Going low with large format CD

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
My question is this:
With lots of large format compression drivers around, why can't we use them down to 150hz on a long enough horn.
Lets look at WE 12a with WE 555 driver. Just an example...

What can we use from the current CD offerings (JBL or Radian 4" diaphragm) to substitute the WE 555?
Why can't the current large format drivers be used the same way and what can we do to modify them for this application, so that they CAN be used?

Thanks,
Herman
 
Sorry, i have no idea on that, the only compression driver i have used for my Sato Horn are the JBL 2482 with genuine 16 ohms phenolic diapraghm rated at 300hz that you can stretch down to 150hz, and a friend who owns a pair of Altec 290 with genuine diapraghm rated at 300hz can play low like the jbl 2482.

If you already have the compression driver, you can try it and you can start experimenting with different crossover cut off point (paasive or active) but this will cost a lot.
 
Last edited:
The problem with most modern CDs...

...including the large format ones, is that they are designed to be TWEETERS, not MID-RANGE drivers.

By that I mean that several design parameters are optimized to yield the best possible high frequency extension AND dispersion, to the detriment of low frequency loading and extension, even when coupled to large horns.

Among these design parameters are: a rapid-flare phase plug design, a rapid-flare internal throat (or even none at all), a very small back chamber, a very small gap between diaphragm and phase plug (which yields a tiny "front chamber" volume and better high-freq. extension, but less Xmax), etc.

Virtually all modern compression drivers are designed that way, because their intended applications are 2-way high-power PA 'speakers or (sometimes) monitors, with a 12" or 15" Woofer crossed over at around 1-2kHz and a small constant-directivity "waveguide" that itself does not load the driver much at all below 800Hz or so.

This way, one can dispense with a separate Tweeter... but it's basically a trade-off.

There are still some (FEW) exceptions, like the "old school" JBL drivers like the 2446 and 2450, and the TAD TD-4001, which still feature slower-flare phase plugs and internal throats, larger back-chanmbers etc.

Typically, though, these are expensive "relics" (and I do NOT mean this disparagingly, just historically: they are still among the finest CDs money can buy, IMHO...), and even they struggle below 500Hz or so because the typically one-piece diaphragm & surround construction does not allow the necessary linear excursion to go much lower.

One possible improvement in low-frequency extension with these latter drivers could be obtained by employing replacement diaphragms with a more compliant (e.g. plastic) surround. Truextent for instance produce Beryllium dias with mylar surrounds that are compatible with the JBL large format drivers.

But it gets VERY expensive quickly... (I know, I have these myself!)

I hope this helps to answer your question - even if not - alas - to find an easy modern solution for your problem.

Cheers,
Marco
 
My question is this:
With lots of large format compression drivers around, why can't we use them down to 150hz on a long enough horn.
Lets look at WE 12a with WE 555 driver. Just an example...

What can we use from the current CD offerings (JBL or Radian 4" diaphragm) to substitute the WE 555?
Why can't the current large format drivers be used the same way and what can we do to modify them for this application, so that they CAN be used?

Thanks,
Herman

Ever been stuck in a traffic jam? That's fluid dynamics, basically there are too many cars trying to squeeze through a narrow gap.

Air (and sound) works the same way. If you try to cram a large volume of air through a small throat, it doesn't work.

Check out the zillion threads on Unity horns, those horns are designed to address this. Basically mount the larger drivers at a point in the horn where the compression ratio is appropriate. Small driver at the apex, medium sized driver at a point where the diameter of the horn is larger, and a larger driver at a point where the horn is even larger.
 
Ever been stuck in a traffic jam? That's fluid dynamics, basically there are too many cars trying to squeeze through a narrow gap...

I always considered cars a semisolid but nowadays your probably right. :)

In modern compression drivers how low they can play is relative to how loud you play them.

What’s the excursion rule? One octave lower requires four times the diaphragm travel to play at the same level.

When the diaphragm is only about 0.040” from the phase plug, there’s just not a lot to work with excursion wise.

On my planewave tube, I sweep drivers through zero Hz and the large format compression drivers have measurable output below 10Hz.

When I install and dial in / center up diaphragms I buzz them at 300Hz for large format drivers. With some drive signal, usually less than a Watt you can tell when you have no coil to plate contact. Then with increasing power I can usually get them up to around 8 to 10 Watts before the dipragm start to buzz on the phase plug. I chase this until its as good as I can get.

So again, how low you can use them is volume / SPL dependent.

Hope that helps.

Barry.
 
I buzz them at 300Hz for large format drivers. With some drive signal, usually less than a Watt you can tell when you have no coil to plate contact. Then with increasing power I can usually get them up to around 8 to 10 Watts before the diaphragm start to buzz on the phase plug. I chase this until its as good as I can get.

"Buzzing" a diaphragm against the phase plug is risky, and even if you don't damage it straight away, you are still causing unnecessary fatigue.
I wouldn't recommend this practice, especially with Beryllium diaphragms...
 
I am not recommending anyone do anything.

A couple of notes:

Using these drivers in sound reinforcement requires that they play loud, like 140+ dB loud, often crossed over in the 700-800 Hz range and if they are not properly centered and height matched to the phase plug they wont get there.

Most anyone can drop a diaphragm in a driver and make them work. Getting them to sound and measure their best requires some tools.

Diaphagms and phase plug physical measurement for proper mating, or tailored mating to extend the LF or HF, is beyond the capability of most as is a proper PWT and a real measurment platform. I made the investment and these tools have taught me a lot.

I am very careful with my Be components but even these can usually benefit from some careful tweaking.

This stuffs not a fragile as you might think.

Barry.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.