Compression drivers - sound characteristics.
I was looking to buy a Audio Nirvana 15" Co-ax that come with a compression drive or to use a normal Audio Nirvana 15" with an external ribbon tweeter.
Ideally the compression driver would be a better match than the ribbon tweeter, since it is built into the speaker driver. However I am clueless about how the sound characteristics of a compression driver?
How is the sound like, harsh, detailed, airy, smooth?
If you have made a comparison of compression driver with other types of drivers domes, metal or ribbons, I would very much like to hear what you say on them.
Many thanks ahead.
What sounds the best to your ears will be determined primarily by your listening room, enclosure placement, the quality of the driver units used, and not particularly the choice of one set of system design trade-offs over another set.
A compression driver, by design, improves electrical to acoustical conversion efficiency and requires use of a horn to transform the acoustical high impedance of the diaphragm to a low one of the surrounding air. This type of baffle focuses sound into a narrowed dispersion pattern that reduces the relative intensity of early reflections from adjoining room walls. When used as the center element of a coaxial driver, the assemblage avoids source bifurcation and behaves as an ideal point source in the near as well as far field.
Use of a separate ribbon tweeter and cone woofer drivers, trades these characteristics for a low mass tweeter diaphragm, sans voice coil, totally immersed in a magnetic field. Here in the near field it behaves as a line source; however, at the low crossover frequency that is most likely, the dispersion patterns of both drivers will remain sufficiently matched.
apart from that, a woofer cone is not the ideal waveguide for a compression driver
is it enough advantage to be a good point source ? maybe it is in some cases
but usually the driver/waveguide match appears to be the key to a good result
even the best driver in the world wont help you one bit if not set up properly
I would also consider to use a dedicated tweeter amp
which again could cause some other 'issues'
See P-Audio  for an example of a coaxial driver with horn loaded compression driver tweeter.
 P-Audio SN Series II SN15-500CX 600W 15" Low Frequency Coaxial Driver with 3.28 Inch Voice Coil[/url]
What makes "the sound" from a compression driver?
Large Dome tweeter with massive magnet
Compression Chamber leading into a Phase Plug
In short, if you look at the coax itself:
Its surround and cone profiles, and the baffle it will be mounted in, will all affect the sound in terms of on AND off-axis frequency response.
Then you have to consider the transition from the throat to the coaxial. Since the cone moves, there is a sure discontinuity. Whether it matters is debatable. The size of the throat determines the ability to produce high frequency response
Finally you're down to the dome itself. It will have its own resonances, for example diaphragm breakup. A 1.8" dome probably breaks up lower than a 0.75" dome, and has its displacement limitations too. It has to be stiff to deal with the high level of compression it is under. It has to have good control over magnetic flux to ensure a very consistent sound.
Last but not least you have the crossover. it affects what you're asking of the compression driver, what you're asking of the cone it's mated to, etc.
So the answer is... just like any other driver type it totally is case-specific. There is no one compression driver `"sound" just like there's no one ribbon sound or one flushmount dome sound. Like with any of those, it's possible that colorations can be introduced through poor driver design or poor system design. But it's also possible for a CD to be very transparent - but often gets expensive with respect to exotic diaphragm materials. Generally you can squeeze about a decade of bandwidth out of a compression driver with a very consistent dispersion pattern - which often makes them a good choice for upper mids and the treble. They may not have the exact reach of a ribbon in the top octave as a result though. Still, ~15khz extension out of a 1-inch CD is perfectly adequate for most people. If you do have a lot of hearing above 14khz, then perhaps a cheap ribbon supertweeter with a steep crossover slope could be an option - IE Hi-Vi RT1.3C - assuming you can match sensitivity or use an active crossover. A compression driver could be made with higher frequency reach, but then it wouldn't extend as a low. It's a pick-your-bandwidth poison. Arguably two-ways are just inherently too zealous.
A 12" Coaxial like the BMS 12C362 can still have a very wideband response. I don't know what to say about the Audio Nirvana unit. Not much information is given.
From another perspective, here are a couple of reviews regarding the sound of compression drivers vs ribbons and AMTs. It might give you some ideas of the virtues of each:
B&C HF drivers DE400TN & DE35 - [English]
[Review] ESS Heil Air Motion Transformer - [English]
On a good horn, even at a very loud level, a properly equalized compression driver sounds very much like the source material.
I did a series of tests which allows you to hear the sound of a variety of compression drivers at various levels.
The “Full Monty” has been posted Here:
A shorter version (only 3000 rather than 8000 words, and a lot less pictures) can be found here:
High Frequency Compression Driver Evaluation
The sound files in the soundforums.net posts do not have the low frequency portion of the music mixed in, listening to the HF horn alone makes it easier to hear the difference in sound quality and distortion between the drivers.
The recordings afford an insight in compression driver comparison (as far as I know) never undertaken before this study.
Hope you will enjoy the opportunity to compare the different driver’s sound without all the usual problems associated with A/B/C/D/E/F testing.
Thank you for all the lengthy replies. Will take me a bit of time to digest everything. Could i summarise everything into a few sentence
1. Sound quality could have quite a bit of variaance. Depending on well the horn was designed.
2. A 2 inch compression driver is unlikely to work well in the very high frequency region ..as a supertweeter of sorts. Really needs a much smaller one.
3. The sound will be highly directional rather than dispersed.
4. Unlikely the compression driver in a coax will sound good because of the lack of control over the size and shape of the horn.
Would that be correct?
I would say:
3: The sound of what?
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