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The Low End Blues

One of the biggest challenges in recording studio design is getting a great low-end sound. But the problem isn't limited to studio designers. It has become increasingly common for AV professionals working on corporate board-rooms, presentation facilities, and other venues to struggle with clear, consistent, low-frequency reproduction.

One of the biggest challenges in recording studio design is getting a great low-end sound. But the problem isn't limited to studio designers. It has become increasingly common for AV professionals working on corporate board-rooms, presentation facilities, and other venues to struggle with clear, consistent, low-frequency reproduction.

Recently, our firm worked on a pair of rooms with nagging low-end issues. In each case we took steps to correct the rooms' performance that any design engineer could take in his own projects. The engineer's challenge, we found, is as much about hearing an accurate low end as reproducing it.

First, a little background on trends in sound studios. For many years, people in large and small studios monitored on two different sets of loudspeakers: large monitors, which were typically soffit-mounted, and small, near-field monitors that sat on a consoles' meter bridge. The smaller, near-field monitors often had roll-off frequencies around 100 Hz. The idea was to use the smaller speakers, located closer to the mixer, to reduce the influence of the room. But in doing so, people gave up an accurate low end.

More common today is the use of what we call “semi-near field monitors.” These monitors are typically used in close proximity to the mixer and are essentially full-range devices, with roll-off frequencies usually below 50 Hz. In dedicated mixing and mastering rooms, it's increasingly common to see large-format monitors standing on the floor 11 to 13 feet from the mixer. In this case, the tonal balance of the room is especially critical because there is no attempt to reduce the influence of the room.

Both of the rooms we worked on recently fell into this latter category.

UNHAPPY FROM THE GET-GO

Bruce Botnick is a well known sound engineer, with a resume that spans work in film, rock music (including all The Doors albums), Broadway show soundtracks, and television. In late 2003, Botnick built a mixing room north of Los Angeles, out-fitted for both stereo and 5.1 mixing. From the time it opened, though, Botnick was unhappy with the low-end performance of the room.

The complaint was actually one we hear often. Botnick found that the low end was uneven, with certain frequencies booming while others were seemingly missing. There was also a great deal of variation in the low end at the mix position as compared to a producers' couch located 10 feet behind the mixer. These problems left Botnick unhappy with his facility and unable to trust that the low end of mixes he was working on would translate to other systems.

About the same time, we were called into Tangerine Mastering, a new mastering facility in Weehawkin, N.J. Their complaint was almost identical to Botnick's. Facility owner Roger Johanson and his engineers were concerned that low-end sound in the room was inaccurate and lacked impact and clarity. Like Botnick, Johanson needed his room to both sound great and perform accurately in the low end.

Both rooms—Botnick's and Tangerine Mastering's—feature very high-end loudspeaker systems. Bruce uses B&W Nautilus series, while Tangerine features Dunlevy SC4a monitors. Both sets of loudspeakers were designed to deliver a tight, clean, and even low-frequency response. Trusting that to be true, we started to look at the rooms themselves.

In evaluating any room, our approach is always the same: critical listening and careful acoustic measurement. The critical listening part simply requires playing back music that we know well and which provides a clean, tight low end. Though straightforward, the importance of listening can not be overstated. We use a wide range of rock, jazz, hip-hop, and classical music (see “Musical Samples for Critical Listening” for sample selections) to appreciate a room's low-end (and other) sound characteristics.

For acoustic measurement, we use two features of our firm's own SIA-Smaart 5 software, the Fixed Point Per Octave (FPPO) transfer function and the impulse response. We use the transfer function to see which frequencies are being supported and which are being canceled at both the mix position and other positions around the room. The impulse response is used to look at the overall decay of sound in the room.

When we perform our measurements, whether in a sound studio or other room, we're looking for two critical issues: the relative decay times for mid and low frequencies, and the evenness of the decay at low frequencies. Both of these measurements could indicate the room is acting modally—and that there is a need for treatment beyond simple acoustical panels.

PAIN IN THE REAR

In both Botnick's and Tangerine's rooms, our recommendations focused on the design of the respective facilities' rear walls.

For Botnick, we rebuilt the wall, creating asymmetrically cut corners and a large soffit at the top of the wall. While asymmetry is often considered undesirable in studio design, carefully designed asymmetry in the rear wall (particularly at the corners) can help to reduce modal behavior and create a more even low-frequency response. The new wall featured two types of bass absorbers from RPG Diffusor Systems and two types of diffusive treatment, as well as a large amount of 6-pound-density glass fiber.

The configuration of the soffits allowed us to place absorption with a large airspace behind it and along the rear wall. Botnick was concerned that adding so much absorption would create a dead feel in the room. However, much of the absorptive material was mounted behind diffusive treatment, which scatters mid and high frequencies and allows the low frequencies to reach the absorptive materials. This resulted in a room that is both acoustically live at mid and high frequencies, and well controlled at the low end.

Another problem we found in Botnick's studio was existing, cotton-based acoustical panel material. While this specific version of the cotton-based material is certainly absorptive at the upper-mid frequencies, with absorption coefficients of 0.9 and above, at 125 Hz its ability to absorb sound energy is limited to an absorption coefficient of less than 0.10. In general, coefficient values greater than 0.85 reflect very good performance, but values less than 0.25 indicate poor sound absorption.

When designing or retrofitting a room, it's critical for designers to ensure that the materials and techniques they select address the entire audio spectrum.

At Tangerine both our listening and acoustic measurements confirmed that the low end was behaving modally. There appeared to be two types of frequencies in the room: those that were resonating and those that were getting cancelled out at the same time. This is typical of rooms where modal behavior dominates the low end.

An inspection of the acoustical treatment of the room showed that although there were many acoustical panels and a large area of foam diffusers on the rear wall, all the acoustical panels were just 2-inch-thick glass fiber, mounted on or near the walls. Panels that were angled from the walls were not backfilled with lower density fiberglass, which is one way to improve low-end performance.

Furthermore, the diffusers selected were mounted directly to the rear wall, without placing materials behind them, allowing for effective low frequency absorption without overly deadening of the space. We added the missing fiberglass, which improved the ability of the existing and new materials to control low frequency energy (particularly in the 125 Hz range).

We also recommended using two corner bass traps from RPG Diffuser Systems and additional acoustical panels in both rear corners of the room. We also designed and installed an acoustically absorptive and diffusive soffit at the rear upper portion of the room, where the side and rear walls meet the ceiling. By creating a soffit, sound-absorbing materials could be placed away from the rear wall by as much as 2 feet, allowing several layers of material to be used while cost-effectively controlling low frequencies.

In both the Botnick and Tangerine rooms we were ultimately able to improve the evenness and accuracy of the low-frequency response by substantially reducing modal behavior. Moreover, in both cases the modifications represented a small fraction of the cost of the studios' original construction while resulting in dramatically better sound.

In retrospect, the initial acoustical designs in these rooms did not fully meet the users' expectations because not enough attention was paid to controlling mid-low and low frequency energy. We actually find this is a common mistake not only in mixing and mastering rooms, but also in home theaters, screening rooms, and corporate boardrooms where speech can be degraded by “boomy” acoustics.

As these two cases illustrate, though, careful selection of materials and attention to mounting details can help resolve problems for even the most demanding situations—and result in great sounding rooms and happy clients.

Sam Berkow is the founding partner of SIA Acoustics, an acoustical design firm with offices in New York and Hollywood, Calif. Recent projects include the Studios of the Clive Davis School for Recording Arts at New York University and the concert halls and jazz club for Jazz at Lincoln Center.



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