Many audio field technicians are now in possession of measurement systems that can be used to assist the listening process in equalizing sound reinforcement systems. But, they’re often surprised to find that the measured system response correlates poorly with subjective impression of how the system sounds.
In other words, the system can sound good when it looks bad on the analyzer, and it can sound bad when it looks good on the analyzer. As a result, some users have become frustrated and distrustful of analysis systems in general.
Let’s look at why the eye and ear do not always agree on what is best regarding the response of the sound system. First, consider the most popular methods of measuring the response of the sound system.
By “response,” I am referring to the magnitude of the frequency response as displayed on a dB (vertical) vs. logarithmic (horizontal) scale. The goal of technical system equalization is to produce a “flat” horizontal line on this display.
Working In Real Time
The real-time analyzer (RTA) is essentially a bank of meters, each driven with a 1/n-octave constant percentage bandwidth filter so that only the level of a limited range of frequencies is displayed by each meter.
The original RTAs used analog meters, but current versions use a vertical row of LEDs for each 1/n-octave band. One-third octave resolution is the most popular, and correlates well with the response of the human auditory system.
The measured vs. “ideal” response for the direct field of a loudspeaker.
The RTA input is fed from an omnidirectional test microphone located at a listener position. Omnis are used because they typically have a very flat, “benign” frequency response over most of their band pass.
RTAs can also be software-based, utilizing the sound card on a personal computer to provide the A/D conversion of the microphone output voltage. A mathematical algorithm (the FFT) is used to produce the previously described dB vs. frequency display.
These “digital” analyzers emulate their analog counterparts in how the information is displayed, but differ in that the filters and display is the product of a computer algorithm rather than analog filters. This type of RTA is more versatile, as the octave-fractions, colors, etc. are under software control.
Regardless of which type is used, the standard method-of-use is to drive the sound system with pink noise (equal energy per 1/n octave) and adjust the system equalizer for a “flat” magnitude response on the analyzer display.
RTAs are powerful tools when certain guidelines are followed, but indoors they can indicate a system response with poor correlation to what the listener is hearing. The major consideration is the placement of the measurement microphone.
The effect of increasing distance outdoors (top) versus indoors.
If the mic is placed in the near field of the loudspeaker (typically less than 10 feet), the correlation with human hearing is pretty good. At this position, the direct energy from the loudspeaker dominates what is being observed on the analyzer and very little of the reflected energy from the room is included in the displayed response. Adjustment of the equalizer for a flat direct sound field on the analyzer produces a desirable result.
The down side to the near-field placement is that the measured response is very sensitive to small vertical movements of the microphone when the loudspeaker has offset vertical components (as most do). This sensitivity can be reduced if the microphone is moved to a greater distance from the loudspeaker (into the far field) since the path-length difference back to the individual components becomes more equal.
But, as the microphone is moved further away, the reflected energy from the room begins to dominate the displayed response.