In this paper we will discuss how and where to mount your in-wall speakers for best performance. We will explain how the wall cavity can strong impact their performance, and what to do about it.

Maximize the Performance of your In-wall Speakers

David Smith | Snell Acoustics

Maximize the Performance of your In-wall Speakers
by David Smith, Snell Acoustics

In this paper we will discuss how and where to mount your in-wall speakers for best performance. We will explain how the wall cavity can strong impact their performance, and what to do about it.


www.snellacoustics.com


People love in-wall speakers. As whole house systems put audio in every room homeowners are, more than ever, looking for an invisible solution for their speakers. In the past, in-wall speakers were the "poor relation" to conventional speakers, not to be taken seriously. As the performance potential of in-wall speakers rises through better design they can approach the performance of the better freestanding systems, as long as careful mounting rules are adhered to.

In this paper we will discuss how and where to mount your in-wall speakers for best performance. We will explain how the wall cavity can strong impact their performance, and what to do about it.

Where do I put them?

For best stereo effect your in-walls should be mounted at a certain spacing and height. A lot of people will place a pair of ceiling speakers in the kitchen wherever they can fit them with little regard to sound quality. That's all right; there are some locations where you are just happy to get a little background music. In other rooms you may want to listen more seriously and here careful speaker placement will pay off. Stereo effect is strongly determined by the spacing between the speakers. If they are too close the sound will lack dimension and space, too wide and you'll get a "hole in the middle" effect. We recommend that the speakers are from 50 to 60 degrees apart (relative to the listener, when viewed from above). This will be achieved when the spacing from left to right speaker is 85% to 100% of the spacing from the wall to the listener.

The sound will be most natural when the speakers are at a natural height of 40 to 60 inches off the floor. This is in the range of seated or standing ear heights. The temptation is great to make the speakers less visible by placing them much lower or much higher (down behind that cabinet). Resist this if you want to achieve best quality sound!

The rules don't change for multichannel or home theater systems. Front left and front right speakers should be in the same positions as described above and, of course, equally spaced around your television screen. The center channel should be half way between them and just above or just below the screen. Try and get all three front speakers roughly in a line, rather than having left and right above the top of the screen and the center below. Placement within a foot or so of the same height will suffice.

A lot of speakers have some features for the angling of sound. Some have tweeters that pivot as if in an "eye socket". Others have a midrange and tweeter assembly that can be rotated such that the units are vertically aligned. Follow the manufacturers instructions. Snell in-wall speakers use what we call a "Directed Power" tweeter. This is a wide range wave-guide that angles all of the treble 20 degrees inward. Another model, the AMC800 has two woofers and a switch that determines which woofer is the midrange. This feature assures the flattest response in the inward direction of the listeners, irrespective of the systems orientation.

The best position for surrounds is straight left and right of the seating position. This means on the side walls rather than on a back wall behind the listener. (To the sides and a little to the back of the listeners is okay.) Unlike the front speakers, surrounds might sound better if ceiling mounted. There are two reasons for this. Front speakers are called upon to create a definite sonic image. We want agreement between the screen image we see and the sonic location we sense. More frequently, surrounds are called upon to create a diffuse environment to be immersed in. Whether it is the crowd at the baseball stadium or the Turkish bazaar in an Indiana Jones movie, we want to the surround channels to create a broad sound field that is fairly undefined in direction. This is best with surrounds mounted farthest from the listener and at higher angles. In most rooms this comes from mounting the surrounds in a region extending from 2 feet below the ceiling up to just onto the ceiling within a foot or two of the side walls. In other words, close to the junction between side wall and ceiling.

If you are using a Dolby EX or similar setup (with left surround, right surround and center back surround) then mount the center back surround at a similar height and position, straight behind the main listening position.

Effects of the Wall

Conventional (non in-wall) speakers use carefully designed and constructed enclosures. These are specifically built with rigid and heavy walls and carefully specified damping material to absorb the internal sound energy. The enclosure will provide the woofer with the right air volume. It will contain enough absorptive material to absorb the back radiation of the cone. It will also be very non-resonant.

Most in wall speakers use the wall cavity as their enclosure. In the case of our in-wall speaker the cavity of the wall should also absorb and contain sound from the backside of the drivers so that that sound can't cancel or interfere with the front radiation. Wall construction being what it is, your walls will not be as rigid and non-resonant as an expensive high-end loudspeaker enclosure. Still, some judiciously placed absorptive material will eat up a lot of the back energy. Fiberglass is always the best sonic absorber. Use standard insulation grade fiberglass. Peel off the paper backing if present. Having the right density of material is important. If you can lightly stuff the in-wall cavity above and below the system for perhaps 18" up and 18" down, this will absorb a lot of the energy that would go up and down the cavity. Apply the insulation without compressing it at all. To compress too much reduces flow through the material and constricts the volume effectively creating a solid barrier, reducing the air volume the woofer sees. Material should also be behind the system to reduce reflections off of the rear dry-wall surface. Again, be careful not too overly compress the fiberglass. You should compensate for the depth of the speaker components. It is easy to split the material into half its thickness. You might also want to cut out a hole for the woofers magnet structure (usually the deepest point) so that you might be able to leave a little more material around it.

Subwoofers

Manufacturers are just now coming up with effective in-wall subwoofers. Getting low bass with an in-wall is even more of a challenge than upper frequencies. Subwoofers rely even more on their enclosure volume for optimum performance. Also, low frequencies are more likely to leak through typical wall construction and reduce the bass energy. Still, careful mounting will help you get good performance from an in-wall subwoofer.

Placement wise you have a little more freedom in locating the subwoofer. If the crossover point is low enough (100Hz or lower, with a steep crossover slope) your ears will be relatively insensitive to subwoofer location. Keep the sub in the front half of the room and you will be okay. Be aware that subwoofer location will impact the standing waves of the room. These are resonances determined by room dimensions that give unevenness to bass reproduction. Some subwoofer locations will exacerbate these standing waves while other locations will give a more even bass. With conventional subs you can experiment by moving the sub to various positions while listening for bass smoothness. This is obviously not practical with in-wall subwoofers!

It is practical to place a conventional subwoofer facing the wall (leave a 1 or 2" gap) and try the sub at various potential positions. When you find a good position, cut and mount your in-wall subwoofer there.

Unless fully enclosed, your in-wall subwoofer will need very careful mounting for best bass performance. Anything that can be done to improve wall rigidity and air seal will pay rewards. If you are finishing out the room (rather than retrofitting) you might consider facing both sides of the wall in the vicinity of the subwoofer with heavier and stiffer material than drywall, or with double layers of drywall. You can guarantee a better air seal along the active wall channel with caulking. Follow the manufacturers instructions for mounting and volume requirements carefully.

The cavity between the studs and from floor to ceiling can have a specific resonance due to its length. This makes it act as a closed end organ pipe that actually strongly reduces bass output at the frequency of its resonance. This seems to be worst when the woofer is at either end of a long between-wall channel. I've seen in-wall subwoofers loose a lot of energy due to this resonance, and over a wide enough bandwidth to kill their usefulness. If you run up against this you should try the fiberglass damping as described above, but you might even try to alter the frequency of resonance by altering the effective channel length. Less may be more. If you cut the length of the cavity in half you may find that the resonance moves out of band or gets to a frequency where the fiberglass works more effectively. You might be able to squeeze a footer or header stud half way up the cavity to do this. Alternatively you might tightly stuff fiberglass or foam as far up the cavity as you can reach. Having a tightly packed section will redefine the effective cavity to the length within the tightly stuffed endpoints. Lightly fill the remaining cavity and try again.

Wiring

You'll need to run a pair of wires back from each speaker to your amplifier or multichannel receiver. Use a heavy gauge two conductor cable of perhaps 16 gauge or 14 gauge to prevent power loss, especially for longer runs. If the room is of new construction then run the wires before the drywall is up. This is infinitely easier! If it is of pre-existing construction then things are more difficult. One easy halfway measure is to drop a wire from each speaker, within the wall, to a spot near floor level. At that point you can install and connect to a two conductor loudspeaker terminal plate and run the wires the rest of the way at floor level around the perimeter of the room. This will reduce their visibility and is relatively easy to do.

Good luck with your in-wall speaker project. Follow the guidelines for placement and wall treatment and you will be pleasantly surprised with the quality of the results.


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