Digital Organ Subwoofers

[Keys]

Hello,

I have a project going and need some advice. Tech support referred me to this forum. Their advice seems trustworthy, so here goes. I have a digital organ built from a computer and a couple of multi-channel audio interfaces equaling about 40 audio channels in all.

The organ is in a room that is 30' X 50' with a 15' ceiling. The organ--like many--has a lowest pitch of 16 hertz which has a wave that is just over 68 feet long. There are two small transept-like extensions but they are on the 30' sides of the room.

Probably the first question should be whether it's possible to get that pitch in that room. The organ does well down to low G, which is 24.5 hertz. That wave is just under 46 feet.

I have four boxes available, all built by an organ company. Two are each 25 ft3 and two are 17 ft3. These are in addition to other speakers, of course.

How many of the 18" Ultimax subwoofers do I need? These subwoofers consume a good bit of power, but an adequate amplifier won't be a problem. Do you recommend anything beside the Ultimax?

Would one of the larger boxes be enough? Should it have two subwoofers in it? Would two of the larger boxes be better? Do I need all four boxes? IF I need the two smaller boxes too, should they each have one or two subwoofers? Does any or all of this need to be ported? Be sealed? Have passive radiators?

The boxes will not be visible, so adding ports would not be a problem, even if the ports are outside the boxes. If they should have ports, how many, how long and what diameter? I can probably make whatever you tell me needs to be done. It won't be pretty or have much wife acceptance factor, but it won't be seen.

I apologize for not trying to work this out myself, but I don't want errors. I can use the computer to make an organ, but for some reason the T/S parameters sort of make my eyes glaze over. Maybe I've spent too much time doing minor electrical work or on the bench practicing.

An unusual thing about organ sound at that pitch is this: On many pipe organs, the sound can still be heard/felt even at the fullest organ sound, while still not overpowering quiet sounds. So, while I'm not looking to fill Notre Dame with sound--may she be restored soon--I still want a good rumble, if you know what I mean.

The system for the organ is of custom design, and one can easily direct which sounds--even which notes--go to which channel(s).

Thank you in advance for your help.

Sincerely,


Keys

[billfitzmaurice]

Probably the first question should be whether it's possible to get that pitch in that room.

Why wouldn't it be?
The Ultimax 18 works best in a 10 cu ft sealed box. How many you need depends on the SPL you want to realize.

[fpitas]

Be cautious about porting. If the port is tuned above 16Hz, it will unload the woofer below its tuning frequency, and you'll get a lot of cone motion. The danger is exceeding Xmax. You might try tuning the port to 16Hz and EQ the result.

Another note, you'll need a pretty large port area to minimize chuffing at 16Hz. WinISD will help you get an idea.

[LOUT]

I think a pair of Ultimax18's inside a single 25ft-cu box with a pair of 3ft-long X 8inch-diameter ports will reach F3~15hz and keep pretty low port-velocity (23-29m/s) up to 120-123db.
This should also avoid unloading unless there's content below that 15-16hz bottom.

[djg]

https://www.youtube.com/watch?v=F9r9G0CS0BM

[Chris Roemer]

LOUT's numbers seem valid.
Bill will tell you that you're better off using 2 different boxes, positioned to fill in each other's nulls.

One UM18 in a 17cf box using a pair of 6"id x 3' long vents will give similar tuning and results (F3 of 14Hz).

[Keys]

Mr. Fitzmaurice,

Thank you for responding. I must have misinterpreted your question, reading, "Why wouldn't it be [the first question]?" instead of "Why wouldn't it be [possible to get that pitch in that room]."

The reason I wondered whether that pitch could be made in the room is because of what a guy who builds pipe organs has told me. He says can rarely get some of those lowest notes to sound if the room isn't big enough. He figures that the lowest note has wavelength of about 68 feet. If no dimension of the room is that long, then the note won't sound. I'm sure home theater folk would dispute that, but it's why I asked.

Of course, with pipes that size--16' if a stopped pipe and 32' if an open pipe for a single note--they have to go where they will physically fit and not where the optimum place is. They decrease in length geometrically to half that length by the next octave, but that's still a lot of real estate, especially since the longer the pipe the wider it has to be to keep the same tone quality.

You have a very interesting web site and forum. If these cabinets weren't already made, and if I were the kind of woodworker my father was, I'd build a tuba instead.

Thank you for sharing your expertise.

Blessings on thee, sir.


John Maher

[billfitzmaurice]

He's wrong. He's so wrong that he's got it totally backwards. When the longest room dimension is a half wavelength speaker sensitivity rises as wavelength increases. In a perfectly sealed room sensitivity rises at a rate of 12dB with each octave decrease in frequency. If he was correct headphones and auto sound wouldn't work, let alone single digit home theater. It's also why pro-touring concert sound doesn't even attempt going lower than 35Hz. The room sizes are too large. The reason why pipe organ pipes don't go as low as their length might indicate they should is that their radiating area is too small.

[wogg]

Mr. Fitzmaurice,
...The reason I wondered whether that pitch could be made in the room is because of what a guy who builds pipe organs has told me. He says can rarely get some of those lowest notes to sound if the room isn't big enough. He figures that the lowest note has wavelength of about 68 feet. If no dimension of the room is that long, then the note won't sound. I'm sure home theater folk would dispute that, but it's why I asked.
...

The way the wave works definitely changes, but the sound can still be created. Case in point: headphones create a perception of deep bass with only mm between the driver and your eardrums and car audio easily produces low frequencies way longer than any interior dimension.

When the wavelength becomes larger than the largest room dimension the physics change. Instead of the wave propagating over distance, the whole room airspace is pressurized so you still feel and / or hear the sound. The acoustics change and you have to deal with standing waves, cancellations, room modes and all that.

Of course with an actual pipe resonating, the way the sound is created may be a bit different than a moving cone so the pipe organ guy may be absolutely correct in that instance. Not an expert on that at all...

[Keys]

Mr. Fitzmaurice,

Thank you. That makes perfect sense after you put it in writing. Intuition told me the pipe guy was mistaken, but I didn't have enough background to say why. (I also don't need to lose any friends.)

I can play the blamed things, but can't always explain exactly how they work beyond the basics.

John

[Keys]

Guys,

I've been thinking I'd use concrete forms for the vent tubes, since they're pretty large, and PVC type tubes that size are pricey.

Our local building supply doesn't carry forms smaller than 12". Is there a reason not to use one tube that's 12" by 16"? According to my calculations the volume of the tube is the same as two 8" by 36" tubes. I could check other suppliers that are out of town if necessary, but I like to shop locally if at all possible.

Thank you in advance for your advice.

John aka Keys

[Keys]

Correction

I just re-read Lout's note. Make that one tube that's 12" wide and 8" long to equal two tubes that are 6" by 36"

[djg]

8" dia. 48" long concrete forms are available in my neighborhood, not too pricey. If you can't find them, make square ports from wood.

[Chris Roemer]

If you use a FREE box modeler ( like linearteam's WinISD from linearteam.org ) you'll see that Louts ports tune the box to 15Hz. YOUR 1st (16" long) port raises the tuning to 22Hz (about 40% higher) giving a nasty "boom" @ 25Hz (or, MAYbe you could live w/that, it's a +5dB "bump"). Your "8in long" "revision" raises the tuning to 27Hz boosting 30Hz by +8dB.

Problem is that the output @ 15Hz is then down -18 to -20dB (not what you wanted).
It's not strictly the port volume that sets the tuning, it's the relationship between it's cross-sectional area and the volume - not the same thing.

You CAN use a 12" port (and it IS close to the same area as a pair of 8"ers), but it should still be about 3' long.

[Keys]

THANK YOU, guys for the clarifications. I'll find the 8" forms. I'm about an hour from Ft. Wayne and 90 minutes from Indianapolis. I'll find them and do it right.

.John

[djg]

Be aware these forms are approximate dia. They make them in slightly different sizes so they can "nest" 3 or 4 together. Bring a measuring device.

[Wolf]

Ah, another Hoosier! I'm just north of FW myself....

[Keys]

Ah, another Hoosier! I'm just north of FW myself..

Wolf,

Hi! I'm near Marion, though I can't claim to be Hoosier born and bred. I moved here when I was 15, and that was a loooong time ago.

John aka Keys