Multiple Cabinet Combined Response

[weltersys]

After noting differences between measured results of single cabinets of various types (TH, FLH, and BR) compared to multiple cabinet simulations using Hornresp, I conducted an extensive series of outdoor tests using one through eight bass reflex (BR) cabinets in various arrays.

Coherent summation of multiple cabinets should yield a 6 dB addition per doubling, +3 dB for doubling radiation area, and +3 dB for doubling power.
The closest to coherent summation was achieved with the speakers placed in an arc with all cabinets ten meters from the measurement mic, resulting in a net even response between one and 8 cabinets from F3 to around 250 Hz.

Incoherent summation results in only an average +3 dB addition per doubling of power and cabinets.

The graphs have been “normalized” with a 6 dB reduction for 2 cabinets, 12 dB reduction for 4, 18 dB reduction for 8 cabinets so that differences from coherent summation can be seen. Series cabinets have no normalization, as the net result of less power per cabinet and increased radiation net a 0 dB change.

The results of the tests show how different arrays result in quite different upper response from single units. Although David McBean is considering some changes to Hornresp, they still will not be able to reflect each type of array response, a complicated combination of many factors beyond the present model.

Similar upper frequency response reduction and general smoothing as is seen and heard in FLH and TH response occurs in the BR cabinets also.

The frontal area of the stack makes a significant difference in response shape, adding “wings”, flat pieces of plywood (AKA “barn doors”) to the stacks add 2 to 3 dB to the LF response when the frontal area is tripled.

Art Welter

[18Hurts]

I like the "barn doors"

So when I build my tapped horn--would adding barn doors to it give me about 2 to 3 extra dB also? If the horn is tuned for 18Hz, would it be advisable to use heavy bracing--say pieces of 2x2 lumber to frame the doors?

I'm always interested in "free dBs" and the barn doors would help when dragging the speakers outside for BBQs and such.

[David McBean]

Originally Posted by weltersys

The results of the tests show how different arrays result in quite different upper response from single units. Although David McBean is considering some changes to Hornresp, they still will not be able to reflect each type of array response, a complicated combination of many factors beyond the present model.

Hi Art,

Many thanks for taking the time and effort to conduct these very comprehensive tests.

Your measurements now make it quite clear to me that unfortunately, I am probably wasting my time trying to develop a new multiple speakers model that will ultimately be any better overall, than the current one. There are just too many variables to take into account.

In the circumstances, perhaps a more sensible approach may be to simply keep the current model, but to document known limitations in the Hornresp Help file so that users are aware of the shortcomings.

A couple of comments on speaker testing in general, in relation to Hornresp:

Hornresp assumes that the outputs from multiple speakers are in effect combined into one "concentrated" sound source. This means that to most accurately reflect the simulation assumption, four speakers should ideally be configured in a 2 x 2 square array - rather than a 1 x 4 linear array, for example. Eight speakers should be configured in a 3 x 3 array, but with only two speakers in the top row, centrally positioned. Not sure how much of a difference, if any, this would actually make to measured results though :-).

Tests should ideally be done under anechoic conditions, meaning that to replicate 2 x Pi half space conditions outdoors, it would be necessary to place the speakers on solid ground, pointing vertically towards the sky with the microphone suspended an appropriate distance above the geometric centre of the array, suitably shielded from the wind. Not a very practical proposition in your case :-).

Thanks again for all your assistance - it is greatly appreciated!

Kind regards,

David

[sebDIY]

An other intested PDF from LinearX about arrays sum in LEAP : http://www.linearx.com/files/pdf/EncShopApp_02.pdf

[salzburgsoundsystem]

some barndoors of my own!

Regards, Ben

[weltersys]

Originally Posted by 18Hurts

So when I build my tapped horn--would adding barn doors to it give me about 2 to 3 extra dB also? If the horn is tuned for 18Hz, would it be advisable to use heavy bracing--say pieces of 2x2 lumber to frame the doors?

Getting an extra 2 to 3 dB down to 18Hz would probably require much larger "barn doors" than what I have used, the extenders used on my TH and FLH provide an increase in level down to about 35 Hz.

If the panels are vibrating much, some acoustical energy is being wasted.
That said, flopping panels will loose more energy in the upper bass range than lower.

My panels have a 1.5" x 2" brace on the mouth side, that seems adequate to keep them from flexing.

[18Hurts]

Thank you Art,

So if triple-wide is good to 35Hz--that is good enough for outside use for BBQs. Since I've done some DIY stuff, there is a dedicated pile of wood for bracing so I'll give it a shot.

Once I finish my line array mod, I'll have spare wood to make some doors to attempt to get the most bass I can out of 24 five inch woofers. Ironically, I'll have enough wood and 24 feet of 2x2's to make one and check it out. If it's ugly, it has to work!

[weltersys]

Originally Posted by David McBean

Hi Art,
Your measurements now make it quite clear to me that unfortunately, I am probably wasting my time trying to develop a new multiple speakers model that will ultimately be any better overall, than the current one. There are just too many variables to take into account.

In the circumstances, perhaps a more sensible approach may be to simply keep the current model, but to document known limitations in the Hornresp Help file so that users are aware of the shortcomings.

A couple of comments on speaker testing in general, in relation to Hornresp:

Hornresp assumes that the outputs from multiple speakers are in effect combined into one "concentrated" sound source. This means that to most accurately reflect the simulation assumption, four speakers should ideally be configured in a 2 x 2 square array - rather than a 1 x 4 linear array, for example. Eight speakers should be configured in a 3 x 3 array, but with only two speakers in the top row, centrally positioned. Not sure how much of a difference, if any, this would actually make to measured results though :-).

Kind regards,

David

Documenting known limitations in the Hornresp Help file so that users are aware of the shortcomings would be a good solution.

It is worth noting that the ground plane reflection made the 2x4 and 4x2 stacks both fairly equal, and 1-2 dB louder in the LF than the vertical or horizontal line of eight cabinets.

The addition of the wings adds a bit more level down low than I would have expected on BR cabinets, almost as much as the 3 dB they did when added as a 90 degree "waveguide" to TH or FLH horns.

Art

[mdocod]

Originally Posted by 18Hurts

... If the horn is tuned for 18Hz, would it be advisable to use heavy bracing--say pieces of 2x2 lumber to frame the doors?...

I'd imagine you'd have to have an effective baffle width/height of ~60ft to eliminate 4PI transition at 18hz.... uh.... I think?

[18Hurts]

Originally Posted by mdocod

I'd imagine you'd have to have an effective baffle width/height of ~60ft to eliminate 4PI transition at 18hz.... uh.... I think?

I have about 20 feet of garage wall, figured I could use it to give a boost down to a usable level if I do it right. Inside the garage I have plywood walls with corner loading--outside I lose that advantage.

Since I have 4 x 8 sheets of plywood, I'll screw around with putting them on either side of the line arrays to hear if it would increase the bass level enough so a subwoofer is not necessary--much easier for BBQs that way.