Post by fbczar on Jan 14, 2016 11:25:35 GMT -5
Standing waves are indeed a property of both the room and the speaker. The location of room modes (standing waves), and where the nulls and peaks occur, depend on the dimensions of the room itself, and really aren't going to move unless you move the walls. The solution there is to move your chair. However, the way the energy from your speaker is coupled into that room changes when you move the speaker. The reason you get more bass when you put a speaker in a corner is that doing so improves the coupling between the speaker and the air - it makes the speaker more efficient - and that change has more effect at lower frequencies (the corner basically acts like a horn and improves the coupling between the speaker cone and the air in the room). So, basically, you can't change where the standing waves occur in a room; but, by moving the speaker, you can affect how much energy it "pumps" into them, and at what frequencies, and corner placement provides for the best efficiency in transferring energy into the room at all lower frequencies. (The reason many speakers sound really bass-heavy when located in a corner is simply that you have to account for the coupling when you design a speaker. So, you can design a speaker to be flat floating in the middle of a room, or against a wall, or in a corner, but no single design will be flat for more than one of those options.... so either you have to provide some sort of adjustment that allows you to compensate for placement, or you have to position the speaker more or less where it is intended to be - in terms of how many major surfaces it's near. However, overall, you will get the MOST bass if you put the speaker in a corner, which makes that a reasonable starting point. )
The one thing I don't agree with is your comment about "doubling the distance between the listening position and the sub reducing the perceived output by 6 dB". In an open space, with no walls, that is basically true - and at all frequencies. If you shine a typical flashlight on a wall you get a round spot of a given brightness, double the distance and the spot will be twice the diameter (because it has further to spread out), but it will be 1/4 as bright at any given location (because the same power is now lighting four times as much wall area - area is a function of the square of the diameter of the spot). In open space sound acts very much the same way - so the level drops off as you get further from the speaker.
However, in an enclosed room, when the wavelength of the sound wave starts to be of the same magnitude as the dimensions of the room, things STOP working this way, and start seeming more like the pressure of the room is simply going up and down. For reference, the wavelength of a sound wave at 20 Hz is about 55 feet (the wavelength at 10 kHz is about one inch). The overall result of all this complication is that; when the wavelength is much smaller than the size of the room, sound acts sort of like a flashlight beam, spreading out and dropping in level as you get further from the speaker; and, when the wavelength is much larger than the size of the room (subwoofer in a six foot square room), it acts more like pressure, and you'll get almost exactly the same sound level anywhere in the room; and, when the wavelength is similar to the size of the room, you'll get standing waves, in which case whether you happen to be sitting in a node or a null is going to be the controlling factor (if you're sitting at a node, the bass will be loud; if you're sitting at a null, it will be almost gone, and this will almost totally override any difference due to how far you are from the speaker). Unfortunately, because of that huge difference in wavelengths between different frequencies, with most room sizes you have all three of those situations - but at different frequencies. In a ten foot square room, a 10 Hz sound is going to act like pressure, and so sound about the same anywhere in the room; a 10 kHz sound is going to "shine" out of the speaker in some radiation pattern, gradually losing level as you get further from the speaker; and sounds in the 50 Hz or so range are going to excite resonance modes in the room, so how loud they sound will depend mostly on whether you're sitting relative to room nodes and nulls. (And, to complicate matters even more, depending on how reflective your walls are, those higher frequencies are going to bounce around, and the reflections are going to add to the level you actually hear; sort of how that flashlight beam would act if your walls were lined with tinfoil.)
However, none of this has anything to do with sound LOCALIZATION.....
Localization is simply based on the fact that the ability of our human ears and brains to tell what direction a sound is coming from isn't very good below about 100 Hz (the details about why don't matter). So, if we have a subwoofer producing only sounds below about 80 Hz, we can theoretically put it anywhere, and a human listener won't be able to tell where it is. (And, if we have a complex sound, with components above and below that frequency, as long as the components above that frequency come from the correct location, we won't notice that the low frequencies are coming from somewhere else - and we'll perceive that sound source as coming from where the higher frequency components are positioned.
The main problems people usually have with sound localization are usually based on simple physical and practical limitations:
1) Crossovers are gradual. Let's assume you have a subwoofer with an 80 Hz crossover at 24 dB/octave... Assuming the subwoofer itself was totally flat, it would be flat from 0 Hz to about 60 Hz, about -3 dB at 80 Hz, and -27 dB at 160 Hz. As you can see, if you have a really loud sound at 160 Hz, a significant amount of it will in fact still be coming from the sub. Since your ears can localize sound at 160 Hz to some degree, that means that the part of it "leaking" from the sub will "interfere" with the part coming from the main speaker (it will sound like the sound is coming from NEAR your main speaker, but that apparent location will "pull" a tiny bit towards the sub). Note here that, in a home theater system, the information in the LFE channel should only consist of stuff that can cheerfully be handled by the sub, so you shouldn't have to worry about limiting it with a high-cut filter. However, for the information sent to the sub from your other channels, you want to make sure to avoid sending frequencies where the "wrong" location will be audible. This is why bass management usually defaults to 24 dB/octave - because it's been found that, at a crossover frequency of 80 Hz, 24 dB/octave is sharp enough to prevent this from being a problem. (Which explains both why choosing crossover frequencies above 80 Hz or 100 Hz is usually a bad idea, and why choosing a crossover slope lower than 24 dB/octave - like 12 dB/octave - is usually not a great idea - at least unless you use a lower crossover frequency.
2) Subwoofers themselves often make extraneous noises. Sometimes you can hear the cabinet itself vibrating; sometimes you can hear distortion from the driver; sometimes you can hear air blowing and chuffing through the port. If any of these things happens, then those sounds are audible and, since they only happen when the sub is playing loud, they draw attention to it.
3) Sometimes you can hear the floor itself, or the walls, or the pipes in the walls, vibrating or rattling. Now, how much or a problem this is sort of depends on the specific situation. If you're watching a scene in a building, and good old T-Rex walks by the window, it's sort of natural to hear a few rattles coming from the room - so you probably wouldn't notice. However, if you're watching T-Rex stomp by a mossy log, deep in the woods, the rattle of a nearby pipe is going to be very out of place - and very distracting. (And, depending on the specifics, it may simply be distracting, or it may trick your mind into thinking that the low frequency sound that's supposed to be coming from somewhere else is coming from the sub.)
One final note, and one that many people seem to forget, is that you should NOT be able to "hear" a sub. A sub should make what you're listening to sound "solid", and it should sound like "something's missing" when you turn the sub off, but you shouldn't be able to hear where the sub is located, or even be able to tell for sure that there is a separate speaker making low bass - if you can, then either it's turned up too high, or it's otherwise configured wrong. (A standard bit of guidance is "turn your sub up until you can just notice that it's there - then turn it back down a tiny bit".)
In general, a corner is the best place to begin the process of determining the optimum placement of a subwoofer. Closed corner placement excites all room modals which results in the flattest frequency response at all listening positions. A closed corner being defined as a corner with at least 5’ of linear wall on either side of the subwoofer. It is also more likely to yield the best phase and signal timing response. Other positions do not excite all modals, resulting in holes in the frequency response. Of course, all corners are not equal. A corner symmetrically across from a room opening is not ideal, whereas the opposite is.
Closed corner placement lowers bass extension. Closed corner placement maximizes output. Obviously, you want your subwoofer to play as low as it can and as loud as it can with the least effort possible from the subwoofers amplifier. Placing a subwoofer next to a wall will yield a 3db increase in output versus a sub placed out in a room. Placing a subwoofer in a corner will provide an additional 3db increase so placement in a corner yields a 6db increase versus a placement well away from all walls.
Another consideration is distance. Doubling the distance of a subwoofer from the main listening position will decrease the perceived output of that sub by 6db. Halving the distance from the listening position to a subwoofer will result in a perceived increase in output of 6db.
In general the best listening position is usually about 1/3 of the way into a room from the back wall. Therefore, the best place to start with subwoofer placement is one of the two corners behind the listening position because those corners are the closest ones to the listening position.
When you consider that doubling amplifier power only results in a 3db increase in output it is obvious that subwoofer placement is critical.
Placing the subwoofer in the closest closed corner to the listening position will allow the subwoofer to play as low as it can and as loud as it can while allowing the subwoofers amplifier to work with less effort. All you need to do after such a placement is adjust your subwoofer levels at the subwoofer or with your processor to achieve the balance and output levels necessary to provide a non-localizable result.
If you have access to something like Dirac you should be cooking with gas.
In fact, Dirac is a real key to outstanding subwoofer integration. Of course, bass traps and other room treatments are all part of achieving the best result with a subwoofer.
I use stacked Kreisel Sound DXD-12012 subwoofers in a back corner nearest my listening position, equalized with Dirac. The subs are totally impossible to localize for home theater and stereo music. Stacking the subs results in a 6db increase. Increasing the overall height of the subwoofer may also be a positive factor depending on your room.
Tom Nousaine and Ken Kreisel have both written extensively on subwoofers. Check them out.
