Sealed test enclosure

[Ricci]

I have built a 10 cu ft sealed enclosure with a removable or changeable baffle, so I can easily remove extremely heavy drivers, or switch driver sizes. This will allow me to more accurately measure a drivers TSP's with the WFT3, to see the effects of stuffing, different volumes by adding material and to have a control enclosure for testing drivers. It's nothing pretty, or deadly accurate as it's not something that will be used permanently. Built it with a jigsaw, sabresaw and a power drill in 2 afternoons. Dimensions are 24.5" x 24.5" x 36.75". The driver mounts in with 12 1/4" x 3.5" bolts into a 2x2 with t-nuts and is sealed with weather stripping.

[thxgoon]

Neat! I'm interested in hearing the results of your testing.

[Ricci]

So far I had a small air leak in the front baffle and some more air leaks from the driver mounting holes, but other than that things work ok. I've taken the baffle off probably 5 or 6 times so far with no problems.


This is an impedence measurement of a XXX driver in the box with no stuffing. 24.9hz resonance peak.





This is the same test with stuffing added (11 Walmart pillows). 23.5hz peak.







This is a close mic fr also with no stuffing and no crossover. Notice the nastiness at 170hz. This is a box resonance apparently. The level seems high but these measurements are with the mic 3" from the cone.










And the same sweep after the pillow stuffing has been added...Notice the 170hz nasty's have been diminished greatly.










Here are the before and after overlaid on each other. Orange is with stuffing.

[avaserfi]

Considering the box dimensions it seems as if the peak at 170Hz is related to the 1/4 wavelength internal reflection. It seems as if your introduction of the pillows was sufficient to stop this reflection. This is one of the many reasons I recommend using a high grade acoustically absorbent (OC705 or 8lb mineral wool) material in subwoofers and loudspeakers. Materials such as polyfill or eggcrate foam are not sufficient to properly absorb waves of this length.

Do note this isn't typically an issue with smaller subwoofers, but with units such as LLTs it easily becomes an issue due to their massive size.

[Ricci]

Yep. That's what I thought it was too. I'm kind of surprised that the pillows helped out with it so much.

[avaserfi]

Ricci wrote:

Yep. That's what I thought it was too. I'm kind of surprised that the pillows helped out with it so much.

This is likely due to the large amount of pillows you added. A more efficient material would require less to have the same results.

Also, note that in the higher octaves, based on your cabinet design it is likely that there are audible cabinet panel resonances which do not actually show up on such measurements. The only solution to these resonances would be decoupling of the driver or a more efficient/dense bracing matrix. Of course, this is not an issue with most subwoofers, but is one with midbass modules or loudspeakers.

[Ricci]

Actually the resonances aren't too bad. There are a few that showed up when I did a loud full range sweep, but they were out of the SW range that this will be operated in. The box could use more bracing up front but I needed to keep it open for possibilities down the road. This is quick, dirty and rough around the edges. For sw testing below 200hz.

How would you decouple a big powerful driver?

[Mike P.]

The 1/4 wave length of 117 hz is 29 inches so is that the internal length of the cabinet?

[avaserfi]

Ricci wrote:

Actually the resonances aren't too bad. There are a few that showed up when I did a loud full range sweep, but they were out of the SW range that this will be operated in. The box could use more bracing up front but I needed to keep it open for possibilities down the road. This is quick, dirty and rough around the edges. For sw testing below 200hz.

Actually, cabinet resonances will not show up in a typical measurement. These other resonances you saw are likely either other internal reflections or cone breakup. Large resonant peaks will normally show up as blips in the impedance plot (as the peak at 170Hz did), but other audible resonances can still be present and not show up on the plot.

There are two ways to measure cabinet resonance the first is using an accelerometer. The other, far more accurate method, is to take multiple far field impulse response measurements at varying angles in an anechoic chamber then average the results and waterfall.

How would you decouple a big powerful driver?

Such a large driver isn't really reasonable to decouple. The method B&W uses would work though:
The driver is installed from the rear of the front baffle with a ring of low Fs foam attached to it. A piece of threaded rod is connected to the rear of the driver and inserted into a hole in the rear baffle where foam is placed. This tension holds the driver up while ensuring it is decoupled.

This would probably result in about a 6dB drop in cabinet resonances alone (not sufficient to remove audibility fully). Now, if the intended passband it just up to 80-100Hz there is no need as typical subwoofer bracing techniques are sufficient to attenuate resonance this low. Cabinet resonance is a more an issue from 150Hz-1kHz with most damping technics.

Mike P. wrote:

The 1/4 wave length of 117 hz is 29 inches so is that the internal length of the cabinet?

The resonance is at about 170Hz 1/4 wavelength is about 20 inches .

-Andrew

[Ricci]

Internal front to back measurement is 34.5". I think that the 170hz blip ( it was actually in between 165 and 170hz) was most likely due to a side to side resonance as it is 23.25" x 23.25" square and this is where the least amount of bracing is up front.

I didn't mean that I saw resonances like the one at 170 on the full range sweep graph. (Yes that is not a pretty graph either! I didn't expect it to be though and I just had to know.) I heard them or felt them with my hand resting lightly on the top during a loud full range sweep. My best guess is that there is another around 340hz and seemed like there was another one higher up. These are way out of the range of this driver though, which could never be confused with a woofer or MBM. It'll be crossed at 80hz or lower and anything past 150hz should be well rolled off.

I was interested what you would say about de-coupling, because I just don't see any way to do it effectively with a driver capable of exerting force like this one. I was all ears though if you did have a solid idea.