Peaks & Valleys
Resonant problems can actually be improved rather easily, but only if you can accurately identify the amplitude and Q (the ratio of the center frequency divided by the bandwidth) of the resonance.
A high-resolution analyzer is essential for this task. You’re not going to see the precise characteristics of the various resonant modes on a 1/3-octave RTA (real-time analyzer); it’s simply too coarse.
Conversely, an FFT (Fast Fourier Transform) is ideal for measuring the response of a loudspeaker system in a room, as long as it has very fine resolution in the lowest five octaves of the audible spectrum ~20 Hz to ~640 Hz, because that’s where you’ll detect and defeat room resonance.
Of course, a room can’t actually be stopped from resonating with an equalizer alone, but the energy from the loudspeakers can be attenuated at the resonant frequencies. Doing so will radically improve the clarity, intelligibility, and musicality of the event.
Typically, there will be one dominant LF peak, followed by several harmonically related peaks (Figure 2). The frequencies of these peaks are a direct function of how much air volume is in the room. Other peaks, at other frequencies, may also be present in one or more compartmentalized acoustic areas, such as under or over a balcony. These areas need to be examined individually, but only after the main system has been flattened.
Figure 2: The top graph (A) depicts the three primary room resonant modes; note that they are harmonically related. The middle graph (B) shows the response of the parametric filters, accurately adjusted to attenuate the resonant peaks. The bottom graph (C) shows the final result of the correction.(click to enlarge)
Every resonant peak that is identified needs an attenuating filter that precisely cancels-out that peak. The filter should be set dead-on, on the center frequency of the room peak and as narrow in Q as possible. The goal is to surgically cancel the room peaks. Larger rooms usually require four, five, or more filters to remove the primary room resonant frequencies.
Typical amplitudes of room resonant peaks range from just +3 or +4 dB in relatively small rooms to as much as +20 dB (or more) in large rooms. Don’t be afraid to use the full extent of an equalizer’s capability to cancel the peaks. Cutting filters (attenuation) are safe, generally speaking, and will not harm the phase response of the system; in fact, if the amplitude, Q, and band-center are accurately dialed-in, the measurable result will be an improvement in the phase response of the system in those spectral regions where the cutting filters have been applied.
On rare occasions it’s acceptable to use an accentuation filter (boost) to “fill-in” a hole in the system’s response, but this should be done carefully, and with enough time to critically listen to the results with familiar tracks.
In most cases, some aspect of the loudspeaker system’s response, not a room-related issue, is being corrected. Rooms don’t often exhibit acoustic cancellations that need to be boosted, but if the architectural elements are complex enough, or the building materials unusual enough, then it’s possible.