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Old 7th October 2012, 06:14 AM   #371
Djim is offline Djim  Netherlands
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"the Philips system does not do such."

Lol, I can assure they do :-) (Philips SW945 and Philips SW8000SA for instance)
Actually, there is also a Danish company (SA) that calls it "Hybrid-reflex".

"I have used a PR with the PPSL it's fine, but not suitable for transport"

Not sure what you have build but they become smaller than the usual PPSL's.

Last edited by Djim; 7th October 2012 at 06:19 AM.
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Old 7th October 2012, 07:30 AM   #372
Djim is offline Djim  Netherlands
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"Using a PR and a port in the same box makes no sense to me"

Maybe I should give you a hint, bassdrums. The skin you hit is your active membrane, the front skin is the passive membrane. In order to tune it lower and give it an extra boost they start to cut a whole in the front skin, years later they even put a basreflex port in…;
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Old 7th October 2012, 09:33 AM   #373
djk is offline djk
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"Not sure what you have build but they become smaller than the usual PPSL's. "

They are already 6th order with 6dB of boost. The only way they could be smaller and go as low would be with more boost.

Not suitable for transport refers to the fact that a PR tuned low in a small box will have very high mass. Without a mechanical stop on the PR for transport it will damage the woofer cones. I had this happen with some 6th order EV that the seller forgot to take the weight off the passive before transporting. The original version had a board screwed across the front of the cabinet and into the front of the PR, you had to remove to use, and replace for transit.

I have some Beyma 12LX60 to make a very small PPSL with a huge and heavy PR. I'm still thinking about some of the details on this (securing the PR for transit).

"The skin you hit is your active membrane, the front skin is the passive membrane. "

I think we are comparing apples and kiwi fruit here. I am not trying to make a one note ringing device. Drums can be hard to get mic'd up for good sound through a PA. I can usually get it to sound better than it does on stage, but it can be a challenge. Most drummers kit is sadly out-of-tune and they don't even have a clue.

My friend Steve carries a complete backline when he does a large job, that way he has some good instruments, amps and drums available if needed (five different snare drums).

http://i56.photobucket.com/albums/g1...ch/COE08-3.jpg

(Steve with 1/3 of his PA, although you would have a hard time recognizing him today after he cut his hair and lost 50 lbs. The JBL 2380s have been replaced with EV HP640s and he has six Crest amplifiers in two racks now)
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Old 7th October 2012, 10:55 AM   #374
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djm,

yup, my math was waaayyyyy off.
I have to remember not do numbers so close to bed-time


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Old 7th October 2012, 12:08 PM   #375
djk is offline djk
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"The plenum acts as a cavity which itself can act as a low pass filter. Depending on the frequency region it might be lowering some odd harmoic (eg 3rd) distortion "

Yes, the low-pass of the plenum cavity does roll off the higher odd harmonics, at about a 12dB/oct rate above the depth of plenum cavity corner. The 'gargle' is also reduced as the cones are moving sideways rather than-towards-and-away from you (think of how it sounds to talk through a fan).
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Old 7th October 2012, 04:39 PM   #376
Djim is offline Djim  Netherlands
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"Not suitable for transport refers to the fact that a PR tuned low in a small box will have very high mass. Without a mechanical stop on the PR for transport it will damage the woofer cones. I had this happen with some 6th order EV that the seller forgot to take the weight off the passive before transporting. The original version had a board screwed across the front of the cabinet and into the front of the PR, you had to remove to use, and replace for transit."

Ah, I see the difference now. We experimented with larger PR’s instead of same diameters and in 4th order to obtain a less steep roll-off from smaller cabinets at costs of gain of course. The PR’s were tuned above the ports, which allows them to be lighter. Our problem was/is to get enough excursion from them, since we used modified PR's made from standard drivers. I wish some Italian PA brand would jump into this because I still see some benefits for this application. The PR's should also benefit from your slot loaded push-pull idea, although we haven't tried that.
What kind of PR’s did you use?

Last edited by Djim; 7th October 2012 at 04:50 PM.
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Old 7th October 2012, 06:24 PM   #377
djk is offline djk
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A high mass 18 with 3" P-P travel (the Beyma design required a minimum of 54mm P-P at 40hz with their rated power).
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Old 7th October 2012, 06:47 PM   #378
kees52 is offline kees52  Netherlands
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I did not ask yet but how is the spl and low end from this box type? like dB 1watt.

A normal vented box does most 90 db watt, but a ppsl has a lot more muscles in the low end?.

I now if I search here on the forum I will find it, but I am lazy this time.

Het vallen van het blad ;-)
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Old 8th October 2012, 01:28 AM   #379
djk is offline djk
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Errata:

#366 should read "I see this more as a transition from 2Pi to 1Pi," not "I see this more as a transition from 2Pi to 4Pi, "

" but a ppsl has a lot more muscles in the low end?."

Read #366 please.

A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).

The theoretical limit to efficiency from coupling is 25%, plus the increase in the Di (Directivity index) from the large front baffle surface (transition from 2Pi to 1Pi).

With four cabinets a side the stacked pair of 12s mid-bass/mid-range horns have a hard time keeping up, as does the HF compression driver.

People unfamiliar with the performance of these cabinets may have problems from feedback from their use of compression on drum mics and bass boost at the port tuning frequency (around 30hz for these).

I had to come to the rescue of a FOH engineer for a national act that was using an eight bass cabinet system indoors and was making people run for the door in a club that held about 1300 people. He got it into feedback at 30hz and because these have such low distortion he couldn't tell what was causing it. I stepped in, turned the EQ back to flat on his drum mics, and backed-off the drum mic compressors from 15dB~20dB over threshold to about 6dB and adjusted the mix for him. The amount of 'punch' and 'slam' on the drums was unreal!

Another time on an outdoor stage a mic got disconnected and then plugged back in (without muting the channel) with the system gain at full volume! It felt like the cannon shots in the 1812 overture (the system being time-aligned), and had people running for cover. The system owner (Steve) got this sick look on his face and timidly started checking out the system. The 6th order filter (built into the crossover) protected the bass cabinets and an ultrasonic filter (also built into the crossover) protected the HF drivers.
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Old 11th October 2012, 01:54 PM   #380
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I have a couple of question to ask.

Is there a upper limit for compression ratio for the plenum? I am working out the compression ratio by dividing the combined SD of the drivers by the front area of the plenum. I am working on a design using two RCF 15p200AK and could easily get a very high compression ratio of 5:1 now building it with a compression ratio of 2.7:1. I would imagine like horns the comp ratio depends on the size of the driver and how strong the cone is.

Next question, does the air mass in the plenum lower the cone resonance? I think it does. Can this lowering of resonance be calculated or only measured?
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