Acoustic Geometry: CornerSorber

The CornerSorber is a membrane-based low-frequency (LF) absorber from Acoustic Geometry. Designed to work in conjunction with preexisting treatments, such as the LF membrane Curve Diffusors [Tape Op #88 and #122], each unit is comprised of two panel-wedges that resemble giant chocolate bars. The units butt together at 90 degrees, forming a freestanding L-shape. Small locking plates keep the pair mated. Simply set in a pressure zone (aka corner), make sure the membranes are parallel with, facing, and at 3-inches from the corner walls, and that's it for set up. The review model was matte black, about 42-inches tall, 24-inches wide, and 6-inches deep, and reminiscent of something out of Darth Vader's meditation chamber. In no time they seem to disappear into your room's background, going unnoticed to guests and visitors.

One of the biggest acoustic challenges concerns what are known as modes. In a paper for the 143rd AES Convention (#9886), Acoustic Geometry's John Calder explained, "Modes result from sound resonances in rooms at frequencies with wavelengths matching the room's dimensions (length, width, and height). For example, a room with dimensions of 20 feet, 14 feet, and 8 feet will resonate at – and near – 56.5 Hz, 80.7 Hz, and 141.3 Hz. When a mode occurs at a resonant frequency we observe nodes, which are sound energy cancellations in some places, and anti-nodes, which are energy additions at other places." So, about every room we've been in has some form of mode issue. It's nothing to be ashamed of, but it is important to understand. Short of moving the walls, changing the room geometry, or otherwise changing the area of your room, acoustic treatments are the best option.

Two key things need to be mentioned regarding evaluation of low frequency treatments. The first deals with sound velocity versus pressure. When most people talk about bass traps, they refer to foam or fiber units. Foam and fiber, however, respond to velocity only. Conversely membrane absorbers, like the CornerSorber, respond to sound pressure. Relying on velocity treatments alone will not address some of a room's major low-frequency acoustic issues.

The second issue regards reliance on specifications provided by the manufacturer. For instance, if we have a problem at 80 Hz, and we buy a product that claims to work that low, how do we know it works? Well, we check the spec sheet from the maker! My concern is that most acoustical testing laboratories have small reverberant chambers, with the larger ones typically being about 300 cubic meters in volume. This is too small to test absorption at the lowest frequencies. A 300-cubic meter room can only accurately measure down to about 160 Hz. In Elma, Washington, NWAA Labs has the largest test room in North America, measuring 738 cubic meters and accurate down to 40 Hz. From now on I'm going to ask the manufacturers: a) how they tested their low frequency response and b) what laboratory they used. I know several makers who will provide independent lab results, validated by a third party. Even then, it's important to know if the lab is large enough to support accurate low frequency testing, or if they did theory-based interpolation calculations. There exists statistical assumptions that support the legitimacy of interpolation calculations, but not everyone agrees. According to Acoustic Geometry, they had preliminary specifications for the CornerSorber based on calculations. After doing real tests, they found the theory-based assumptions and calculations were wrong. Had they released the products without using NWAA Labs, their published specs would have been incorrect.

In my test room, I used only one CornerSorber pair in tandem with eight Medium Curve Diffusors (version 2). Remember, the Curves have bass absorption membranes built-in. Acoustic Geometry states that six Medium Curves are approximately equal in efficiency to one pair of CornerSorbers. Before installing the treatments, the room was really not usable for critical listening. While the Curves smoothed out reflections and tightened the response, having a pair of CornerSorbers and eight Curves locked down the low end, especially the lower areas at 50 Hz. In my present case, moving the walls was not an option so LF treatment that manages room modes is reassuring. In the past, I've built bass traps with fiber and foam: they do not work as well, take up much more space, and look terribly obtrusive compared to the lovely, Vader-esque CornerSorbers. At $799 (per pair), it is a small price to pay for peace of mind, and it's a heck of a lot cheaper than buying a new building or getting renovations.