How much difference does stuffing make in a sealed subwoofer cabinet?

[swechsler]

Or: how can I improve this sub?

I recently purchased an Aperion Audio Bravus 8D sub (original, not series II) with a dead amp. The sub has two 8" aluminum woofers with good excursion and very large magnets on opposite sides of an approx 1' cube (internal volume is a bit less). The woofers are connected in phase and in parallel. The original (BASH) amp was rated at 150 watts and included an internal adjustable parametric EQ (which, according to the manual, is turned off when initially shipped). Construction quality is good, with the internally braced cabinet made of HDF with a high gloss piano-black finish. The original amp had too many missing custom parts, so repair is out of the question.

I'm currently driving the sub with a 100 watt (into 4Ω) commercial Crown amp. I've now got the cabinet sealed up good and tight, and with a frequency generator app, the output starts to drop off below about 60hz. Now, I'm aware of Hoffman's Iron law, so I'm not expecting super efficiency, but originally the manufacturer rated it to 35hz at -3dB. I couldn't find any reviews online of anyone who had tested this, but 60hz and 35hz is a huge difference. One thing I did notice, when opening up the sub to fix the leaks, is that there was no stuffing inside. I couldn't find any indication that there was any before, but I knew that the previous owner had modified it to be used as a passive sub, so there was a decent chance that he removed the stuffing for whatever reason. Every acoustic suspension speaker I've ever opened was fully stuffed, so this seemed strange to me.

So I guess my questions are:

1. How likely is it that this sub was originally stuffed?
2. How much would re-stuffing it improve the lower end?
3. With only a 150 watt amp, is it possible that it originally used equalization to obtain that 35hz -3dB point (the manual doesn't mention)?
4. Anything else I can try?

This is an exercise rather than a necessity, since it's really an extra sub at this point, so I don't want to put much money into (although I did consider the Dayton DSP-LF). However, I do like the way it looks, since it's unobtrusive, and matches not only with the entertainment unit we have in our media room, but also most of my NHT speakers (although with it starting to drop off at around 60hz, it's not much of an improvement to my 1.5s).

 

[whoaru99]

Well, comparing a measurement e.g. -3dB @ 35Hz first requires testing under same/similar conditions as where the spec came from. I don't know what that is for this sub.

Far as the amp, some commercial/pro amps have high pass filters. Most my Crowns and QSCs do. If so, is it turned off?

On box stuffing don't know if it would have been or not. But, I understand stuffing the box with polyfill makes it appear larger to the driver. Seems I recall it can increase apparent size by up to ~20%. Been a while since I looked into it though.

 

[GordonW]

Many of the BASH amps, come with built-in bass boost, as part of their rumble filter.

It's a design called a fourth order assisted alignment- a sealed enclosure, with low Q and low resonance, combined with a high Q, highpass filter (which provides a calculated "bump" in the response, before rolling off), which acts to "prop up" the response of the subwoofer, to a lower frequency that it can do naturally.

When the amp was replaced- the new amp would not have the same EQ...

Regards,
Gordon.

 

[Powertech]

The difference between 35Hz and 60Hz is less than an octave (about 2.8dB/octave rolloff) In a small sealed box, that is actually pretty good. I would have expected the rolloff to start higher than that.
With regard to stuffing - What I would do personally is to line the enclosure with 1/2" of natural felt (well glued and stapled) to reduce internal reflections and leave the volume clear of wadding. The volume is very small, so filling it would probably have little effect anyway.

 

[baco99]

Agree with all of the above:

1) Your testing environment and equipment differs from those used in the advertised specifications. Room gain/cancelation will play a role in response, even in a close-mic scenario. You could try to measure the sub outside in a field, placed as high above the ground as possible, which will come close to mimicking that of an anechoic chamber. Curious what you measure under those conditions.

2) Different amps, EQ, etc.. As mentioned.

3) I say yes to polyfill. Originally this speaker would have been stuffed to a reasonable level with polyfill.

 

[swechsler]

Thanks for the responses so far. I'll have to work on doing some accurate measurements to see exactly what was going on.

To respond to some of the points made:

@whoaru99 No high pass filter on this amp. In fact, when I set the FG app to about 5 hz you could still the the woofers moving in what appeared to be a sinewave pattern.
@GordonW Typically what sort of boost is used with these? With only 150 watts to play with, ISTM you can't do too much without running out of power.
@whoaru99 @baco99 I was under the impression that FR measurements of speakers were done in an anechoic chamber. At least, that's how it was done back in the 1970s.
@Powertech @baco99 OK, different opinions regarding polyfill. I would tend to agree that it was originally stuffed. Will have to get some and do some testing and see how much difference it makes.

 

[GordonW]

It would only take maybe 3dB of boost, to get from 50 to 35Hz (roughly 1/2 octave extension).

Regards,
Gordon.

 

[baco99]

From my experience *most* of these BASH amps equipped with the bass boost circuit was +6dB at 40Hz (theoretically). But again, they could all vary. And not all BASH amps had the bass boost circuit added or equipped.

 

[baco99]

@whoaru99 @baco99 I was under the impression that FR measurements of speakers were done in an anechoic chamber. At least, that's how it was done back in the 1970s.

It's possible. But I am assuming you don't have access to one, so the point is if you want to come close, measure the woofer outside.