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Old 13th May 2008, 08:46 PM   #141
gedlee is offline gedlee  United States
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Quote:
Originally posted by graaf


can We calculate reflections' delay the way I did it in my first post using law of reflection?
or is anthing wrong with that?

best,
graaf
No there is nothing wrong with that, but did you consider ALL rays in ALL directions impinging on ALL surfaces? I looked at what you had done and it seemed to me that you were ignoring a few key ones. Draw a map of the sources and listeners and show the rays and the distances traveled. That will tell you (and me) what we need to know. Don't forget about the floor, ceiling and back walls. Just because the source is CLOSE to the wall does not mean that it won't refelct off of the wall. Only if it is flush with the wall surface can you assume that.
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Old 13th May 2008, 08:59 PM   #142
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Quote:
Originally posted by gedlee


No there is nothing wrong with that, but did you consider ALL rays in ALL directions impinging on ALL surfaces? I looked at what you had done and it seemed to me that you were ignoring a few key ones. Draw a map of the sources and listeners and show the rays and the distances traveled. That will tell you (and me) what we need to know. Don't forget about the floor, ceiling and back walls. Just because the source is CLOSE to the wall does not mean that it won't refelct off of the wall. Only if it is flush with the wall surface can you assume that.
Including 2ndary reflections
spkr>wall>spkr cabinet>ears
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Old 13th May 2008, 09:16 PM   #143
graaf is offline graaf  Poland
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Quote:
Originally posted by gedlee

No there is nothing wrong with that, but did you consider ALL rays in ALL directions impinging on ALL surfaces? I looked at what you had done and it seemed to me that you were ignoring a few key ones. Draw a map of the sources and listeners and show the rays and the distances traveled. That will tell you (and me) what we need to know. Don't forget about the floor, ceiling and back walls. Just because the source is CLOSE to the wall does not mean that it won't refelct off of the wall. Only if it is flush with the wall surface can you assume that.
yes, as I have said:
Quote:
the positioning would give very early reflections (VER) <1 ms from adjacent wall. I hope that they would not be strong enough to become audible problem because the Fostex is quite beaming (I donít know for sure but 8-inchers are typically 7-8 dB down at 1 kHz at 75-90 degrees and 12-15 dB down at 2 kHz). I am also considering using small Carlsson style absorbers effective for midrange and high frequencies as an option.
I drew the maps although I didn't post them. I had to draw them to be able to calculate the path lenghts and delays.
I didn't forget about the floor, ceiling and back walls:
Quote:
- floor reflection would not reach the listener
- first reflections off the front wall and off the back wall would reach the listener 9.3 ms after the first wavefront
- first reflections off opposite walls would reach the listener 8.3 ms after the first wavefront
- first reflections off the ceiling would reach the listener 8.8 ms after the first wavefront (ear at height of 90 cm, ceiling at height of 300 cm)
It seems that apart from those earliest reflections <1 ms all other reflections are delayed more than 8 ms

all of this assuming that the room
Quote:
is 350 cm wide and 550 cm long.
(...) positioning the speakers against the opposite longer walls (scheme attached). Stereo basis would be 330 cm. The listener is to be located 200 cm from stereo basis, 250 cm from both speakers
my room is small and hence the delays are in the range of 8-9 ms
in a bigger room they would be easily above 10 ms

if those earliest reflections < 1 ms are problematic, I don't know, then they probably can be absorbed with Carlsson style wall absorbers (images attached earlier in the thread)

these reflections are very specific
are we sure that they are problematic? I don't know
they come from almost exactly the same direction as the direct sound, very early <1 ms (the speaker axis is <13 cm from the wall), they are low pass filtered because of the directivity of the speaker and their contribution to the overall SPL is very small

(in case of Etienne88 setup they are absorbed by the couches)

tell me - are You sure that they are audible and detrimental to the sound?
I understand that in the light of Your research we can say that they probably can be detrimental to the sound but the real question is - are they?
what if not?

best,
graaf
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Old 14th May 2008, 06:32 AM   #144
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Is there any, even simple, "beams and reflection" SW calculator or the geometry is my friend?
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Old 14th May 2008, 10:21 AM   #145
graaf is offline graaf  Poland
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Quote:
Originally posted by MethMan
Is there any, even simple, "beams and reflection" SW calculator or the geometry is my friend?
I don't know of any such calculator but fortunately the calculations are not overly complicated

just Pythagorean theorem

best,
graaf
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Old 14th May 2008, 03:14 PM   #146
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Graaf, now that I read both Beveridge's and Carlsson's papers, I can say that your loudspeaker set up is much more closer to Carlsson than Beveridge. Beveridge idea seems great in theory put in the practice, I don't know...
In our case, with a unique driver firing upward, we get an omnidirectional speaker at the low frequencies. Then, the higher it gets, the closer to a thin cone you get. I don't see any line source in that. Could you explain where you see a line source?

What I will remember from both papers is first, the importance of having a reflected sound with almost the same frequency response as the direct sound (omni or dipole being possible solutions). Second, to take into account the time delay.

I did some delay calculations with the speakers at the armrest level (driver at 60cm above the floor). By the way, it is better like this than with the lower positioning, I get more medium-highs. But then the delay calculation showed that the roof reflection arrives at 4,5ms which is not very nice since it is a difficult one to damp (WAF oblige...).
Some readings (from Olive and Toole) I did told me that: depending on the level and the delay of the reflected sound, the reflection can be perceived as an echo, a broadening of the image, spaciousness or not perceived at all. This means that if you can sufficiently damp a reflection in an undesirable time domain, it will be perceived as spaciousness or not be perceived at all!
Back to delay calculations: I have a short delay from the wall close to the LS at 0,5ms. If this reflexion is 10dB lower than direct sound it will be heard as spaciousness, if it is 20dB down, it will be inaudible. I now get why Carlsson had acoustic foam on the wall side of its speakers. I did the same!
The second short reflection is at 1,1ms from the floor. I have a sofa on both sides so that I guess that these reflections are well damped.
I already talk about the roof...
Then we've got the front one (the one hitting the wall you look at when you listen, ok!). The delay is 7,5ms. If the reflection is at least 3dB below the direct sound it is heard as spaciousness, if it is 15dB below, it is not heard at all. I put cushions in front of the window!
The reflexion from the right speaker coming on the left is at 8,6ms. I did not do anything.
I also calculated the secondary reflections, they are really hard to describe with my "poor" English level... 2 are are below 5ms, 5 are between 5 and 10 ms, the rest is above. Since they hit 2 surfaces their level is most probably lower than the direct sound (how much is another question...). Some more damping of the side walls as well as of the roof would be needed. 2 secondary reflections involving the floor are around 8 ms, the sofas are there to tame them!

Going back to Briggs, non directional and natural results are the advantages he named of upward firing units. Remember that it was the beginning of stereo and that most of the book consider mono reproduction. Nevertheless, at the end he writes about column speakers giving good results (subjectively) for stereo listening. Both type have upward firing drivers and a diffusing cone is added above the drivers. The cone can be improved to a circular parabolic diffuser which is turned from hard wood and coated with hard cellulose or shellac. The diffuser might be looking like this.

The last link, shows very nice looking LS which principle remove the need of treating the roof reflections. From that I fully understand how Earl went into horns! But I think that it is a mistake (I am young and foolish so that I can do mistakes by myself! ) since you don't get any direct reflections from the back wall and less from the side walls, floor and roof.

Regards,
Etienne
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Old 14th May 2008, 03:53 PM   #147
graaf is offline graaf  Poland
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Quote:
Originally posted by Etienne88
Graaf, now that I read both Beveridge's and Carlsson's papers, I can say that your loudspeaker set up is much more closer to Carlsson than Beveridge.
(...)
In our case, with a unique driver firing upward, we get an omnidirectional speaker at the low frequencies. Then, the higher it gets, the closer to a thin cone you get. I don't see any line source in that. Could you explain where you see a line source?
not a line source but I see the beneficial effects of a line source, as I have said my aim was to "emulate" Beveridge setup with conventional cone driver
first - wide and even dispersion in horizontal plane
second - with positioning just above the floor - the problem of floor and ceiling reflection is pratically gone

my loudspeakers are closer to Carlsson but setup is very specific and invented by Beveridge
nowhere in Carlsson paper You can read about speaker positioning against opposite walls - and this is most important in the setup I proposed
only with positioning against opposite walls You can get this much better reflection pattern and longer delays

Quote:
Originally posted by Etienne88

What I will remember from both papers is first, the importance of having a reflected sound with almost the same frequency response as the direct sound (omni or dipole being possible solutions). Second, to take into account the time delay.
that's right

Quote:
Originally posted by Etienne88

I did some delay calculations with the speakers at the armrest level (driver at 60cm above the floor). By the way, it is better like this than with the lower positioning, I get more medium-highs.
well, theoretically it should be worst because You get worst reflections
why not trying some frequency equalization instead of elevating the speakers?

Quote:
Originally posted by Etienne88

I now get why Carlsson had acoustic foam on the wall side of its speakers. I did the same!
can You hear any difference with or without those absorbers?

Quote:
Originally posted by Etienne88

Then we've got the front one (the one hitting the wall you look at when you listen, ok!). The delay is 7,5ms.
move the stereo triangle back a little and it will shift beyond 8 ms

Quote:
Originally posted by Etienne88

I also calculated the secondary reflections, they are really hard to describe with my "poor" English level... 2 are are below 5ms, 5 are between 5 and 10 ms, the rest is above.
which of them are below 5 ms?

there seem to be three time limits separating "very bad" early reflections from "just bad" and then from "not so bad" and "the good" in the end
these time limits seem to be: 5 ms, 10 ms and 20 ms
this is probably why for some the most important is to delay the reflections above 6 ms, such is for example Linkwitz's minimum recommendation

those below 5 ms are really bad

Quote:
Originally posted by Etienne88

a diffusing cone is added above the drivers. The cone can be improved to a circular parabolic diffuser
I don't know - diffusing cone means a lot of frequency dependent VER
presumably something "not right"

Quote:
Originally posted by Etienne88

But I think that it is a mistake (I am young and foolish so that I can do mistakes by myself! ) since you don't get any direct reflections from the back wall and less from the side walls, floor and roof.
sorry I don't understand this last senstence - what is a mistake?
a waveguide approach?

best,
graaf
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Old 14th May 2008, 03:58 PM   #148
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Quote:
Originally posted by graaf


I don't know of any such calculator but fortunately the calculations are not overly complicated

just Pythagorean theorem

best,
graaf

But it is about orthogonal triangle, isn't it? Tens of them. I am not convinced of practical usefulness of it. Even worse for hand calculation when two borders are included. Grafical solution could be more suitable.
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Old 14th May 2008, 04:10 PM   #149
gedlee is offline gedlee  United States
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Quote:
Originally posted by Etienne88

Some readings (from Olive and Toole) I did told me that: depending on the level and the delay of the reflected sound, the reflection can be perceived as an echo, a broadening of the image, spaciousness or not perceived at all. This means that if you can sufficiently damp a reflection in an undesirable time domain, it will be perceived as spaciousness or not be perceived at all!

This is an incorrect conclusion from what Toole said. You NEVER get a spaciousness perception from reflections below 20 ms. So you must either get rid of them below about 20 dB or they will have a negative effect.

Quote:
The second short reflection is at 1,1ms from the floor. I have a sofa on both sides so that I guess that these reflections are well damped.
I highly doubt that!

Quote:
Then we've got the front one (the one hitting the wall you look at when you listen, ok!). The delay is 7,5ms. If the reflection is at least 3dB below the direct sound it is heard as spaciousness, if it is 15dB below, it is not heard at all. I put cushions in front of the window!
As I said before this is not correct, at only 3 dB down you will get a bad coloration effect. And cusions will not decrease this reflection very much at all. You need to do actual measurements and not just guess at the effects.

Quote:
From that I fully understand how Earl went into horns! But I think that it is a mistake (I am young and foolish so that I can do mistakes by myself! ) since you don't get any direct reflections from the back wall and less from the side walls, floor and roof.
BUT I DON'T WANT side wall, floor or roof refelections thats just the point!! I DO GET backwall reflections and plenty of them since the rear of the room is highly reflective. This sets up exactly the situation that I want. No significant early reflections, with lots of delayed reflections coming mostly from the sides and rear. You aren't thinking this through.
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Old 14th May 2008, 04:18 PM   #150
graaf is offline graaf  Poland
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Quote:
Originally posted by MethMan

But it is about orthogonal triangle, isn't it? Tens of them. I am not convinced of practical usefulness of it. Even worse for hand calculation when two borders are included. Grafical solution could be more suitable.
well, yes, tens of them
this is how I did it being rather poor matematician
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