Listening Room Dynamics & Control

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We are all familiar with the chronic listening room (or any other enclosure) problem provoked by the room low frequency resonances or standing waves (modes) and reflections. Any enclosure, especially the ones with opposing walls, including listening rooms, large vehicles such as minivans and SUVs are susceptible to this problem.

Free Listening Room Evaluation

Standing waves, especially the low frequency ones, cause wildly uneven distribution of bass energy, yielding boomy bass in some locations (e.g., near the side walls and the middle of the room along the longest dimension of the room) and inaudible bass at other spots (e.g., about 1/3^rd of the way from each wall along the longest dimension of the room). They will also cause imbalance between left and right speakers which can be as high as 20 dBs at some frequencies from 30 to 500 Hz, causing a blurring of the image. Resonant frequencies, i.e., the frequencies at which standing waves develop, depend on the dimensions of the listening room. Slide show depicting the first 4 standing waves (modes) of a room

Mode Shapes and Resonant Frequencies
At certain frequencies the dimensions of a room are integer multiples of the wavelength of the tones corresponding to those frequencies. This causes the reflection of the wave from the opposing walls reinforce each other and establish standing waves between the parallel walls. These frequencies are called resonant frequencies (known also as characteristic frequencies) of the room and their corresponding standing wave pattern are called mode shapes of the room. The resonance frequencies and the corresponding mode shapes depend primarily on the shape and size of the room. The damping depend mainly on the boundary conditions, i.e., on either acoustic impedance or the absorption at the walls. Stiff walls keep more energy in the room and make the distribution of energy highly uneven at low frequencies.

We are all The solid line in Figure 1 depicts a sound transmission from a source located in one corner of a room to a microphone located in a diagonally opposite corner. The dimensions of the room are: 10x15x20 ft. The dashed line in Figure 1 corresponds to a similarly determined sound transmission as measured in a anechoic chamber. The marked influence of the room on sound transmission is quite apparent. This is why the acousticians note that the performance of a subwoofers are dominated by the room.

Acoustic frequency response of a rectangular room (solid line) and an anechoic chamber (dashed line), at low frequencies

Control of Bass in a Listening Room
Due to the short wavelength of sound at higher frequencies the modal intensity (the number of modes in a frequency interval) at high frequencies is by far higher than that at low frequencies. This along with the high absorption effectiveness of the furniture and walls make the variation in sound intensity less noticeable at higher frequencies.
Acoustic treatment walls or having large chairs or sofas and other plush furniture in the room will not change the standing wave patterns of the bass (low frequency acoustics) in the room. Note that the absorptive ability of materials decreases with decreasing frequency (increasing wavelength) and thus damping the low frequency standing waves using sound absorption is not effective at bass frequencies. Compliant walls and ceilings on the other hand do increase absorption in the modal range to some degree and make the model peaks less obvious. Another effective technique for adding damping to the bass modes is the use of bass traps (resonator devices tuned to the resonance frequency of the mode to be damped). Such devices intended to work at frequencies below 100 Hz are large and unattractive.
An alternative to the use of bass traps is incorporating a feedback control scheme into the design of the subwoofer to add damping to the low frequency modes and smooth out the frequency response. DEICON has developed an electronic feedback control solution for reducing the boominess of sound at frequencies corresponding to the first or first few standing waves of a listening room. This control solution which will be custom tuned to a listening room, is implemented either through an existing subwoofer or an stand-alone subwoofer. In addition to the subwoofer, the control solution will only add a microphone and a small circuit into the construction of the subwoofer. Thus the additional cost of incorporating feedback control into a subwoofer over a traditional subwoofer is minimal.
Please contact DEICON for a free evaluation of your listening room bass dynamics and recommendations on how to enhance it.