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Lowering The Boom (Literally): Keeping Bass Energy Under Control As Part Of A Semi-Silent Stage

A look into the physics of low-frequency sound and why it’s so hard to control.

Bass players provide the foundation for nearly all contemporary music. Without them, our mixes sound thin and nobody wants to dance. Add the right amount of bass and all of the sudden it’s a show. But while we love bass, it often leads to trouble, particularly in small rooms.

Are there ways to reduce bass levels on stage that are spilling into the room while keeping the musicians happy? You bet. But first we need to understand the physics of low-frequency sound and why it’s so hard to control.

The Numbers

Let’s start by defining the wavelengths of various frequencies. Figure 1 shows that a frequency of 30 Hz (cycles per second) has a wavelength of 37 feet. That’s a really big wave in the air that’s long enough to move around any normal-sized obstacle (like the person standing in front of you), and it will easily bend around corners. In fact, it’s really difficult to stop bass energy from wrapping around everything in its way.

On the other hand, frequencies above 10 kHz have very short wavelengths, on the order of 1 inch or so. That implies that a relatively small object placed between you and the high-frequency sound source can block the sound relatively easily.

Figure 1

As a quick experiment, hook up a single audio reference monitor and have someone stand between it and your own ears. Now play back a recording of a drum kit or anything else with a lot of high and low frequencies while the person moves in and out of the line of sight. You’ll note that the highs from the cymbals are easily blocked while the kick drum and bass guitar keep passing around the blocking object.

This also suggests that while high frequencies are relatively easy to control simply by pointing a loudspeaker cabinet to the left or right, low frequencies (below 100 Hz) from a woofer can’t be directed away from an area simply by aiming the cabinet one way or another. It really is omnidirectional below 100 Hz or so.

And once low frequencies get started, they’re like water and will roll throughout the room and go everywhere you don’t want them. At that point, stopping bass that’s already filling a room is pretty difficult, requiring technologies like bass traps and absorption panels. As a result, it’s much cheaper to stop bass energy before it gets started.

Also note that the physical size of each instrument is directly related to the wavelengths of the sounds they produce (Figure 2). Take, for example, a flute. It’s about two feet long, which implies a lowest possible frequency of 250 Hz or so since it’s a half-wavelength, dipole resonator (sound from both ends of the same tube that’s energized by blowing across the mouthpiece).

Now, consider a tuba, which is basically a bent tube of around 20 feet in length. This forms a mono-pole radiator, so it can reach frequencies down to 50 Hz or so since that’s the actual wavelength of those low notes.

Figure 2

The process of producing low-frequency energy from loudspeakers follows the same formula. In a nutshell, high frequencies are typically reproduced by a tweeter or compression driver with around a 1-inch diameter diaphragm or throat, while low frequencies need to be reproduced by large cones of 15, 18 or even 21 inches in diameter that can move a lot of air.

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