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Multi-Way Conventional loudspeakers with crossovers

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Old 7th November 2008, 02:46 PM   #1
pjpoes is offline pjpoes  United States
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Default My most recent tower design

I will try and post some pictures and such later. Probably won't be able to post any graphs or schematics until my laptop returns next week. None the less, I'm wasting some time at the office, and thought I would at least start the thread.

Ok so this tower is a 3-way design using a Focal W-cone 6W4452 with additional magnet added to back. The tweeter is a Focal TC120tdx with some changes. The set I had were slightly damaged, so new td5 diaphragms were ordered and made to fit. The ferrofluid was replaced with a slightly thicker viscosity ferrofluid purchased from ferrotec (made for use in tweeters), which helped tame some of the resonances and slightly lower distortion levels. Midbass/bass is being handeled by a pair of Mpyre Audio 6.5" woofers. These are very simliar to the adire audio 6.5" driver with xbl2 motor, but uses a carbon fiber cone (heavly dampened on the back with some black goop (I wish it didn't have this), making for a driver with a linear bl curve out to 12mm one way xmax. The efficiency is quite low, around 84db's in the range I use them in, which is unfortunate. They use a neodymium magnet that is quite large and heavy for a driver of this size.

Ok first, all I have done are models, individual driver measurements, and the enclosures are now at the final finishing stages, so I can't say anything about final design yet, as things may change. The reason for the driver choice is nothing more than this, its what I had laying around.

The design thus far models out with roughly 89-90db's efficiency. It is going to be wired so that the two bass drivers could be used as either a normal passive system with passive crossover for all drivers, or with a separate amp powering the bass speakers actively. Doing this will allow greater efficiency as the midrange with added magnet is around 91 db's efficient with 1 watt at 1 meter in its used range (350hz on up) and the tweeter is around 92/93, so again, is not a real issue. The 350hz point was chosen because it coincides with the baffle step point. The upper crossover point is a little high for my taste, but was chosen to minimize the tweeter's distortion at high output levels which seem to be caused by driver power compression. That point is around 3.2khz or so. The midrange lowpass and tweeter highpass are basically LR4 types, but the highpass on the midrange and low pass on the woofers are 2nd order B-W.

While these will be used for both music and movies, my goal with this was to test how different crossover points and types can improve or worsen the sound when used with highly dynamic sources. While I will be very interested to see how they handle well recorded highly dynamic music, I will probably be doing much of my testing with movies. One goal was to have a speaker which could play easily to over 100db's without even the remotest amount of compression distortions, which to me meant, able to play to 115db's or more. Not that I will listen that loud, but for the headroom.

Now, on the negative, these speakers model as being very power hungry. The tweeter and midrange aren't too bad, the 91db efficiency means that I can hit my output goals with a reasonable 50-100 watts of power. The woofers actually need around 300-500 watts in order to do this (based on models, actual results are unknown, but these drivers are notoriously power hungry). While I understand the whole, watts are cheap thing, amps capable of producing 500+ watts into an 8ohm load cleanly are not that cheap. All of these is of course based on ideal crossovers with zero losses, something, at these frequencies, is really best achieved with active crossovers and a separate amp. However, even with the bass taken care of, and the increased efficiency I managed with the midbass, the mid/high section still needs a lot of clean power, which, as we know, exponentially increases with even small increases in output.

Also of note, but really somewhat unimportant, according to the Mpyre guys, these drivers were tested to handle sustained power peaks (I don't recall the iec standard for this measure) in excess of 2000 watts. They also tested them with 500 watts for even long periods, and found that, while the magnets get quite hot and eventually saturate, they do handle these without damage to the voice coil for quite some time. Based on this, I have modeled the bass section to be capable of over 120db's from 50hz to 350hz, but would require in excess of 1000 watts to do this. Again, this is just too loud, and not something I would likely ever see or try, but for a couple of 6.5" drivers, this impresses me. In fact, since the bass is largely non-directional in the range they are operating, the drivers can be taken as a group of 4, and then total output really increases even more. The models suggest that a likely output of 127db's would be possible with 1500 watts of power (750 to each group of drivers), and xmax would still not be reached as long as a first order highpass is used at 40hz.

I also did some real world testing of these drivers to see if they really could do what the models and claims would suggest. I have 8 of these drivers as well as some spare parts as a result of Mpyre's demise. I took one driver, stuck it in a sealed enclosure with a Qts of around .805. I then used a Crest amplifier capable of in excess of 2000 watts into this 4ohm load (amp bridged), and using an oscilliscope, current problem, volt meter, test tone generator, and high spl mic, took measurements. With around 1000 watts of power and no filter I began to notice the first signs of power compression with output (2 meters measuring point) of 115db's at 150hz. The magnet became very warm to the touch, but not so hot that it would burn me.

While I don't have them finished enough for actual listening tests, I'm pretty sure they will achieve my clean output goals. However, thus far my conclusion has been that, while I can achieve a flat response on axis like this, and even a decent response at up to 30-45 degrees off axis, controlled dispersion really has a lot of advantages. Second, efficiency is really important, and the market move in the 60's and 70's toward ever less efficient speakers seems to have put us in a situation now with many flawed speakers. I really think that a speaker needs to be able to reach its maximum needed output with just a small amount of power. Dealing with the thermal and logistical problems of a speaker that needs 500-1000 watts to reach 110db's far outweigh any advantages they may have in compactness, bass extension, or even Fr response. However, I also look forward to confirming this feeling when my Abbey kit comes from Dr. Geddes, which should fall into a polar opposite end of the design spectrum, and will have both high efficiency and controlled dispersion.
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Old 7th November 2008, 05:44 PM   #2
pjpoes is offline pjpoes  United States
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Default Here are some early pictures

Ok here are some pictures

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Old 7th November 2008, 05:46 PM   #3
pjpoes is offline pjpoes  United States
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The tweeter routes did come out terrible, but I actually have been effectively fixing them with fill, so maybe they won't be so bad in the end.
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Old 7th November 2008, 07:16 PM   #4
Taco is offline Taco  Netherlands
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Wow, great write up. Do you have measurements of the tweeter with the different ferro fluid?

You describe that the spl is rather low of these bass woofers, how are you gonna compensate for this. As the BS will also be handled by these babies.

Another thing I was wondering is the difference in timbre between the lightweight W-cone of the Focal and the heavy dampened carbon ones. You already listened to these woofers, what about the dynamics? Are the dynamics on par with the Focal?

p.s. you use a lot of glue .
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Old 7th November 2008, 09:49 PM   #5
pjpoes is offline pjpoes  United States
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I don't hear noticable timbre differences between driver brands at frequencies that low, but we shall see. I haven't used this drastically a different driver combination before over this wide a range, but I have a feeling that timbre differences at 350hz won't be overly noticeable. Using 2nd order slopes did allow for a decent amount of overlap, so maybe that will help too. Like I said, its an experiment, so we shall see.

I don't have measurements I can post for the differences per say. I have old measurements and could take new ones in maybe a week or so when my laptop gets back. I will put it up if I can. I will warn you, this isn't an experiment you all can easily try, as Ferrotec won't sell their fluid to non-business, and also only sells in large quantities. I purchased it through my university department as if it was for an independent study.
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