why do larger drivers beam upper frequencies?

[mrkahn]

I am wondering if anyone could explain the principle behind why a larger driver seems to beam upper frequencies. I just want to understand technically what this effect comes from!
I have been playing with a Stephens Tru sonic FR120 full range driver (12") running with a 15" OB and I have enjoyed what it does in the midrange (sounds lovely, smooth and flat) but it beams the highs. It sort of gives the cupped hands effect...
It rolls off steeply at 10k, nothing much after that so I am looking for recommendations on filling in the top end (tweeter? supertweeter?)
Is this rolloff contributing to the beaming?? not sure...

[Geek]

I'm a little fuzzy on the "why", so I'll let a guru answer that part.

But these might help:
http://www.webervst.com/blocker.html

Cheers!

[mrkahn]

Thanks Geek, I'll check it out...

[tinitus]

Originally posted by mrkahn


It rolls off steeply at 10k, nothing much after that so I am looking for recommendations on filling in the top end (tweeter? supertweeter?)
Is this rolloff contributing to the beaming?? not sure...

Only directly on axiss
If you listen off axiss it will roll off long before that
Im no expert, but beaming is loss of dispersion, which ofcourse is offaxiss, and could also be seen as a change in the polar response pattern
It would cause offaxiss phase issues
Its closely related to size
And shape and weight
Also size of voicecoil seems to be important
Voicecoil inductanse matters

Mind you, beaming and the unpleasant "shouting" is not the same thing...well, I dont think so, or maybe it is related

[Drew Eckhardt]

Originally posted by mrkahn
I am wondering if anyone could explain the principle behind why a larger driver seems to beam upper frequencies. I just want to understand technically what this effect comes from!

As the frequency increases and the wave lengths shrink relative to the driver diameter, there's more phase difference in off-axis positions so they sum to lower magnitude than when in-phase.

For example, think about what happens when two points on the driver are half a wave length apart. If you were infinitely far from the speaker 90 degrees off-axis the sound would cancel completely.

[ashok]

FWIW... Beaming is mainly due to the size of the driver. The driver starts beaming when it's size becomes comparable to the wavelength of the frequency being radiated. That's why tweeters are small .

Shouting should be due to cone breakup or other mecahnisms where the response breaks up with narrow peaks that cause exaggeration of certain (usually) upper harmonics . Wide band resonance could be tamed but narrow band break up is hard to compensate or remove.

[454Casull]

Originally posted by tinitus

Its closely related to size
And shape and weight
Also size of voicecoil seems to be important
Voicecoil inductanse matters

Yes, sort of, no, no, no

[planet10]

That cylimder in front of the driver isn't going to help... you should champher the front of the holse so that the driver holes opens up at 45 degrees. Or front mount the mid-tweeter.

Highs start to beam at higher frequqcies because of combing effects. When the size of the driver is large compared to the wavelength and coming from all surfaces of the cone you get lots of varting path lengths hich cause cancellation. On axis path lenths are symettrical.

Similar to this:



dave

[HK26147]

Google Book preview this:
Loudspeakers By Philip Newell, Philip Richard Newell, Keith Holland
for the phrase "beam"...
( p10 - 12 )
and this book as well
Loudspeaker and Headphone Handbook By John Borwick

and you will also find discussion of this topic

[mrkahn]

Thank you all for your knowledge and help!
It seems to me that a large fullrange has problems... since the FR120 rolls off the highs anyway, the best solution to all the problems it presents is to run a tweeter above the fullrange (so much for fullrange)
Not exactly what I wished for but to solve this drivers faults, I must not ask it to reproduce anything above ???hz
What a strange driver to manufacture as a fullrange huh? What are its ideal applications?
There is definitely an awful off axis rolloff... I will measure this weekend to see what it is.
You really have an opportunity to learn more from a problematic driver than an ideal one... I am having a blast!