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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: concept for crosoverless speaker posted by ChasLieb on November 25, 2010 at 11:05:05:
Even this crossoverless speaker will have 2nd order acoustic rolloffs that would require the tweeter to be wired reverse polarity if it meets the criterion for an LR2 design. Crossover or no crossover it's the final acoustic rolloffs that matter, and how they sum in the crossover region. You may as well add 2nd order electric filters to the natural acoustic rolloffs and aim for a 4th order acoustic design. Sure, the group delay is not as good, but the drivers polarities are the same, and the response is in phase at all frequencies. You just don't get perfect impulse response - but neither 2nd or 4th order have perfect impulse response, save for the 2nd order transient perfect design but that's a whole other enchilada. Also, you need to realize that even if both drivers have a 2nd order rolloff, they also need to have the same corner frequency if they are to sum correctly. If their corner frequencies differ, you'll either get a big hump or a big dip in the response. There is more to it than just finding drivers with good looking natural rolloffs. Further to that, all conventional tweeters have a natural resonant frequency. Even WITH crossovers, if you cross too close to this frequency or don't cross steep enough, the effects can be far from subtle. And I mean ranging from "too analytical" sounding to downright offensive. Tweeter Fs considerations and often impedance bump and rise compensation are a big part of the design process. Last but probably most important is distortion. When you try and run devices like tweeters full range and just let them roll off "naturally" you're going to be operating almost ever tweeter in existence (if not all) in a region where distortion is going to skyrocket. Not to mention with a ribbon, as you go lower in frequency, you also go lower in AC impedance. Ribbon transducers use transformers, which will result in a near dead-short when you try to apply a full range signal to them. This can be seen on their impedance plots IF the manufacturer wants to show you the impedance down low in the first place. Most do not - they don't want to advertise this feature of the transducers. So, even if you DID find a ribbon transducer that could handle full range, you'd still need at least ONE CAP (1st order electric filter) to protect not the tweeter per se, but the amp driving it!This "tweeter" is the closest thing on the planet to a true ribbon full range, considering electrostats are not, by definition, true ribbon transducers.
http://www.orcadesign.com/products/raven/ravenr3.htm#
In a nutshell, tweeter low end rolloff, by itself, is just not usable like the rolloff of certain well-behaved midbass drivers. Some designs DO have no crossover on the midbass but still use some sort of filtering and equalization (passive) to get the tweeter acoustic response to align with the midbass natural rolloff.
Read this:
http://www.musicanddesign.com/Speaker_Transient_Analysis.html
John Kreskovsky has written a lot of stuff about transient-perfect crossovers such as subtractive or filler-driver and quasi-transient perfect designs as well.
Another path is digital. You can use the Thuneau Allocator and Thuneau Phase Arbitrator to essentially eliminate the group delay of a 4th order LR crossover using forward-reverse processing. How this works is that digital filters are used to mangle phase in the exact opposite way that the desired acoustic response will mangle phase. Then, the speakers don't mangle phase, they actually unmangle the mangle, resulting in near-perfect group delay and a perfect impulse response (on the chosen design axis of course). And you still need to eq the drivers flat in their passbands and match the target acoustic response curves for the chosen fitler types. Think of it as "real time phase compensation" and it works. And it's not a "room correction / impulse EQ" Measurement is mandatory here. "Published" FRD data, bode plots, frequency response, impedance plots, etc. are generally good for theoretical work only - not really useful in the actual design process.
Go here: www.thuneau.com
But don't forget you're also going to need a measurement suite (software program) Speakerworkshop is free but a b*stard to figure out how to use. For a beginner I would high recommend something else. Soundeasy is the definitive "DIYers choice" for speaker design using passive crossovers, but still has a steep learning curve. ETF is good for measurements for active systems where you don't need to model impedance and passive elements like you do with passive designs. ETF can be used for passive designs too, but you would need a separate passive crossover design suite. Soundeasy is both a measurement suite and a crossover development tool.
I am currently working on a WMTMW with all digital crossovers right now that will be transient accurate, with no phase distortion. Essentially, a speaker system that is both time and phase coherent. The big trick with speaker design is not just getting a flat on axis-response, but understanding and being able to implement concepts of power response, dispersion, preventing lobing, and baffle step compensation are all pivotal in getting a speaker that sounds good. Making pretty graphs is easy. Making a great sounding speaker that also happens to conform to design ideals is not easy - and much experience is required to get the correct measurements, but more importantly, being able to interpret what you have measured.
If you do go the digital/active route be prepared to spend $250 - $500 for an entry level audio interface and as much as $2k or $3k if you want the best that's out there that will rival most high end components. You'd be surprised, however, what a decent $250 or $300 audio interface can do in the right hands.
No offense, but you've vastly over simplified the design process here. Your idea is neat in theory but in practice faces far too many barriers to be possible with real-world drivers at sensible listening levels. Also, 3-3.5K is quite high to cross over to a 7" midbass. Driver center spacing would indicate a much lower frequency, such as 1.8 - 2.5k, else you're going to run into lobing problems. Lobing is a necessary evil for ribbon designs that require high crossover points combined with the large center spacing distance of a large ribbon unit used with a larger midbass. You see a few 7"/ribbon designs out there and really, they start out with a few serious design drawbacks which might explain why they are not all that popular and not the be-all end-all. Getting a seamless integration between a dynamic 7" driver and a ribbon transducer is a very very tall order. Often, the drivers that are "fast" enough to keep up with a ribbon are not well known for low frequency extension. You need mass to get low Fs and low bass - and light quick drivers can't have a lot of mass.
Cheers,
Presto
Edits: 11/30/10
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Follow Ups
- Why would you want a 2nd order acoustic anyways? - Presto 10:21:17 11/30/10 (1)
- Dude, you make it sound so intimidating! - caspian@peak.org 09:21:01 12/01/10 (0)
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