The result of studying these plots might lead you to conclude that in order to make measurements that correlate well with our listening experience, we must use very short time windows that isolate the direct sound at high frequencies, and increasingly longer time windows as we look at lower frequencies.
At first glance this idea might seem to violate the often quoted phrase, “One can only affect the direct sound with processing.”
However this is not the case. At mid-low and low frequencies, the interaction of a sound system and a room can be affected and optimized by signal processing.
In other words, at low frequencies (long wavelengths) the direct sound and reflections from nearby surfaces combine to form a composite response. It is this composite response that a listener hears.
The ability to measure several time windows simultaneously provides a measurement that both correlates well with human hearing and provides insight into how the signal being sent to the loudspeaker can be tailored (via equalizers, or other processing) to optimize the loudspeaker/room interaction.
Figure 5: The frequency response of the direct sound portion of an impulse response of a 1,250-seat multi-purpose hall. The response was calculated using a 8192 point FFT (which equals a 8192/48000 or ~107 msec). As you can see the frequency response shows low frequency energy that is much more pronounced than seen with the shorter time window.
Our last figure shows a measurement of a loudspeaker system that includes multiple time windows and displays both the magnitude and phase response of the “system.”
The use of multiple time windows allows one to isolate the direct sound of a loudspeaker in a real-world situation at high frequencies.
However, at lower frequencies, longer time windows that include the loudspeaker/room interaction have been found to correlate well with our listening experience.
Multiple time windows in a single measurement is an extremely interesting way to measure and optimize the response of a sound system in a room.
Sam Berkow has completed a wide variety of acoustical design projects including: concert halls, recording studios, broadcast facilities, production facilities, house of worship facilities, large multi-purpose venues, amphitheaters and stadiums. His educational background includes a masters degree in Engineering from the Stevens Institute of Technology, where he specialized in acoustic measurement and design. He is also the original developer of Smaart acoustic measurement & system optimization software.