Placing sub 4ft above ground. Good or bad idea? - AVS Forum
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post #1 of 36 Old 09-07-2013, 08:37 AM - Thread Starter
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My niece and nephew are about 2 years old. And they just love to climb on top of the subwoofer. My nephew fell off the sub landing face first. And now I am afraid they will get hurt. Both my subs are in each corner of the wall. I was thinking of building base about 4ft tall off the corner of each wall. But I am afraid of how it may sound. I would like to get some opinions before I start building the base. Oh and I own 2 outlaw lfm-ex.
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post #2 of 36 Old 09-07-2013, 12:01 PM
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I think you would lose some corner-loading effects halfway up the wall?

You could just stack the two subs on top of each other so they can't climb it because it's so tall, then buy another two for the other corner so you're symmetrical wink.gifbiggrin.gif
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post #3 of 36 Old 09-07-2013, 04:46 PM
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Originally Posted by joe801 View Post

I was thinking of building base about 4ft tall off the corner of each wall. But I am afraid of how it may sound. .
It shouldn't bother anything. For that matter it may be better, as the closer distance to the ceiling will raise the frequency of ceiling bounce.
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post #4 of 36 Old 09-09-2013, 10:11 AM
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A friend of mine uses fireplace guards to keep his cat off his sub those mesh trifolds seem to do the trick just a thought.
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post #5 of 36 Old 09-09-2013, 11:45 AM
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Teach the kids the subs are off limits. It is a no, no to touch them. I have 2 Outlaw Audio LFM-1 Plus's and I have a 2 year old daughter and she already has learned that all daddy's stuff in the front of the room is off limits.
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post #6 of 36 Old 09-11-2013, 08:46 AM
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In my opinion it's best to not raise the sub unless you have to. The low frequencies of the sub often resonate with the level that they are placed on. Meaning most subs that are placed on the floor react together, giving you that common "rumble" feeling that sub users seeks. Raising the sub can also cause frequency nodes (or Standing Wave forms). This can Live'n parts of the room you may not want resonating, such as, the corners of the room and shallow, less dense areas in your rooms walls. If you must raise it try to physically keep the sub grounded as much as possible to replicate the same sound and feeling. Let me know if that helps, I'd be glad to help give more feedback.

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post #7 of 36 Old 09-11-2013, 12:30 PM
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Originally Posted by tanveers View Post

The low frequencies of the sub often resonate with the level that they are placed on. Meaning most subs that are placed on the floor react together, giving you that common "rumble" feeling that sub users seeks.
I believe you're referring to boundary reinforcement. It fully occurs when the source is less than 1/4 wavelength from a boundary. With an 80Hz signal 1/4 wavelength is 3.5 feet, so a stand of reasonable height will not be a problem. OTOH a floor placed sub will be well over 3.5 feet from the ceiling, so there will be a fair amount of cancellation from the ceiling reflection. By reducing the distance to the ceiling the frequency of the cancellation will be pushed higher, so there are potential gains to be realized by elevating subs. You don't see it done, mainly because most people would see a sub falling off of a stand to be a lot more dangerous to a child than said child falling off of a sub, and of course there are the esthetic issues.
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post #8 of 36 Old 09-13-2013, 03:47 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

I believe you're referring to boundary reinforcement. It fully occurs when the source is less than 1/4 wavelength from a boundary. With an 80Hz signal 1/4 wavelength is 3.5 feet, so a stand of reasonable height will not be a problem. OTOH a floor placed sub will be well over 3.5 feet from the ceiling, so there will be a fair amount of cancellation from the ceiling reflection. By reducing the distance to the ceiling the frequency of the cancellation will be pushed higher, so there are potential gains to be realized by elevating subs. You don't see it done, mainly because most people would see a sub falling off of a stand to be a lot more dangerous to a child than said child falling off of a sub, and of course there are the esthetic issues.
Thanks Bill, I was wondering about the ceiling bounce thing but that adds clarity! smile.gif
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post #9 of 36 Old 09-14-2013, 12:47 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

By reducing the distance to the ceiling the frequency of the cancellation will be pushed higher, so there are potential gains to be realized by elevating subs. You won't see it done, mainly because most people would see a sub falling off of a stand to be a lot more dangerous to a child than said child falling off of a sub, and of course there are the esthetic issues.

FWIW, I frequently read that elevating a sub is not recommended practice, but rarely do I get a good explanation as to why. Then I read a Geddes paper describing a method for getting good bass response that involved three subs where the third is to be raised off the floor, iirc. And not only that, the idea he was positing was to INCREASE the number of modes in-room, not eliminate them as is usually proposed. I've been busy and haven't had the time to look into this further, but it interests me because he's a known quantity and respected heavy-hitter who proposes the opposite of what is commonly recommended.

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post #10 of 36 Old 09-14-2013, 04:56 AM
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If you have a flat ceiling, raising the sub will probably help with the sound since all your subs won't be equidistant from the ceiling. If you raise both of them to the same level, then this point is moot. Just my opinion and I think no one can state with any certainty whether or not it's a good idea. At least with respect to how it will sound. if you do build a platform, make sure you put a decent lip around it. You don't want your subs to vibrate off the edge.
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post #11 of 36 Old 09-14-2013, 06:14 AM
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Originally Posted by defmoot View Post

FWIW, I frequently read that elevating a sub is not recommended practice, but rarely do I get a good explanation as to why. Then I read a Geddes paper describing a method for getting good bass response that involved three subs where the third is to be raised off the floor, iirc. And not only that, the idea he was positing was to INCREASE the number of modes in-room, not eliminate them as is usually proposed.
You can't eliminate modes. What you want to do is introduce many modes, at different frequencies, each with small effect, rather than just a few, each with large effect. If all the subs are on the floor then they'll all excite the same vertical mode, just as having two in the front of the room will excite the same mode off the wall in the rear of the room. Sound waves operate in three dimensions, and that's how you have to view them with respect to the room boundaries.

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post #12 of 36 Old 09-14-2013, 09:17 AM
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Originally Posted by Bill Fitzmaurice View Post

You can't eliminate modes. What you want to do is introduce many modes, at different frequencies, each with small effect, rather than just a few, each with large effect.

This is as succinct a description as I've come across, and at odds with the way I've interpreted a lot of what I've read. Or perhaps what I've read wasn't expressed very accurately. Okay, I'll take the blame. biggrin.gif

In the Audyssey thread, there is an excerpt from a Toole paper describing the placement of a sub in a null to smooth (?) the response for a particular frequency. I read this to mean "eliminate" but I can see now that the subtlety in my interpretation had greater implications than I understood at the time.

That modes occur in three dimensions should be pretty intuitive even if it's not always stated explicitly. I guess it's enough work to even out the amplitudes of modes front to back and side to side, let alone vertically. Having good bass across a row of theatre seats is probably enough for most enthusiasts. Maybe this.is the source of comments regarding "not raising the sub off the floor." Trying to grok that third dimension in practice may be too headache inducing, or practical, for that matter.

Greddes, using the method he described, indicated decreasing benefit from the addition of the number of sources (subs) beyond three. Three subs, three dimensions. Kind of an elegant way to approach the problem.

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post #13 of 36 Old 09-14-2013, 01:33 PM
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2 - 4 sub is needed to get a smooth response in a room. More than 4 subs has no real benefit for HT in most cases. As was stated earlier, there is some benefit to a sub being elevated off the floor. If done right, bass traps can be made to go under the shelving to help clean up the sound if needed.

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post #14 of 36 Old 09-14-2013, 03:12 PM
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Quote:
Originally Posted by defmoot View Post

In the Audyssey thread, there is an excerpt from a Toole paper describing the placement of a sub in a null to smooth (?) the response for a particular frequency. I read this to mean "eliminate" but I can see now that the subtlety in my interpretation had greater implications than I understood at the time.

Grrr... Just saw this. 'Supposed to read: "but given the above my interpretation is likely wrong, and the implications less subtle than I would have thought."

Just me trying to edit on one of these e-readers, scratching away fat fingers and all instead of using a real keyboard, and not using the preview function. Really slows down the thinking, too. mad.gif

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post #15 of 36 Old 09-14-2013, 03:51 PM
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Quote:
Originally Posted by defmoot View Post

In the Audyssey thread, there is an excerpt from a Toole paper describing the placement of a sub in a null to smooth (?) the response for a particular frequency. I read this to mean "eliminate" but I can see now that the subtlety in my interpretation had greater implications than I understood at the time.
So other can see what you're describing:



Toole does use the word "eliminated".

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post #16 of 36 Old 09-14-2013, 06:10 PM
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A sub falling 4 feet is going to kill someone.
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post #17 of 36 Old 09-15-2013, 12:36 AM
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Sanjay, would it be possible for you to provide a link to the Toole paper?

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post #18 of 36 Old 09-15-2013, 07:02 AM
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Originally Posted by sdurani View Post

Toole does use the word "eliminated".
Not correctly. The modes remain, they're only eliminated if there are no boundaries. Their effect is not eliminated with multiple subs, but to a great degree alleviated. His figure isn't all that accurate either, as it shows only one frequency, but it need not be to get the point across. A more accurate representation of the effect of adding subs would be to start with a full plot of the response of one sub at the LP, then that with two, three, four, etc. You'd see a gradual smoothing of response with each added sub.

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post #19 of 36 Old 09-15-2013, 09:41 AM
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Originally Posted by defmoot View Post

Sanjay, would it be possible for you to provide a link to the Toole paper?
Sure:

http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White%20Papers/LoudspeakersandRoomsPt3.pdf

The snippet I posted is from page 14.

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post #20 of 36 Old 09-15-2013, 09:56 AM
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Originally Posted by Bill Fitzmaurice View Post

A more accurate representation of the effect of adding subs would be to start with a full plot of the response of one sub at the LP, then that with two, three, four, etc. You'd see a gradual smoothing of response with each added sub.
Where would you put the subs? For example, where would you start the first sub? And would you move it when you introduced the second sub or leave it in place to build on the response of the first sub?

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post #21 of 36 Old 09-15-2013, 11:23 AM
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Quote:
Originally Posted by sdurani View Post

Sure:

http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White%20Papers/LoudspeakersandRoomsPt3.pdf

The snippet I posted is from page 14.

Thanks!

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post #22 of 36 Old 09-15-2013, 01:52 PM
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Originally Posted by sdurani View Post

Where would you put the subs? For example, where would you start the first sub? And would you move it when you introduced the second sub or leave it in place to build on the response of the first sub?
I'd put the first sub where it gives the best result at the LP. I'd put the second where it does the best job of smoothing response at the LP, the same for any additional. By all means use guidelines to aid in the process, but remember that guidelines are only that, and aren't definitive for every room, or even the same room if you move the LP.

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post #23 of 36 Old 09-15-2013, 03:00 PM
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Originally Posted by Bill Fitzmaurice View Post

I'd put the first sub where it gives the best result at the LP. I'd put the second where it does the best job of smoothing response at the LP, the same for any additional. By all means use guidelines to aid in the process, but remember that guidelines are only that, and aren't definitive for every room, or even the same room if you move the LP.
Thanx. That's like the approach Geddes used to recommend. By comparison, the Harman braintrust (Toole, Welti, et al) typically recommends placement at 'mode cancelling' locations (if you believe in that stuff), which tend to be even divisions (halves, quarters) of room dimensions. The idea being that placing a subwoofer in a modal null will cancel that mode (again, assuming you believe that can happen). Hence the snippet from Toole's paper recommending placement at the quarter points of room width (null locations for the 2nd width mode).

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post #24 of 36 Old 09-16-2013, 04:44 AM
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Originally Posted by sdurani View Post

Thanx. That's like the approach Geddes used to recommend. By comparison, the Harman braintrust (Toole, Welti, et al) typically recommends placement at 'mode cancelling' locations (if you believe in that stuff), which tend to be even divisions (halves, quarters) of room dimensions. The idea being that placing a subwoofer in a modal null will cancel that mode (again, assuming you believe that can happen). Hence the snippet from Toole's paper recommending placement at the quarter points of room width (null locations for the 2nd width mode).

That Harman placement only helps at specific frequencies. And the room has to be ideal in that it's rectangular with no openings and no objects in the room.

I would follow Bills advice and place the first one at a location that sounds best at the primary seating location and then place the second so that the response is even. Where those locations are can really only be found through experimentation. A good way to determine the best spot is to place the sub where you sit and walk around the room and see where in the room the sub sounds best. In reality, you can't place the sub anywhere so go to the few possible locations and see which one sounds best. Not a perfect solution, but a good starting point.
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post #25 of 36 Old 09-16-2013, 11:58 AM
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That Harman placement only helps at specific frequencies.
True, that placement only helps at resonant frequencies, where you'll find peaks & nulls. No reason to help at frequencies that don't need help.
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And the room has to be ideal in that it's rectangular with no openings and no objects in the room.
Doesn't have to be ideal, as long as there are parallel walls between the listener and subs. Frequencies typically in the subwoofer range have wavelengths large enough to go around most furnishings, so I don't see why there needs to be "no objects in the room".

Strange for me to bounce between this thread and the REW measurement thread in the Audio Theory section of this forum. The same technique that folks here believe doesn't work (mode cancelling by subwoofer placement) is used routinely in the other thread to keep modes from being excited (in rooms with asymmetrical openings no less). It's like meeting people that tell you that plants can't flourish indoors but then you visit people's homes that have plants flourishing indoors, even when placed away from a window.

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post #26 of 36 Old 09-16-2013, 09:57 PM
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Originally Posted by sdurani View Post

Strange for me to bounce between this thread and the REW measurement thread in the Audio Theory section of this forum. The same technique that folks here believe doesn't work (mode cancelling by subwoofer placement) is used routinely in the other thread to keep modes from being excited (in rooms with asymmetrical openings no less).

I'm thankful you did. Information was exchanged and people can follow it as they see fit, no matter their knowledge level, experience, or beliefs. The REW thread, and Audyssey thread for that matter, are big and a little unwieldy to keep up with sometimes, at least for me. It's a time thing. This thread, as it stands, is a nice little primer on the competing theories.

Thanks to all who constructively contributed.

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post #27 of 36 Old 09-17-2013, 05:50 AM
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Originally Posted by sdurani View Post

Doesn't have to be ideal, as long as there are parallel walls between the listener and subs. Frequencies typically in the subwoofer range have wavelengths large enough to go around most furnishings, so I don't see why there needs to be "no objects in the room"

The wavelengths can't go around solid objects completely unaffected. Maybe if the object is porous it may go through it somewhat. I do agree that at sub frequencies the effect of furniture, wall treatments, etc... Is less so than with other frequencies since sub frequencies tend to pressurize rather than excite the air molecules.

Their models don't take into account room openings and anything being in the room. The effect of these may or may not have much effect on optimum placement depending on the particulars.
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post #28 of 36 Old 09-17-2013, 10:16 AM
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Originally Posted by sdurani View Post

Frequencies typically in the subwoofer range have wavelengths large enough to go around most furnishings, so I don't see why there needs to be "no objects in the room".
.
+1. For an object to be an obstacle it must be a goodly percentage of a wavelength in size. An 80Hz wavelength is 14 feet, a 20Hz wavelength 56 feet.

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post #29 of 36 Old 09-17-2013, 11:14 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post

+1. For an object to be an obstacle it must be a goodly percentage of a wavelength in size. An 80Hz wavelength is 14 feet, a 20Hz wavelength 56 feet.

If you're talking about an object by itself, say a sofa, in an open field, then what you state is true in that if you're sufficiently behind the sofa, it will sound the same as if the sofa was never there. But most rooms have walls and many pieces of furniture and people are sitting on the sofa, which makes things different. For example, if you're sitting on the sofa and the sub is behind you so the sofa is between your ears and the sub, the sofa will definitely effect what you hear from that sub.

I'm not saying having objects in the room will have a huge effect. It's just one thing that separates the room conditions in the paper from a real room. Not the biggest thing, but something none the less.

Here's a link that explains this in more detail...
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html
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post #30 of 36 Old 09-17-2013, 12:28 PM
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Quote:
Originally Posted by KidHorn View Post

If you're talking about an object by itself, say a sofa, in an open field, then what you state is true in that if you're sufficiently behind the sofa, it will sound the same as if the sofa was never there. But most rooms have walls and many pieces of furniture and people are sitting on the sofa, which makes things different. For example, if you're sitting on the sofa and the sub is behind you so the sofa is between your ears and the sub, the sofa will definitely effect what you hear from that sub.

I'm not saying having objects in the room will have a huge effect. It's just one thing that separates the room conditions in the paper from a real room. Not the biggest thing, but something none the less.

Here's a link that explains this in more detail...
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/diffrac.html

Take a look at the first paragraph and the last. It appears that objects you would find in most rooms will have little to no impact on LF. Do I have that right?

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