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1.4" vs 2" for high power?

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roborg View Drop Down
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    Posted: 02 March 2008 at 10:43pm
Still on the 3" diaphragms case!  Anybody measured/seen the HD from a decent selection of 1.4" & 2" exit drivers with 3" diaphragms when driven at near full rating?  Curious to know how the 2HD scales as throat size on these drivers.  At the moment my driver of choice would be 2" exit ~1k to about 8k rather than 1.4.
  I invested in a beyma cp750ti rather than 755 cos i'm kinda suspicious of 1.4" exit for flat out use & crossed over less than 1k.  If i was going to run absolutel full range then i'd prolly go fo 1.4.  cp750ti is a v nice driver BTW, not overly harsh when full range (to my perfectly calibrated hearing, or was so, b4 i listened to the driver close up heh)
 
cheers,
  Rob.
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ceharden View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2008 at 12:30am
I don't have anything to back this up but my thoughts are that the 1.4" driver came about because it's a better match acoustically to a 3" diaphragm without needing a massively long phase plug.  Note that drivers like the DH1A actually has around a 1.4" exit, then a huge lump of aluminium which expands it out to 2" before going into the horn.

Given that B&C are now producing co-axial midrange drivers which go down to 400Hz on a 1.4" throat I don't think there's an issue in that department.

Also look at drivers like the PD-C2 and PD-C14, essentially the same driver just with a different phase plug expansion.  I don't think there will be much in the way of performance differences between the two.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote TONY.A.S.S. Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2008 at 9:08am
Power wise, a 1.4" will take the same power as a 2", as it is the same diaphragm and voice coil, It's only the response that will be slightly different because of throat size. In spite of the low end spec on some of these drivers, most people, including me, would think that 1k is a bit low for a standard compression driver. Response on paper is one thing, but sound is another issue.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote roborg Quote  Post ReplyReply Direct Link To This Post Posted: 03 March 2008 at 3:08pm
@ceharden:  I've noticed alot of 'excess' volume in the throat area of 2" drivers, always seems a bit crazy having a nice resonant tube in the throat when it could be left to the horn design to decide what to do.
 
@tony: I agree about 1k being too low.  I usually x-over at 1k or lower if i'm listening at low volume, then up to ~1.5k for loud PA use (slightly muddier sound sometimes, but more headroom before failure.)
 
What i'm really interested in, is how much distortion the smaller constriction causes at high outputs (say 80w RMS.)  For me the distortion figures at low power levels are great to assess the quality of the construction, but don't help in determining the distortion at rated power.  My suspicion is the 1.4" exit drivers will produce about 30+% more 2HD than a 2" with the same VC phase plug etc..  Here's a little table using Beraneks distortion for a expo horn:
 
 
This ignores the rear compliance which is prolly smaller for a 1.4 driver, so a higher relative distortion than a 2" driver.  Obviously these are idealizations, but it's food 4 thought.
  Any1 here got any measurements for rated power & a decent selection of drivers?
 
cheers,
  Rob.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TONY.A.S.S. Quote  Post ReplyReply Direct Link To This Post Posted: 04 March 2008 at 9:26am
While we're on the subject of 1.4/2 drivers. I discovered long ago that flat fronted drivers with the phase plug in the front were better for constant directivity than a driver with a snout. I.E. JBL 2446. I guessed the reason was that the steep angle in the 2446 or other drivers of the same design made the extreme hf directional. There is around 50mm of angle which affects the related wave length, so any wave length of 50mm or shorter would exit the driver at the determined angle. Only the longer waves would defract and follow the angle of the horn.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote _djk_ Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2008 at 6:41am
Also remember that the throat area is actually the area of all the slots in the phase plug right next to the diaphragm, nothing to do with the exit hole to the horn.
 
Beranek's chart is assuming a 2.84" diaphragm for a 1.4" exit, a 4" for a 2" exit, and a 2" for a 1" exit (incorrect, as most 1" exit have 1.75" diaphragms). In the past these were fixed ratios, not so today. Also, these all had roughly 10:1 compression ratios in the phase plug (giving rise to high distortion levels), today there are very low compression ratio 2" drivers with much lower levels of distortion (CLS-M200A at 2:1 for example).


Edited by _djk_ - 10 March 2008 at 6:42am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote roborg Quote  Post ReplyReply Direct Link To This Post Posted: 11 March 2008 at 3:16pm
Yep good point, the graph then is the minimum expected 2HD.  Also the throat power density used above would be for 2xrating @50% efficiency,  IE program power rating (50% is a pretty bold assumption for the 1850 horn Wink)  For the moment my preferrence (for high power usage) is for a 2" exit to about 5-10Khz then something else above, piezos anybody hehe.
 
PS the chart is simply a tabulation of the forumla there.  It dispenses with the actual geometry of the horn by assuming the expansion is exponential with the specified cut-off.... easier to show the page!.....
 
 
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