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  • Frank Leonard Walker

15 FLUSH! Ah Ahhh: Savior of the SBIR

Updated: Apr 14

This week we're turning our attention back to the front wall design, the how, and why, I mounted speakers flush into the wall.


Why? What are the benefits of doing this, and do they outweigh the negatives?


First, the positives:

Mounting speakers into a wall removes potential SBIR issues. SBIR stands for Speaker Boundry Interference Response; this is the phasing effect you get from placing your speakers near a wall. As sound radiates from a speaker in multiple directions, especially lower frequencies which radiate in an omnidirectional sphere, the sound will reflect off of nearby surfaces. What that means for us listening is that we hear the sound direct from the speaker, followed by that same signal, reflected off the wall behind the speakers. This results in some frequencies being in-phase (sounding louder) while other frequencies may be out of phase (quieter), creating an unbalanced response. When mounting speakers into the wall, this essentially turns the wall into an infinite baffle, removing the reflection points. This is similar to the way in which many speaker manufacturers measure their speaker responses, by digging a hole and burying the speaker flush with the ground for greater accuracy in their measurements.


Image from audiojudgement.com

Another issue in speaker design is Edge Diffraction. As the speaker generates sound waves from the diaphragm they expand in a hemispherical fashion. However, once the sound wave reaches the edge of a speaker baffle, the pressure drops, because the waves expand from a hemisphere to a full sphere. This difference in pressure creates a new sound wave that originates from the edge of the speaker cabinet which will cause more phasing issues, typically in the midrange. Speaker manufacturers will design speaker cabinets to greatly reduce this effect, but... you've guessed it, flush mounting speakers will reduce this phenomenon even further. With reduced phasing effects we can achieve a better transient response and clearer stereo imaging.






The third acoustic benefit of flush-mounted speakers is the increased acoustical loading, meaning the speakers don't have to work as 'hard' to generate low frequencies.


Other positives include saving space, rather than building a wall and placing the speakers in front of the wall, why not inside? Takes up less room, can hide cables, and, let's be honest... looks pretty impressive! That's not to say it's all roses though, there are some downsides, so let's look at...


The Negatives:


Once it's built, it's built! There's no moving position if the speakers aren't in the right place! Obviously, this just means meticulous effort needs to go into the design and planning of not just the speaker position, but everything else in the room.


Want a new set of monitors? Forget it! Or at least you have to seriously think about how you're going to change the speakers out, and it is going to be a much larger job!


An increase in bass frequencies will occur, because of the acoustical loading. Not necessarily a bad thing, but it's crucial to have some controls on your monitor to adjust the EQ as you will typically get a bass boost of around 6dB when flush mounting speakers.


Finally, it could all go wrong! If it's not built correctly or measurements are off, it might make your room sound worse... but I like to take a risk, so here we are!


The how:

There are potentially three types of Soffit or Flush mounted speaker systems to choose from.

  1. Solid

  2. Fake

  3. Semi-Solid/Absorptive

The best system is a solid wall, the heavier the better. Ideally concrete or brick, this brings all the benefits mentioned above but can take more work to construct. A fake system is just for the visual design and the ability to hide cables and acoustic treatment but with none of the acoustical benefits associated with flush mounting. A Semi-Solid/Absorptive front wall is a half-way house, providing the overall acoustical benefits from the solid wall build, to a lesser extent, but also providing space for further acoustic treatment for the room, as some low frequencies will still be omnidirectional with this type of construction.


Then comes the issue of how to 'mount' the speakers. Either decouple the speakers completely from the wall, or fit the speakers so rigidly that they become a part of the wall, then decoupling the wall from the rest of the structure. The former requires a few calculations to get the speaker to truly 'float' within the wall, whereas I opted for the latter, based on John Sayers flush-mounted system design (pictured below), providing room for ventilation and acoustic treatment behind the wall.


The Build:

Let's look at how the construction of the wall progressed, the last we left the front wall, the structure and the speakers stands were built but I had yet to get the speakers in situ. This is them roughly in place, but they have no been secured yet.


At this stage, I also turned my attention to the centre wall, where my TV monitor would be mounted. Installing the TV mount at the right height and distance so once the front wall was finished it would sit flush with the paneling.


The speakers were then placed on an MDF sheet that was screwed to the frame, while the rest of the MDF box was built around the speaker, glued and screwed in place ensuring a completely air-tight fit.


Next, a layer of MDF could be screwed to the timber frame, creating the first layer of the solid wall construction.


Some close-up images of the sealed speaker box. You'll notice that the speakers are sticking out by a bit, so they will be flush when the wood paneling is installed.


You may remember I was creating some access panels within the walls so I could get to the controls on the back of the speaker and my gas meter. These were designed to be 'hidden' and involved some push-to-open latches to avoid handles or hinges visible on the wall.


Finally some shots of the walls in place, doors on and off...


Obviously, there is still a way to go before the front wall is finished and working properly, but that's for another day!


Next time, we'll be looking at installing the acoustic hangers, for even more low-end treatment.


Frank Leonard Walker


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