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post #1 of 18 Old 08-22-2012, 12:07 PM - Thread Starter
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Hello all,


I have been having a problem with vibrations the subwoofers in my theater, most of which come from frequencies below 45Hz. I have tried several subwoofers, all with the same problem. I'm currently using a pair of F-20 horns. There is usually a dip in the frequency response from ~25Hz to 40Hz by about 8-15dB. Moving the subs helped a little, but not by much. The majority of the vibrations come from the floor, which is carpeted and on the second floor of the building. Is there anything I can do to reduce this effect? And possibly even out the room modes?

The room is 28ft L x 23ft W x 10ft H, carpeted and untreated.

Thanks

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post #2 of 18 Old 08-22-2012, 12:24 PM
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The majority of the vibrations come from the floor, which is carpeted and on the second floor of the building. Is there anything I can do to reduce this effect?
Probably not, short of rebuilding the house. The thirty foot and longer wavelengths of very low frequencies will cause all but the most solid structures to vibrate, and pass through walls and floors with impunity. Evening out room modes requires using more subs.

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post #3 of 18 Old 08-22-2012, 12:35 PM - Thread Starter
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Probably not, short of rebuilding the house. The thirty foot and longer wavelengths of very low frequencies will cause all but the most solid structures to vibrate, and pass through walls and floors with impunity. Evening out room modes requires using more subs.

I'm wondering if the floor is not supported well enough for that room. I did try one of the subs in another room on the other side of the second floor and while it did still vibrate, it was tolerable. In the theater room I can feel the floor bounce from someone walking across it. Movies at reference could almost throw a person off balance if they're walking. Would putting foam pads under the subs help tame this a little or is the only way to add more support?

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post #4 of 18 Old 08-22-2012, 12:36 PM
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have you experimented with moving the listening position?

modes are based on room dimensions and are worse the more rigid the structure. the vibrating floor could cause a suckout, but i kind of doubt that is what you've got.

you are most likely dealing with simple nulls. the easiest solution might be to move your listening position out of the null. sometimes it doesn't take much moving to make a large difference.

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post #5 of 18 Old 08-22-2012, 12:39 PM - Thread Starter
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The frequency response does smooth out by moving forward 8ft, but that's in the path of the projector at that point.

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post #6 of 18 Old 08-22-2012, 04:19 PM
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well, your room sucks...

...just kidding.

how about the back corners, could you plop some great big bass traps there?

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post #7 of 18 Old 08-22-2012, 04:29 PM - Thread Starter
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well, your room sucks...
...just kidding.
how about the back corners, could you plop some great big bass traps there?

It really does. I plan on moving in a year or two and plan on building a dedicated theater. Until then...

How large would the traps need to be? The center of the room is 10ft high, but the ceiling is sloped and drops to about 4' 6" at the walls. It definitely needs some acoustic treatments.

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post #8 of 18 Old 08-22-2012, 08:12 PM
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How large would the traps need to be?
Bass traps are only effective roughly an octave higher than your problem frequencies.

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post #9 of 18 Old 08-22-2012, 11:20 PM
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Pull up the carpet and use deck screws to tighten up the floor?

If that doesn't work, put down thin set, then hardibacker board, and screw that down.
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post #10 of 18 Old 08-22-2012, 11:50 PM
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"Bass traps are only effective roughly an octave higher than your problem frequencies."

what about those perforated helmholtz resonator absorbers?

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post #11 of 18 Old 08-23-2012, 06:49 AM
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Pull up the carpet and use deck screws to tighten up the floor?
If that doesn't work, put down thin set, then hardibacker board, and screw that down.
If the floor is 'squeeky' then screwing it to the joists would help. If the floor is just plain soft adding another layer of sheathing, applied with adhesive and screws, would be in order. 1/2" concrete board with a topper of 1/4" plywood underlay would be sweet, assuming the joists will handle the load. But this isn't at all a minor undertaking.
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what about those perforated helmholtz resonator absorbers?
Absorbers and traps help tame response bumps in the room, but they do nothing to stop low frequencies from going through the walls, floor and ceiling. That's only accomplished with the combination of high mass and stiffness.

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post #12 of 18 Old 08-23-2012, 09:42 AM - Thread Starter
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Since I will be moving in the next year or so I don't want to start a big project. My primary concern is the frequency null in the bass response. The rattling I can bare until I have a new place and I can build a dedicated theater room from scratch. Is there something I can try to correct the null short of rebuilding the room? Maybe a different placement? I have both behind the screen about 8ft apart right now.

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post #13 of 18 Old 08-23-2012, 10:09 AM
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can you put one of the f-20's up front, and one in the back of the room with the polarity switched? Might help some...

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post #14 of 18 Old 08-23-2012, 10:57 AM
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Quote:
Originally Posted by cmgunn View Post

Is there something I can try to correct the null short of rebuilding the room?
The combination of placement of multiple subs and EQ should do it. Most rooms will work well with two subs.

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post #15 of 18 Old 08-23-2012, 08:29 PM
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So, one question.. How is the sub touching the floor?

Do you have sound isolation spikes on it?

I wonder if that would help.
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post #16 of 18 Old 08-24-2012, 12:11 AM
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So, one question.. How is the sub touching the floor?
Do you have sound isolation spikes on it?
I wonder if that would help.
They won't. Sound is only transmitted directly from the speaker to the floor when the cabinet panels are too thin and/or insufficiently braced, allowing them to vibrate, which means the cabinet is defective. Even in that case the vast majority of floor vibration would still be caused by the acoustic output of the sub, not vibrating panels.

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post #17 of 18 Old 08-24-2012, 01:12 AM
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A dip in response can have a couple of causes and it's best to understand what is going on before you move to a solution. One cause is due the phase cancellation. You can eliminate that one by measuring the sub on its own with the crossover running it up high enough to see if the dip disappears.

Sometimes a dip is simply related to the modal issues of your room, and this is the most likely. In that case, the solution is to measure the transfer function of your room. You place your sub with the mouth around the listening position, then move the mic and measure every spot in which you could use a sub. Even consider unusual locations, because with DIY you can put a sub in the ceiling potentially, if that location works. Now once you've done that you will see a stack of plots, and you can look over them to see if you can find positions that when used together, will fill in your dip.

One way to integrate a big sub like the F20, is to place it for maximum LFE gain, and ideally avoiding dips below 40 Hz. Then you place other smaller subs around the room to cover above 40 Hz, as many as you need for a smooth response.

I've written about this at length in a 3 part series on bass integration:

http://redspade-audio.blogspot.com/2011/05/bass-integration-guide.html

As Bill says, bass traps won't fix the problem. Even with very large ones that show an improvement down to 25 Hz, you still need to EQ because they tend to do a better job at fixing the time domain, than the frequency response. Why? My best theory on this so far is that bass traps tend to have a billiard table kind of effect. Bass traps are like the padded sides of the billiard table. EQ is a bit like the speed at which you fire off the ball. The bass traps get many chances to damp the standing waves, which would seem to explain why they can actually work better than expected. If you try to treat speaker boundary interference with traps, what do you find? It does very little, most likely because the absorbers only get one shot at it. However, let's consider that you have a 44 Hz peak from two walls 4m apart. It's a primary room mode. In 150 ms it will have done 13 laps, so you might say that the trap is now 13 times more effective at damping the mode, compared to SBIR. Over that period of time with decent bass traps, you can expect to see quite a bit of improvement.

Someone may argue that the reduction in ringing is a reflection of the EQ effect of the bass trap. I suspect this is not the case, but it's an idea worth testing.
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post #18 of 18 Old 08-26-2012, 04:28 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post

The combination of placement of multiple subs and EQ should do it. Most rooms will work well with two subs.

I tried several different positions, as well as placing one at the listening position and measuring around the room with no improvement. Looks like I'll just have to wait until I can build a dedicated theater.

Thanks everyone for the advise and help!

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