"Solving Low-Frequency Modes with Real World-Inspired Modeling Software"

By Keith Yates

This article appeared in the November 24, 2014 edition of Residential Systems.

The BassCAMP report includes before-and-after color pressure slices to let the eye see what the ear will hear.

The interaction of private theater audio with the room’s shape, dimensions, and construction can turn even the best sound systems into low-frequency “sludge pumps” that boom oppressively at one seat and are weak and vague at another seat a few feet away. These low-frequency resonances (sometimes referred to as eigenfrequencies, stationary waves, or standing waves) build up at frequencies related to the specific combination of the sound frequency’s wavelength; the room’s shape, dimensions, and wall construction make-up; acoustic elements (e.g., sofas, engineered bass traps, etc.); quantity and positions of pressure sources (woofers and subwoofers); and locations of listeners within the space.

When long-wavelength (bass) energy is pumped into the relatively small confines of a home theater or media room, standing waves slosh back and forth in self-reinforcing and self-cancelling ways, making a patchwork of the room’s sound pressure map. It’s not the subwoofer’s fault; in an anechoic chamber or the great outdoors the chosen sub may generate ruler-flat bass throughout its typical 20 to 80 Hertz range. But try parking the same sub in a corner of a home theater and feed it a steady sine wave at any of the roughly two dozen room eigenfrequencies while meandering around the room with a simple SPL meter in hand. (A $30 Radio Shack unit will do just fine; set it to “Slow” and “C” weighting.) What you’ll likely find is that if it reads, say, 90dB in one of the boom zones, it’ll register something like 50 or 55dB in the null zones (“nodes”). High-pressure areas (“antinodes”) tend to occur in corners (though not only there), while low-pressure nodes can turn up most anywhere. If you’ve got a two-row theater with the second row head positions approximately a foot off the rear wall, you may well find that the maximum seat-to-seat variation in the room exceeds 30 decibels at one or more modal frequencies. This is a disparity that would have been considered “lo-fi” even by your grandfather’s standards, back before the invention of the transistor radio.

One seat may have relatively smooth, flat, low-frequency response, while another seat a few feet away may exhibit response problems that seem intractable, no matter how much you fiddle with equalizer settings. Such variations can prove beyond the ability of the room correction solution in the surround pre-pro, or even a downstream equalizer/DSP box to fix.

Some of the industry’s best and brightest have devoted years to understanding and addressing the problem. In a series of papers in the Journal of the Audio Engineering Society beginning in 2006, Harman researchers, led by Drs. Floyd Toole and Todd Welti, modeled the problem, compared the predictions with actual measurements, and suggested practical solutions. Among them was placing a subwoofer in each of the room’s four corners, a layout that exploits what Toole termed natural “mode canceling.” Alternatively, if all four corners are not available, and if the wall’s acoustic impedances are similar (their construction makeups are the same) the four subwoofers can be installed at the walls’ midpoints.

It should be noted that the intent of Harman recommendations is not to “flatten” roller-coaster bass response, but rather to drive down seat-to-seat variation throughout the notoriously difficult 20-150Hz range in most home environments. Why? Because if you can get seat-to-seat variation low enough, when you equalizer/flatten the response at one seat, you’re naturally flattening it at all seats.

The BassCAMP report points out specific wall and ceiling areas that would be particularly effective locations for low-frequency absorbers to further tighten bass response.

Now widely considered a “best practice” among media room and home theater designers, both Harman layouts apply most fully to perfectly rectangular rooms (parallelepipeds) devoid of ceiling soffits, seating platforms, alcoves, or other architectural “features,” all of which, it hardly needs to be said, are fairly typical of residential theater design.

Not All Theaters are Perfect Rectangles

In 2006, my company (Keith Yates Design) initiated an in-house research program to build on Harman’s work and to take what the Harman team considered to be the next logical step. Instead of basing the modeling and optimization on a perfect parallelepiped, we wanted it to reflect a room’s real-world shape, size, and construction makeup, replete with all its architectural idiosyncrasies, diverse constructions, and even its furniture layout and acoustic treatments.

Subwoofers of whatever size and type–free-standing, in-cabinet, in/ on wall, in/on ceiling or below the floor–are put into the 3D model only if they can be concealed or at least made architecturally acceptable to the homeowner. The idea is to increase the number of allowable subwoofer positions, and thereby the number of viable combinations, for the computer to simulate and rank.

Once reviewed and OK’d by the integrator and homeowner, the 3D CAD file is brought into industrial-grade finite element analysis (FEA) and brute-force computational fluid dynamics (CFD) to simulate low-frequency acoustic behavior throughout the room. The data is post-processed in mathematical optimization tools to reveal the layout combination–the optimum quantity, positions, and electronic settings (gain, polarity, delay) of individual subwoofers–that will create the highest possible seat-to-seat uniformity throughout the bass range.

We call the tool “BassCAMP” (Computational Acoustic Modeling Program). Uniquely, it allows the modeling and optimization of rooms of any size, shape, construction, or degree of geometric complexity. It “sees” doors, windows, seating platforms, soffits, pony walls, alcoves, openings into hallways, and other contiguous spaces and other features that influence low-frequency performance.

Two levels of analysis were developed, depending on budget and the number of available subwoofer positions. BassCAMP Standard reveals the best-performing configuration (combination) of up to four subwoofers from among 15 possible positions. BassCAMP Advanced reveals the best combination of up to eight subs from among 30 possible positions.

The Results

The results of the simulation are compiled in a bound, approximately 60-page BassCAMP Report that identifies the specific subwoofer configuration, with dimensions, for use by the dealer and his installation team. The report also ranks the top-performing combinations in terms of their mean spatial deviation (MSD); lists the optimum settings for each subwoofer (gain, polarity, delay); points out specific wall and ceiling areas that would be particularly effective locations for low-frequency absorbers to further tighten bass response; and includes before-and-after color pressure slices to let the eye see what the ear will hear (see image).

Created from modern, physics-based modeling and custom optimization algorithms, BassCAMP empowers dealers and integrators to offer a powerful, room-specific solution to a problem that has been “reverberating” since audio’s beginnings a century ago. Brand-agnostic, BassCAMP is compatible with any subwoofer type, make or model with sufficiently flat response and low distortion. It’s a purpose-built tool for revealing the subwoofer combination, among thousands, tens of thousands, or hundreds of thousands that will drive seat-to-seat variation down to a level that can be neatly dealt with through built-in room correction circuits or outboard equalization. While every room is different, over the years we’ve found that what would have reached 30 decibels or more of seat-to-seat disparity at various frequencies throughout the bass range can be reduced to 3 or 4 decibels or less with BassCAMP subwoofer layouts.

Dealer Access

Coming full circle, BassCAMP now has a Harman connection in the form of its JBL Synthesis-branded cousins, FLO Gold and FLO Platinum, which KYD developed to take maximum advantage of the sound field management feature built into the Synthesis SDEC digital processors.

Strong, clear bass is the foundation for movie realism. It’s now possible to predictably achieve it at every seat. With BassCAMP and FLO, the CEDIA-affiliated dealer channel has access to science-based services that expand the application of subwoofers to predictably and palpably enhance the entertainment experience for clients and their families.