Home Theater Acoustics: Subwoofer Placement

[Eggybasket]

Hi all,
I'd like some advice on subwoofer placement for new home theater.
It is a downward firing sub, with the port also being downward and has plastic/rubber feet.
It is currently placed on carpet, which is over wooden floorboards as it's on the second floor. I'm trying to minimise the vibrations in the floorboards since these are very annoying for anyone underneath.
I will be ordering a Subdude, which is an isolation platform consisting of velour covered MDF on top of some platfoam.
I've also heard advice saying that Granite can help isolate the sub. If I got some Granite, would this be better placed above the Subdude (sandwiched between the subdude and subwoofer) or below (sandwiched between the subdude and carpet)? The Granite will also have feet attached.
Any advice appreciated :)
Thanks.

 

[sophiecentaur]

Hi. Welcome to PF.
The wavelength of the sounds from a subwoofer is so low that position in the room can't have much of an effect, imo. The purpose of a S/W is mainly to give an 'effect', rather than to get high fidelity. Nothing wrong with that but I am being realistic.

If you want the 'best' results from your S/W, then I would advise you to ask the designers the test conditions (if any) that were used in the development. The acoustics of your average living room make it a bit pointless to get too fastidious about the sound systems you use. If you want to spend loadsamoney on the equipment then you should really expect to spend even more on having the room properly acoustically treated and have suitable furniture and furnishings. I always reckon that, with Home Cinema, you should just lay back and enjoy it, rather than worry overmuch about quality. For a start, the signal processing that gives you the surround sound is not likely to give you accurate images.

 

[nsaspook]

The proper room treatments for "surround' home theater are different from a 'music' room because you want to reduce reflections to create a 'dead room' to improve imaging from each separate speaker. Dealing with SW vibrations is something that should be included in a proper design.

Sound Isolation link: http://www.audioholics.com/room-acoustics/a-guide-to-sound-isolation-and-noise-control
Subdude: http://www.avsforum.com/forum/113-subwoofers-bass-transducers/644330-auralex-subdude-owners-thread.html

 

[olivermsun]

The wavelength of the sounds from a subwoofer is so low that position in the room can't have much of an effect, imo. The purpose of a S/W is mainly to give an 'effect', rather than to get high fidelity. Nothing wrong with that but I am being realistic.

The excitation of low-frequency room modes can be a significant factor in the 'heard' sound at different locations in the room. Addressing this problem in some 'optimal' (or, perhaps, 'less-bad') way has been an active research topic over the past few decades.

Experimenting with placement of the sub (preferably not right in the corner) could potentially help the OP with both the room modes and also with loose floorboards.

 

[nsaspook]

The excitation of low-frequency room modes can be a significant factor in the 'heard' sound at different locations in the room. Addressing this problem in some 'optimal' (or, perhaps, 'less-bad') way has been an active research topic over the past few decades.

The Harman paper is one of the classics on the subject.

 

[sophiecentaur]

The Harman paper is one of the classics on the subject.

That paper was interesting but I was a bit disappointed at the amount of simulation compared with actual listening tests. How many rooms would actually correspond to the idealised room? In real life there is a massive range of wall and floor structures with many different impedances and resonances. I reckon it would be very difficult to predict what would be best for a random householder's listening conditions.

I liked the comments about the relevance of SW system linearity. That could be very relevant to the perception of lf sounds.

Overall, the paper seems to be typical of 'seller's literature' - encouraging people to spend as much as possible on the system, after which, they hear with their wallet.

 

[enorbet]

Since this is a real-world problem please allow me to respond with some real-world advice. I submit that the highest priority is isolation, IOW, keeping your neighbors happy and friendly, not to mention avoid breaking any covenants regarding sound and time of day. Community associations only rarely site actual SPL and leave it up to people's ears (and sometimes, sensibilities) to be the arbiter.

Your problem is twofold in that not only can low frequencies be transmitted as OP seems to understand by direct contact with a less solid than granite floor, but also once the air in your closed space begins to vibrate at 40Hz and below, it will create sympathetic vibrations in the walls and ceilings, some of which may cancel but others which may amplify in the oddest and most undesirable places.

I sincerely doubt you (or your neighbor) will hear much difference between granite/carpet placement, especially since your SW apparently has some isolation of it's own via the feet. For ease of installation I would just place the slab on top of the carpet and the SW on the slab. You can always experiment by adding absorbent materials on top of that "sandwich" to isolate the SW from the slab and the slab from the carpet, but I doubt that will be needed or have significant effect.

With all practicality in mind I urge you to first talk to your neighbor. Explain that you are a music lover and have installed a SW but that you also like good relationships with your neighbors and wish to confer with them with your results. Get them to see your friendly concern. Make them feel some part of the project. Make a bargain that they don't go immediately screaming to authorities and simply talk to you since you will do whatever is necessary to be a good neighbor. Occasionally you will be cursed with a neighbor that just can't be satisfied but this "prep work" is your best defense and tool to get at least close to what I assume you want - awesome sound!

One problem that faces you is that you probably don't have proper instrumentation such as a Spectrum Analyzer or Fast Fourier Transform equipment so you can't predict standing waves or even frequency response all throughout your abode. Don't try to get it to sound awesome everywhere. Pick a common listening area and focus on that. Getting the sound "right" without instrumentation in an apartment is a daunting job and will take much trial and error. Just spread that good will before you spread the sound.

 

[AlephZero]

I suspect the main effect of putting a down firing sub on a smooth reflective surface like granite is that it will reflect more sound energy into the room instead of absorbing it into the carpet underneath the sub.

Potentially, it will also create a standing wave between the sub and the reflector which will screw up the flatness of the frequency response and "color" the sound with some relatively high frequency nonlinear distortion. But hey, home cinema subs are more about making a lot of noise than about hi-fi.

To stop low frequency sound from being transmitted through the building structure, you will need a lot more than a bit of granite, or an expensive-looking box made from 3/4 inch MDF and filled with plastic.

 

[dschlink]

The length of the sound-waves put out by a sub-woofer make them very difficult to block. As an example, at 50 Hz the wavelength is 6-7 meters. That's the main reason that the placement of a sub-woofer in a room isn't too critical. The amount of energy going into the air far exceeds the amount transmitted through the feet; so hanging a sub-woofer from your ceiling would probably help the most; that way the amount of energy hitting a any section of the floor would be minimized.

 

[Eggybasket]

Thanks for the responses guys. My wording may have been misleading, I am not so much looking at where I should place the Sub in the room, I have decided this already. I am also not super serious about spending loads of money on fitting out the room for optimum sound. My issue is just the simple matter of the floorboards vibrating.

@enorbet , Thanks for your reply, I should have clarified, I am in a detached house and it is the rest of my family I am trying not to disturb. For this reason, the low frequencies are not so much the problem, instead it is the rattle of the floorboards that can be annoying for those below. I think I will do similar to what you mention, placing it on the slab, and then experimenting with other materials in addition to this before buying a Subdude or similar.

@Alephzero , From what I've heard from other people on the AVForums, it is as you say, that the Slab reflects more of the sound to the room with less of it being transferred to the floor. I have not heard anyone mention any negative aspects to this such as distortion but I think I follow your thought. I appreciate that I will not stop any of this transference, but my aim is to limit it as much as possible. I also appreciate that the Subdudes are expensive for what they are and may not bring about much more in the way of isolation when paired with the Slab. But if I were to purchase both, I was wondering about which stacking order of the slab/subdude would be most efficient.

 

[rcgldr]

so hanging a sub-woofer from your ceiling would probably help the most

I'm old enough to recall this is how discos / night clubs with high end audio systems dealt with with the powerful speakers (the good ones had acoustic suspension 15" woofers with a very tight base sound (plus 6" or so mid range speakers and dome or horn type tweaters)). The speakers were suspended by chains from beams on the ceiling and located away from walls, and the speaker cabinets were padded inside and outside.

Since the main concern is the sound traveling to the floor below, suspending the sub-woofer from beams in your ceiling may be a good idea, if there's a location for it that's convenient and a way to mount suspension cables to the sub-woofer.

 

[dschlink]

Re stacking order: put the heaviest layer closest to the sub-woofer. The higher mass will move less and therefor transmit less energy to the next layer.

 

[enorbet]

AlephZero made a good point regarding standing wave problems with down firing SWs. I didn't address that issue as I was more concerned with your "neighbors" first. While some manufacturers claim to have researched speakers that are expected to employ reflections, this is generally malarky and the only serious ones, like corner horns, have precise specs as to placement. It is probably worthwhile to try the SW flipped on a side to eliminate that particular standing wave issue. This will likely sound better and may, if Murphy is engaged elsewhere (odds of that are pretty high since he apparently often hangs out at my house <grin>) serve to reduce a resonant spike that presently increases floorboard rattle.

 

[olivermsun]

I don't expect the forward-facing arrangement to be that different from the down-firing sub for the formation of modes, just due to the wavelengths involved—the entire speaker baffle width is surely only a small fraction of a wavelength (> 5 m below 70 Hz)! On the other hand, the OP would lose the use of the nice built-in feet.

The vertical coupling of the cone to the floor board might be more of a problem when down firing vs side/forward firing, but this would tend to be an issue mainly when the sub is playing very loud (big excursions of the moving mass!). Your family downstairs would surely be hearing the huge BASS coming from upstairs and transmitted through the vents, the floors, the frame of your house, etc. :big grin:

In that case, maybe the granite slab would help after all. I'm not sure having the heaviest layer closest to the sub (as dschlink suggests) is really the goal, but you definitely want to couple the sub tightly to as much mass possible.

 

[enorbet]

Aside from obvious option of moving the feet, the more important issue is close proximity reflections. Yes, of course it is accurate that at 35Hz the full wavelength is 32 feet, roughly 10 meters. Just because few homes have a listening room with such dimensions does not mean 35Hz is inaudible or useless (or problematic) in common room sizes.

Perhaps few here have had the distinct pleasure of hearing one of these gems

Here's a detail of a derivative enclosure showing the spacers used to get a precise distance from a reflecting surface, preferably a corner

These, I assure you, could and did produce extremely serious frequency response below 35Hz with even moderately decent amplification since the sensitivity is 105db 1W/1M.

Granted, my concept of "decent amplification" may be a bit jaded since I still prefer valve/tube amps and even though it looks like a phasing problem requiring some tweaking I have to admit I'd REALLY like to experience THIS!

It is certainly easy and useful to try the SW on it's side to see just how much orientation and reflection affects not only sound quality in the desired location, but levels in the undesired locations as well. I hope we hear progress updates.

 

[olivermsun]

Perhaps few here have had the distinct pleasure of hearing one of these gems…

Yes, indeed I have. Fun stuff!

 

[nsaspook]

That paper was interesting but I was a bit disappointed at the amount of simulation compared with actual listening tests. How many rooms would actually correspond to the idealised room? In real life there is a massive range of wall and floor structures with many different impedances and resonances. I reckon it would be very difficult to predict what would be best for a random householder's listening conditions.

I agree that the papers idea of a idealized room and what's usually available for a typical Home theater will differ but there are some low cost and simple treatments that can make it sound much cleaner if you have at least two subs that are not placed symmetrical so the peaks and nulls don't overlap. Corner traps, wall panels, thick carpet or rugs with lots of cushions on furniture for absorption to damp room modes and reflections to make the frequency response as flat as possible without electronics, then a good parametric equalizer to bring down the peaks. I made corner traps from concrete forms stuffed with fiberglass, reflection panels from thick cotton batting covered with cloth and large wall panels from sound board suspended on tracks from the walls. I don't think I could have done it in the living room but my 'Mancave' theater with almost 2kW of audio power is a different story for experimenting over the years. Lucky for me the room is mainly isolated from the main house by a garage.

https://www.flickr.com/photos/nsaspook/sets/72157645387002841/

Most of the guys who are really serious about home theater sound isolation but have a pre-built space build a room inside a room with at door seal system that's an airlock for sound. That can get mighty expensive if you need total sound suppression but some techniques can be used to reduce escaped sound.

http://www.soundproofingcompany.com/soundproofing101/room-within-a-room/
http://www.soundproofingcompany.com/soundproofing101/flanking/

 

[verty]

The obvious solution is to place the sub on its side, the vibrations will be parallel to the floor and will cause no vibrations. And probably it'll sound better besides. I think any down-firing arrangement is going sound like an echo chamber on the floor below.

Lay it on some of those cheap rubberized carpet tiles perhaps. I'm thinking that two tiles sandwiched with the rubber side outwards would be perfect.

 

[verty]

I want to add that it may seem that putting the subwoofer on carpet tiles is a bad idea because there would be some movement, the sub wouldn't be kept rigid, and we know that many speakers have needle-like feet to eliminate vibrations. So wouldn't this make the sound worse?

Probably not and it may make the sound better. The main reason is that most subwoofers will play higher frequencies more loudly, this is the woody or punchy sound that one often hears from home audio subs. And the slight give in the carpet will affect higher frequencies more, having a greater dampening effect with higher frequencies. So if anything, this will be slight corrective to that skew on the frequency response. Also our ears are more sensitive to higher frequencies, so even if the response is flat, dB(A) weighting will be skewed towards higher frequencies, so it can improve the sound.

And having the sub play sideways could mean that there is less distortion. The main concern is how it'll look lying on its side, but you can easily put a screen in front.

 

[sophiecentaur]

From most of the comments I read, it seems to be assumed that the SW you buy has not been 'designed' and that you can do better by tinkering with it. I realize that a cheapo version will just have been built to look nice but are there not decent SWs available that have been designed by someone who knows about these things? These should come with recommended installation information and it's not likely that your average user could improve on that. It isn't hard to squeak a room and to get a reasonable idea about low frequency response. If you risk squeaking your front room + speakers at higher frequencies, you are unlikely to be impressed with this objective assessment. HiFi descriptions are a bit like the legendary Eskimo words for snow.

 

[nsaspook]

It isn't hard to squeak a room and to get a reasonable idea about low frequency response. If you risk squeaking your front room + speakers at higher frequencies, you are unlikely to be impressed with this objective assessment. HiFi descriptions are a bit like the legendary Eskimo words for snow.

I know the thread is mainly about isolation but usually that goes hand in hand with room response in small rooms.

Yes, nothing can compete with simple testing to generate real data that can be used to evaluate speaker/room response. If you're willing to invest thousands for HiFI, why not invest a few hundred in the tools needed to make it sound its best.

A simple sound meter with line out, a computer with a suitable sound card and software is all you need.
Low frequency response of a 12" SW install using ETF5 software

 

[AlephZero]

I suspect the main motivation for designing down-firing subs is to minimize accidental damage from inquisitive kids, etc. Not to mention pet mice etc thinking the port tube is a nice place to live!

The bottom line is that you can't get a reliable flat frequency response below the fundamental resonance of the room (determined by its size) whatever you do. Any attempts at a "theoretical" design solution will be defeated by the fact that the room is not airtight. Even small openings can have a large effect on the resonance frequencies and sound energy distribution through the room. And if you can hear low frequency sounds from outside coming through closed windows, the window is effectively a "hole in the wall" so far as the low frequency acoustics are concerned.

If you don't have issues with neighbours, a good solution is to use a wall or ceiling as an genuine "infinite baffle".

Here's a picture of an 18 in speaker used like that:

And the measured frequency response in the room:

As Enorbet's pictures show, if you want real low frequency sound you need to move a lot of air around. The speaker efficiency is roughly proportional to the square of the area. So an 18 inch speaker is about five times more efficient than a 12 inch, aside from the fact that its self-resonant frequency will probably be lower as well.

Cheap and cheerful "small" subwoofers try to repeal these laws of physics with a design that has a huge peak in response at one (low) frequency, which meets the basic criterion of making a lot of noise, but at the expense of everything else.

 

[nsaspook]

Bigger is better but be sure it fits in the door.

 

[Eggybasket]

Thanks for all the suggestions, got more help than I expected. I will be unable to experiment and give feedback for a while though but will post up when I do.