Beaming effect on polar response at crossover

[tiroth]

What kind of effect does driver "beaming" have on the expected polar response at crossover? I have always seen simulated phase plots calculated with the assumption that each driver is a point source radiator. This is not at all true for some popular drivers, with long ribbons as the extreme example.

I know that there is no definitive general answer, but what about this case: a beamy mid (planar or large cone) crosses over to a long thin ribbon fairly high up. What differences from the perfect driver pictured below will occur in the real world?

[SY]

This is a real can of worms. One of the most common faults of crossover design is exactly what you're saying here- neglect of the effect of actual driver dimensions and placement on polar pattern. What you end up with with is an irregular horizontal dispersion with frequency and at worst, the speaker never sounds balanced no matter how much you jiggle the xover and eq, at best you have a speaker which is HIGHLY critical of placement and room treatment. The ear senses not only the direct sound, but the reverberant field- optimize one and you screw up the other unless driver dimensions are taken into account. And when you factor in break-up modes, it gets even worse.

A common "bad" two-way that can really illustrate the problem is the classic 8" crossed over to a 1" tweeter. If you make the on-axis response flat, the off axis response will almost always have a dip near the top of the woofer passband, then a peak as the tweeter rolls in. That can only be avoided by setting the crossover quite low, like 1200 Hz, which will NOT make a 1" tweeter very happy.

[tiroth]

Not an easy thing, designing loudspeakers.

Here is a ripple tank simulator that I tried to use to emulate line sources...a true line source should be a good model for the situation in the vertical axis, right? It looks like the center lobe is not much altered but the off-axis lobes, especially the lower one, are seriously attenuated.



Does this connect with reality?

Here is the ripple tank I used. I didn't play with the wavelength yet.
http://webphysics.davidson.edu/Apple...le/Ripple.html

Still looking for a simulator that supports line sources.

[Bricolo]

SY: and what would be a "good" 2 way?

[tiroth]

It isn't that 8" + 1" is "bad", just that it is a very challenging design problem. Most designs fall short because of the woofer beaming, breaking up, and/or rolling off before xover; and the tweeter distorting, limiting dynamics, or peaking at resonance due to low crossover point.

[Svante]

Tiroth:
I don't have a definite answer to the question, but as you say the response will be different. My intention with the XDir program was mainly to understand the effects that can occur, in principle, not the details in a specific case. In order to understand something, I find it useful to start with simple models, and then to refine them if needed. I might do that some time. It is not very simple, though, modelling the driver surfaces with a bunch of point sources would be one way, but at frequencies where the cone does not move as a rigid piston simulations become very difficult.

Now the question was not about simulation techniques, so I guess I have answered nothing whit the above. However I still think simplified simulations can help the understanding of more complicated systems. I think you would find some of the effects in the real world similar to what XDir predicts, even if not identical.

[DanWiggins]

Originally posted by Bricolo
SY: and what would be a "good" 2 way?

IMHO, you need a high order crossover to make it work. A good 4+ order XO, around 2 kHz can definitely successfully integrate an 8" woofer an 1" tweeter.

The premise is to shut off the woofer as quickly as possible, and likewise the tweeter. Use the woofer as high as you possibly can (most 8" woofers are OK - from an off-axis standpoint - up to 2 kHz), then cut in to the tweeter. Roll off the low end of tweeter to reduce distortion. Plus, with a high order XO, you have reduced overlap, so other interference issues are reduced as well.

Dan Wiggins
Adire Audio

[tiroth]

Originally posted by Svante
Tiroth:
I don't have a definite answer to the question, but as you say the response will be different. My intention with the XDir program was mainly to understand the effects that can occur, in principle, not the details in a specific case.

Svante, I must thank you for your excellant little apps. I think they are very useful and there is a lot to be said for acheiving an 60% match to reality...especially when the next 20% requires FEA and the final 20% can't be simulated at all.

I think it is a testament to the quality of your programs that I am always wanting more: line sources in xdir, sources of multiple sizes in The Edge, etc. The work is never done.

[Jimmy154]

Wouldn't a ribbon tweeter, if in the long part going vertically up and down beam in the vertical direction and not in the horizontal, which I wouldn't see to be that bad?

I built a Eton 8.1 type 2-way (except with MB Quart tweeter and little different x-over, also sealed enclosure). It has a 6 db/octive on the woofer and 12 db/octive on the tweeter and I'm happy with it although it might beam a little, I'm not sure.

[SY]

One "funny" approach to mating an 8" with a 1" was an old British design by (I think) Rogers. The woofer fired through a vertical slot 4" or so wide, which caused it to have a dispersion pattern that looked like a much smaller driver. The cavity resonance was reduced by appropriate eq, albeit imperfectly.

A fine example of a well-done two-way was the cheapo Super Zero from NHT. And of course, the classic LS3/5A. And just about anything from PSB in the last 7 or 8 years.

Wouldn't a ribbon tweeter, if in the long part going vertically up and down (standard placement I'm assuming), beam in the vertical direction and not in the horizontal, which I wouldn't see to be that bad?

Yes, that's right. The limited vertical dispersion may or may not be a bad thing, depending on the design goal, but it WILL be noticeable, as will the change in spectral balance with distance in the near field due to the 1/r power law of the ribbon contrasted with the inverse square power law of the woofer.