Compression driver size and dispersion characteristics

Defo

Member
2008-11-06 9:15 pm
I need some help to clarify what I've read about dispersion characteristics relative to compression driver size.

1. People say 2 inch compression drivers beam more at higher frequencies then 1 inch drivers. This seems kind of a contradiction, since 2 inch horns tend to be shorter then one inch horns. How is this?

2. The BMS 2 inch coax drivers seems to have less beaming at higher frequencies then a normal 2 inch drivers. How so?

Thanks :)
 

ScottG

Member
2003-02-04 12:23 am
US
Looking at ONLY the compression driver (not the horn):

The larger the exit diameter, the lower in freq. the driver will start to "beam".

Wavelength

Look to the wave's length. (..a little lower than 7000 Hz for 2", and about 13000 Hz for 1".)


The coaxial unit in the BMS uses a smaller concentric ring radiator and exit dimension to achieve a higher freq. before it starts to beam. (..like having a 1" exit in the center of a 2" exit.)



Adding in the horn adds some "dimension" to this..
 

ScottG

Member
2003-02-04 12:23 am
US
Scott, do you have any off-axis curves on the BMS coax? I've heard a lot of talk from BMS and on the forums, but never seen any measurement on a real horn. And the BMS folks don't publish anything other than an on axis response.

No - and I wouldn't suggesting using a coaxial compression driver. :eek:

The problem is that the center higher freq. driver isn't "loaded" to the horn, only the lower freq. outer-exit driver portion is.

This basically means that for most of it's bandwidth the higher freq. driver portion is just *reflecting* off of the interior of the horn. (..imagine connecting a 1" exit driver to a 2" entry horn without any phase-plug expansion adapter - that's pretty much what the interior high freq. unit of coaxial is doing.)

The only real way "around" this is a diffraction lens in front of the driver, which is something else I wouldn't suggest.
 
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mayhem13

Member
2008-09-22 4:37 am
Agreed......if using a 2" driver, a seperate, 1" driver above crossed high is the way to go, unless high output pro system use is the goal.

Not sure where the trend of 2" horns being 'shorter'? Than 1" horns. If I were designing around a 2", my goal would be lower extension and matched directivity....which in contradiction requires a much larger lens. 1" is just right for the home IMO.

......and IMO a quality ribbon sounds better than both of the above.
 

POOH

Member
2010-10-05 9:06 pm
Ohio
The ultimate home systems use large 2" exit drivers in big horns handed off to a good, extended 1" driver in a much smaller horn. They also use horn loaded cone lower midrange and bass. Beaming is not an issue when you do this correctly. All the BMS drivers I've had, never the coax, sound very unnatural to me compared to most other quality drivers.
 

dumptruck

Member
2010-05-04 5:02 pm
MN
The problem is that the center higher freq. driver isn't "loaded" to the horn, only the lower freq. outer-exit driver portion is.

This basically means that for most of it's bandwidth the higher freq. driver portion is just *reflecting* off of the interior of the horn. (..imagine connecting a 1" exit driver to a 2" entry horn without any phase-plug expansion adapter - that's pretty much what the interior high freq. unit of coaxial is doing.)
Are you sure about that? I've never looked into drivers like that before, just wondering. If you look at the HF portion's response and impedance on the spec sheet graph of the 2" on a 90°x60°, it looks loaded to me :confused:.
 

tomtom

Member
2006-12-18 10:33 am
No - and I wouldn't suggesting using a coaxial compression driver. :eek:

The problem is that the center higher freq. driver isn't "loaded" to the horn, only the lower freq. outer-exit driver portion is.

This basically means that for most of it's bandwidth the higher freq. driver portion is just *reflecting* off of the interior of the horn. (..imagine connecting a 1" exit driver to a 2" entry horn without any phase-plug expansion adapter - that's pretty much what the interior high freq. unit of coaxial is doing.)

The only real way "around" this is a diffraction lens in front of the driver, which is something else I wouldn't suggest.


It isn work like that - LF and HF are "connected" prior phase plug. Nobody will be so stupid do it like you propose... Anyway this driver has other problem - connection chamber is too big so there are HF resonances and LF can modulate HF. Both these problem are adressed and solved in new JBL D2 driver
 

ScottG

Member
2003-02-04 12:23 am
US
Are you sure about that? I've never looked into drivers like that before, just wondering. If you look at the HF portion's response and impedance on the spec sheet graph of the 2" on a 90°x60°, it looks loaded to me :confused:.

No. I think it's actually "riding" off of the central phase-plug. Same sort of problem though - the wavelength's aren't large enough to couple to the exit properly.

It isn work like that - LF and HF are "connected" prior phase plug. Nobody will be so stupid do it like you propose... Anyway this driver has other problem - connection chamber is too big so there are HF resonances and LF can modulate HF. Both these problem are adressed and solved in new JBL D2 driver


That's the way it looked on the thread "Testing the Big Waveguide".. Which unfortunately doesn't have its "curves" anymore. Both with respect to its off-axis measurements and it's distortion profile.

(I'm also pretty sure that the B&C DCX50 did do this - it's a compromise, like anything else.)

http://kiirojbl.exblog.jp/10037965/
 
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ScottG

Member
2003-02-04 12:23 am
US
I haven't looked in a while, but I thought that the exit/wavefront from the two radiators in the "coaxial" compression driver were combined before they exited the driver's throat.

There is this rendered image of the 4590:

Google Image Result for http://cdn.avsforum.com/e/e7/e7276bad_vbattach206740.jpeg

Which doesn't have the concentric design.



Then there is the replacement assembly that certainly *looks* concentric:

BMS 4590 - Mittelfrequenzkalotte für BMS4590H 2" Coaxial Treiber - Showtechnik & Veranstaltungstechnik - ATLD Ton und Licht Service GmbH



So I don't know what to make of it.. :scratch1:
 
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ScottG

Member
2003-02-04 12:23 am
US
The first image is of the 4594, not a 4590.
What is your definition of concentric?

Yes, that makes a bit more sense.

Yes, they are both concentric.. should have said "partitioned" (or separated) concentric. :eek:


Here is the 4590 (post 33):

JBL 2360 - new install - Page 3

-and it looks like it's not partitioned, both the super-tweeter ring and the mid-tweeter ring use the same phase-path system.
 
According to what I've learned from some posts elsewhere by Tom Danley, the tweeter's radiation pattern at the upper part of its band is already determined in the first inch or two of the horn, some even before it gets into the horn. The higher in frequency, the closer to the driver the beamwidth is determined.

(That's for non-diffraction horns - diffraction horns which squeeze the output through a narrow slot further toward the mouth can widen beamwidth, though introducing some other problems).

In general, a waveguide type horn (such as Conical, OS, or others that don't squeeze down the aperture at a distance from the driver diaphragm) doesn't widen the beamwidth at high frequencies, it just narrows it at lower frequencies to reduce the variation. So at the very high end, the driver aperture size essentially defines it.

Or, so I understand (I'm happy to be corrected or have this modified by someone who knows the horns or their mathematics more completely -- Tom?)
 
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Adding to what Bill wrote. The way you can think about it is physical wavelength sizes. When the circumference of the pathway is equal to or smaller than the wavelength passing through the pathway, its dispersion is controlled by the pathway shape. When the pathway is acoustically large in comparison to the wavelength, pattern control is set and the horn you attach has little to no effect. The higher frequencies are just beaming through the acoustically large pathway. If the crossover is set to 6KHz, when the internal cross sectional area increases to 2.596cm^2 the dispersion pattern is set. For the BMS coaxial this means the high frequency pattern is set well within the driver.
 
. For the BMS coaxial this means the high frequency pattern is set well within the driver.

Or available pattern. If it is to go on a 40º horn it would use less than the available pattern.

Additionally, the 4592 has a wider exit angle than the 4590.
The 4594 is shorter, and has an even wider exit angle. So its available HF distribution angle is bigger.
 
Hi
Both Bill and John have touched on the “bits” but since I have been writing about this and it’s fresh in my mind, I would also reply.

Everyone who has looked at making a bass horn has seen the reference to the ideal mouth size, a daunting 1wl in circumference. If you look at the acoustic radiation resistance curve for a radiator / horn, you see that that point is approximately where the curve goes from slanted up to flat.
This curve has Frequency on the horizontal and resistance on the vertical. The “impedance transformation” part of a horn is the coupling of the large end where the resistance is greatest, to the small end which has very little loading. If one makes the horn mouth that ideal size, one has insured that at any frequency above that, it will always present a constant load (the flat part of the radiation resistance curve). Also, making the horn much larger, has no further effect on loading but can control directivity.

It follows then that at 20KHz where the wavelength is only 5/8 inch, that ALL of the impedance transformation takes place well before reaching even a 1 inch exit.
In a wide band horn, one can picture that the active portion is the entire length at the low cutoff but the “active” region moves towards the compression as the frequency climbs.
Past the active (impedance transformation) portion of the horn is a part where the horn is still confining the radiation angle. Once one is in that region, if you change the horn wall angle or have a discontinuity, it is possible to radiate from that location like with edge diffraction etc. Earl Geddes waveguides are a case where the horn wall angle is changed at a precise rate which avoids that problem.

As you move closer to the horn mouth, you reach a dimension large enough relative to the frequency where the wave can be “launched” free form the horn but still maintain that shape. Don Keele discovered this and through several of his papers (beginning with “what’s so scared about exponential horns) arrived at the “pattern loss frequency” thumb rule. Given the dimensions, the frequency and horn wall angle, one can find the frequency down to which that horn angle will maintain that radiation angle.
So in addition to the acoustic transformation region moving towards the driver as the frequency climbs, so does the pattern control point.

This is why an exponential (normal curved wall horn) has a narrowing radiation angle with increasing frequency.
Also, what you want to avoid is doing anything to the wave front, once it is large enough to have it’s own directivity because that can easily cause a secondary radiation from that point and radiation from more than one point (points more than ¼ wl apart), this produces an interference pattern.
To “fill” out a horn, you do not want a source which already defines the radiation angle, that is what you want the horn to do. So, one can consider how the driver radiates as a simple source before adding the horn to it to make sure that it is the horn, not the driver that is defining the pattern. This plot shows the radiation angle produced by a simple source give different acoustic dimensions.

http://www.soundandcommunications.com/archive_site/audio/images/2005_pics/sc05_09_audio10.gif

On the BMS drivers (Hi Jack), I like these a lot because many radiate an expanding wavefront into the conical horns I use. This makes the acoustic path more like a continuous horn down to a small dimension and then what radiates is as much like a spherical patch as possible. They have other advantages in output and an usually low resonant frequency (more like a TAD) which makes the Synergy crossover easier into 5 in mids.

I use the 1.4 inch coax drivers too, they are really powerful but to be clear, they are projecting a well defined beam at 20Khz due to the dimensions and configuration and it is difficult to make the same phase shift free Synergy horn xover passively. That aside, it is a really powerful and good sounding driver.
Best,
Tom Danley
 
Tom what happens when you are using a horn which is compromised in mouth size and or in length, what then happens to pattern control at low frequency range of the horn? I have in mind here typical mid horns used in three ways and two ways. Lets say a Cornwall with a K700 crossing at around 650 Hz with a fifteen inch woofer below. There is the idea of running the woofer up to the point where its polar matches that of the mid horn but this will result with the woofer generating multiple points of output or radiation of the same frequency and so spatial sources. Thanks for any comments on this. Best regards Moray James.
 

MaVo

Member
2006-01-07 12:40 pm
Concerning the crossover difficulties with a 1.4 inch Coax: Would it be possible to leave the cone mids out of the speaker and use just the woofer and coax, or even make a speaker similar to the SM60M with just the compression driver? BMS suggests a 300hz crossover, which should be in the area of the woofer crossover of the sh50. On the other hand, i have no clue how much of the "synergy" concept would be left in such a horn.