Seen any driver beaming studies?

[Drew Eckhardt]

Originally Posted by J.R.Freeman

A bit off topic, but what are the reasons for avoiding 'beaming'?

Monotonically increasing directivity is required for natural sound in domestic rooms.

The speaker's timbre comes from both direct sound and reverberant field. When you mate an acoustically small tweeter to an acoustically large mid-range there's a decrease in power response as the mid-range beams followed by a jump back up.

I'm confused as some call 12 inch drivers as 'mid-bass' and sight the narrowed dispersion as a positive. Is this a case of two different system goals and two schools of thought?

Where a wave guide or horn is used the high frequencies start out with a directivity index higher than 6dB and preserving monotonically increasing directivity requires the same from the mid-range.

However you get there uniform directivity sounds similar; although with increased directivity the speaker is less sensitive to room and placement issues and retains clarity farther into the room.

[planet10]

Originally Posted by cuibono

My ideal set of info would show me a 10" driver, an 8", 6" and 4", at angles up to 90deg off axis

You would also need to somehow account for cone shape, stiffness, how well (or not) the cone decouples the outer edges as the frequency goes up.

dave

[Pano]

Originally Posted by Drew Eckhardt

When you mate an acoustically small tweeter to an acoustically large mid-range there's a decrease in power response as the mid-range beams followed by a jump back up.

Which I have heard used to great advantage. Can help reduce the forwardness or shout that some speakers have. I does seem like a strange thing to do, but can sound quite good if done right.

.

[grantnsw]

Hi,
Please excuse my uninformed question, but could you Gurus please advise at 'about' what frequency (given various constructional parameters as mentioned) most woofers would start beaming? say 8, 10 12 and 15 inch?
Grant.. I'd really appreciate this info...

[Pano]

Hey Grant, it's all in the charts above. Or look at the specs for Selenium drivers, they provide good polar plots.
The baffle is only going to make a difference where the pattern gets out to 180º and beyond. There will always be some bounce of the baffle, but less and less as the pattern tightens.

[Ron E]

Originally Posted by grantnsw

Hi,
Please excuse my uninformed question, but could you Gurus please advise at 'about' what frequency (given various constructional parameters as mentioned) most woofers would start beaming? say 8, 10 12 and 15 inch?
Grant.. I'd really appreciate this info...

Look at post#2. IT depends on your definition of beaming.
an 8" driver is -6dB at 90 degrees off axis at ~1440Hz
a 10" at ~1440*8/10
a 12" at ~1440*8/12
a 15" at ~1440*8/15
a 6.5" at ~1440*8/6.5

For an 8" driver to be 3dB down at 90 degrees off axis, the freq. is 1050Hz
You can calculate for the other driver sizes the same way as above, although you would want to use piston size rather than frame diameter, using the frame size is a ballpark figure.

These are theoretical values for perfect pistons, but are actually fairly close to real driver behavior in all but very sloppy cone/dome drivers.

[Robh3606]

advise at 'about' what frequency (given various constructional parameters as mentioned) most woofers would start beaming?

If you look at the nomograph you can see like Panomaniac has suggested. That said many drivers don't work well up to thier beaming -6db point. You need to have good smooth response up to and beyond to cross them over at that frequency. An example is a 128H-1 woofer vs a 2214H both JBL 12's.

The 128H is good up to about 900hz or so before it rolls off. A 2214 goes on above 1.2K. If you look at how JBL used them the 128 is in the 4412 monitor which is a 3 way and the 2214 is used in the 4425 which is a 2 way CD design and crosses over at the -6db point at about 1.2-1.4K. You can't use a 128H in a 4425 so you won't match the directivity at crossover with the horn used in the 4425.

Rob

[grantnsw]

Pano, Ron and Rob,
I made a mistake, oops!
I'm using a lowish resolution plasma TV as my monitor and when I originally clicked those plots the detail was very hard to read. Now, after downloading and magnifying in an image editor.. all is revealed. Thank you all for your great advice.