Measuring Loudspeakers, Part Three Page 8
Unlike other audio components, the perceived sound of a loudspeaker is affected by factors other than its intrinsic performance. All the previous measured acoustic parameters that have been discussed examine the loudspeaker in isolation. But loudspeakers are used in reverberant rooms rather than anechoic chambers, and the interaction between the two is complicated [69, 70, 71, 72, 73, 74]. However, my experience has been that different models of loudspeakers sound different in a consistent manner when similarly set-up and auditioned in different rooms. But it is clear that the sound perceived by the listener consists of a combination of the direct sound of the loudspeaker (the first-arrival sound, which correlates with the anechoic response, at least in the midrange and treble) and the room's reverberant field (which is affected by the loudspeaker's off-axis behavior or overall power response) [75, 76].
The question is, What is the balance between the two factors in that combination? For many years, the reviewers at Consumer Reports have held that the power response—the overall power put out by the speaker, summed over a complete sphere—dominates the perceived balance. However, this will only be true in a very large room with the listener sitting a long distance away from the speaker. However, several loudspeakers that I have measured have had the tweeter and midrange units electrically connected so that the units were 180 degrees out of phase in the crossover region between them. This results in a large suckout in that region in the measured anechoic response; this is also audible as a "hollow"-sounding coloration when the loudspeakers are listened to at a relatively close distance. However, upon investigation of some of the circumstances underlying some of the designs, it appeared that the designers were listening to their speakers at least 15' away in very large rooms. Their perception of the loudspeaker's balance was therefore mainly related to its overall power response in the room.
I have measured the in-room response of a subset of 60 loudspeakers. For historical reasons, but also to act as a check on the MLS measurements, I use both a different technique and a different test setup. The rectangular room is my own dedicated listening room. This measures around 19' by 15.5', with a 9.5' ceiling broken up by 9" vigas—raw pine logs. The room is carpeted, and there are patches of Sonex foam on the ceiling to damp primary reflections of the sound. The other wall has RPG Abffusors behind the listening seat to absorb and diffuse what would otherwise be early rear-wall reflections of the sound that might blur the stereo imaging precision. ASC Tube Traps are used in the room corners to even out the effect of the room's upper-bass resonant modes, the result being a relatively uniform reverberation time of around 200ms from the upper bass to the middle treble, falling to 150ms above 10kHz.
For this in-room spectral analysis I use an Audio Control Industrial SA-3050A spectrum analyzer with its own microphone. I average six measurements at each of 10 separate microphone positions for left and right speakers individually. These positions are arranged in a rectangular grid 8' wide by 18" high, centered on the position of my ears in the listening chair, 36" from the floor and around 9' from the loudspeaker positions. The 120 original spectra are averaged to reduce the effect of room resonant modes. What you're left with is basically a snapshot of the balance that the listener hears. Fig.35 shows a typical curve (again it's the loudspeaker whose anechoic response was shown in fig.24). This measurement has proved to give a good correlation with a loudspeaker's perceived balance in my room.
Fig.35 Typical spatially averaged, 1/3-octave response of loudspeaker in JA listening room.