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Perceptual Effects of Room Reflections

josh358

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"good for analytical hearing, not good for listening" is paradox imo.
why would you create an acoustical room if there is one in the recording?
I would agree if the recordings were totaly dry.

no matter how you construct your reflections, it will always be coloration. obviously there is no law saying "you shall listen to the cleanest reproduction possible", but in a strict was it is not HiFi
Well, that would be true if two channel stereo could reproduce what's in the original venue. But it can't.

In an anechoic chamber, two channel stereo sounds like a line of sound between the two speakers. So you need to use the room reflections to compensate partly for the missing reflections/ambiance, because having the recorded ambiance come from those two speakers doesn't cut it -- it doesn't sound real at all.

But when you're mixing or mastering, you're usually more concerned with hearing every glitch and detail on the recording, so studios usually have a dry acoustic. In my experience, it can be spookily real for a single sound or instrument, because it sounds like it's in the room with you. But it doesn't sound realistic with recordings of actual performances.
 

josh358

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Great to know so then Anthony Grimanni's comment has merit in small rooms as his work is usually for small acoustic spaces. Good to know. So the goal here is to just have a slightly different time delays from left to right? Did I understand that correctly...?
Unfortunately, it isn't that easy -- a fixed time delay would just sound weird. You have to scramble the arrival times/phase of the reflections.
Yes but I am guessing at the cost of spatial representation?
No, it shouldn't do that. You're just delaying the reflection. If you go out to 20 ms on Toole's chart, you'll see that it's still within the zone of spatial reflection (and coloration).
I wish there was chart that can give us an approximation of the general time delay of a reflection according to the distance of the speaker to the side wall that way we have a way to start designing(redesigning in my case) our rooms.
It's simple geometry. If you want to know empirically, you can find the first wall reflection by putting a mirror on the side wall so that you can see the speaker from your seat. Then stretch a string from the speaker to the reflection to your seat. The total length of the string is the distance of the path.
 

Trdat

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Trdat

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Unfortunately, it isn't that easy -- a fixed time delay would just sound weird. You have to scramble the arrival times/phase of the reflections.
Any link or article on how to do that? Or any advice...

No, it shouldn't do that. You're just delaying the reflection. If you go out to 20 ms on Toole's chart, you'll see that it's still within the zone of spatial reflection (and coloration).
Great. I'll analyse the chart more closely.

It's simple geometry. If you want to know empirically, you can find the first wall reflection by putting a mirror on the side wall so that you can see the speaker from your seat. Then stretch a string from the speaker to the reflection to your seat. The total length of the string is the distance of the path
I think what I meant is that if I knew 1.5 metres to the side gives you about 10ms delay and 2 metres gives 15ms delay that way I can choose which delay I prefer and orient my room before I set up and measure a hundred times which is all fun and games till your patients runs out. Unless of course what you mean is that there is a way to calculate the delay from the distance to path? Or I just have to measure.
 

josh358

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Any link or article on how to do that? Or any advice...



Great. I'll analyse the chart more closely.



I think what I meant is that if I knew 1.5 metres to the side gives you about 10ms delay and 2 metres gives 15ms delay that way I can choose which delay I prefer and orient my room before I set up and measure a hundred times which is all fun and games till your patients runs out. Unless of course what you mean is that there is a way to calculate the delay from the distance to path? Or I just have to measure.
You can try diffusers -- QRD or skyline, the polycylindricals don't change phase. But there really isn't an easy way to do it right. I saw a post by one guy who said that he had used numerous speakers to do it with some kind of diffusing arrangement, not sure what it was. You can also do convolving reverb with multiple small speakers in a line, but needless to say, it's elaborate. I think that would give you the most realistic concert hall sound.
 

josh358

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I think what I meant is that if I knew 1.5 metres to the side gives you about 10ms delay and 2 metres gives 15ms delay that way I can choose which delay I prefer and orient my room before I set up and measure a hundred times which is all fun and games till your patients runs out. Unless of course what you mean is that there is a way to calculate the delay from the distance to path? Or I just have to measure.
Yes, you can calculate it. It's simple geometry -- above the bass, the wall acts like a mirror. And for angles, the angle of incidence equals the angle of reflection, just as with a mirror.
 

Spkrdctr

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I think so much of this audio stuff is due to preference unknown bias, and the brain of each individual doing wonky stuff. It is amazing to me that we can still get studies where maybe the results are 80/20 . It shows that even with all the known issues, we can still get great sound or at least agree on decent sound in general. It is amazing.
 

Duke

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Sorry for coming in late -- I'm catching up on the thread, so I hope I'm not going over old ground -- but did they compare an RFZ arrangement? That would pretty much be the state of the art for control room design. It uses reflection, but with walls angled in such a way that the early reflections arrive at the mix position no less than 20 ms after the direct sound. I'd expect it to sound better than most untreated reflective spaces, as well as those that meet the ITU specs with absorption or diffusion.
It seems to me that 20 milliseconds ballpark time gap is hard to achieve outside of a dedicated RFZ-type room. Imo there is still arguably worthwhile benefit from a 10 milliseconds ballpark gap, which is the target Geddes shoots for (he cites Griesinger as his source).

Imo the ideal role of the in-room reflections is to be carriers of the venue cues on the recording, but without drawing attention to their own inherent "small room signature" cues. I don't think this ends up being radically different in practice from what you have described in your posts in this thread, though it's arguably looking at room reflections through another lens.
 
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josh358

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It seems to me that 20 milliseconds ballpark time gap is hard to achieve outside of a dedicated RFZ-type room. Imo there is still arguably worthwhile benefit from a 10 milliseconds ballpark gap, which is the target Geddes shoots for (he cites Griesinger as his source).

Imo the ideal role of the in-room reflections is to be carriers of the venue cues on the recording, but without drawing attention to their own inherent "small room signature" cues. I don't think this ends up being radically different in practice from what you have described in your posts in this thread, though it's arguably looking at room reflections through another lens.
I agree -- it's similar to what I've said and even more similar to what I haven't, because I also toe in my speakers to cross in front of the listening position. In my case, they're dipoles, but I think the general principle is the same -- the dipole null is at the first reflection point on the proximate all, so you're hearing the second reflection from the opposite wall. It also reduces the level of first reflections from the front wall, which for better or worse are an issue with dipoles. (Proximate corner reflections are an issue as well with dipoles -- they pretty much demand diffusion in the front corners!)
 

Duke

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I agree -- it's similar to what I've said and even more similar to what I haven't, because I also toe in my speakers to cross in front of the listening position. In my case, they're dipoles, but I think the general principle is the same -- the dipole null is at the first reflection point on the proximate all, so you're hearing the second reflection from the opposite wall. It also reduces the level of first reflections from the front wall, which for better or worse are an issue with dipoles. (Proximate corner reflections are an issue as well with dipoles -- they pretty much demand diffusion in the front corners!)
Very interesting!

Imo cross-firing dipoles are potentially an improvement over cross-firing monopoles if the backwave energy has a fairly long reflection path, such that it arrives after a decent amount of time delay. I used to also diffuse the backwave of SoundLabs, typically with a few fake Ficus trees.

These days I'm working with controlled-pattern monopoles, augmented by adjustable rear-firing drivers which contribute some relatively late-onset reverberant energy. So not exactly dipoles, but still polydirectionals.
 
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Here's a crude drawing of my room and what I believe are the direct speaker reflections. I believe this works mainly due to speaker design but do not know for sure. And I think it may work in a larger room provided speakers have reflector panels in place at an optimal distance. This however would require experimentation which I am not able to do.

My Room Reflections.jpg
 

josh358

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Very interesting!

Imo cross-firing dipoles are potentially an improvement over cross-firing monopoles if the backwave energy has a fairly long reflection path, such that it arrives after a decent amount of time delay. I used to also diffuse the backwave of SoundLabs, typically with a few fake Ficus trees.

These days I'm working with controlled-pattern monopoles, augmented by adjustable rear-firing drivers which contribute some relatively late-onset reverberant energy. So not exactly dipoles, but still polydirectionals.
The old ficus tree trick, LOL. One thing many don't realize is that the corner reflection can be louder than the front wall reflection with dipoles. A lot of us have discovered the benefits of diffusion in the corner independently.

Are you delaying the rear-firing drivers? That seems to me like it would be ideal.
 

josh358

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Here's a crude drawing of my room and what I believe are the direct speaker reflections. I believe this works mainly due to speaker design but do not know for sure. And I think it may work in a larger room provided speakers have reflector panels in place at an optimal distance. This however would require experimentation which I am not able to do.

View attachment 127542
That's an interesting arrangement. Have you ever seen this? It's a great way of visualizing room reflections:
slide11c.jpg
 

Duke

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Are you delaying the rear-firing drivers? That seems to me like it would be ideal.
I'm not using electronic delay in order to keep the system from requiring that much complexity, but separate inputs for the rear-firing drivers are provided so DSP delay could be added if desired, and I have at least one customer who has done so. The rear-firing drivers are aimed up-and-back such that, combined with the recommend toe-in of the main speakers, there is typically a multiple-bounce path-length-induced delay of at least ten milliseconds without taking up as much real estate as properly positioned dipoles. You can see a stand with a built-in rear-firing coaxial here, scroll down a bit.
 
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"good for analytical hearing, not good for listening" is paradox imo.
why would you create an acoustical room if there is one in the recording?
I would agree if the recordings were totaly dry.

no matter how you construct your reflections, it will always be coloration. obviously there is no law saying "you shall listen to the cleanest reproduction possible", but in a strict way it is not HiFi
What you are saying makes total sense. When I think of analytical, headphone or anechoic chamber, and an acoustic space as a properly treated room to not color the sound (least) but get clear instruments and placement in 3D with speakers not appearing to be emitting sound. It is like having the band there, you expect to be able to walk around d the band some. I also like to maintain as much SPL in the room as possible so you really have to pay attention to reflection time, absorption and diffusion.

I do not like anechoic chambers for playback but great for hearing all the good and bad of a speaker, or system, or recording.

Perhaps that makes more sence regarding real HIFI in a (semi-live) room vs analytic.
 

josh358

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I'm not using electronic delay in order to keep the system from requiring that much complexity, but separate inputs for the rear-firing drivers are provided so DSP delay could be added if desired, and I have at least one customer who has done so. The rear-firing drivers are aimed up-and-back such that, combined with the recommend toe-in of the main speakers, there is typically a multiple-bounce path-length-induced delay of at least ten milliseconds without taking up as much real estate as properly positioned dipoles. You can see a stand with a built-in rear-firing coaxial here, scroll down a bit.
Clever! I like the idea of delayed rear-firing drivers since they could be placed close to the front wall without losing a sense of depth.
 

Duke

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Clever! I like the idea of delayed rear-firing drivers since they could be placed close to the front wall without losing a sense of depth.
Thanks! Credit to my partner James Romeyn for using the vertical plane like that. Based on various things I've read I would have said that keeping the reflections in the horizontal plane was the higher priority, but Jim didn't know any better so he just tried it, and the additional time delay made a big improvement. I went from designing bipolar speakers to designing polydirectionals which deliberately use the ceiling.

We can get within a foot or so of the front wall and still hit our ten milliseconds backwave delay target in most rooms, without electronic delay. The spatial impression (including the sense of depth) changes significantly from one recording to the next so I think it is dominated by the recording rather than the playback room. I think this is because the "playback room" cues are rendered inconsistent by early reflections off some of the walls and the ceiling arriving considerably later than they "should", thereby weakening the "small room signature" package of cues, which in turn offers the recording's package of "venue cues" (whether real or engineered or both) a window of opportunity, assuming they are effectively presented. The later-arriving room reflections have the job of effectively presenting the venue cues which are on the recording (in particular the reverberation tails), so imo we don't want an overdamped room.

The claimed theoretical benefit of this approach is good imaging because the early reflections have been minimized, AND good spaciousness because we have a lot of relatively late-arrival reflections. Like a well-set-up pair of dipoles, our approach arguably offers both qualities at the same time, though with less real estate required because the backwave energy uses the room's vertical dimension.
 
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