Bass Traps
Last Edited: Dec 25, 2023
What Is the Purpose of Bass Traps?
The main purpose of the bass traps is to improve the response of the low frequency in a room by reducing peaks, raising nulls, and reducing modal decay lines. Peaks and nulls always vary in level around the room. Therefore, adding the bass traps makes the room more consistent for the listeners at different locations. Music producers and sound engineers use bass traps to reduce the low-frequency reverb time in large spaces. However, they are more commonly used in large studios and small rooms to reduce modal ringing and flatten the low-frequency response. Small rooms don't have a true reverb below their Schroeder frequency. This is because the ringing at the individual mode frequencies dominates. But bass traps are essential for reducing the low-frequency reverb in large studios, churches, or auditoriums.
Categorization
The two ways that categorize the bass traps are pressure versus velocity and broadband versus tuned. Porous materials are broadband and act on velocity, while pressure absorbers are tuned to a specific frequency. Most rooms benefit from broadband bass trapping. This is because peaks and nulls can occur at all low frequencies, not just those related to the room dimensions. A broadband bass trap could sound like an oxymoron. But, it just means it absorbs over the wide range of bass frequencies. Most tuned bass traps are useful over about one octave and highly effective over a range of half an octave or even less. Tuned bass traps help target problematic frequencies that are too low for porous absorbers to handle effectively. However, you would also need broadband traps. Bass traps are built from porous materials such as rigid fiberglass and work over a broad range, falling off at some lower frequency depending on their thickness. Tuned traps use some mass-spring resonance to establish their center frequency and add damping. The mass does not continue to vibrate after the source sound in the room ceases. Common materials for tuned pressure bass traps are wood and vinyl sheets for the mass and air trapped inside a sealed cavity to serve as a spring. Rigid fiberglass is often used inside the pressure traps to add damping.
Low-Frequency Resonances
Since low-frequency resonances in a room have points of maximum or minimum pressure in the corners, resonant bass traps in these positions will be the most efficient. At the same time, porous traps are most efficient at the points of high particle velocity, like 1/4 of the desired wavelength away from the wall. Bass traps typically attenuate modal resonances. The exact placement depends on which room mode one is trying to target. They usually combine structural mechanisms that can work at high particle velocity/low pressure (thick fiberglass) and high pressure/low particle velocity (membranes).
Foam Wedge
One type of bass trap many companies sell is a foam wedge in the room corners. Most foam wedges have a front face 15 inches across, wide enough to absorb well down to about 200 Hz. High-quality acoustic foam absorbs well, but most commercial products are too small to absorb at low-bass frequencies.
Additional resources and source texts
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