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Domes on Horns vs Compression Drivers on Horns
Domes on Horns vs Compression Drivers on Horns
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Old 25th November 2017, 12:30 AM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default Domes on Horns vs Compression Drivers on Horns

For the past couple years I've been getting good results with domes on horns, so I thought I'd post some data that demonstrates why they work surprisingly well.

Click the image to open in full size.
In this corner, we have a Tymphany DFM2535R00-08 compression driver. It sells for $42. Here's some specs:

diaphragm diameter: 3.72cm / 1.46"
VC diameter: 3.55cm / 1.39"
"sensitivity" on spec sheet: 107.8dB
power handling: 30W

Click the image to open in full size.
In the other corner, we have a SB Acoustics 3/4" dome tweeter. It sells for twenty bucks. Here's some specs:

diaphragm diameter: 2.2cm / 0.86"
VC diameter: 1.9cm / 0.75"
"sensitivity" on spec sheet: 88.5dB
power handling: 30W

Click the image to open in full size.
Here's a measurement comparing the Tymphany DFM-2535R00-08 on an 18Sound XT1086 vs a SB Acoustics SB19 on the same horn.

Same volume level, same horn, different drivers.
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Old 25th November 2017, 12:38 AM   #2
Patrick Bateman is offline Patrick Bateman  United States
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Here's some things that I notice in the measurement:

1) The lowly $20 dome tweeter has lower distortion!
2) The lowly $20 dome tweeter produces nearly as much output!
3) IMHO, the dome tweeter performs better on the low end, the compression driver performs better on the high end.

So... Why is this?

Click the image to open in full size.

Click the image to open in full size.

I think the easiest way to make sense of these results is to compare high efficiency woofers with low efficiency woofers. A high efficiency woofer may have an efficiency as much as 95-100dB. It will have low xmax, under 8mm. A low efficiency woofer may have an efficiency as low as 85-90dB. It will have high xmax, over 12mm.

The key here is that the low efficiency woofer is often capable of more undistorted output at low frequency.

The reason why the low efficiency woofer can produce more output at low frequency than the high efficiency woofer is because the high efficiency woofer is limited by xmax. For instance, if you have an 8" woofer with 12mm of xmax and a high efficiency 12" woofer with 6mm of xmax, the low efficiency woofer will win at low frequencies. This is just physics; even though the high efficiency woofer can produce more output with a single watt, when it hits it's xmax that's going to prevent it from getting any louder without distortion.
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Old 25th November 2017, 12:46 AM   #3
Patrick Bateman is offline Patrick Bateman  United States
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So...

If what I've posted is true, why would anyone use a compression driver?

The answer to this question is easy: compression drivers generally have way WAY higher power handling.

For instance, according to the spec sheet, the Tymphany compression driver and the SB Acoustics tweeter have identical power handling. (30 watts.) Realistically, this is impossible. The voice coil on the Tymphany has a diameter that's nearly twice as large. Realistically, the folks at Tymphany are being incredibly conservative about the thermal power handling of their compression driver.

The next question you might have is why is the efficiency of the SB19 so much higher than the spec sheet? I think this is largely because it's horn loaded. On a flat baffle, the SB19 is good for 89dB of output. Horn loading the dome tweeter raises it's efficiency tremendously.
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Old 25th November 2017, 12:50 AM   #4
Patrick Bateman is offline Patrick Bateman  United States
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Click the image to open in full size.

If I may editorialize for a moment, I'd argue that the advantage of the dome tweeter on a horn is at low frequency. As frequencies get higher and higher, the relatively low motor force of a dome tweeter is lowering output levels. You can see this in the measurement; at 10khz, the Tymphany has 6dB more output than the SB Acoustics SB19. At the same time, the SB Acoustics has wider bandwidth overall.

So this becomes a catch-22. There is no "perfect" tweeter. The SB19 has wider bandwidth, but lower output at higher frequency. The compression driver has more output at higher frequency, due to it's higher motor force. But at low frequency, it's xmax limited.
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Old 25th November 2017, 12:56 AM   #5
Patrick Bateman is offline Patrick Bateman  United States
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On a random side note, here's what an impulse response is supposed to look like:

Click the image to open in full size.

Here's the impulse response of the SB Acoustics SB19 on an XT1086 vs a JBL 2470 compression driver on an XT1086 horn:

Click the image to open in full size.
In the impulse response pictured above, you can see some weird squiggles and blips in the impulse response of the compression drive. I can only speculate on what's causing this, but I would guess that we're seeing reflections inside of the phase plug.

Click the image to open in full size.
IE, a compression driver has a phase plug. A dome tweeter doesn't. In a compression driver phase plug, we're probably going to see reflections that occur when the wavefront crosses from the phase plug into the horn. Basically an impedance mismatch.

Domes don't have this issue; there's no phase plug whatsoever.

This may explain why the impulse response on the dome tweeter is arguably superior.
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Old 25th November 2017, 01:11 AM   #6
Patrick Bateman is offline Patrick Bateman  United States
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IMHO, I think it's possible to get really great performance from a compression driver, but it gets expensive. Here's an example of what I mean.

Click the image to open in full size.
In this corner, we have a JBL 2470 compression driver. This thing is a relic, hasn't been produced in nearly 40 years. It has all kinds of crazy features, like an alnico magnet and a silver (yes silver) shorting ring. Back in the day it sold for hundreds of dollars. I bought a pair used for about $300 on eBay. It probably would have cost more if it was collectible.

diaphragm diameter: ?
VC diameter: 4.4cm / 1.75"
"sensitivity" on spec sheet: 117dB
power handling: 50W

Click the image to open in full size.
In the other corner, we have a SB Acoustics 3/4" dome tweeter. It sells for twenty bucks. Here's some specs:

diaphragm diameter: 2.2cm / 0.86"
VC diameter: 1.9cm / 0.75"
"sensitivity" on spec sheet: 88.5dB
power handling: 30W

Click the image to open in full size.
Here's a measurement comparing the JBL 2470 on an 18Sound XT1086 vs a SB Acoustics SB19 on the same horn.

Same volume level, same horn, different drivers.

In this measurement, you can see that the JBL is basically crushing the SB Acoustics SB19 dome tweeter. And considering that it retailed for more than six times as much, it better! The JBL is a technological tour-de-force, with an alnico motor, gobs of efficiency, silver shorting rings and every technology that JBL had at their disposal.

The JBL 2470 isn't a TAD 2001, but it's close. In the measurement, you can see that the JBL is more efficient across the entire frequency range. If you equalized the two drivers to the same SPL, I'm confident that the JBL would have lower distortion.

But the point here, is that it takes a TON of technology to increase the bandwidth of a compression driver. We're seeing the 'law of diminishing returns' here. Basically the JBL is more efficient and it has wider bandwidth than the SB19, but it takes a remarkable amount of money and technology to exceed the lowly 3/4" dome tweeter.

Which was the point of this entire thread

If anyone would like to buy these JBL 2470s off of me, they're for sale These measurements use radian diaphragms, which I used to replace the JBL phenolic diaphragms.

Last edited by Patrick Bateman; 25th November 2017 at 01:13 AM.
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Old 25th November 2017, 02:20 AM   #7
bwaslo is offline bwaslo  United States
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Domes on Horns vs Compression Drivers on Horns
Patrick -- one (possibly the main) reason the compression driver's impulse response is more wiggly is that it has 6 to 8dB more HF output relative to the LF. Wiggliness of short time duration = HF energy.
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Old 25th November 2017, 04:31 AM   #8
ScottG is offline ScottG  United States
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Quote:
Originally Posted by Patrick Bateman View Post
..Basically the JBL is more efficient and it has wider bandwidth than the SB19, but it takes a remarkable amount of money and technology to exceed the lowly 3/4" dome tweeter.

Try scaling-up the input/power levels on the two: look at the distortion differences, the impulse response settling time, and the linear response withOUT averaging (on optimal axis). Take it up to a dynamic peak at *listening position* of about 102-105 db.

You shouldn't see much change in the compression driver (other than those related to the horn), but with a soft dome you should start seeing some more problems crop-up.


-don't get me wrong though, other than sensitivity I'll mostly prefer the dome tweeter for treble in a domestic use-case (vs. a compression driver). ..I've said as much in another thread and a few people thought I was screwed-in-the-head.
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Last edited by ScottG; 25th November 2017 at 04:34 AM.
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Old 25th November 2017, 02:37 PM   #9
Pano is offline Pano  United States
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Domes on Horns vs Compression Drivers on Horns
Thanks Patrick. Nice plots, the little dome does well at low levels. But 10dB is a big difference in sensitivity.
There was another thread some time back with small cone drivers on horns and they did remarkably well. That's something I'd like to try, and have gotten has far as Hornsresp sims.
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Old 25th November 2017, 02:51 PM   #10
marco_gea is offline marco_gea  Italy
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This seems to confirm my suspicion, i.e., that if you're on a tight budget then you're better off sticking to conventional tweeters, but if you're after the ultimate performance and are willing to pay for it, then compression drivers on good horns are still the superior option.

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