"We are convinced that room modes are in fact a subset of the more general case of acoustic interference."
Click the images below to see them full size.
MODES VS. ACOUSTIC INTERFERENCE
This article was assembled from web forum posts describing original research performed by RealTraps partners Ethan Winer and Doug Ferrara. Many acousticians believe that modes are the single defining property of all rooms. However, we are convinced that room modes are in fact a subset of the more general case of acoustic interference. Acoustic interference occurs outdoors against a single boundary, and so doesn't even require a room. As you close off one wall, then two, and so forth until the room is completely enclosed, the interference patterns simply become stronger and more complex due to interaction, reinforcement, and resonance within the enclosed space. But simple boundary interference is the basis - the parent property, if you will - and the additional subset properties of room modes develop when the space becomes enclosed.
In the explanations that follow, the front wall is the wall at the front of the room and the rear wall is the wall in the back of the room behind the listeners.
DO ROOM MODES EVEN MATTER?
Doug Ferrara and I ran a series of low frequency response measurements in my living room to determine the effect of moving the listening position. This room is 25 feet long by 16 feet wide, with a vaulted ceiling that varies in height from eight feet at the front and rear walls to 11 feet in the center. The room also has a large number of MiniTraps - 19 in total - and most are mounted in corners. I used only the two main loudspeakers for this test, intentionally turning off my subwoofer to avoid additional variables. The ETF program from www.acoustisoft.com was used to measure the response at each location. The results of these tests are described in detail following, with graphs, and they confirm what we believe is the primary cause of the peaks and deep nulls that occur in all small rooms.
The first graph shows the change in frequency response when moving from the front toward the back of the room. In all cases the measuring microphone was centered left and right at ear height when seated. We started about one foot forward of the listening position, then moved farther and farther back in increments of about one foot. The graph's line colors are set to become progressively darker, which shows the trend more clearly than using blue, red, yellow, and so forth. Top
So what does this graph tell us? Most important, of all the peaks and deep nulls only one of them is related to a room mode! All of the others are directly related to the quarter wavelength distance from the rear wall, as annotated on the graph. The peak at 45 Hz is modal, and is due to the 25 foot room length - its frequency does not change with distance, though its level does. All subsequent peaks and nulls shift upward in frequency in direct relation to moving the microphone closer to the rear wall. Note also in the graph above that the modal peak at 45 Hz is not much of a problem near the front of the room (lightest line color). At least not in a room that has a serious amount of acoustic treatment and bass trapping.
This next graph was taken at the listening position, and shows the change in response when moving from two feet left of center through two feet right of center. As you can see, when moving left to right the major peaks and nulls do not change frequency, only level. This further confirms that the distance from the rear wall is the primary influence on low frequency response.
The last graph shows the result of changing height at the listening position, as when moving from sitting to standing. Although there is a small change in the center frequencies, they remain closely related to the distance from the rear wall. Top
The most important information we get from these tests is that a room's modal behavior is the secondary mechanism that defines its behavior. The primary mechanism is the quarter wavelength effect - boundary-induced comb filtering - that occurs at a predictable distance from the rear wall. Why the rear wall? Because that's the main direction the wavefront travels as it leaves the loudspeaker.
The notion that boundary-induced comb filtering is even more important than modal response makes a lot of sense once you think about it. The only difference between modal and non-modal frequencies is that modal frequencies fit exactly between the walls, and so have the potential to ring longer when excited. So it's clear to us, anyway, that the primary cause of peaks and nulls at all low frequencies is the quarter wavelength distance from the rear wall.
One other thing this data shows is that frequency-specific absorption and even equalization can indeed be useful at very low modal frequencies. Since the single mode-induced 45 Hz peak remains at that frequency no matter how the listening position is moved, that means it can be targeted successfully with narrow band absorption and/or EQ. Top
MODES PART DEUX
About a month ago, in a post titled "Do room modes even matter?" I reported that peaks and nulls in my living room shifted frequency in direct relation to the listener's distance from the rear wall. This contradicts the accepted theory that rooms modes are solely responsible for all peaks and nulls in enclosed spaces.
Several people in the various audio forums suggested the test was not valid because there are so many bass traps in my living room. One theory put forth was that all those traps damped the room modes so much that only the non-modal peaks and nulls remained. Another criticism was that the angled ceiling could support an infinite number of modes, so all that our test showed was that different height modes were being excited.
These are certainly valid concerns, so last weekend Doug and I repeated the test in my empty garage. This garage is bounded by a lot of cement, and is mostly a plain rectangle. We ran the tests with the room completely empty, and then again with a dozen sheets of 1-inch thick 703 rigid fiberglass scattered around to simulate a more normal room having carpet and a few furnishings. The results were identical, which makes sense because 1-inch 703 does very little at the low frequencies we tested. Top
What we found was that at very low frequencies - below about 160 Hz - the room is dominated mainly by modal response. Above that virtually all of the peaks and nulls shifted in relation to the listener's distance from the rear wall. So I guess we can conclude that both theories are correct! That is, room modes do matter, but the quarter wavelength effects are equally significant.
At left are screen shots of our results, photos of my garage showing the test setup, plus a drawing showing the exact details and dimensions and the first few modal frequencies for each dimension. Click the images to see full-size versions. Top
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