Constant Beam Width Transducers line arrays

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T

thune

Member
Joined 2010
The measured CBT prototype in the AES paper was individually shaded (if I'm reading it correctly.) The presentation slides have measurements of the step-shaded array, and nowhere do I see a comparison.

From AES paper:
4.3. Experimental Arrays
Two experimental line arrays were measured: a straight-line array with no shading (equal levels to each driver) and no delays, and a circular-arc 45o curved-line array with Legendre shading and no delays.
...
All the drivers were driven individually using three DSP-programmable twelve-channel automotive amplifiers. Individual amplifier drives to each speaker allowed each to be driven with different gains if required. No equalization was used other than the required differences in gain.
Click to expand...
 
Bill F.

Bill F.

Member
Joined 2001
How about a multi-tap transformer instead of resistors?

I have a number of Tannoy THP-60 and THP-30 transformers out of ceiling speakers. They have taps for -3, -6, and -9dB. Might make for a more efficient array and more flexibility to scale it and/or use a wider variety of drivers...
 
J

jeno

Member
Joined 2006
Hello!

I've been working on a cbt design for å while, but I find it to be very complex. Especially doing the tapering correct with passive parts is beyond my skill level, and multiple amplifier channels adds too much cost at this point.

My question:

Would a straight floor to ceiling line array eliminate the pros of the cbt? With a acoustically solid floor and ceiling the straight line array would be virtually infinite, and hopefully work as a theoreticly perfect line array. How well does this work in practice? Any thoughts on this?

Jan
 
TerryO

TerryO

Member
Joined 2002
jeno said:
Hello!

~~~~~~~~~~~~~~~~snip~~~~~~~~~~~~~~.

My question:

Would a straight floor to ceiling line array eliminate the pros of the cbt? With a acoustically solid floor and ceiling the straight line array would be virtually infinite, and hopefully work as a theoreticly perfect line array. How well does this work in practice? Any thoughts on this?

Jan
Click to expand...

How does it work in practice?
Much, much better than a short straight line array!
 
bear

bear

Member
Joined 2002
bearlabs.com
jeno said:
Hello!

I've been working on a cbt design for å while, but I find it to be very complex. Especially doing the tapering correct with passive parts is beyond my skill level, and multiple amplifier channels adds too much cost at this point.

My question:

Would a straight floor to ceiling line array eliminate the pros of the cbt? With a acoustically solid floor and ceiling the straight line array would be virtually infinite, and hopefully work as a theoreticly perfect line array. How well does this work in practice? Any thoughts on this?

Jan
Click to expand...

The constant beam width thing as far as I can tell is talking about the VERTICAL dispersion.

The horizontal dispersion is more or less the same as any line source - except to the extent that it is possible to seriously limit the output of all but a single center driver, and so have an array at HF have the same dispersion as say a single tweeter. That would imply frequency tapering, and not amplitude tapering, if I am correct on this count. Keele uses amplitude tapering only, iirc.

Keele's own paper shows that there is no performance difference between his curved array and a straight line source under two conditions:
- you are either short (sit low) or close in (and below the top of the array by a bit)
- the array is floor to ceiling

So, imo the straight vertical array, floor to ceiling is superior to the curved array on three counts:
- it is simpler to build and has equal or better VERTICAL dispersion characteristcs
- it has higher max SPL, and/or lower THD for a given SPL
- it is easier to build and wire

(one can have a discussion as to tapering a full height array...)

The CBT, as far as I can tell is intended to be able to provide even HF coverage above and near the top of the array in situations where a floor to ceiling speaker is not feasible or practical.

This is what I came away with...

_-_-bear
 
G

Generic George

Member
Joined 2011
Bill F. said:
I just noticed a CBT kit now available at Parts Express.

Here's the selling page.
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Interestingly the kit apparently does NOT include a DSP unit for the active crossover necessary. Nor do they suggest one either.

If I understand this type of speaker it does depend on power shading, anyone know if the resistors included in the kit are for this. I had kind of been under the impression that it would by handled by the DSP.
 
R

RockLeeEV

Member
Joined 2010
Generic George said:
Interestingly the kit apparently does NOT include a DSP unit for the active crossover necessary. Nor do they suggest one either.
Click to expand...

The DCX2496 is suggested in the manual, although you could use a miniDSP or DEQX as well (being careful with the transfer functions).

If I understand this type of speaker it does depend on power shading, anyone know if the resistors included in the kit are for this. I had kind of been under the impression that it would by handled by the DSP.
Click to expand...

Everything except the crossover and amp are included in the kit.
 
Bill F.

Bill F.

Member
Joined 2001
It's not clear to me why it requires a digital crossover. I believe the shading is accomplished with the kit resistors, so all it would need is standard crossover functions that could just as easily be an analog prosound xo with the right filters and 4 amp channels, right?
 
J

Jim Griffin

Member
Joined 2003
Bill F,

The CBT array prototypes that Don Keele and Marshall Kay demonstrated at the 2010 Midwest Audio Fest (sponsored by Parts Express) used a Behringer DCX2496 (three way crossover/EQ) and had a separate subwoofer. Thus it needed 5 channels of amplifiers. The DCX2496 provided the necessary crossover and EQ functionality.

You have no crossover in the P-E CBT kit. This array 'system' essentially needs a three way crossover. One way for the tweeter lines, one way for the woofer lines, and a subwoofer cross for adequate bass. Thus you will use up to 6 channels of amplification if you had dual subs. Now the array tower can be biamp'ed with a four channel (2-way) crossover and a separate subwoofer amp (or two for 2 subs) with a plate amp/crossover.

The tapering (or shading) contours the SPL output of the woofer and tweeter lines to attain the specific directivity of Don Keele's design. The shading is done in 4 specific steps within each line. Now you could use separate gain variable amps for the shading but you would need 8 additional amps per array tower line for a total of 16 gain channels. Thus, those resistors are a neat simplification after all.

Jim
 
Bill F.

Bill F.

Member
Joined 2001
My pastor has been hinting that he would like my help upgrading our church PA system. This PE kit has me seriously considering clamping this speaker pair bottom-to-bottom for a flown full-space center array. Horizontal coverage looks good out to the 120 degrees we need.
 
J

Jim Griffin

Member
Joined 2003
If you haven't reviewed the assembly instructions of the CBT yet, do so and most questions will be addressed. Marshall and Don did a great job in preparation of the document. It is an excellent treatment of the details and some insight to how the array works.

Specific setup details are included for the DCX2496.

I notice that the shading function is approximated by 5 discrete steps of attenuation vs. the 4 that I mentioned in Post #176.

http://www.parts-express.com/pedocs/...36k-manual.pdf
 
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