This article originally appeared in the February 2018 issue of Pro Sound News. Innovations is a monthly column in which different pro audio manufacturers are invited to discuss the thought process behind creating their products of note.
Acoustics are a vital component of every room, especially in professional audio and recording studio environments. The science of acoustics is based on analysis of complex sound waves and a deep understanding on how sound travels based on the specific environment. No two rooms are alike, so the acoustic properties of each space vary. There are many factors that affect sound in rooms, including the size and shape of a room, where sound leakage exists, the number of doors and windows, and the contents of the room itself, from audio gear to furniture. All of these aspects need to be considered when diagnosing a space. The art and science of “acoustics” is essentially simple ... the tricky part is figuring out how it works in your space.
Acoustic Geometry has been offering acoustical sound-control products for nearly a decade, for a wide range of applications. A major component of creating room solutions is testing our products to ensure they will perform as designed. We don’t take this obligation lightly, which is why we test our products at NWAA Labs, the only facility in the United States that can accurately test sound absorber products down to 40 Hz.
Know Your Room (Modes)
Room modes are one of the greatest problems in accurate sound recording and reproduction. Modes result from sound resonances at frequencies with wavelengths matching the room’s dimensions: length, width and height. Modes at resonant frequencies—also called standing waves or eigentones—consist of nodes, which cause large energy cancellations in wavelength-dependent locations in a room, and anti-nodes, which cause large energy additions in different wavelength-dependent locations. Also, at frequency wavelengths longer than the room dimensions, called Schroeder frequencies, the room will “cross over” and sound becomes pressure-based instead of velocity-based.
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To solve this room mode problem, low-frequency (LF) absorbers, sometimes called “bass absorbers” or “bass traps,” can be used to improve modes’ destructive effects in professional and consumer audio spaces. However, absorbers made of fiber and foam act only on sound velocity, whereas membrane absorbers act on sound pressure. Velocity, or the speed of sound in air, is about 1,130 feet per second, but it becomes 0 feet per second at hard reflective surface boundaries because sound energy is stopped by the boundary before reflecting; pressure becomes maximum at hard, reflective surfaces because the sound energy builds up before reflecting. This is why velocity-based fiber- and foam-based absorbers work far less effectively at boundaries than pressure-based membrane absorbers for frequencies below the Schroeder frequency room crossover, usually between 160 Hz and 200 Hz in small rooms.
To address this disparity, Acoustic Geometry developed a membrane-based low-frequency absorber, the CornerSorber, to complement the effective frequency range of our original LF membrane absorber, the Curve Diffusor, which was designed primarily as a phase-coherent diffusor above 300 Hz. The initial CornerSorber design goal was to use different membrane densities to offer different frequency ranges for the same enclosure, making them easily changeable in manufacturing, but when we tested our prototypes, there were some surprising results.
Putting Products to the Test
When testing sound-absorbing products, nearly all independent acoustical testing laboratories have reverberant chambers of under 300 cubic meters in volume. This is large enough to measure absorption results above 160 Hz, but it becomes very inaccurate below 160 Hz, again per Schroeder frequencies. There is only one test lab large enough, at 738 cubic meters, to be accurate for measuring absorption down to 40 Hz: NWAA Labs, in Elma, Wash.
To test the effects at NWAA Labs of different mass-loaded vinyl (MLV) membrane densities in CornerSorber enclosures, slide-in frame retainers were made to hold frames with different sets of membrane densities: 1 lb., 1/2 lb. and 1/4 lb. per square foot.
Some of the test results differed from our predictions, which were based on various resonance theories and subsequent logical assumptions. Notably, we found that absorption ranges and center frequencies, as well as effectiveness, did not change significantly with changes in membrane densities; the most effective membrane distance from the wall for CornerSorbers was not the same as for the earlier-tested Curve Diffusor design; and absorption profiles for the new design in “semi-free space” (more than 1 meter from any wall) was approximately an octave higher in range and center frequency than when in the “pressure zone,” parallel with, facing, and close to the walls. Also, when the enclosures were reversed, with membranes facing into the test chamber instead of facing the walls, the results were also nearly an octave higher in range and center frequency than when parallel with, facing, and close to the walls.
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We also tested our design premise that the most effective location for membrane absorbers is closely coupled to, in parallel with, and facing the wall surfaces in corners, standing on the floor. This concept was proven to be accurate. Acoustic Geometry did not test with the membranes oriented horizontally (parallel with the floor), as membranes are less effective when at a right angle to gravity because of membrane displacement.
In summary, the type of absorber, and its location and orientation in a room, are critical to LF absorber effectiveness. Additionally, the set of 10 tests conducted by Acoustic Geometry proves the value of laboratory testing. If these products had been offered without accurate standardized laboratory absorption testing in a lab capable of accurately testing down to 40 Hz, our claims of product ranges and efficiencies would have been unfounded. This assertion is applicable to other LF absorber product designs and categories as well. Therefore, we feel it would be wise for consumers to check with manufacturers for results from an accurate LF testing lab before purchasing.
We strive to create products and solutions that are proven to work through accurate testing, ensuring that customers’ rooms will actually “sound better,” which is the goal for any critical-listening space.
John Calder is the director of Retail Sales for Acoustic Geometry.
Acoustic Geometry • www.acousticgeometry.com