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Speaker Boundary Interference

Where you place your speakers matters. However, you’re probably not in the most ideal room. For this reason, you have to put your speakers where they can go. In this article, I’m going to talk about the consequences of where you place your speakers as it relates to nearby surfaces.

To put it as simply as possible, your speakers put out sound in all directions. While higher frequencies are more directional, bass ones aren’t. What happens is the sound bounces off walls and things like your desk then combine with the original signal to cause comb filtering distortion. The sum of these distortions is called boundary interference. Let’s get into a bit more detail.

Speaker-Boundary Interference Response

Speaker-boundary interference is what happens when your lower frequencies go behind your speakers, or to the ceiling and the floor then come back and join in with the acoustic energy moving toward your ears. This is how your room functions as a giant filter. Acoustic treatment helps with this by absorbing the frequencies so they can’t come back to wreak havoc.

You can take individual measurements of each of your loudspeakers to confirm this. Ordinarily, you’d expect them to have the same response but that’s rarely the case. The reason for this is the degree of asymmetry your room or speaker placement has.

Luckily, with an acoustic measurement, we can maneuver the speaker’s placement so that the two responses ideally compliment each other. This is the reason that many audiophile recommend having more than one subwoofer. In any case, I consider SBIR to be more of a bass issue than a high frequency one and therefore one you defeat with placement in addition to treatment.

Listener-Boundary Interference Response

Listener-boundary interference is a bass issue which results in where your head is located relative to the pressure nulls in a given frequency’s wavelength. For example, if your head is in the center of the room the it’s right at the quarter wavelength of the room’s fundamental modes. By extension, if your head is at 1/4 distance from any two resonance-supporting boundaries (like walls) then you’re in the null for the first harmonic of that mode.

The best way to take care of this variant of boundary interference is to choose your listening position carefully. Anthony Grimani has done presentations where he says 20%, 32%, and 45% the distance between any two resonance-supporting surfaces gives the most even modal response. Previously, I believed that 38% was best – and for a given room it might be, due to construction factors – but I recommend you start with those three and measure from there.

Therefore, if you want the best acoustics then you need find the best place to work. The caveat to this is that sometimes large surfaces can help your response but you’ll have to figure that out on a case-by-case basis as it mostly pertains to subwoofers and speakers and that’s related to SBIR. In general, though, if you keep your speakers more-or-less in an equilateral triangle and within manufacturer distance specifications then you have to work within those parameters to find where your speakers will live. First find your listening position then place your speakers relative to that.

Battle Plan

Like I’ve already mentioned, the best way to handle both SBIR and LBIR is the trifecta of treatment, placement, and minimization. Any one of these will help but in combination there’s almost a synergistic effect. Let’s briefly recap our options.

Your acoustic treatment needs to be in the form of absorption. I’ve done a video on why you don’t need velocity absorbers. Make sure you have thick treatment in your corners, like soffits or super chunks, and feel free to place your subwoofer near them. In addition, you need to create a reflection-free zone so you don’t get early reflections. Or at the very least you need to consider the design of your speakers and balance that against your preferences for a live or dead listening area.

Placement means putting your speakers in the room symmetrically and complimentary to any subwoofers and each other. Do this the best you can then take measurements of each one and then both together. See if the individual measurements are the same or complimentary and compare that to your stereo measurement. You can also adjust your subwoofer’s location as well as your listening position to find the most optimal arrangement.

Finally, minimization means that feng shui isn’t bull crap after all. Clear out your clutter. All those old beer cans are actually making your room sound worse. If you don’t need it every day then it shouldn’t be on your desk. Remember that the best room has nothing but speakers, absorption and you in it. The further you go from that, the more you’re fighting an uphill battle.

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