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In Reply to: RE: Echo threshold posted by josh358 on August 04, 2010 at 05:59:14
>>I don’t know of any research that shows that first reflections affect tonal character and alter imaging, other than corresponding cues being used for determining perception thresholds for the single reflection scenario.<<
>I believe that Haas demonstrated this in 1949.<
I don’t have the Haas paper with, so I can’t comment.
>I've noticed three main phenomena as speakers were moved away from room boundaries: changes in the bass and midbass response, clearer sound (correlating to intelligibility), and better (or sometimes just different) imaging (correlating in part to differences in image spread and location).<
When moving the speakers away from room boundaries you are changing several parameters simultaneously (i.e. boundary reinforcement, room mode coupling, delay and possibly spectrum of early reflections, probably also the interaural cross-correlation), so you don’t know for sure that it’s the early reflections that are responsible for the perceived effect. In my experiments I added absorption at lateral reflection locations (Klein+Hummel) and I placed the Genelecs on a table in the middle of a large room, and placed room doors against that table to generate lateral reflections. In the first case reverberation time might have changed slightly due to the absorbers, in the second case the reflections were generated geometrically, hence without any changes to the acoustics of the setup.
Klaus
Follow Ups:
> > I've noticed three main phenomena as speakers were moved away from room boundaries: changes in the bass and midbass response, clearer sound (correlating to intelligibility), and better (or sometimes just different) imaging (correlating in part to differences in image spread and location). < <
> When moving the speakers away from room boundaries you are changing several parameters simultaneously (i.e. boundary reinforcement, room mode coupling, delay and possibly spectrum of early reflections, probably also the interaural cross-correlation), so you don’t know for sure that it’s the early reflections that are responsible for the perceived effect. <
Not all of the are due to early reflections. The bass changes are a consequence of room modes. The midbass variations are a consequence of the Allison effect. Interaural cross-correlation of the direct sound isn't an issue, since the angle subtended by the speakers is constant. That leaves early and late reflections. By rotating a pair of dipoles, it's possible to tune out the first sidewall reflections. The effect is immediately apparent. Also -- and I'm not sure why you didn't hear this -- the effect of adding diffusion or absorption at the first reflection points. I'm particularly puzzled as to why you didn't hear the presence of the doors, assuming they were at the reflection point from the perspective of your listening position. I've heard the effects of proximate boundaries many times. For example, console bounce is a common problem when setting up mini monitors.
> Also -- and I'm not sure why you didn't hear this -- the effect of adding diffusion or absorption at the first reflection points. <
As I said, these experiments were without any controls. My speakers are on the long room wall, so maybe the reflection level is already close to audibility threshold so that any additional measure won’t have audible effects.
> I'm particularly puzzled as to why you didn't hear the presence of the doors, assuming they were at the reflection point from the perspective of your listening position. <
I checked the reflection points with a mirror. As far as not hearing the presence of the doors is concerned, any image shift could have been “masked” by localisation blur, the generated comb filters could have been below detection threshold. I’m not generalizing here, but in my case reflections don’t seem to have a detrimental effect.
> I've heard the effects of proximate boundaries many times. For example, console bounce is a common problem when setting up mini monitors. <
If the desktop reflection is the only reflection, which probably is the case for nearfield listening with desktop monitors, one could, in view of the corresponding statements in Salomons’ thesis, expect an audible problem. Genelec probably has taken that particular boundary into account (http://www.aes.org/e-lib/browse.cfm?elib=13683).
> > I don’t know of any research that shows that first reflections affect tonal character and alter imaging, other than corresponding cues being used for determining perception thresholds for the single reflection scenario. < <
> I believe that Haas demonstrated this in 1949. <
I’ve looked at the original paper and the setup Haas was using was two loudspeakers at 45º, half to the left and half to the right, one as direct sound source, the other as source for the single reflection. Both loudspeakers played at the same SPL. Once again, the single source/single reflection scenario. What happens in 2-channel stereo (phantom source, see also http://www.aes.org/e-lib/browse.cfm?elib=12718) with multiple natural reflections, has not yet been thoroughly investigated, to the best of my knowledge that is.
Klaus
> I’ve looked at the original paper and the setup Haas was using was two loudspeakers at 45º, half to the left and half to the right, one as direct sound source, the other as source for the single reflection. Both loudspeakers played at the same SPL. Once again, the single source/single reflection scenario. What happens in 2-channel stereo (phantom source, see also http://www.aes.org/e-lib/browse.cfm?elib=12718) with multiple natural reflections, has not yet been thoroughly investigated, to the best of my knowledge that is. <Mine either. As far as I know, all we have are the fusion experiments and the practical experience and lore, which suggests
- Side reflections -- all reflections -- add a sense of spaciousness to a stereophonic image, but alter timbre and broaden the image when within the 10-30 msec fusion range
- Rear reflections are important to the recreation of depth and distance cues (this has been established by informal experiment)
- Linkwitz's assertion that room reflections should be spectrally similar to the original so that the brain interprets them as reverberation, a formalization I think of some very old observations regarding the spatial fidelity of dipoles and omnidirectional loudspeakers but also I think an excellent result. Again, there have been informal experiments here, e.g., the observation that adding a rear-firing tweeter to a loudspeaker with limited high frequency dispersion improves spatial rendition.
Linkwitz did do an interesting experiment and make some measurements, but his papers on the subject are more in the manner of throwing out some hypotheses and a simple experiment than a systematic investigation. The design and implementation of the sort of experiment that would significantly expand our knowledge in the manner of Haas's fusion and intelligibility experiments would I think be fairly challenging.
Edits: 08/21/10
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