The placement of one or several subwoofers.
By Ingvar Öhman.
First published in "Musik och Ljudteknik", no.1, 1997, Sweden.
The loudspeaker is usually considered the weakest point in the HiFi chain. There may be many good reasons to claim that a loudspeaker is the link with the most influence on the sound. Consequently loudspeakers could be the dominating investment in a qualified music reproduction system, but if you move forward in the chain you will find yet another link; the room - a link at least as important to consider as the loudspeaker, maybe even more so. Regardless of the order of importance between the speaker and the room, it is obvious that as well as the choice of a loudspeaker should be made with the greatest care, the same is valid for the attention to the acoustics of the room and the proper placement of the loudspeakers therein. This article addresses the problem of finding the best placement for a subwoofer.
lnterference and resonance
There are two phenomena to consider when you chose a place for a subwoofer, one is interferences, which in this case means that the direct radiation from the subwoofer is mixed with reflections from adjacent surfaces (especially floor, rear wall and side wall), in or out of phase. What is coming in phase with the direct radiation will reinforce the sound and what is coming out of phase will weaken or cancel the sound.
A good starting point is to try to get as many reflections as possible in phase (preferably all three; floor, rear wall and side wall), to arrive in phase with the direct radiation for the entire working range of the subwoofer (typically 20- 80 Hz). This means that no path via a reflecting surface should be more than a quarter of a wavelength longer than the direct path.
I practice, this means that no adjacent surface should be at a longer distance than 1/8 of a wavelength from the driver in the subwoofer. (Since the sound then will travel a quarter of a wavelength to go there and back). The wavelength at 80 Hz is 340/80 = 4,25 meters, and 1/8 of a wavelength becomes 53 cm.
The subwoofer driver should be placed within this 1/8 wavelength sphere.
A subwoofer can be considered corner placed for the entire working range if the driver is placed less than 53 cm from the corner.
For a difference of sound paths of 1/3 of a wavelength (i.e. a placement 1/6 wavelength from a wall, 71cm) the intersection between reinforcement and cancelling, i.e. the sound pressure from the driver together with the reflected sound is no stronger than the sound pressure from the driver itself. We have no support from the wall, but no weakening or cancellation either. For distances greater than 71 cm the summed sound pressure driver +- wall reflection becomes weaker than from the driver alone. All applies for the frequency of 80Hz. For lower frequencies the walls will give support at inversely proportional longer distances from the surfaces.
The second phenomena to consider are room resonances. The "best" way to excite room resonances is to place the loudspeaker close to a corner. Corner placement may initially appear the worst possible place considering room resonances, but this is to jump to conclusions. By choosing a corner placement you excite all existing resonances in the room and the frequency response will not become appreciable inferior compared to a completely free placement. The share of "true" direct radiation versus stored energy in the room resonances will in fact remain in the same magnitude as for a properly freely placed loudspeaker, but it really takes that the subwoofer is close to the corner in the entire working range, because when the reflexes from the corner begin to arrive in opposite phase at higher frequencies, the sound will be immensely distorted. A full range speaker should never be placed in a corner.
A full range loudspeaker placed in a corner generally sounds terrible.
Multiple subwoofers are easier to place
By using two subwoofers instead of one, and placing them in opposite corners on the same distance from the listener, (it is best done in each corner behind the main speakers) there will be an obvious reduction of the room resonances, while the directly radiated sound pressure will increase. This is because two corners in a room will have opposite phase effects of the fundamental (half wavelength) room resonances acting in this direction.
If we have two subwoofers placed on the floor against the front wall the horizontal standing wave will disappear. If we have two subwoofers in the same horizontal direction instead, but one placed on the floor and one up against the ceiling, the vertical standing wave will disappear. If we have four subwoofers (down left, up left, down right, up right) then we avoid both the horizontal and the vertical standing waves in those two directions (I then assume mono connected subwoofers). If you want to lessen the action of even higher order multiple standing waves, i.e. full wave resonance, 1.5 wave resonance and others, even more subwoofers may be added to the system. One example of an excellent placement of four subwoofers is an even distribution horizontally behind the main speakers.
When the four real subwoofers come to an end,
the reflections from the side walls will continue,
creating a line source of infinite length.
A single subwoofer
If you use only one subwoofer, the placement will take some experimenting to get the most out of it. There is a classical method based on a principle saying that the acoustical transmission between two points in a room is equal in both directions. This method consists of placing the subwoofer on the sofa in the best listening position (with the woofer where the head is normally placed. Then you play some good (well mixed, with bass content) recording, and finally you crawl around on the floor to find the place where it sounds the best, because that's where the subwoofer should stand! This method has been cherished in most HiFi magazines, but it has several weaknesses that might be helpful to know.
1. It is not at all certain that what is appointed, almost axiomatically, "the best listening position" in the first phase of the experiment, is really the best. It may even be a disastrous listening place, where bass of good quality is almost impossible to get. Try to rearrange the furniture, it may be rewarding. Avoid listening places in the middle of the room.
2. An unexperienced listener may have a hard time judging what bass sound is best/most true, especially when the parameter of adapting them to the sound of the main speakers, and establish correct completeness is included. To separately judge just a part of something complex is often difficult.
3. How do you know how the recorded bassnotes should sound when they are reproduced correctly? This is maybe the greatest problem with all the manipulated recordings today.
4. When placing more than one subwoofer, the method is useless since you cannot reverse the process, i.e. placing the subwoofer on the sofa and simultaneously be in several places and listen at the same time. It may still be meaningful to listen in one place at the time to check that it does not sound too bad. It is however essential that they are approximately at the same distance from the listening position.
5. The method is completely subjective, and the risk to choose a sound that at the time feels groovy, but eventually, maybe after playing 100 records, appears obviously colored and makes all sources sound the same. And then you have to put on the knee protectors again...
6. The thesis of equal sound transmission in both directions is not true, at least not without reservation! This is because a subwoofer as a sound generator does not have infinitely high mechanical impedance, it will therefore play the room resonances a little weaker when it is placed in the corner itself, than when it is placed in the sofa and we listen in the corner.
As a sum up, I would recommend trying to choose a placement from acoustical considerations/scientifical analysis, a placement you may check out and maybe fine tune with the sub-in-the-sofa method.
A corner placement gives lower distortion!
An interesting side effect of placing the subwoofer in a corner is that the distortion is lowered significantly. The cause for this is that you get a reinforcement of fundamental tones from all the reflections in phase from the adjacent surfaces, while harmonics (which is distortion when it comes to a steeply low pass filtered subwoofer) are lacking the corner support, and become weaker in amplitude compared to the wanted fundamentals. In addition, you also get increased efficiency which means that the input power to the subwoofer can be decreased, which also contributes to less distortion and increased headroom. There are many good reasons to consider a corner placement if you have only one subwoofer.
Integration with the main speakers
The subwoofer shall produce a good sound, not only alone, but together with the main speakers. There are lots of things to consider here. The less problem you want with the cooperation, the steeper filters you should use, because the range where both are contributing together becomes minimized. On the other hand, the demands on precision in filters are proportional to the steepness of the filter.
Commercial speakers with a common passive filter of the first or second order often use components with a tolerance of 10 or 20%. In speakers with a third or fourth order filter you must use components with tighter tolerance or else resulting phase jerks and resonant wiggles in the frequency response will be sure to occur. Problems with an unstable and flat sound image would be the result. The spokespersons for filters of low order often maintain, incorrectly, that higher order filters will give a flat soundstage, but the truth is that the soundstage is not affected at all if the tolerance in the filter is good enough. Higher order filters make heavy demands upon the manufacturing. But you should not forget that they also make less demand on the tolerance of the drivers (which are often worst in the extreme ends of their working range, i.e. the parts cut off by the filter).
If you use passive filters there are also significant problems with the thermical change of voice coil resistance, changes that make it impossible to get a flat frequency response other than at a specific level if you don't use some smart tricks. Passive filters of higher order than third, should not be used under any circumstances if you do not know very thoroughly how filter works and are capable to design for great immunity to temperature changes due to input power heating. This is how it may look if you use a passive high order filter without taking special care of the heating of voice coils:
Frequency response with cool voice coils.
Frequency response with hot voice coils.
How does one connect a subwoofer in the right phase?
When you want to check if the phasing is correct between the subwoofer and the main system, you need, if you lack measuring instruments, to know what to listen for when phase is changed from wrong to wright. The answer is: Much and full-toned, opposed to a hollow/thin sound, as the wrong phase will produce. The best test signal is pink noise, preferably octave band filtered centered to 80 Hz (or to the cut off frequency of your subwoofer filter). One person is sitting close to the subwoofer and connecting and the other in the favourite sofa and listening. When the sound is strongest, the cooperation with the main system is best. You may need to adjust the sound level of the subwoofer compared to the main system when the right phase has been detected, but the relative phase between them is not affected when the relative level is changed, so there is no need to go back and check phasing again. When the phasing is right, all you need to do is to adjust the subwoofer level. If you move the main speaker considerable in relation to the subwoofer, the phasing would have to be rechecked however.
The proper order to do things
My advise is that you first try to find the best possible listening place, avoid the center of the room. Then place the subwoofer, use a lot of experimenting. Now you search the optimum place for the main speakers, often less fine tuning is needed here than with the subwoofer. Then you check what phasing will be best. Finally you adjust the level between the main system and the subwoofer. Ready! Listen and enjoy.
Translated by Per Arne Almeflo, Sonic Design, with permission from the author.
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