Audio Engineering Society

Chicago Section

Meeting Review, February 1998

other meeting reports 2/24/98 Meeting Highlights
by Brad Olson

On February 24, 1998 David Prince of International Jensen spoke to the Chicago Section on his work in room acoustics. He noted that we must measure rooms since rooms themselves distort or alter musical instrument sounds. Clearly it is the goal of acousticians to determine what characteristics in a new room will correspond with a space listeners will like, or determine why an existing room sounds the way it does.

Dave gave a conceptually simple way to analyze this problem. He recommended one finds "good" rooms and measures them. Then one can determine the subjective parameters and try to correlate measurements to people's subjective responses. Next one can determine any deficiencies which exist, set up criteria and modify the room. Finally after the room is altered, it can be measured again to verify that the improvement has occurred as planned. Mr. Prince noted that traditional analytical measurements often use omnidirectional sources and omnidirectional microphones to derive objective parameters from impulse responses. Common examples are parameters like T60, EDT, C80 and IACC. It is also possible to do subjective measurements by having artists play instruments into a room and measure responses via dummy head recordings or trained listener evaluations.

The tradeoff here is clear. One method is repeatable and objective in terms of source material, etc. but one method is much more highly correlated with a real concert hall experience including the subjectivity of such an event. David talked about prior work in instrument directivity and how it affects the way we hear music in a hall. He noted that part of this directivity is simply frequency dependent but part of it is complex and very instrument dependent, with the instrument position onstage being part of this equation as well as type of instrument. Prince did an exhaustive study of one hall instead of studying many casually. He also looked at source location and directivity, not at many receiver locations like many acousticians have done. He used a directional source instead of an omnidirectional one, to more closely approximate a real musical instrument. In this case, early reflections can be equal to or greater than the direct sound in level, and more sound may be heard by one ear than the other. He saw that a real instrument tends to have a much more "evened out" response at a seat location than one from a loudspeaker, as it sets up a radiation pattern over a longer time and slowly decays.

The problem that David ran into in his research is the fact that when he attempted to boil his work down in to quantitative parameters which could be analyzed statistically, he saw very high standard deviations in the data. This suggested that seat to seat variation is on the order of hall to hall variation making qualitative assessments difficult. However, David suggested that in reality there may be parameters which he is failing to capture which make humans able to more accurately discern hall to hall comparisons than his data suggests. In the end he concluded that there is no perfect solution to his problem of the best way to analyze the room. However, he claimed that the most promising solutions may lie ahead in the creation of virtual lab environments as advancing DSP technology and improved transducer quality give the ability to easily transport one listener to multiple acoustic environments instantly, without visual biases and other factors which may be clouding current studies. However, for now he recommended studying problems with complex stimuli to better simulate complexities of real performing groups.

He also recommended the development of a repeatable presentation which can be done in A-B fashion with trained listeners to give best responses. He also noted that traditional specs such as RT60 and C80 can correlate with perceptions, but only in a very general way since many subtleties are being lumped together in such simplified indicators. The attendees enjoyed David's presentation and we thank him for presenting.