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Home theaters often have an
uneven low frequency response. Room
resonant modes are widely spaced in audible low
frequencies, right within range of your subwoofer. This can create
strong peaks, which cause your sound to "boom" at these frequencies.
Bigger rooms, such as commercial movie theaters, have more closely spaced, overlapping low frequency resonant modes. These modes merge together to produce a smoother frequency response. An effective and economical way to treat home theater room mode problems is to insert a parametric equalizer before the subwoofer. A parametric equalizer has multiple "second order" filters, each of which is controlled by 3 independent parameters: center frequency, gain, and bandwidth. If these parameters are set correctly, they can effectively cancel out the effects of room modes for a listening area. The idea is to place an attenuating (or "cut") filter at the same frequency as a problem room mode. If the gain and bandwidth are matched to the room mode resonance, it will neutralize that mode. This is called "inverse filtering." Because both the room and the equalizer are linear systems, their effects are additive. Our "Parametric Equalizer Simulator" allows you to see the frequency response of a 5-channel parametric equalizer. You can observe the effects of adjusting each parameter separately. The "Frequency" parameter is the peak frequency for the boost or cut. The "dB Boost" or "dB Cut" parameter is the peak dB level for this band alone. The "Q" parameter is the sharpness of the filter band. The "bandwidth in octaves" is given next to the Q. It decreases as the Q increases, and vice-versa. For an actual room resonant mode, the Q and dB levels are linked together. A higher Q automatically gives a higher peak decibel level. But for accurate control of the characteristics of a filter band, these parameters are left independent. To download this free simulator, click here. Why do the frequencies only go as high as 100 Hz? While equalizers generally cover the entire audio spectrum, up to 20,000 Hz, this equalizer simulator is designed to explore the canceling of low frequency room resonances. These are generally confined to below 100 Hz. Why is there boost or cut mode? Why can't I boost some frequencies and cut others, just like a real parametric equalizer? Again, the idea is to confine this simulator to room mode problems. Since you are neutralizing room mode resonances, the "cut" mode should be used. This mode is also healthiest for your subwoofer. It could be overdriven if you attempted to boost some frequencies. Nulls can occur in the frequency response -- never attempt to fill them in with an equalizer. SECRET: With the simulator "Cut" check box unchecked, you can model the shapes of up to five room modes in combination, one for each filter band. Then, check the "Cut" box, and the equal and opposite inverse filter will be shown. This is the equalizer which will just cancel the effects of the room modes. back to Alpha Certification
All material copyright 2003 by Terry Montlick Laboratories LLC. All rights reserved. |
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