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Multi-Way Conventional loudspeakers with crossovers

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Old 9th January 2008, 07:12 AM   #2981
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Since I already have the superb RAAL tweeters, I intend to use them in the region where horns start to become directive (throat circumference approaches a wavelength) and compression drivers rely on bandwidth-extending tricks like broad regions of resonance (which also greatly increases distortion). The extended HF is a difficult region for horn/compression drivers, unless it is a dedicated supertweeter - another breed of cat entirely.

By relieving the horn system above 5~7 kHz, a larger throat diameter is acceptable. This allows either a small cone driver with a 3.5" throat (similar to what Magnetar is using), or a large-format (1.4, 1.5, or 2") compression driver. Both have substantially improved power-handling, and more importantly, lower IM distortion compared to a 1" small-format compression driver, particularly in the critical 1~3 kHz region, where the ear reaches peak sensitivity for distortion and time-domain errors.

The 1~3 kHz region is the most critical part of the spectrum, and design choices in this region make the difference between a loudspeaker that is high fidelity and one that is not. Unfortunately for direct-radiators, it is at the top of the working range, and the region where cone breakup is starting to make its presence known. Similarly, for small-format compression drivers, it is at the bottom of the working range, and a region where distortion is rising rapidly below 1 kHz. Trust me, when a driver is working at the end of its frequency range, it is easily audible - as a sense of "stress", unease, and a lurking potential for sudden harshness at unexpected times. Systems that don't do this are systems we call "relaxed" and confident-sounding.

This is the primary reason I am interested in a large-format horn with an additional HF driver. The 1~3 kHz region needs to be served with the highest-quality technology available - the lowest IM distortion, and the cleanest time-domain decay characteristics (more than 30 dB down in less than 1 mSec is desirable).

There are not many choices here - and all of them will have a clear and distinct "personality". I do not know how the 1.4" compression drivers will compare to prosound cone drivers - that will take extended listening to make an assessment.
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Old 9th January 2008, 08:18 AM   #2982
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Default Simulator tool

Hello SunRa,

witch software do you use to simulate the horns? I intent to build a OS waveguide for a 6' cone driver. So some simulation would be helpfull before creating "hard facts".

Best regards

Zelter
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Old 9th January 2008, 01:30 PM   #2983
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Default Re: Re: More AH-550 Information

Quote:
Originally posted by Lynn Olson

Here's Martin's measurement of the frequency response of a JBL 2420 (aluminum diaphragm, 1" exit, no longer in production) mounted on a AH-550 with a 350 Hz 24 dB/oct highpass filter - microphone at horn mouth, 1/24th octave smoothing.

Quote:
Originally posted by ScottG



Nice! But what is the response like off-axis?


Forget off axis. What doe sthe response look like at 1 M or at a reasonble listening distance?
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Old 9th January 2008, 04:09 PM   #2984
ScottG is offline ScottG  United States
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Default Re: Re: Re: More AH-550 Information

Quote:
Originally posted by john k...

Forget off axis. What doe sthe response look like at 1 M or at a reasonble listening distance?

When it comes to horns it would be a GROSS error to "forget off axis".

As to the response at 1m or 2m - no doubt what you would expect, a "falling" high freq. response as distance increases. Still important though! (..something Lynn will have to sort out when crossing over to the RAAL.)
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Old 9th January 2008, 09:09 PM   #2985
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Neither the RAAL nor the mid-horn will be following the inverse-square distance law above 7 kHz, so output at a listening distance of 3~5 meters will be substantially higher than an equivalent dome tweeter radiating into a hemisphere. With a 1.4" throat, the horn will be at the top of its resistive-radiation range (just starting to beam), and the RAAL will have very wide horizontal (approaching 180 degrees) and fairly narrow vertical dispersion. Both result in additional mid and HF output at greater listening distances.

The narrow vertical dispersion of the RAAL will require some kind of system to point it up and down - maybe nothing more than a set of wooden shims at the rear of the magnet assembly.

I will be using autoformers with 1 dB taps (in the same style as Klipsch, if not the actual Klipsch parts) for trimming mid/HF levels to listening distance. As ScottG mentions, the mid horn may require mild shelf equalization - this circuit would precede the autoformers and be part of the mid/high crossover.

The crossover/autoformer topology is: LP/HP/Shelf filter -> 8 ohm shunt resistor -> high Z primary -> low Z secondary -> compression driver. The autoformer multiplies the Z curve of the compression driver many times, and the 8 ohm parallel shunt resistor flattens out the variations to a value only slightly above 8 ohms. The autoformer also provides a low source resistance to the compression driver, effectively making the source Z of the crossover disappear from the view of the compression driver.

I'll be taking a brief break from DIYAudio and writing an article for the Clarisonus blog entitled "A Different View of Dispersion". Rather than tediously explaining my view over and over again in this thread, I'll write the thing once and just point to John Atwood's blog, where it'll stay. This is a contrarian view largely at odds with the industry-prevailing Floyd Toole approach.
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Old 9th January 2008, 10:21 PM   #2986
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Default Re: Re: Re: Re: More AH-550 Information

Quote:
Originally posted by ScottG



When it comes to horns it would be a GROSS error to "forget off axis".

I'm not saying forget about it. Just making the point that a measuremenet at the mouth isn't eaxctly what I would expect at 1M, or further out, and I don't mean just roll off of high frequencies.
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Old 9th January 2008, 11:22 PM   #2987
AJinFLA is offline AJinFLA  United States
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Default Re: Re: Re: More AH-550 Information

5

Quote:
Originally posted by john k...

Forget off axis.
No. Forget on and off axis, just EnaBL the darn thing(s).
Same for the RAAL. EnaBL it.
If such stellar cone designs as Fostex, Lowther, Radioshack, etc. can possibly be improved upon by added mass stiffening/damping, certainly the ribbon here can also be taken to the next level (up of course). Supposedly even horns and baffles (as to be used here) are not immune to its benefits?
I'm now thinking of placing some EnaBling directly upon my ears, to help me actually hear better?
The off axis, DI, etc of this design is definitely not going to follow the Toole, JBL, etc. research. Some serious wavelength spacing between AC's too.

cheers,

AJ
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Old 10th January 2008, 12:40 AM   #2988
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Hello Lynn

"I am in agreement with Newell and Dr. Holland on essentially all of the points. I'm primarily interested in smooth FR, time/phase response, and directivity (most importantly, without narrow spikes only a few degrees wide). I'm less interested in a specified degree of directivity than I am in freedom from narrow artifacts in frequency, time, or spatial domains - thus the interest in drivers that are free from breakup in the working band, horn/waveguides that are free of internal obstructions, and phase-plugs that emit coherent wavefronts. Smooth driver response without a lot of wacko equalization is a major advantage in my book - and I'm aware this isn't a fashionable viewpoint in the pro and audiophile worlds these days."

If something is fashionable or not is really not important. The reason the SR community at large embraces Constant Directivity and CD type wave guides is because they make so much sense in that application.

I think you should be looking at what's happening off axis and particularly what the directivity is around your proposed crossover points. All the phase, frequency and time anomalies you are trying to avoid potentially will be present is a 3 way using midrange and tweeter components that have narrowing directivity patterns on axis. Just the diameter of the horn and the voice coils offsets will almost guarantee comb filtering through the proposed crossover range. All you need to do is look at the various system measurements from the classic JBL 3 and 4 way monitors.

If you look at the 2420/2307/2312 compression driver horn combo the directivity of the chosen horn has to be close to them to flatten the on axis response of the 2420. The directivity of the 2405 is similar to what the ribbons are far a directivity is concerned with narrow in the vertical and wide horizontal axis. Both the horn and 2405's directivity were very limited through the crossover range and above.

If you look at the monitors the 2405 and 2307 are side by side. Looking at the directivity it easy to see why. These systems sound the most balanced when your ears are on axis with these drivers. Putting them side by side cure this does not effect the comb filtering through the crossover range.

The best way to avoid this in simply not add a tweeter. It's name your poison though as you don't want to run a 1" driver to low. If you look at the 4 ways, which are arguably the best sounding a 10" midrange covers the 300hz-1.2K range. It's all about compromises.

If you have not seen these take a minute to read them. You will enjoy them just for the read

http://audioheritage.org/vbulletin/s...ead.php?t=7852

http://audioheritage.org/vbulletin/s...ead.php?t=6687

http://audioheritage.org/vbulletin/s...ead.php?t=4410

http://audioheritage.org/vbulletin/s...ead.php?t=4408

Attached is a system plot for the 4333.

Just food for thought.

Rob
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File Type: jpg jbl-4333b-fr&i.jpg (49.9 KB, 769 views)
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Old 10th January 2008, 05:43 AM   #2989
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Since the target application is not a soffit-mounted studio monitor, there is no requirement for the midbass driver mounting flange, mid-horn mouth, and ribbon tweeter flange to be on a common plane. Although physically convenient for a conventional box speaker, there's no reason to do this when the mid and HF drivers are mounted in free space (instead of a front baffle). The preliminary system will have the acoustic centers of the drivers at a common distance from the listening location - to be determined by measurement.

In my experience, nulls in response at frequencies of 7 kHz or above are surprisingly difficult to hear (unlike 3 kHz or lower). This is what makes supertweeters practical; although the supertweeter and the mid or tweeter below it are spaced several wavelengths apart at the crossover frequency, audibility of the null as you move up and down is surprisingly small - even with pink-noise stimulus, the most sensitive test material. With music, audibility is even lower.

That luxury does not exist at the lower-frequency crossover in the 640~800 Hz range, where lobing is quite audible. Thus the provision of a single 15" driver to cover the 50~800 Hz range, with a mid-horn mounted directly above it. The other 15" drivers are only active at much longer wavelengths.

The mouth of the mid-horn is probably not going to be on the same plane as the midbass baffle; it will probably be several inches in front or in back of the midbass baffle - depending on the alignment of the acoustic centers of the midbass driver and the mid-horn (which is affected in the turn by the electrical delay of the lowpass crossover for the midbass driver). To be determined by measurement, although I expect the acoustic centers to be fairly close to the voice-coils.

My goals, as mentioned earlier, are not compliant with Toole (et al) directivity criteria, and are not suitable for studio-monitor or THX applications. I am aiming for the first-arrival sound to have flat frequency response, rapid time decay, and low IM distortion across a horizontal arc aimed at the listening area. The overall power response into a sphere, or 90-degree cone, although important, is less important than the first-arrival performance.

I am grateful every day I don't have to report to Harmon International, a corporate marketing department, or apply for THX certification. Frankly, if I had to report to any of these organizations, I'd get out of hifi altogether, and do something completely different.
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Old 10th January 2008, 06:38 AM   #2990
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As mentioned earlier, I don't know if the Great Plains Audio 416 and 515 Alnico drivers will available any time soon. I'll be buying one or the other when they become available. I have 18Sound 15NMB420's on their way, and these are akin to Altec 416's and 515's - although it's a safe guess they probably sound different.

The 18Sound 15MB700 appears a bit more akin to the 515, with a noticeably rising response, unlike the 15NMB420. You'd think they would almost be identical twins, with 73-gram cones, but the cones are evidently different based on the photos and measurements.

Regarding ribbons as supertweeters, others have reported good results with compression drivers. If they can work successfully with a driver as awkward as a Lowther, a compression driver should be more straightforward, especially if it is aligned in the time domain. (If the drivers are misaligned in the time domain by a millisecond or more, as is typical in conventional enclosure multihorn systems, then crossover design gets a lot more difficult.)
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