Site Updates

A Tale of Two Enclosures

Bennett Prescott
Posted onMar 30, 2011 at 11:07 PM
Updated onUpdated on Feb 22, 2018 at 12:26 PM

I recently had the chance to measure and build presets for, in quick succession, two subwoofers using identical drivers in different enclosures. Both enclosures were by the same manufacturer but from different lines, one touring grade and one in the mysterious region between MI and Pro. The drivers are decidedly pro.

What was interesting to me was how a seemingly small change in enclosure volume could make such a big difference. On paper, there is a small difference between the low frequency extension (35Hz versus 33Hz) and physical size (about 12%) of the two boxes. The larger box claims 6dB (!) more output.

In the real world the differences are obvious and enormous. The bigger box has [I]at least[/I] 6dB more output, and sounds good doing it. The smaller box has 1/3 octave less LF extension, requires a 3dB boost to achieve that, doesn’t sound as good, and hits its excursion limit before the specs say it should.

Here is a measurement I took of the raw, unequalized driver response in front of one woofer, both driven, box on the floor, of both cabinets.

[ATTACH=CONFIG]190258.vB5-legacyid=967[/ATTACH]

In short, enclosures are incredibly important. Same drivers. Same manufacturer. One box is not very good, and one is touring grade in every sense of the word. Interesting week, to be sure!

A
It also looks like the better box is a lot more consistant in response in the range you would probably cross it over to some tops.
D
Hard to say whether that's real or not, J. This is an indoor measurement, 4" from the cone. Usually somewhat representative of the box's final response, but not always. Also ignores some port contributions. The vertical scale is also just 3dB per division, which makes it look lumpier than it actually is.
M
The response differences looks like the microphone may be hearing more port on the "big bottom" cabinet, and more cone on the other.

If the "big bottom" is a bandpass type, the port contribution will outweigh the speaker contribution.

If I measure my little rear ported Tannoy studio monitor four inches from the front, then turn it around and measure the back side at four inches, the same cabinet will show a difference more extreme, but similar to the difference you posted between the two different cabinets.

Four inches from a cone on a large cabinet indoors does not give a useful response curve, unless you are going to use a pair as "Texas headphones".
Why would you use such a distance?

What was the orientation of the cone, port, floor and mic for each cabinet test?

How do they measure at 2 or ten meters distant outdoors?
Did you short out the unused cabinet?
D
They are both direct radiators with similar port location and geometry. These measurements are only useful as a comparative measure between the two boxes, and give a rough idea of the largely room-excluded on axis response of the driver.

Believe it or not, Art, I've done this a time or two.
M
Bennett, I'm certainly aware you have done more than a few tests.

That is why I asked the four questions I did, of which you partially answered one.

A four inch distant measurement of a (presumably) large enclosure is not generally predictive of "real world" behavior. Even on a sealed enclosure four inch measurement at different portions of the cone will show quite different response curves.

Cone and port size and proximity make a very large difference when doing a four inch measurement, at Fb the port may be putting out far more SPL than the cone, a seemingly minor variation in measurement location could easily account for the variations you noted.

You wrote:
"What was interesting to me was how a seemingly small change in enclosure volume could make such a big difference."

A 12% change in cabinet volume should not have as near as much effect on a standard BR cabinet's response using the same speakers and similar Fb as you measured, hence my questions. The boxes appear to be tuned around 50 Hz, quite high to be considered "touring grade" this century.

The Fb is an important determination for BR cabinet HP presets and any corrective EQ. Did you determine the Fb frequency for each box ?

The screenshots at 4" may be representative of the cabinet response, or may only be representative of that particular measurement position.

If both cabinets are actually simple bass reflex, I am inclined to think that four inch measurement position may not be representative of the actual response.

An outdoor measurement at a reasonable distance from the cabinet and away from buildings would confirm any "real world" validity of the four inch tests.
Mike Gjesdal
I would imagine build quality could have a great deal to do with the difference between traces. Bracing and what not ...
B