SBIR Matters for Subs Too!
By Matthew J Poes
This is a quick write-up to show the effect of SBIR on low frequencies, highlighting something I found by accident, and how I addressed it. Recently, I was testing a large 18” subwoofer in my theater and temporarily had it placed in front of my screen, which is 30” from the actual front wall. This created a scenario where I had a subwoofer out into the room (a situation some believe leads to better sub performance). I decided to use this as a case study of why subwoofers should probably not be placed out into a room and should always be placed against at least one wall.
Speaker Boundary Interference Response
What is SBIR? SBIR stands for speaker boundary interference response and refers to the interference caused by low frequency sound waves bouncing off boundaries (e.g. walls, ceilings, and floors) and reflecting against the other waves from the source, causing either peaks or dips. Because of these responses, one unique attribute of SBIR (as compared to typical modal interference) is that resulting peaks and dips will always remain constant regardless of the listener’s position. The only thing that modifies the peaks and dips are moving the speaker.
The cause of the interference has to do with the way that acoustic waves interact with each other when their phase aligns or not. Look at this graphic to help understand how interference works in perfect 0-degree and 180-degree phase to each other:
Figure 1: Interference graphic
As can be seen, when two waves are perfectly in phase (0 degrees or 360 degrees) the result is perfect constructive interference. In other words, the amplitude increases. When the two waves are completely out of phase (180 degrees) then the amplitude is decreased and the resulting sound is cancelled. Since a reflected wave travels a further distance, the phase of the wave will be different from the wave coming directly from the subwoofer. When the reflected and direct waves meet up they interfere with each other, sometimes constructively and sometimes destructively. Because low frequencies are steady state in small rooms by the time they hit our ears, we hear the bass with all the room reflections. This cancelation effect isn’t delayed, it’s a part of the fundamental signal we hear.
What does SBIR look like?
Let me first describe the setup in my room that caused this measurement. I turned off all correction, all EQ, and measured the 18” subwoofer along with my two 12” B&C Bandpass subs and my single 12” Dayton reference sub. When I saw the cancelation illustrated in Figure 2 (below), I assumed all four subs were interfering with each other and phase had been reversed. Because my front wall is a false wall, I forgot how far the subwoofer was from the front wall. I proceeded to switch each sub’s phase, singularly and in pairs, to see if that effected output measurements. While the response changed, a noticeable notch at 32hz never went away. Then I turned off the new subwoofer and the notch was gone. Of course! The 18” subwoofer measures roughly 28” long, and because of its in-room position, was actually sitting nearly 60” away from my room’s actual physical wall.
Figure 2: Uncorrected system response subwoofer front and center
Now you may be asking yourself: how did I know there was nothing wrong with the subwoofer? How did I know this was SBIR? The answer: simple calculations (but we need to keep a few things in mind). This interference effect depends on the acoustic distance to the walls, which can differ from the actual distance for a few reasons. One is that materials in the room can have an impact on the exact position of these interferences, but more commonly, the wall in our room may not be the hard barrier causing the reflection. It may be both a wall’s surface in a normal 2x4 wall (drywall on both sides) or it may be the cement outer wall in a basement room. In my case the cement front wall of the basement foundation is about 24 inches from the theater room’s inner wall. Due to some pipes my side walls are actually 36” from the cement foundation walls. The rear wall is a double stud wall with 4 layers of drywall that measures about 14” thick. All of this impacts the room’s acoustic dimensions, as compared to its internal actual dimensions. Remember, low frequency waves can travel through materials like drywall as a hot knife through butter.
Figure 3: Room simulation of SBIR
In Figure 3, I have created a simulation that seems to somewhat closely match the response I measured in my actual room. This is with the subwoofer placed dead center of the screen and with the LF source roughly 50-60”from the interior front wall. However, with a dip at 32Hz, it suggests the subwoofer is much farther from the walls, as if the room is much larger. I used the dimensions of the foundation walls instead and had a very close match. The proximity of the sub to each barrier caused a bunch of dips to all fall very close to each other. This compounded the dip and made it both very wide and very deep. Room modes caused the peak in that measurement, specifically the 70hz mode is a length axial mode in the room.
How did I fix the SBIR effect?
I was in fact able to largely fix this problem without moving the subwoofer behind the screen. I moved the subwoofer to the right side wall and oriented it to face the front wall. Why did this help? By moving the sub to a barrier, the distance between surfaces is no longer equal and the interference effects are spread out. This reduces the significance of their effect. Additionally, some of the interference effect is raised above the operating range by placing them against a wall. Turning the subwoofer toward the front wall placed it closer to the front wall, again moving the interference to a higher frequency. This is how it looked after I moved the subwoofer. To be clear, I did not change any settings in the DSP, nor did I adjust anything in the electronics. I only moved the subwoofer to the side of the room, and turned it around.
Figure 4: Uncorrected Systems Response with sub on right side wall
As you can see the response is much flatter. The dips and peaks are no longer on top of each other and the previously measured deep valley in the response is removed. Instead, the bass response is relatively flat and smooth.
Now let’s look at what a little EQ does:
Figure 5: Corrected Systems Response with sub on right side wall
As you can see, once some EQ is applied, the bass response can be made much flatter. The response falls within a +/- 5db window at nearly all frequencies relative to my intended house curve. It’s important to note that no amount of EQ could have filled in that dip in the bass response. That dip was a wide cancelation effect and the only solution was to move the subwoofer to a more optimal position. What was that more optimal position? It was pushing the subwoofer against the side wall and closer to the front wall.
Two things have been demonstrated in this exercise. First, subwoofers should not be placed far away from room boundaries. Second, the distance between a low frequency source and surrounding boundary surfaces should not be equal or multiples of each other. These compound cancelation effects, resulting in a large measurable trough in bass response. Hopefully this also helps to explain why full range speakers (and their typical position in a room) are perpetually suboptimal low frequency sources. The ideal placement of main speakers is rarely if ever the best location for bass reproduction. This is why I frequently say, subwoofers are audiophile. It’s a lack of subwoofers (separate LF sources which can be optimally placed in a room) which is not audiophile.