Will these horns/CD's cross well at 8Khz? (Altec 288/MR-64 & B&C DE10/ME10)

[steve71]

I’m currently using Altec 288 8K drivers on MR-64 (constant directivity) horns and I'm looking to add a B&C DE10 or DE250 + ME10 horn since the Altec's are rolling off by 8K Hz. By the way, this is for a home theater system.

I'll be using a 24db/octave active x-over and I need some help deciding if these two driver/horns are an OK match.

Frequency Response?

The B&C DE250 looks to be a little smoother than the DE10 in the 7-18K bandpass, but not by much. DE10 is a lot cheaper… Both should mate up ok?

DE10 FR plot
http://www.partsexpress.com/pdf/294-600.pdf

DE250 FR plot
http://www.partsexpress.com/pdf/294-605.pdf

Altec 288 / MR-64 FR plot
http://www.altecpro.com/pdfs/vintage/SpeakerAndMics/horns/MR64B HF Horn.pdf


Dispersion?

http://steve71.fileave.com/AltecBChornpolarplot.jpg

Looking at the Polar plots it looks the dispersion characteristics are not that close - the Altec MR-64 horn having tighter directivity control.

But how close is close enough? What would be the consequence of the dissimilar dispersion at the 7-9K hz crossover point?

Power handling?

Altec 288/MR-64 is 114db/w/m (max 15 watts RMS) = 126db @ 1meter (500-8Khz)
B&C DE10/ME10 is 107db/w/m (max 20 watts RMS) = 120db @ 1meter (8-18Khz)
B&C DE250/ME10 is 108db/w/m (max 60 watts RMS) = 125 @ 1 meter (8-18Khz)

Clearly the DE250 is a better match, but 5db isn’t that much because..
(1) I would think that the DE10 could handle more power since I’m crossing it so high (or is it thermally limited to 20watts?)
(2) Music generally has less energy in the 8K-18K range, so no need to match power requirements, right?

Phase?

"The distance between the axes of the two drivers cannot exceed the wavelength at the crossover frequency or lobbing and cancellations will occur."

Wavelength (m) = 345/frequency (hz)

So at say 8K, the wavelength is = 4.3cm

Obviously then I will have to mount the B&C horn inside the Altec MR horn, which means I can’t time align the voice coils. I could invert the polarity to get close though, or maybe x-over a little lower, like 6K hz? Thoughts?


Sorry if these questions are dumb… I’ve been lurking a ton here on DIYAudio this past six months and this is my first DIY speaker endeavor.

In the future I’ll be adding a conical mid-bass horn and tapped bass horn to replace the direct radiator I’m currently using to cover those frequencies.

 

[steve71]

OK, no responses, so maybe my post was a bit long winded 😀

The cliff notes version:

(1) common wisdom states that driver/horn dispersion needs to be the same at the crossover point. The B&C ME10 horn has a wider dispersion than the Altec Mantaray horn. Is this really an issue at 7-8K Hz?

See polar plots here
http://steve71.fileave.com/AltecBChornpolarplot.jpg


(2) Am I able to spec a driver with less max SPL (relative to the rest of the system) at 8K Hz? Looking at pink noise frequency response, HF content is down 10db compared to 1Khz.

Any help is much appreciated. 🙂

 

[kepa1]

I'll try 🙂

Why go for a 1" compression driver when a tweeter can do the job pretty well and will logically perform better above 10k?
Let's take the DE250, people use it from 1,5 or even lower, so it will be under employed if crossed at 8k.

 

[steve71]

I'll try 🙂

Why go for a 1" compression driver when a tweeter can do the job pretty well and will logically perform better above 10k?
Let's take the DE250, people use it from 1,5 or even lower, so it will be under employed if crossed at 8k.

Thanks for the response Kepa1.

When you say tweeter, do you mean a conventional dome tweeter?

Most of those are about 92/db/w/m so they won't be able to keep up with the rest of the speaker.... they'd need over 1Kw of power to get to 120db and of course they would melt, compress and distort long before they'd take that.

Ribbon tweeters are about 102db/w/m which isn't a bad match, but they are expensive and they "look" like they'd have a wide dispersion... not good since I have to mount them inside the huge MR-64 horn (2.5 foot x 3 foot).

I can't hear much above 16K anyway, so covering 7/8K to 16K is all I need.

I could use a bullet tweeter (great sensitivity), but I haven't heared much about them and most of the ones in the Parts Express website look pretty cheap and nasty.

 

[kepa1]

Just for the fun of it, I used a JBL 2426 comp in HF (without horn) and it sounded good. But when I replace it with a Beyma 104db horn tweeter I experienced way better highs, more detailed and going higher.
Maybe you should check the JBL pro tweeter line, or the ones from Fostex, Beyma being a cheaper alternative.

 

[steve71]

Just for the fun of it, I used a JBL 2426 comp in HF (without horn) and it sounded good. But when I replace it with a Beyma 104db horn tweeter I experienced way better highs, more detailed and going higher.
Maybe you should check the JBL pro tweeter line, or the ones from Fostex, Beyma being a cheaper alternative.

Thanks Kepa1 I appreciate the suggestions and I'll check em' out.

However I'm still in the dark as far as matching dispersion properties at the 7-8K Hz cross over freq. Until I know what's an acceptable variation (if any), I have no way to judge the suitability of any solution.

I guess I'm either asking a really dumb question, or a really hard one 😕

As I understand it, different radiation patterns introduce Inter Modulation Distortion or doppler distortion, but it's unclear to me if that only occurs in the off axis area where the radiation levels differ.

 

[BHTX]

I guess I'm either asking a really dumb question, or a really hard one 😕

Hopefully it's not too dumb, as I've been interested in this as well. 🙂

I'm much too exhausted at the moment to say much right now, and I guess I don't really have anything useful to add at this point either.

Anyway, I'll be waiting for more replies in the meantime..

 

[steve71]

Hopefully it's not too dumb, as I've been interested in this as well. 🙂

Well it appears that Lynn has inadvertently addressed my question in his Beyond the Ariel 'mega thread'. Hope he doesn't mind me quoting him here.

The HF crossover is another matter. The drivers are separated by 6 wavelengths - a lot. Not only that, the dispersion pattern shifts from conical with a soft edge (Tractrix & Le Cleac'h pattern) to very wide horizontal and narrow vertical. This would be a disaster at a lower frequency, say to 2 to 3 kHz, which falls right in the region of peak sensitivity of the ear.

However, in my previous experience at Audionics, supertweeter crossovers have much lower audibility than expected. The one thing to avoid is a "shelf" transition, where the supertweeter is 1 or 2 dB higher in level than the midrange driver. The rising edge of the shelf subjectively sounds like at peak centered around that frequency, and draws attention to itself. On the other hand, having the "shelf" slightly depressed in level compared to the mid-frequency driver leads to it not being audible at all. So the design tolerance is all in one direction - never raise the supertweeter higher in level than the mid driver!

This is also a very good reason to use a supertweeter with flat and level response, and avoid any type of peaking or response irregularity near the crossover region. This is a common problem with horn supertweeters, which can rather rough in the 5~7 kHz region - this makes setting the level on the supertweeter crossover much more difficult.

The rule-of-thumb is to choose a supertweeter crossover high enough that switching the supertweeter on and off is only slightly audible, select a supertweeter with very flat response close to the crossover frequency, and be careful to never set the level higher than the mid driver. The last rule-of-thumb is commonly violated in many audiophile speakers in the desire to give the speaker more aggressive and obvious HF - more "zing", more "detail", more sparkle, particularly to draw attention away from a peaky midrange.