how do u prevent mechanical transmission with a flown PA in a solid concrete building - diyAudio
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Old 13th May 2004, 04:51 AM   #1
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Default how do u prevent mechanical transmission with a flown PA in a solid concrete building

I am currently trying to solve the problem of the upstairs floors shaking when the PA is used in a basement club. It is quite a big system (5K) and all the enclosures are bolted to the solid concrete walls. There is no treatment on the walls either so you can imagine that it sounds pretty blastingly bad in there when a DJ cranks it up to try to get it to sound alright. Typical DJ style "ooh it sounds really nasty, it obviously isn't loud enough!". I have reccomended large quantities of absorbtive materials on the walls. I estimate that they must get 500th order reflections in there. OWWW the comb filter! Anyway I now have the issue of how to isolate the speakers from the walls to prevent the mechanical transmission of sound (about 12dBs of S/N @ 250Hz inside the shop and about 9m/s added vibration of the floor), and I have considered springs, foam, rubber hosepipe, bicycle inner tubes, ultra pricey GERB mounts, netting and so on and I was rather hoping that some lovely characters on this user group might be able to suggest some bright ideas which could be implemented with low cost and low fire and health and safety risk. I do not think that rubber blocks is enough here.
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Old 20th May 2004, 10:39 PM   #2
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You can calculate the 1/4 and 3/4 resonante freqs for the floor to the ceiling and maybe add a band reject filter. Another thing to do is to simply add more bracing to the ceiling, possibly in the center of the room from floor to ceiling (maybe create an island) and put more speakers, lol, that should help quite a bit with the floor traveling up/down and will also raise the resonante freq.
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Old 21st May 2004, 02:06 AM   #3
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Having concrete walls and floors leave the bass to be absorbed mostly by the ceiling. You need some good bass traps. This will improve the sound dramatically and increase the apparent amount of bass by reducing cancellation. This will help in 2 ways. Total driver output can be reduced, so there's less to head upward. Also the traps will absorb alot of the stuff that's just wasted through cancellation anyway, so again less is left to go vertical.

Also, with the cabs bolted to the walls, the cab vibrations are being transferred upward through the walls. Try suspending the cabs from the wall mounts via cables with some kind of rubber bushing further shielding the mount (wall) from the cabinet vibrations.
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Old 21st May 2004, 01:56 PM   #4
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Default Re: how do u prevent mechanical transmission with a flown PA in a solid concrete building

Quote:
[i]I do not think that rubber blocks is enough here. [/B]
Car engine mounts (if you choose carefully) can be fairly inexpensive, wide variety, drilled metal bracket on each end, and strong as all get-out.
Click the image to open in full size.
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Old 21st May 2004, 08:02 PM   #5
djdan is offline djdan  Romania
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Can somebody help me to find more information about bass traps ?
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Old 22nd May 2004, 04:10 AM   #7
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Quote:
Originally posted by eric93se
Another thing to do is to simply add more bracing to the ceiling, possibly in the center of the room from floor to ceiling, that should help quite a bit with the floor traveling up/down and will also raise the resonant freq.
Interesting point actually. The club in question has been going for about 10 years with no complaints from upstairs. What they did recently was knock out 2 pillars that went between the ceiling and the floor about 2/3 of the way across the room. Now they have complaints. The floor is very very solid (6ft thickness of steel reinforced concrete and clay for the next hundred metres) so it would damp the ceiling quite considerably. Also the ceiling panel has now got a lower resonant frequency and a more perfect geometric shape. Lower frequency = more excursion @ equal RMS volume. Enlightening!

Also they have the subwoofer resting on the floor! naughty naughty!

The club has also called in a civil engineering company called ATKINS who are putting in a competing bid. They have recommended that a large wall to wall panel of double thickness plasterboard filled with 5kg/m2 of loose mineral behind it and an air gap of 30cm. This would lose about 1 1/2 feet of ceiling space, cost a bloody fortune, and shut the club for weeks. However their word counts for more than mine in the eyes of the owner since I am a freelancer and they are a massive international corporate conglomerate. What they suggest would certainly work, but I don't think it would do much for the sound of the room! (it would make the room uncomfortably close to 2:1 wall:ceiling) My idea is to eliminate direct mechanical transmission and to sort out the sound of the room so they don't have to have the music at deafening volume to compensate for the terrible sound quality. Much cheaper and much better in my opinion. Thanks for the posts and I like the look of those tube traps!
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Old 22nd May 2004, 04:59 AM   #8
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If you go with tube traps, the placement should probably be along the corners formed by the walls and ceiling. When the club is packed the bodies will cause the problem area to be above everyone's heads.

Also, I'd go talk to the upstairs neighbors and tell them the club wants to be a good neighbor. Foind out if the problem started with the removal of the columns or is it just worse now. See if you can coordinate a test to see if there is just a narrow problem frequency. If so, then a costly fix that addresses the whole frequency spectrum would be unwarranted. Being closed for a period can hurt business long-term, not just while it is closed. Also, that fix could even further degrade sound quality inside.

Maybe just putting a column back, but in a less obtrusive location would solve the problem and change the harmonic that was created with the removal of the original columns.

If it's a narrow frequency problem then even just a notch filter in the system might work and not affect the sound too much. Also, some ceiling mounted helmholts resonators could address exactly what frequencies are needed and could be built outside and installed during the day and not force closing the club at all.

A little research and testing is what's called for before closing the club for an extended period of time. Talk to their pocket book and you'll get the contract instead of the big guys who don't care about their business.
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Old 22nd May 2004, 02:17 PM   #9
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Quote:
Originally posted by johninCR
Also, some ceiling mounted helmholtz resonators could address exactly what frequencies are needed
Is that some kind of spring-and-mass assembly that you would hang from the centre of the ceiling? At the natural resonant frequency of something like this, it would damp any ceiling movement very strongly. Same principle as bass reflex speaker box reducing cone movement at port-box resonance.
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Old 22nd May 2004, 02:53 PM   #10
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No, another name is a slat resonator. From the front it would look kind of like wood panelling. It doesn't absorb the frequency, it dissipates it. Variations of this are used to block highway noise from going into residential areas with a design that's only 2 inches thick. It's the most cost effective way to deal with problem mid and bass frequencies. Here's the link:
http://www.saecollege.de/reference_m...requencies.htm
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