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when about all the available T/S info on the compression driver is it's mouth area, flux density and weight? Which of course aren't really T/S parameters at all. Now, given all the available horns, there must be some way to estimate T/S parameters and there must also be some lack of sensitivity to them or all those plastic horns for 1" drivers wouldn't exist. So please........tell me the secret so I can start fiddling in HornResp. TIA
Follow Ups:
Hello,To begin to design compression driver + horn using Hornresp you could start from the T/S parameters of a small electrodynamical loudspeakers (not a compression driver) like the Focal 5V 323 by example if you want to design a low-mid compresssion driver:
SD = 87
CMS = 5.10E-04
MMD = 4.03
RE = 6.60
BL = 8.64
RMS = 0.95
LE = 0.50
ES = 2.83
Then you can design the compression chamber itself (taking in account the area of the diaphragm and its distance to the phase plug)
VTC = 25
ATC = 87
Then you have to calculate the throat area you need according to the compression ratio that will depend on the ratio between the active area of the diaphragm and the active area of the slots of the phase plug.By example for a compression ratio around 10 in this example you can take
S1 = 9Then you can design a horn:
S1 = 9
S2 = 6455.26
EXP = 300
F12 = 60Finally you can design the rear chamber:
VRC = 2
LRC = 4
FR = 40
TAL = 0then calculate... and optimize to your taste.
Well this is a first approximation, others software can give additionnal precision and can take more parameters (shape of the suspension gap...) like AKABAK
Also it exist finite elements software than are used by few manufacturers
Also even if the T/S parametres of that small loudspeaker can be helpfull to begin in designing a compression driver, a true compression driver will have different BL, CMS...Best regards from Paris,
Jean-Michel Le Cléac'h
just what I was looking for.
Hornresp will not be much use for a typical high frequency horn design. It will predict the power response which is not the same thing as on-axis response which is what most people are concerned with. In most bass horns, the size of the horn is too small to greatly influence the horn's polar pattern, so what hornresp shows for low frequencies will be pretty much what you get on-axis.For high frequency horns, the horn is usually large compared the wavelengths you're reproducing, so the horn will effect the polar response, causing the power response to be different than the on-axis response (depending on the design).
The typical approach, I think, is to pick a horn ideology and go from there. Iow, do you want to try for low throat distortion and good polar response? Okay, go conical and realize you will have to correct the frequency response in the crossover. Or go for something like a tractrix horn or better yet a LeCleach horn for low internal and mouth reflections, better low frequency / flatter onaxis response.
In an ideal world, your horn would provide a rho*c load over the bandwidth you wanted to use it in, same as a plane wave tube, and the only thing you would have to consider would be the plane wave tube response and the polar pattern of the horn / driver combination. But unfortunately nobody seems to measure PWT response anymore, and most people have a hard time figuring out a polar pattern and how it will influence the SPL at some given point.
where the properties of the driver enter the equation. Do I just calculate a taper with a 1" diameter throat, build it, and start testing/tweaking? How did Timo Crist do that Hi-Freq horn with the same angle as his midbass horn? Surely there is a little more info that can be calculated before I start cut and try? Thanks,
Or go for something like a tractrix horn or better yet a LeCleach horn for low internal and mouth reflections, better low frequency / flatter onaxis response.After experimenting with a couple of conicals, and finding they did need a fair bit of EQ, I'm looking at making a couple of round midhorns again. I'm interested in the LeCleach, and have the software working, but I have a question;
When I made my first trax, I saw a post by JLH that said to use 0,707 x the lowest frequecy you wanted to use the horn for the flare rate, eg 270Hz FR for a 400Hz xover. This worked really well with a measured freq resp at -3dB at about 320-330Hz iirc. Do I need to add in any 'factor' with the LeCleach to get the same performance? Or do I simply input the flare rate as the xover frequency? I'm after a 400Hz xover again.
Driver will be a BMS 4592 coaxHaving no experience with the LeCleach, any help would be appreciated in the design, as I'm going to build it by a layered laminate method with is very time consuming and labour intensive, so I'd like to only have to do it once.
Thanks
Hello,John Hasquin presented the interesting idea to choose the horn length (and in most of the cases this means the horn cut-off ) according to the resonance of the driver in order to cancel more or less the throat reactance of the horn.
This is roughly, an extrapolation of the conventional method used to design an horn for a conventional electrodynamical loudspeaker ( = not a compression driver) in order to be able to use it until a lower frequency.
About the formula he gave I am less sure this is adequate for compression driver, but it could be interesting to try. Why I am a bit sceptical is because of the phase plug with very different compression ratio between different compression drivers. The amount of cancellation will be more or less efficient according to the compression ratio. Correct me if I am wrong.
The more efficient way to cancel the throat reactance is to do it at the rear of the diaphragm like Iwata San used to do with tuned pipe placed at the rear of the driver. (This is one method , the simpler IMHO, but it exist others).
But that sure that experimenting with John Hasquin’s idea can be interesting.
Then some precision about the shape of the horn calculated my method. The shape in itself is not about just reducing the reflected waves from the mouth toward the diaphragm. The reduction of the reflected waves in that horn can only be obtained for opening angle larger than 180°. We did experiments about different length for the same horn “Le Cléac’h” profile and it is a general conclusion. People are generally reluctant to continue the horn profile above 180° for reasons more related to the visual aspect of the horn. They are wrong, the sound is better (less reflected waves again).
Also on the point of the T factor (hypex generally between 0.5 and 0.8, exponential = 1). The reason why I prefer hypex is that the impedance is less reactive in the middle (well let's say between the cut-off and the middle of the bandwith) of the useful frequency bandwith).In the same manner a conical horn and all the waveguides present a quite high reactance/resistance ratio in the middle of the useful bandwith. When I look at the reactance and resistance curve of different horns according to their T value I guess that for T value over 1 the horn becomes more reactive in the middle of the useful bandwith and in that case behave more as a waveguide (or a conical horn) . Eventually some kind of phase equalization can be useful here to obtain a better phase linearity… (against the common and false view that phase is inaudible please refer to Daisuke Koya's thesis).To come back to the subject of the choice of the cut off of the crossover according to the acoustical cut-off of the horn as John Hasquin said it is not directly related to the shape of the response curve in that frequency zone it is related to the acoustical impedance of the horn being more and more reactive when the frequency decrease toward the acoustical cut-off.
Reactance means variation of phase. You can derive the phase curve from the pulse response measurement of one horn loading a compression driver having a resonance lower than the horn cut-off . Also a phase curve can be obtained doing a simulation like the one performed by David Mac Bean’s HornResp software (excellent BTW even such problem as reactance cancellation cannot perfectly be studied using it).But nor the word reactance nor the word phase are clearly understood by audiophiles. For my own purpose I prefer to turn the phase variation curve in delay variation and then to turn that delay variation in equivalent distance variation ( to delay a source of 30centimeters, this is around 2 feet, means something to everybody). So for the horns I own at home I always derive a delay curve from the phase curve obtained by pulse measurement (performed by Angelo Farina’s method that use a logsweep) and then turn the delays in distance ( a delay of 1ms is equivalent to a distance offset of 34.4 centimeters ).
That’s a real pity that horn manufacturer (and loudspeakers manufacturers in general) don’t give such a delay curve or equivalent offset curve!
As an example my new 320Hz horns that I use with the TAD TD2001 driver (they have BTW a flat response within 3dB from 320Hz to 19.2 kHz) lead to:
frequency (Hz)_____offset (cm)____offset(inches)
___400____________ 50_____________20
___500_____________26_____________10.2
___600_____________13_____________5.1
___700______________8_____________3.1
___800______________4_____________1.6
__> 900____________ <3_____________1.2
__> 2000___________ progressively toward 0
Generally I use the compression driver over a frequency corresponding to an offset (= distance equivalent to the delay) lower than 6 centimeters ( = 2.4 inches). This is to apply correctly my own crossover alignment method that leads to a better impulse response of the whole system (see the pdf document from my presentation at last Triodefestival on www.triodefestival.net ).
For the 320Hz horn this means than normally I use it above 650Hz (this is 1 octave above the acoustical cut-off)But since few weeks I am experimenting with lower cut-off on the crossover and at the moment I even use to listen the TD2001 from and above 401 Hz, with great satisfaction, probably more related to the better sounding of voices, less intermodulation than to the pulse response which one is not so good than when I cut at 650Hz.
Best regards from Paris,
Thank you for your thoughtful and illuminating post. There is a lot in there for me to ponder and digest.
I understand your comments wrt the resistance/reactance variations throughout the horn's range and I know I would ideally like to keep it small and as consistent wrt frequency within the range as possible. But, my old Klipschorns must have pretty poor phase performance near the 400Hz xover, yet experimenting with digital xovers, I didn't hear enough difference with them to make them worthwhile to add into the signal chain. Your later comments about your 320Hz flare and the 2001's was interesting with you prefering the performance of this combination sonically even though theoretically it's performance should have been worse. Before I posted I'd been thinking of using a 320Hz flare based upon the success of another builder with the BMS 4590, using it successfully down to 300Hz.One of the previous flares I experimented with, the Yuichi Arai 290 hypex, is very good, but I have wondered if it's possible to do better and that's why I was looking at your designs and the tractrix. Intuitively I like the idea of the flare extending to 180* or more to limit the amount of energy that can make it's way back into the flare from the abrupt transition at the end.
I think it might be time to cut up some wood and experiment with all 3 designs and make an informed decision based upon hearing and measuring. The designs I like least can always be used in the 'rears' as my system also does HT duty, but is being optimised for music, with movies as a bonus.
I have just been offered a position in another state and will be moving (hence my slow response) very soon. A friend nearby where I will be living also has better measuring gear than me too.Thanks again for taking the time to make such a detailed and considered response. Once I have something made, I'll set up the pictures and measurements on my site and email the URL.
I would go more like 1/2 or 1/3 the desired crossover frequency for the flair frequency of the horn to keep stored energy low. However, I should say this is only based on a few data points, so ymmv. If you only care about frequency response, then yes, some factor like .707 should work okay.
I don’t recall the .707 rule of thumb, but that too works out pretty well for predicting how low your mid range horn will go. Although, it has not worked out for some compression drivers. More experienced now, the first thing I look for in a compression driver is its Plane Wave Tube power response plots and resonance. These two factors will determine how low the driver can go. Of course, at this point, this has nothing to do with fidelity; we are just talking about low frequency capacity of the driver here. When looking at the PWT, I look for the where the driver starts to loose its low frequency power. As soon as this happens, it takes a lot more horn to compensate for the loss in power response. And in my opinion is a losing battle that should not be fought.The resonance of a compression driver can be used to perform reactance annulling to extend the low frequency of the horn. Reactance annulling is mostly used in bass horn design by adjusting the rear chamber to resonate at or slightly above Fc to extend the low frequency output to Fc. However, we adjust the horn size to fit the resonance of the compression driver instead of the other way around. You should make the horn ½ a wavelength in length of the peak resonance frequency. This means if your compression driver has a resonance of 380Hz, like the BMS 4592, the length from the phasing plug to the end of the horn mouth should be approximately 17.75 inches. {(13492 in per sec/(2*Fc)) =~ 17.75 in.} Remember, the machined throat inside a compression driver is very much a part of the overall horn and it must be accounted for. So, I’m guessing the actual horn would be about 15 inches in length. With a 2” throat, this would dictate a 235Hz tractrix horn. The equivalent exponential horn would be a higher flare frequency, and the Le Cleach would lead to a much bigger and lower flare frequency horn.
I never use the exponential horn for mid range horns; too much coloration for me. Some people like the exponential coloration, but I do not. YMMV. I favor the Le Cleach horn for most 1” compression drivers because the initial flare expands faster and helps reduce throat reflections. Of course, the Le Cleach is all about reduction of reflections. I like T= 1.3415 for most 1” Le Cleach horns. Unless you are making a very short 1” throat tractrix horn, (i.e. smaller than 400Hz) the throat reflections can be quite significant. The reason for this is for long tractrix horns, such as a 300Hz Tractrix horn with a 1” throat, the neck of the Tractrix approximates the exponential horn pretty much. This leads to the usual nasal tone, or coloration. For 1.4” and 2” compression drivers I like the Tractrix the best. The mouth size for a Le Cleach horn on a 1.4” or 2” compression driver can get pretty ridiculous. And I find that 1.4” and 2” throats are far enough down the neck of the horn that reflections are not a problem with the tractrix. Having said all this, it all comes down to personal preference. I cannot tell you what you like; you have to determine this for yourself.
Lastly, remember me saying sizing a horn for lowest frequency capacity has nothing to do with fidelity? Usually, a compression driver sounds it best when it is crossed over at least twice the frequency of its resonance. This means for the highest fidelity, you should not use a compression driver for anything under ~800Hz for metal dome drivers, and 500Hz for phenolic dome drivers. The resonance peak screws with the crossover point, and low frequencies stress the driver’s ability to reproduce the high frequencies cleanly. I know there are a lot of people that love their large mid range horns. That’s good. If it sounds good to you, then you have achieved your goal. Everybody has his or her preferences and there is nothing wrong with that.
Hi John,I don’t recall the .707 rule of thumb, but that too works out pretty well for predicting how low your mid range horn will go.
This was based on a post you made here some time ago that I saved. Part of it went
Anyway, even through the throat of the 375/2440 and TD-4001 is on the order of a 160Hz exponential horn, it will not go any lower than about 500Hz on the 340Hz full size tractrix horn. The reason why is as follows: The depth of a full size 340Hz tractrix horn with a 2” throat is 14 7/8” deep. This depth corresponds to half a wavelength at 454Hz respectively. I saw Bruce's LEAP print outs of this compression driver and horn combination at the Midwest Audio show in Lima. Just as I stated above, efficiency was nonexistent below 400Hz, but as soon as it reached 450Hz the horn started to “turn on” and by 500Hz it was up to the zero dB reference line on the graph.Lastly, remember me saying sizing a horn for lowest frequency capacity has nothing to do with fidelity? Usually, a compression driver sounds it best when it is crossed over at least twice the frequency of its resonance. This means for the highest fidelity, you should not use a compression driver for anything under ~800Hz for metal dome drivers, and 500Hz for phenolic dome drivers. The resonance peak screws with the crossover point, and low frequencies stress the driver’s ability to reproduce the high frequencies cleanly.
I understand, but I would still like to keep as much of the overall FR on this driver as possible, without too much compromise. On the rough Arai 290 I made, it measured flat to under 400Hz and sounded very good.
Based on my ability to design and measure, and time constraints, I know I won't get MacLaren F1 type performance, but would still like to hit the 95 percentile band with it doing nothing too bad to my ears, and enjoy listening as much as possible. Once set up, I will likely leave it for a decade, untweaked. It will be crossing to a cone driver midbass horn (not yet designed) from somewhere around 400-500Hz, so you can see I'm looking for something "like" Titan performance and having a bit of fun doing it. If I had the money and lived in the US, I would simply buy the Titan.I've made note of your helpful comments above, and will start making up some flares to experiment with and measure when I get to my new home and will post my findings here and on my website. Thanks for taking the time to post.
Hello!
Is the resonance frequency of BMS 4590 also 380Hz? Is this published on their webpage?
The low frequency resonance appears to be centered at approximately 380Hz too. (See the third graph down) Although, I personally would not use it down that low.
Oh, of course I could look at the impedance graph *blush*. I was only looking at the parameters :)
nt
RogerAs you can see from the astute comments by John S., John H. and Jean-Michel, there is quite a bit of "art" to just the horn design for an existing midrange compression driver. I would strongly recommend starting with an existing driver, and making at least one rough prototype horn out of cardboard/glue/duct tape etc, and then measure it's performance. Even if your finished horn will be round, a square prototype of similar dimensions will give you a good basis for studying the horn's bass loading of the driver. Going from strictly numbers and theory directly to a time consuming wood working project is quite a bit of a crap shoot, so it's advantageous to make the odds as much in your favor as possible.
As to making your own comp. driver, you would probably have to start with someone else's diaphram/voice coil, unless you have precision winding equiptment and the theory to jump right in. The next part would be the magnet, which would be hard to make out of general sized magnetic pieces, and if you buy someone elses driver magnet, you already have most of the driver, plus you get the binding posts! Rolling your own is not realistic in view of the number of good comp. drivers available. Probably would be fun though!
not considering building my own drivers, merely correcting my spelling in the original subject.
plan to do exactly that, but one must start somewhere as to length, taper, throat diameter, etc. and finding a reasonable starting point without inventing a three-sided wheel was what I was hoping to learn from the question. Thanks again,
Prototypes are the only practical way to experiment before building the final example. You sure can burn up a lot of wood, $$$, and time on a crapshoot. All builders should take their time and explore their horn before making the final example. Look at aviation; we didn't go from guys running down the street with a pair of fabricated bird wings to a 747 jetliner over night. We all must have patients. Patients my young grasshopper, patients.
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