Vertical dispersion on planars. How much?

[Armand]

I have searched the forums for this much debated topic but I can not find an answer to my simple question..
What is the vertical dispersion of my planar (NEO8-PDR) ??
I know that it is little, but I find it hard to believe that a soundbeam stays within the 15 cm (6 inch) height for several meters. There must be some dispersion.

The second question is then: Is the vertical dispersion very frequency dependent?

The reason I am asking is that I am going to use NEO8-PDR in my centre speaker together with a 6.5" woofer. My sofa is 4 meters (13 ft) from the speaker and I want to have the option of moving my head a little more that 15 cm (6 inches) without missing out on the dialogue in the movie...

I have more drivers, so another option is maybe to use two drivers in a line and mount the woofer beside them.. (this is a centre speaker)

Armand

[WrineX]

Wellif ur ribbon is narow and tall ? if so, the smaller it is the better the dispersion. in this case horzontal. if the ribbon whas wider then tall the vertical dispersion would be better. so a little dot or square would have in both ways the same dispersion. and its all ferquency depended. the higher the fequency the narrower you ribbon need to be to have the best dispersion.

[bolserst]

Originally Posted by Armand

I have searched the forums for this much debated topic but I can not find an answer to my simple question..
What is the vertical dispersion of my planar (NEO8-PDR) ??
I know that it is little, but I find it hard to believe that a soundbeam stays within the 15 cm (6 inch) height for several meters. There must be some dispersion.

The second question is then: Is the vertical dispersion very frequency dependent?

Hello Armand,

Dispersion is frequency dependent and can be estimated by the sinc function:

sinc[ (pi*f*W)/c * sin(theta) ] = sin[ (pi*f*W)/c * sin(theta) ] / [ (pi*f*W)/c * sin(theta) ]

where:
f = frequency
W= width of sound source (m)
c = speed of sound = 340 (m/s)
theta = listening angle

As you can see a 6" wide sound source is pretty directional,losing the top octave at just 5 degrees off axis. This is pretty close to the 6" @ 2m you mentioned.

For comparison, the 2nd plot shows calculated dispersion for width of a typical dome tweeter.

[Kontra]

try to visualize the acoustic lens. You can calculate a lens, a combined for vertical plane and horizontal plane

[twest820]

I posted some simulations here. They're non-PDR but since PDR's only along on the short axis of the drivers the vertical results should be relevant. To my knowledge no one's ever taken a proper set of Neo8 polar and distortion data, though one can infer a few things from Zaph's Neo10 polars.

[bolserst]

Originally Posted by twest820

I posted some simulations here. They're non-PDR but since PDR's only along on the short axis of the drivers the vertical results should be relevant. To my knowledge no one's ever taken a proper set of Neo8 polar and distortion data, though one can infer a few things from Zaph's Neo10 polars.

Hello twest820,

Thanks for posting the measurements.
Do you happen to have the file for the Neo8 on the long axis? You seem to have two files posted for the Neo3 long axis, but none for the Neo8 long axis.

[twest820]

Fixed.

[bolserst]

Originally Posted by twest820

Fixed.

Thanks!

I took your data for the Neo8 on the long axis and normalized the 15 degree and 30 degree curves relative to the on axis data and plotted it up against the theoretical predictions from post #3.

Plot 1) Compares the measured data vs the predictions from post #3 for a 6" wide radiator in an infinite baffle.
Pretty good agreement, but you can see that the measurements were not infinite baffle as the overall level drops as you get further off axis.

Plot 2) Compares the measured data vs the predictions for a 6" wide dipole radiator
Adding in the Cos(theta) SPL reduction with angle off axis for a dipole radiator and we get even better agreement.

Plot 3) Compares the measured data vs the predictions for a 6.25" wide dipole radiator
I'm not sure what the exact dimensions of the Neo8 diaphragm are, but adding 0.25" to the width of the radiator provides a near perfect match with your measurements.

I'd say theoretical directivity calculations come pretty close to reality

[twest820]

Originally Posted by bolserst

I'd say theoretical directivity calculations come pretty close to reality

Don't think so. What you've determined is Edge's FEM simulation of the Neo8's free air response (no baffle or box) results in agreement with the sinc approximation if you tinker around with the input data. Statistically speaking, the OP's probably using closed box and hence won't see the dipole roll off but will see the ususal directivity reduction from diffraction off the box face. As a reminder, free air is usually a best case for dispersion.

I'm not spotting the thread where I originally posted the attachments in this post, but they're a few other models of the Neo3's free air horizontal dispersion after conversion to dipole operation. Opinions varied as to which best approximated Neo3 measurements but, while they're all in the right direction, none is particularly exact. As I mentioned previously, I'm not aware of any Neo8 polar measurements, but I would expect Neo8 simulations to be of comparable accuracy.

[bolserst]

Originally Posted by twest820

Don't think so. What you've determined is Edge's FEM simulation can results in agreement with the sinc approximation.

How embarrassing
I thought the data you posted was actual measurements...I see now that you stated it was simulation results.
Hope I didn't confuse anybody.