Questions about PA cabinet concepts

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Ben Wheeler

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Dec 29, 2020
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G'day,

I've spent much of the past few months trawling through endless build threads of you kind folks. It's time for me to ask a few questions and get designing and building! In a past life I did a bunch of live sound, purely as an operator. Now I've decided to get back into it in a small way, and in the interim I've acquired some furniture making skills. At this stage I have a very basic/small FOH setup, and want to build some stage monitors. However, I probably need to work backwards from a future FOH build, as I'd like the front cabinets & monitors to match to at least some extent.

So, here are some thoughts and things I've gleaned.. I'd love to have any responses that anyone would care to give.

- For mid/high FOH boxes, I'm hoping to plan for something that will be as loud and sound as good as possible. The only caveat is that it must be able to be moved by one person.

- I'm fascinated by Peter Morris's PM90 boxes, but owning megabucks processing is not on the cards at this stage, so that probably counts these boxes out?

- The main concepts I've been looking at and modeling in WinISD are based on either a 12 or 14" coaxial such as a B&C 14CXN88, or a 12 & horn such as Beyma 12MC700Nd & appropriate compression driver.

- The consensus I've noted is that multiple cabinets = phase problems = bad. Is that right? I'm not considering line arrays, so is it correct to be looking at the most powerful single cabinet I can?

- In the case of the Beyma 12MC700Nd, that claims a sensitivity of 100dB (which seems somewhat reasonable from their graph) and the AES power rating is 700W. I believe this equates to a calculated peak of 128dB, not that this is what WinISD shows in my modeling. Is that a decent place to start for a small PA?

Thanks in advance for your thoughts.
 
Others will be along with more detailed answers, but here's initial thoughts.

PM90 - you don't need to spend mega-bucks to make them sound great. A Powersoft Duecanali DSP amp (for example) will do FIR processing and isn't that expensive. Even without FIR processing, such as with a Behringer NX-D amp - they still sound very good.

The most powerful single source (cabinet) is better than multiple smaller, discrete sources, yes, at least from an audio perspective.

Small PA - how small? A front loaded 12" or 14" coax can be made to sound nice, but it's never going to be loud. If you're working with a couple hundred people, then it should be fine. Much larger than that - the PM90's make a lot more sense.
 
Thanks for this..

In terms of how small, yes probably a couple of hundred people, but I'd prefer to have plenty of capacity to spare.

Regarding amps, I don't know how foolish or unethical this would be considered to be, but I'm experimenting with a Chinese amp at the moment (not one that claims to be another brand!). A separate processor, if that would work, could be a possibility. That's a good tip about the Behringer though - that's a very reasonable price.

The RCF 12" Peter has used two of in the PM90 has similar headline specs to the Beyma 12" I mentioned. Going by my crude understanding of things, I'd expect two of either of them to produce around 131dB peak. Do the PM90s have other magical qualities in terms of directionality or frequency response that would make them a ton better than a 2x12+horn or MTM?
 
Ben Wheeler said:
The RCF 12" Peter has used two of in the PM90 has similar headline specs to the Beyma 12" I mentioned. Going by my crude understanding of things, I'd expect two of either of them to produce around 131dB peak. Do the PM90s have other magical qualities in terms of directionality or frequency response that would make them a ton better than a 2x12+horn or MTM?
Click to expand...

The PM60/90 designs use horn loading rather than having the drivers radiate directly.
This has the effect of working a bit like a transformer (or gearbox if you prefer that analogy) to make them more efficient, which is why the projected max SPL's mentioned in Peter's threads are higher than the 131 you've calculated. IIRC the difference is in the order of 6-8dB for the 12" low mid section, equivalent to having at least double the number of direct radiating drivers.
HTH,
David.
 
David Morison said:
The PM60/90 designs use horn loading rather than having the drivers radiate directly.
This has the effect of working a bit like a transformer (or gearbox if you prefer that analogy) to make them more efficient, which is why the projected max SPL's mentioned in Peter's threads are higher than the 131 you've calculated. IIRC the difference is in the order of 6-8dB for the 12" low mid section, equivalent to having at least double the number of direct radiating drivers.
HTH,
David.
Click to expand...

Ah, righto.. I'd better read the second half of that mega-thread and consider them further. Thankyou!
 
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