1" compression drivers with 2" diaphragms

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tinitus

Ex-Moderator R.I.P.
Joined 2005
Obviously the result will be a midrange driver with high end rolloff

A bit strange as there are lots of "normal" and much bigger fullrange drivers ?

It appears that cone break up may be a problem, and might be related to the light cones demanded fore the high efficiency
 
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DSP_Geek

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Joined 2003
454Casull said:
Why not use a .75" or even a .5" throat, then? If I understand correctly, you get better high-frequency dispersion (and less HOM, according to Geddes) that way.
Click to expand...

Power density at the throat increases as the inverse of the area. 1" drivers are already susceptible to second harmonic distortion at high levels, and decreasing throat area will increase distortion.
 
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DSP_Geek

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454Casull said:
Is it only the 2nd-order HD that increases?
Click to expand...

Mostly, although other even products also increase. High levels in a small throat actually start approaching atmospheric pressure, and compression is more linear than rarefaction (awfully tough to get less than 0 psi).

Some numbers: JBL 2425H 1" driver on a 90 degree horn has 110 dB/W/m sensitivity, whereas on a 1" plane wave tube it's 117 dB/mW/m. That's not a typo, the driver will indeed drive 117 dB at 1 milliwatt in a tube the same size as the throat. The aforementioned 110 dB at 1 meter would take 1 watt, which would give 147 dB on the plane wave tube. 1 atmosphere is about 192 dB, so you're only about 45 dB down from an entire atmosphere of pressure. The driver can handle 70 watts, so add another 19 dB of level and you're at 166 dB at the throat! That's less than 30 dB below one atmosphere, so just the air will have at least 4% distortion, let alone any contribution from the diaphragm suspension. And remember many drivers have minimum areas somewhat behind the opening, so the actual throat is smaller than advertised.

Driver for this example:
http://www.jblpro.com/pub/obsolete/2425hj.pdf

This paper has a quote from Beranek:
http://www.meyersound.com/support/papers/REM/


No one sane would use even 1 watt on this driver/horn for home use, however, because most music energy is below 1 kHz, so you'd likely get something like 120 dB at the listening position. These drivers come from the pro sound domain, and there's been little work on a home driver without the pro inheritance of astonishing levels, little surprise since building a driver needs expensive tooling. Home use could get away with a smaller throat if the crossover was 24 dB at about 1 kHz or so, but anything else would need to be considered carefully.
 
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DSP_Geek

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Lynn Olson said:
So following this line of argument, the pressures inside the phasing plug, with its tiny dimensions inside the slots and next to the diaphragm, would be much higher. Maybe that's why 1.4 and 2-inch compression drivers with 3-inch diaphragms sound more "relaxed" in the midrange.
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Ayup, that's precisely it. Good catch, by the way - that means the distortion is even higher, perhaps double or triple, than what's calculated for the throat. The "relaxed" sound with larger diaphragms might also be due to a smaller excursion because of the larger area, and the design of surrounds for larger diaphragms permitting greater excursions.

Larger diaphragm drivers tend to break up lower in the passband, so there's a definite tradeoff.

I'm not a huge fan of horn drivers as currently designed, anyway. Meyer points out the idea of a cylindrical wavefront from a line array is mostly wishful thinking, and phase plug designs also seem to posit wavefronts down the various annular paths will magically create a flat wavefront at the throat, expanding perfectly down the horn, for ever and ever amen -- I suspect things are a lot messier than that. There's also the problem of circumferential phase plugs accentuating certain concentric diaphragm modes; Altec tried addressing the problem with their Tangerine phase plug, which had radial slots.

Meyer on line arrays:
http://www.meyersound.com/support/papers/line_array_theory.htm
 
Lynn Olson

Lynn Olson

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Joined 2004
nutshellhifi.com/
DSP_Geek said:

I'm not a huge fan of horn drivers as currently designed, anyway. Meyer points out the idea of a cylindrical wavefront from a line array is mostly wishful thinking, and phase plug designs also seem to posit wavefronts down the various annular paths will magically create a flat wavefront at the throat, expanding perfectly down the horn, for ever and ever amen -- I suspect things are a lot messier than that. There's also the problem of circumferential phase plugs accentuating certain concentric diaphragm modes; Altec tried addressing the problem with their Tangerine phase plug, which had radial slots.

Meyer on line arrays:
http://www.meyersound.com/support/papers/line_array_theory.htm
Click to expand...

I'm not a horn expert - as everyone on the AAH-E forum gladly reminded me - but as far as I know, horn theory relies on plane waves entering the horn at the throat. If the wave isn't a plane wave (or spherical, or whatever), then all bets are off, since the underlying assumptions are broken.

Horns and waveguides don't magically "straighten out" the wavefront that enters them - it's hard to imagine any device that could make an incoherent wavefront straighten out and become coherent.

So if the wavefront entering the horn is chaotic, it will emerge at the other end in worse shape, not better. This might account for the fine-grained ripples we see in the frequency, polar, and impulse responses. I do consider it significant that horn responses almost always seem to use some kind of smoothing to make the data look nicer.

It would be interesting to know just how far from planar the wave from phasing plugs really are - or is this interpolated from the HF deviations that appear in the frequency response?
 
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DSP_Geek

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Lynn Olson said:

It would be interesting to know just how far from planar the wave from phasing plugs really are - or is this interpolated from the HF deviations that appear in the frequency response?
Click to expand...

Well, look at your typical phase plug: the output comes from a series of concentric annuli, each with a large ratio of length to width [1]. That's a good geometry for sustaining front-to-back resonant modes, and each annulus will have its own quarter-wavelength resonance, or half-wavelength if the diaphragm hits a bending mode - I suspect part of the black magic behind horn driver design is keeping the diaphragm resonances away from the phase plug ones. As you've noted, the sum is nowhere near flat.

[1] The Peavey 22A had a different configuration: instead of slots, the phase plug had a series of 1/8" holes drilled from the back to the front, all joining at the output. Imagine the Q of those resonances - that thing sounded really raspy.

As far as how planar the waves from a phase plug are, a wave tank could show the answer quickly, especially since current phase plugs are symmetric around the axis so 2-D simplification would preserve most of the salient features.
 
djk

djk

R.I.P
Joined 2001
" The Peavey 22A had a different configuration: instead of slots, the phase plug had a series of 1/8" holes drilled from the back to the front, all joining at the output. Imagine the Q of those resonances - that thing sounded really raspy."

Not related to the 'salt shaker' style phase plug.

All the legendary Tannoy use this same type phase plug, as do the legendary RCA mid drivers.

The JBL2415 also used this style.

The newer versions of the Peavey and JBL use the anular concentric slots.

Celestion has a white paper on a much improved phase plug type.

"The technology revolves around a patent-applied-for method, developed by head of group research Mark Dodd and his team, to calculate key dimensions within the compression driver's phase plug. According to the company, this delivers a much lower distortion performance thanks to the suppression of cavity resonances at higher frequencies. The resulting lower compression ratio reduces air nonlinearity and allows for higher maximum SPL. "

JBL is also improving on their older designs. The old 2441 went through three design changes (with no different model designations). The 2445 also went through a similar evolution.

http://professional.celestion.com/pro/media/press_release.asp?ID=89

http://www.aes.org/e-lib/browse.cfm?elib=14316

I read a free copy, but I can't remember the URL.
 
gedlee

gedlee

Member
Joined 2004
gedlee.com
454Casull said:
Why not use a .75" or even a .5" throat, then? If I understand correctly, you get better high-frequency dispersion (and less HOM, according to Geddes) that way.
Click to expand...

Well thats not exactly what I said. The HOM problem moves higher in frequency with smaller throats and of course lower with larger throats. The 1" appears to move them high enough to not be a major problem so smaller throats would not necessarily be better. Larger throats will become a problem if not handled properly.

Lynn
How the non flat throat wavefront assumption affects the wave at the mouth is indeed very complex and in fact cannot even be calculated with Horn theory, only Waveguide theory can do that problem. But even then its very difficult. I have seen some measurements of wavefront coherance in a compression driver at the outlet aperature and its not very flat.

At LF the waveguide will smooth any irregularities at the throat, but at HF it will amplify them. This is a gross oversimplification, but makes the point.
 
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