Earl,
is it in the line of the Clark Experiment (Toole, Sound reproduction, p.147 Fig.9.5 (c))? I have no doubt that this delayed signal, coming from exactly the same point (loudspeaker or earphone) as the original, can only be detrimental. But what about "the issue is arrival time, angle and level", as Markus said. Fig.9.5 (b) was evaluated as "very small effect".
Rudolf
The intelligibility is only degraded at the point where reflections cause modulations at the same range as the original signal has the main modulation bandwidth. In music it's mostly between 50ms to 500ms, corresponding modulation bandwidths of 20Hz to 2 Hz respectively. It's clear that in a small room acoustic space (e.g. 25 m2 livingroom) room reflections do not degrade intelligibility.
- Elias
Of course there can be masking effects by reflections that negatively affect intelligebility.
Sorry Earl, but you're not getting away with that degree of hand waving and accusing me of only seeing what I want to see.
You said very clearly "Since NO reflections < 10 ms. can be achieved in any reasonable sized home room it is a good idea to do two things."
To me that means you are saying it is possible to achieve a room where there are NO reflections in the first 10ms, in other words the first reflection arrives later than 10ms. If this is not what you actually meant then you need to be much more careful with the way you write, as you write in a fashion that is easily misinterpreted.
You went on further to write "2) if one can make the first reflection contra-lateral this is better than it being non-contra-lateral (same ear as signal)"
Again the clear implication is that the first reflection is the contra-lateral reflection from the opposite wall. This is clearly impossible.
Now that you've contested how you're being quoted, what I'm sure you actually mean is the first reflection of any significant amplitude is the contra-lateral reflection, rather than the floor reflection or the near wall reflection. And that you believe there should be no reflections of any significant amplitude in the first 10ms. If this is what you mean you need to be really clear about it.
These are two very different scenarios, as directivity can get the early reflections significantly low in amplitude, whilst only a room of rather large dimensions and suitable speaker placement can get the early reflections completely out of the 10ms time window. (Other than the floor reflection, which will stay stubbornly under ~2ms)
When Linkwitz talks about a minimum recommended time to first reflection of 6ms, he really means it - at least to walls. He apparently doesn't seem to count the floor reflection in his 6ms reflection recommendation.
Your more "practical" solution to floor bounce is a 2 foot thick futon laid on the floor in front of the speakers. I wonder how many wives would let that one fly Earl ? Not many, I'd wager. You either have a dedicated "man cave" listening room, (which from prior posts it appears you do) or a very tolerant wife.
My "impractical" solution is to use an Allison type arrangement where the woofer is right at the floor and the mid/tweeter are at ear level, along with a suitable crossover frequency and adjustment to the network equalization. Doesn't sound that impractical to me, when it doesn't need a 2 foot futon lying on the floor.
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Hello Graaf
You are going to clearly hear the direct sound from those speakers before any reflection. The reflections will likely fall into the 10ms window in any normal sized room. Those speakers are essentially radiating 180 degrees so there are not any discrete reflection points as shown in the diagram. That diagram is a gross over simplification of what's actually happening. You end up with a much higher percentage of reflected sound vs. a convention speaker set-up with "normal/typical" directivity.
IMHO Great for rears in an HT set-up but lousy for a good 2 channel rig. To each their own.
Rob
Attachments
Your room is too small for conventional stereo triangle !
But I have news for you: Do you realise your room is ideal for stereolithic projection with direct sound blocked ?
You can achieve 6ms between first and second ipsilateral image. And more than 9 ms between first ipsilateral image and first contralateral image. Are you not tempted to try ?
- Elias
Thinking again the resent localisation tests with pink noises, and the reason why more than 50 % of the population are not able to hear the loudspeakers as a separete sound sources but they perceive some kind of unreal image in the middle, I have found some light into the matter:
"it has been shown that physiological differences may result in some listeners performing much better than others on auditory localization tasks."
Introduction to Psychoacoustics - Module 08B
Radugazon, this brings some confort to our kind !
It could very well be that those people who cannot locate the speakers as a separate sound sources are just lacking the physiological properties to do so ! Or, it could be that their available processing power is not up to the level of the task !
- Elias
"it has been shown that physiological differences may result in some listeners performing much better than others on auditory localization tasks."
Introduction to Psychoacoustics - Module 08B
Radugazon, this brings some confort to our kind !
It could very well be that those people who cannot locate the speakers as a separate sound sources are just lacking the physiological properties to do so ! Or, it could be that their available processing power is not up to the level of the task !
- Elias
Your room is too small for conventional stereo triangle !
But I have news for you: Do you realise your room is ideal for stereolithic projection with direct sound blocked ?
You can achieve 6ms between first and second ipsilateral image. And more than 9 ms between first ipsilateral image and first contralateral image. Are you not tempted to try ?
- Elias
What does "direct sound blocked" mean? To what frequency is the direct sound blocked? Clearly low frequencies will not be blocked. Does that create a system with large phase shift (time delay) between LF and HF? (answer: yes). Will the midrange diffract around the blocking object? (answer: to some degree). What is the audible effect of a sound path that is only partially blocked? If precedence requires the direct sound to be supressed by 12 to 15dB minimum to not take precedence over the delayed sound, what is the perceived location of the source?
Inquiring minds...etc. etc.
David S.
It means direct sound is absorbed. Please check the details from the stereolith thread, there are measurements of the 'direct sound blocker' etc.
Down to freq still relevant in localisation in small room reverberant acoustic space. I got good results even 500Hz up.
Group delay is meaningless parameter in small room reverberant acoustic space in low midrange freqs.
See the measurements for exact dB values.
No perceived audible effect. It's less than 500Hz that's not blocked. Localisation is overruled by higher freqs.
The speaker is never localised.
I'm happy to hear that !
You got great potential - Elias
If I understand the design correctly the direct path from the woofer and the bounce path from the tweeter will provide a discrepancy much greater than the Blauert and Laws thresholds. You will need to explain the meaninglessness of such delays to the first order network brigade!
I saw the wavelit comparison near the end of the thread. Its hard to pull up dB values from that. Were there some dB curves? How about some in-room response measurements from the listener position with the blackage included?
David S.
Sorry - my typo, that should be "Since No reflections < 10 ms cannot be achieved ... "
Small typos can make for big confusions!
Actually the rest of it doesn't even make sense the way it was first typed, so you should have suspected that something was wrong.
Are those literature thresholds really valid in small room reverberant space with several early reflections before 10ms ? Aren't they normally determined with headphones or in anechoic chamber with clicks or with other artificialities ? And how to define group delay meaningfully at freq range where during one period will be incomming few reflections already ? Group dealy is more of an engineering parameter, but in small room I'm not sure it's applicable to perception at low to low-mid freqs. With small room I mean room size of about 25 m2.
See post #646
http://www.diyaudio.com/forums/multi-way/10962-stereolith-loudspeakers-question-13.html#post2692605
There are wavelet responses measured at listening position without and with the direct sound blocker absorber.
No dB curve, but:
"attenuates direct sound by 5-15dB above 700Hz in a low pass manner."
Note: That is not a perfected design, but a first order prototype ! It was perceptionally very educative project ! Next version ongoing.
- Elias
I don't think intelligibility is the correct term with small room reflections. Masking do lose small details yes, but it hardly affects comprehension of music.
- Elias
"Music comprehension" - even an iPhone lying under a blanket next room will do. That's not the quality in sound reproduction I'm seeking for.
We are going to split off some of the conversation in this thread into a new one. This could take a while and some posts may seem to disappear. They will return in the new thread
Tony.
Whow... that's going to take a lot of someone's spare time! Much appreciated though!
Yes, I think the Blauert and laws thresholds were found with all pass phase shift networks, probably through headphones. Still, live room or not, there are points where time disparity between high and low must be audible.
The first documented problems with time allignment were when John Hilliard, long term VP at Altec Lansing found that the 1930's Fletcher horn system, an early Hollywood 2-way theater system, had enough time delay discrrepency to turn tap dancing into a 2 arrival echo.
"When Eleanor Powell did the tap dance, we heard two taps-8 ms apart. We started to move that high-frequency back until we got to a pointwhere we had no more echo in the tap, which turned out to be somewhere in the first 2 feet. We gradually got down to 1 millisecond where we though nobody could hear the difference, and that's where we tied it off."
It would be interesting to see dB curves. It looks more like 5 to 10 dB to me but it is hard to say for sure. At those sort of differeces I'd be surprised that precedence didn't pull the image back to the blocked drivers, but I'll take your word for it.
David S.
Hello David,
I did test this with a center speaker and 2 wide speakers at 60° to the left and the right. Precedence breaks down for higher frequency instruments like hi-hats pretty early, as low as -5dB below direct signal. If wide and direct signal is the same level, the presentation is room wide and very spacious.
Blocking the direct sound isn't necessary nor do I believe it is beneficial when done as a low pass and not broadband. The trick is simply broadband reflections coming from the right directions (60° lateral and 45°/45° elevated) and at high levels (-5dB and higher).
The problem is implementation of such a bipol center. How to prevent lobing over the listening area? How to achieve the correct dispersion pattern without sending too much useless energy into the room?
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