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Danley BC215 Directivity

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John Hiemburg:
Good morning,


Looking at a design for a new room, and my first intention was 4 TH118XL's in an end-fire configuration (flown). This checks the boxes for a: Total output, b: Some level of directivity; c: Budget.


However, a pair of BC215's would check the same budgetary and output boxes (Direct modeling showing this a little more powerful than the 4 TH's). Just curious about any real world experience or measurements of the actual directivity for a pair of BC215's configured as a pair like recommended.


Thanks in advance.

John Halliburton:
I believe the BC215 really need a boundary to perform to spec, and you state the TH118XL would be flown, so you're likely losing out on some performance going with them.

Best regards,

John

Caleb Dueck:

--- Quote from: John Hiemburg on May 07, 2021, 07:59:19 am ---Good morning,


Looking at a design for a new room, and my first intention was 4 TH118XL's in an end-fire configuration (flown). This checks the boxes for a: Total output, b: Some level of directivity; c: Budget.


However, a pair of BC215's would check the same budgetary and output boxes (Direct modeling showing this a little more powerful than the 4 TH's). Just curious about any real world experience or measurements of the actual directivity for a pair of BC215's configured as a pair like recommended.


Thanks in advance.

--- End quote ---

How about a single BC415?  Or is that too tall?

Art Welter:

--- Quote from: John Hiemburg on May 07, 2021, 07:59:19 am ---Looking at a design for a new room, and my first intention was 4 TH118XL's in an end-fire configuration (flown). This checks the boxes for a: Total output, b: Some level of directivity; c: Budget.


However, a pair of BC215's would check the same budgetary and output boxes (Direct modeling showing this a little more powerful than the 4 TH's).

--- End quote ---
John,

We met about a decade ago when delivering stage curtains you purchased, around the time I tested the directivity of tapped horns (TH), bass reflex (BR), and front loaded horns (FLH), and a TH with an increased boundary coupling area (BC).
Yesterday morning, before seeing your post had thought of posting to see if anyone had measured the directivity of TH compared to other designs in a large open space, as my test was flawed do to proximity to reflective boundaries:

https://www.diyaudio.com/forums/subwoofers/184982-tapped-horn-directivity.html

Though flawed, the test showed that the TH had a much higher directivity index (DI, or forward gain) than FLH and BR, and the BC additionally increased the DI.

Tom Danley current BC (Boundary Coupled) FLH designs are similar to his earlier BDEAP (Boundary Dependent External Air Path) designs, though not dependent on corner boundaries to “be deep” in low frequency response.

Some explanation of the concept :

https://forums.prosoundweb.com/index.php?topic=75746.0

In that thread, on 4/6/2005 Ivan Beaver wrote he measured -10dB at 90 degree off axis around 40-50 Hz with the BDEAP in a quad stack. 
Impressive, though a single 1/3 octave and a single off axis measurement don’t provide enough information to make a decision.

Unfortunately, DSL’s specification sheet claim of “delivering actual directivity” tells even less.
Fulcrum Acoustic provides axial sensitivity response curves for their passive subcardioid designs, which have around -10dB 180 degree response. I’ve walked around the CS118, and can confirm the specs hold up in the ‘real world”.

Without seeing axial measurements for the DSL BC215 subwoofers, I’d tend to favor the flexibility that four TH118XL in a “staircase” end-fire array could provide.
One could “dial in” the angle and frequency that needs most attenuation with the  end-fire array, while the axial response and fixed frontal size of the BC may not work well in the location it is relegated to.

As to direct modeling showing a pair of BC215 a little more “powerful” than four TH118XL, from the spec sheet it appears (as usual per Hofmann’s Iron Law- Low, Loud, Small, pick two..) that the sensitivity is due to the higher and steeper rolloff (and Fb/Fc) of the BC215. 

Both are rated at 108dB sensitivity, but the BC215 is 2 ohms, the TH118XL is 4 ohms. Adjusting their sensitivity from 28.3v at 10 meters to 1watt/1m, additional cabinets added each powered by one watt)

BC215: 30Hz@94dB, 100Hz@107dB
BC215 x2: 30Hz@103dB (+7), 100Hz@112 (+5dB) from single to dual

TH118XL: 30Hz@96dB, 100Hz@103dB
TH118XL x4 : 30Hz@108dB, 100Hz@115dB (+12 dB estimated from specifications)

The frontal boundary area of two BC215 (45” x90”) and four TH118XL are equal.

On 6/8/2006 Ivan posted measurements of 1, 2 and 4 DSL TH115 at 10’ (feet). The SPL from stack of four evidently clipped the test rig above 70Hz, causing a flat line, but below 50Hz an increase of around 2dB forward directivity over the +12dB expected can be seen. If that test is indicative, a quad of TH118XL may have a few dB LF on axis over the single than estimated.

The recommended 30Hz BW24dB HP for the BC215 compared to 26Hz BW24dB HP for the TH118XL would increase the level disparity at 30Hz.

40-50 Hz output should be higher with a pair of BC215, 30-40Hz output should be quite a bit more with four TH118XL.

Art

Ivan Beaver:
The numbers in Direct for the different BC configurations come from actual measurements of the different configurations (1,2 4) as measured outside on the ground without reflections.

They are not simulated.

One way that I like to look at sub directivity (in direct) is to put a mic in front and another behind the subs (at the same distance) to get an idea of the difference.

Here are the actual measurements from the polars

As you can see, the directivity gets greater as you go higher in freq, as would be expected

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