The build-up of low frequency amplified sound, or excessive “boomyness,” is a common problem that occurs in spaces for amplified music such as nightclubs, concert venues, showrooms, and theatres. Performers and presenters want the power and impact of loud bass to add excitement and energy to the show. The driving bass lines enhance the dance rhythms, getting the audience up on its feet and dancing in the aisles. On the other hand, underpowered bass sounds weak and anemic and is the reason for acts adding stacks of subwoofers lining the stage and powering them with thousands of watts of amplifiers.
But in a hall that has poor bass acoustics, the bass line sounds “muddy” and might be described as lacking “punch” or “kick.” Bad bass sound seems to persist and extend in a hall, rolling around the room interminably. The effect not only makes the bass sound lousy, but the excessive bass also covers the mids and highs, setting off a spate of complaints directed at the engineer at the sound console. The hapless mix engineer might try to EQ or turn up the bass to get more punch, but he is fighting himself and only making the situation worse.
So what is really happening here? Why does bass sound tight and punchy in one big room, but in a similar room of about the same size, it sounds mushy and boomy? The problem lies less with the equipment, the rig, and the engineer, and more often with the venue itself. The room designers may never have contemplated hosting these types of events or perhaps may never have focused on the bass acoustics in the original design. More likely, the issues were addressed by the design team, but the client cut the treatment as a cost-saving measure.
What types of treatments solve the boom problem in halls? There are a number of possible options and directions the client and architect might take, depending on the size of the venue and the configuration. Caution: Note that these treatments will make the hall quite dead for the bass frequencies, and therefore classical music and opera will sound dreadful to both performers and audiences in spaces so treated. (How to make the bass absorption adjustable — in for amplified and out for classical — is another subject.)
In these very large spaces, bass build-up is particularly tricky. With the huge air volume in the millions of cubic feet and the long distances between surfaces, the bass sound can persist for three to five seconds and more. Here, using 1“ to 2“-thick acoustic panels made of fiberglass or other soft material mounted on the walls will not do the job. There are not enough wall surfaces on which to mount the panels to meaningfully reduce the reverberation. Increasing the thickness (to 4“ to 6“ thick) will have a minimal effect, as the area for applying the panels is still small compared to the overall volume of the space.
The ceiling surfaces often make the best bass absorbing locations. Obviously, it's the largest surface (after the seating area), and it's well distributed over the entire room. If it's early enough in the design, it is advisable for the team to substitute an “acoustical” metal deck for the roof decking itself. The roof decks span between the roof beams (called purlins) and hold up the insulation and waterproof roofing material on top. Acoustic roof decks have thousands of small perforations in the metal that allow sound to be trapped and absorbed. The thicker the deck (more depth to the corrugations, actually), the more efficient it is at soaking up low frequencies. Using fiberglass rather than thermal insulation board such as “blue board” on top of the deck can further enhance the performance.
If the deck is in place already, or the structural engineer balks at the acoustic deck, there is an efficient and cost-effective solution: catenary banners. These banners are lightweight and typically made of two layers of rip-stop nylon with a 2"-thick filling of fiberglass. They resemble giant quilts and, because they are lightweight, can be readily suspended from the roof. The counterintuitive issue with these banners is that, while it might appear attaching them directly to the underside of the roof would be best acoustically, it is not. This is because of the very long wavelengths, up to 16' to 32' long, when dealing with deep bass. As is true of many things acoustical, it's not intuitive.
The catenary banners should be suspended 18" to 24" below the roof, so that the air space trapped between the banner and the roof acts like a giant bass trap. Draping the banner in such a way that the air space behind it varies for 18" to 24" (or even more) is a good thing, because the variation in the air space improves the banners' performance at the full range of low frequencies.
An entirely new 2,000-seat venue was built for Cirque du Soleil's Love (“Love Is All You Need,” Live Design, August 2006) within the old Siegfried and Roy Theatre at the MGM Mirage in Las Vegas. One of the many acoustic goals that Cirque and sound designer Jonathan Deans demanded from us was a room with a “tight” bass response for the unprecedented quality of The Beatles' new soundtrack. (I learned from Deans that most of the original tracks of The Beatles' music were originally recorded without a bass drum mic, due to limitations of the recording gear at the time, and that AM radios had pretty weak bass performance anyway. As part of the new “mix,” a big, full bass line an octave lower was synthesized from the existing tracks.)
To create the tight bass response in a theatre with over 600,000cu-ft. of air and a ceiling 60' over the stage would take more than a few acoustic panels. Here, we used a combination of thick fiberglass panels, panel absorbers, and catenary bass traps. Creating the right balance of room “liveness” — to keep life and brightness to the sound — and bass control required surgical accuracy on treatment location.
Drywall placed over 6" metal studs, with batt insulation behind it, made the walls diaphragmatic absorbers if minimally braced and supported. Hanging dozens of 30'-long black catenaries in a zigzag pattern caused a few raised eyebrows from the contractor. We located these oversized bass traps behind the huge sidewall projection screens that stretch most of the way around the room. The screen material itself was selected in our lab to be both sound transparent and light reflective, allowing the bass sound to penetrate and be soaked up by the traps. Locating other bass traps in the corners of the room is a technique to increase their bass-soaking capabilities at no additional cost. The bass naturally builds up in corners and thus makes them ideal trap locations.
Reaction to the tight bass response (and overall sound) in the hall has been gratifying and humbling. The credit, of course, goes to the Fab Four, Sir George Martin, and Jonathan Deans.