parameters. When a driver unloads it's basically running in "free air" at that point, so mechanical limiters are no longer effective. If you change the tuning frequency by making the port longer you're likely to cause the driver to extend beyond it's voice coil gap which at best will only make unpleasant sounds, but at worst could physically damage it.

 

[shadyJ]

For it to be dangerous, you would have to widen the port, not extend it. Simply extending the port without widening it would dampen the response, so it would actually lessen the odds of overdriving the woofer. As far as I understand it, that would lower the system Q and therefore extend the tuning but also lessen the port output. It would mangle the frequency response.

 

[Steve81]

For it to be dangerous, you would have to widen the port, not extend it.

Here's a quick and dirty sim which would support Jim's assertion:

Same driver, same box volume, same port diameter, and same power applied; only difference is a tune of 27Hz vs 20Hz and the corresponding difference in port length.

 

[theJman]

For it to be dangerous, you would have to widen the port, not extend it. Simply extending the port without widening it would dampen the response, so it would actually lessen the odds of overdriving the woofer. As far as I understand it, that would lower the system Q and therefore extend the tuning but also lessen the port output. It would mangle the frequency response.

Tuning frequency is determined by length not width, so changing that would be dangerous.

 

[Ed Mullen]

Well above system tuning (Fb), a bass reflex subwoofer behaves more like a sealed subwoofer - cone excursion quadruples at each successively deeper octave in order to maintain the same SPL. So the deeper the system is tuned, the higher the cone excursion will be above tuning, reaching a maximum somewhere around 1/4 octave above tuning.

As the frequency approaches system tuning, cone excursion is heavily damped, and the port starts to shoulder most of the output (Helmholtz resonator behavior).

Below system tuning, the driver will decouple from the port and unload, essentially behaving like it's in free air. The potential for bottoming the driver below tuning is very high. This can be controlled with a combination of a high pass filter and/or a compressor/limiter.

Increasing port length while maintaining the same diameter will lower the system tuning frequency. Increasing the port area while maintaining the same port length will raise the system tuning frequency.

Increasing the port length and lowering the system tune doesn't necessarily mean the subwoofer will extend flat to that deeper frequency, as the enclosure volume Vb will also have an effect.

This can all be seen in Steve's graphs above.

 

[shadyJ]

I wouldn't have guessed the driver would be in danger until below tuning, but yes, I see that if the driver doesn't have the xmax to handle frequencies above tuning, it can bottom out. Thanks for the heads up.

 

[Ed Mullen]

I wouldn't have guessed the driver would be in danger until below tuning, but yes, I see that if the driver doesn't have the xmax to handle frequencies above tuning, it can bottom out. Thanks for the heads up.

Right - and the deeper the tune, the higher the cone excursion becomes above tuning. Our legacy 16-46 PC-Plus was the most bottom-prone of the three PC-Plus models for this reason - it was tuned to 16 Hz. Steve81 should plot the same subwoofer with a 25 Hz, 20 Hz and 16 Hz tune all on the same graph and you'll see how much more cone excursion is required for the deepest tune (before the port takes over). It's already an obvious jump going from 27 Hz to 20 Hz in his above graphs.

 

[Steve81]

Steve81 should plot the same subwoofer with a 25 Hz, 20 Hz and 16 Hz tune all on the same graph and you'll see how much more cone excursion is required for the deepest tune (before the port takes over). It's already an obvious jump going from 27 Hz to 20 Hz in his above graphs.

 

[Ed Mullen]

Thanks Steve81. As you can see cone excursion gets progressively higher with each deeper tune. In addition, excursion exceeds the stated 12 mm of xmax in the 20 Hz and 16 Hz tunes, but more to the point - in the 16 Hz tune xmax is exceeded from 45 Hz to 21 Hz. That's over an entire octave (!) where the driver is vulnerable to overdrive and/or bottoming.

 

[Alex2507]

The SB1000 is indeed a very small subwoofer. For a size comparison check out the review I did on the PB and SB a few months back.

On the above imbedded link if you click on 'the' you get to where you want to be and if you click on 'review' you click on where somebody else wants you to be. I was happy to see Tom's preview of the pb and sb 2000s though. Too bad the gloss black finish is gone.

 

[theJman]

On the above imbedded link if you click on 'the' you get to where you want to be and if you click on 'review' you click on where somebody else wants you to be. I was happy to see Tom's preview of the pb and sb 2000s though. Too bad the gloss black finish is gone.

Thanks for the heads-up; I did notice that after the fact. It seems Audioholics surreptitiously makes the word 'review' point to a link on their website, regardless of what you actually had intended it to be. Since you can't go back and edit a post there's no way to fix it either.

 

[zrtmatos]

I just picked up a SVS SD1000 and have to say I am amazed by its output!