once read of an open baffle subwoofer going down to10Hz. The guy cut a square hole in the floor and mounted a square wooden tube from below. The tube had 3 Adire Shiva on each side, makes 12 Shivas in total.
He reported a -3db point of 10 Hz and still nice sound pressure at 4 Hz. Suits?
Well, if you want to figure what would be necessary, here goes.
According to Richard Small of Thiele-Small fame, in order to produce 112 dB @ 1W/1M, you need to move 6 cubic inches of air. That means your speaker's cone area times it's cone excursion. Excursion is measured from midpoint-to-front, NOT back-to-front.
In my experience, in a closed room, the 1W/1M measurement is close to what you get throughout the room.
If you move twice as much air, your output goes UP 6 dB.
If you go down one half octave, it requires you to move twice as much air to maintain the same SPL.
Extending this down to 10 Hz and 140 dB, we see that we need to move 17,600 cubic inches of air.
If our speaker has an excursion of .5 inch-a pretty long "throw"-we need a surface area of 35,200 square inches.
Allowing an extra 2 inches for the basket and surround, we see we need a 17 3/4 FOOT speaker.
If our ceiling is not that high, we can get away with four 8 foot speakers. Or sixteen 4 foot speakers. Or sixty-four 2 foot speakers.
The Fane Colossus 24 would do well, if you have a room that is 2240 cubic feet behind the wall you are mounting these.
Of course, the wall you are mounting these on would have to be 8 feet high by 64 feet long, but I'm sure you'll think of something.
This ignores the effect of "room gain" which mercifully would be quite substantial. More on that next post.
Now if you really want to shrink your requirements, you can go vented.
Normally, a properly designed vented enclosure cuts your speaker's air-moving requirements by four. However, there are two factors that will cut this advantage in this situation.
A) The larger the enclosure, the less the advantage in air moving requirements. Excursion is cut the most in small vented enclosures. It just works that way-apparently air is not a perfect fluid.
B) The "enclosure" behind the wall you are mounting on is not likely to be known to the rest of the world as an "enclosure". Other people are likely to call this a "room". Rooms are not built of 2 inch thick MDF, generally. They are built of wallboard, and have doors, and are leaky, etc. This "enclosure" is likely to be less-than-perfect from an acoustic standpoint, and therefore cuts our air-moving advantage for venting the enclosure.
We can still probably count on 6 dB coming from the vent, though, so we can cut our number of speakers in half. We are now down to eight Colossi. A room of 280 cubic feet or larger would be required-not that large.
We don't have to use eight Fane Colossi 24's. Thirty-two 12 inchers of your choice-whether Shivas or whatever, can be substituted, as long as the Thiele-Small requirements of the speaker fit the room behind the wall you are mounting these on.
Finally, I would caution you to check with someone who actually has done some wall mounting of multiple speakers. I have not undertaken such a large project. These posts are just using the numbers which I know work for small enclosures and extending them upward to get a ballpark idea of the requirements. Frequently that works; sometimes it doesn't.
But on the basis of what I know works in small enclosures, I think such a project is do-able. It would cost you a few thousand dollars, but then so would a high-quality enclosed subwoofer that could not hope to come close to this project's output.
I love how you've highlighted the (im)practical realities!
Just for fun, I'll mention that I've drawn up a concept for a 30" ELF transducer that should yeild 6" of linear (two-way) excursion. The idea was to anticipate the market niche for compact, transportable large-venue infrasound.
The diaphragm is driven from its periphery by three optical servo switched VCs that traverse a 2.5" deep magnetic gap lined with NdFeB mags (ala the Aura NRTs). The return path encloses an air volume that functions as the diaphragm's suspension, while a linear bearing keeps it centered. In a bipole config, the whole enclosure should still be compact enough to roll through a doorway.
Four of these, (about 17,000 cu. in displacement one-way) should meet Travis' goal .
Thanks for all the number crunching, very interesting. I know it was only a 'what if', but one thing I've learnt to account for with high power systems, is dynamic compression. Depending on the quality of the drivers used, it could easily be 4 or 6 dB or more in your suggested scenario, depending upon the specific drivers. Plus you'd need some HUGE amp power.
The room behind the drivers would not be useful when the system was on, and the listening room would need a supply of Depends and vinyl upolstery.
I more 'turnkey' solution would be a set of Servodrive ContraBasses, say 7 or 8. These were designed by Tom Danley, and don't use normal speakers. Cadge a squizz <a href="http://www.servodrive.com/servo/cb.shtml">here</a>
With an output of 114dB / 200W/ m @ 16Hz and a short term peak of 500W (+3.9dB) they should still be OK at 10Hz I'm guessing. Perhaps you could get Tom to modify the design for you, optimised for lower frequencies. Tom was involved with a project for the US guvmint to design and build some LF (12Hz IIRC) horns to generate avalanches in the skifields (safer than mortars I s'pose), and commented that in testing them in his driveway, everything seemed to 'shimmer' at 12Hz.
BTW, before Travis asked these questions, I always thought a 'brown note' was a bad jazz record.
Start with the first article and note the freq responce on the bottom of the third page.
I have a couple of these and I think they are great. Note that the front AND back of the driver are loaded, unlike most designs. Front and back loading increases effeciency and decreases distortion. When I was building mine I took a 12 inch speaker (an EV 12TRXB) in free space (in my hand, actually) and connected it to an audio oscillator and amplifier. I set the oscillator to 10 Hz. I then turned the amp up to the point where the cone was going up and down like a piston. I made a note of the volume setting on the amp. I then put the speaker in the box. I reconnected the amp and oscillator and set the volume control to where it was before. Standing right in front of the speaker I could not see the speaker move but could feel my pant legs vibrating.
If you have a good three way speaker in the box the sound and dispersion are excellent up to 10KHz or so. If these enclosures are properly placed in a good room and you have a good source, you can walk around in front of these speakers and it seems like you are walking around on the sound stage.
Will it do 140 dB? I dunno. I am not willing to risk my drivers to find out. But you can mount these things "bottom to bottom" the lower one being upside down and the upper one right side up to effectively make a bigger box. And the enclosure will accept 15" speakers without modifications.
I think this is an excellent enclosure. Check it out.
10hz at 140 db...hmmm this would be obtainable...expensive yes but it is obtainable...i know you are thinking about this just theoretically...have a look at the jl w7 drivers...i installed several of these before i got out of car audio and joined the navy...of course this was a smaller area to pressurize (a car) but they made serious sound at low frequencies...assuming you tune your box low enough it could be done...when designing ports for sound output (not really heeding sound quality...but i think 10hz at 140 db will sound good hehehe) i usually shoot for cross sectional area of the vent at 150% of the cone area...then you can plug and play from there...i did a lot of USAC competitions and got pretty high marks...easily hit 158.1 db @ 70hz on the dash with three 15" low budget 15s in 13.5 cubic feet tuned to 44hz...i could pop 150's at 20hz...so here is what we are looking at...perhaps 15 of the jl13w7 in a box somewhere around 125-150 cubic feet tuned out to about 25-30hz...that should get you somewhere close...10hz is no joke though man...talk about making yourself sick...i made my ex puke in the explorer listening to some digeralia (import of digery-doo's did i spell that right)...has crazy low bass...good luck though bro
I propose some infrasonic device that produces insane amounts of infrasound at ultra low frequencys. The key is to have displacement, so a large baffle and long stroke are best. If a electric motor were used they could power some pistonic devices.
I propose something like a boxter motor, with dual apposting pistons, and a very large enclosure. The most impossible part, the surround.
The crankshaft would need be the width of the maximum excursion desired and the maximum SPL would be reached at the maximum frequency that the device could achive.
The motor would of course need a greater amount of power to achieve faster rotation and therefore higher frequency. So with all that in mind.... how many horsepower is 140db?
Well 120db=1watt so 140=100 correct?
If we have a measly 1hp engine then 743watts should be expected, and you might think that would be well able to make 140db with the proper baffle and energy conversion setup. Well nothing is 100% efficient so maybe it wouldnt do the trick.
I propose 10hP should be enough however. And now for the baffle setup well two baffles of 3'x3' on each side with a roatary setup for 2feet p-p excursion it should have over 18cubic feet of displacement one way and 36cubic feet peak.
Now how to create a surround or something to seal the driver..... I want to hear your ideas
140 dB at 10 Hz?! You could probably achieve that by mounting a high excursion driver to each of the portholes in one of these old antique diving helmets, then stickin' your noggin inside it and firing it up.