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In Reply to: RE: Early reflections and comb filters posted by Ethan Winer on December 13, 2007 at 14:31:16
First, Klaus and I have debated a number of issues about treatment of listening rooms over the last couple of years. Klaus has certainly read a lot more of the research than I have but I think in general our debates have tended towards a "the glass is half full/the glass is half empty" sort of thing. Klaus tends to read reports indicating that something is not a serious or major problem as meaning there's no need to improve things. I tend to read some of the same stuff and I tend to read them as providing evidence for the fact that while the issue may not be a major one, things can still be improved with some degree of treatment. What tends to happen is Klaus ends up championing no treatment and I end up championing some treatment, all on the basis of the same info in some cases.
Second, you said "Likewise, he seems to equate "good" but very late reflections in a concert hall that make the orchestra appear wider than it is, to side wall reflections in a home listening room. This defies my own experience, where absorbing those reflections makes the sound stage wider, rather than the other way around."
Concert halls are larger than most listening rooms and the first reflections from surfaces other than the floor in a concert hall arrive significantly later than they do in a listening room because of that fact. If we treat the first reflections in a home listening room and, to take an extreme example, we actually absorb them sufficiently so that they become inaudible, we haven't eliminated 'first reflections", what we've done is to time shift them. Unless we reduce the room to anechoic chamber status which neither you nor I have ever recommended, there will be some audible reflections and the earliest of those to arrive become the 'first reflections' when it comes to how our brain perceives things. If we remove the reflections that would arrive first in our listening rooms, the first reflections to reach us take longer to arrive than would be the case if the room were untreated and our brain is going to interpret that increased delay in the arrival time of the first audible reflection as meaning that the room is larger.
Floyd has published research showing that the level of the earliest reflections has an effect on how we perceive spaciousness and that our perception goes through several stages. If the reflection is low enough in level to be inaudible, no contribution is made. As the reflection becomes audible, it contributes to a sense of spaciousness and assists with imaging until, at a higher level, the imaging starts to degrade as the image starts to spread laterally. Finally, at a higher level again, the reflection can be perceived as a distinct echo. The actual levels involved for these stages vary somewhat with the length of the delay in arrival times between the direct sound and the reflection.
So we have 2 factors here that contribute to our perception of space. The first is the length of the delay in arrival of the first reflection from a room surface and the other is the level of the reflection relative to that of the direct sound. We can certainly control both very effectively with absorption and somewhat less effectively with diffusion.
I suspect that the reason the orchestra in a hall appears wider than it is is simply that the listening position is well into the far field, there's a very strong and diffuse reflected sound field, and what we get is some image spread towards the side walls as a result, making the orchestra appear wider. I noticed a similar effect in my room recently when I added some diffusion to the speaker end of the room and removed the absorption at two of the first reflection points, but my feeling was that while this effect sounds 'normal' and 'good' in a concert hall, it certainly didn't sound either of those things in my room and, in fact, it sounded annoying and distracting to me. I restored the absorption and removed the diffusion after a couple of days.
It may be possible that I could find a level of treatment for the speaker end of my room between the original/later restored level and the interim level which would give some of the effect of what we perceive in a concert hall without becoming problematic but it's rather hard to actually 'dial in' different gradations of absorption and diffusion to find the perfect mix if there is one. It's certainly a lot simpler to simply rely on absorption at the first reflection points to improve the sense of spaciousness and accuracy of imaging, and to rely on the recording to provide a sense of the ambience of the recording space. That produces extremely nice results with a good live recording, much better than I would get if my room were untreated. With good studio recordings made in a relatively 'dead' space it still produces very good imaging without a sense of the space and that, too, can be very acceptable. Bad recordings, whether live or studio, are going to show as bad whatever we do but in some ways they don't sound as bad to me in my treated room as they can do in an untreated room. I suspect that's because in the treated room, the room is adding less to the sound I actually hear and what has been removed from the room sound in my room includes some things which tend to reduce the clarity of the sound. Gains in clarity are always beneficial and that's certainly one feature of room treatment that often gets ignored while we focus on things like room response, reverberation times, and the soundstage/imaging areas.
David Aiken
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