Early diffractions / reflections and EQ

[HiFiNutNut]

There have been many discussions for the past a few months about early diffractions and reflections, some of which were based on earlier studies. We have basically come to an understanding that


Diffractions / reflections arrived within the first 10ms are perceived as part of the original sound therefore cause damage to the original sound and are best avoided.

Diffractions / reflections arrived within the first 2ms are most harmful.

Diffractions / reflections arrived from 10ms to 50ms may be desirable as they artificially create hall effect that simulates live music experience.



But I feel like that we have not had the full picture yet. I am making an assumption that the above theory may only apply to certain frequencies, not the entire audio bandwidth. Most likely it applies to the most critical frequency range, including 1kHz - 4kHz in which our ears are most sensitive.

For example, if we consider the success of Dr. Geddes's multiple sub approach, we would know that there is no way at low frequencies it obeys the 10ms window rule, as our ears are far less sensitive at low frequencies. For 100Hz one cycle is already 10ms. From my memory Dr. Geddes uses 500ms window for low frequency measurements, and that says something.

So to complete the whole picture for our understanding we need to derive a function of the "diffraction window" vs frequencies. Of course, this is based on my proposition that there is such a relationship between the diffraction window and frequencies, and it is likely that the window expands towards lower frequencies.


Note that this is an important piece of knowledge we should acquire. It is the basis to support the pursuit of constant / controlled directivity, low frequency management, etc.

I am not equipped with the knowledge, time, funding to conduct any such scientific studies, but would love to hear the thoughts from the experts and professionals.

Regards,
Bill

[HiFiNutNut]

I raised this because I am facing a new challenge.

I bought my house in probably the last such purchase / change of house for my life, as I am heading to 50's in a few years time. I have now dedicated a room for music. Given that I will unlikely be moving again, I can design and build my (last) pair of speakers for the room.

This is in contrast to most speakers that are designed for perfect response in an anechoic chamber, which may perform badly when room acoustic is combined. For me, I would like to design the room and speakers as one system.

I have been doing some MLS measurements of the tweeters, mid-woofers, woofers and sub-woofers in the room and been amazed to find different graphs while expanding the gated window 1ms by 1ms. It was a revelation.


Based on the theory of 10ms window, for frequencies between 1kHz and 4kHz, I guess the XO design should be based on the measurement using a 6-ms window. My tweeters are crossed at 2kHz. The response should be targeted to that of the anechoic chamber, with no floor, ceiling or wall reflections included. Although the floor and ceiling bounces come within 5ms, I have sufficient acoustic damping to counter the problems.

But for the bass with the XO point at around 180Hz, I suspect that the 10ms window theory does not apply, not to say the data points are insufficient.

I also believe that the XO / EQ should include the near boundary reflections, including floor, ceiling and front wall reflections.

I am struggling on what gated measurements I should use for the design of the low XO / EQ. An answer given to the previous post would provide an answer to my questions here.

[MethMan]

Originally posted by HiFiNutNut

Diffractions / reflections arrived within the first 10ms are perceived as part of the original sound therefore cause damage to the original sound and are best avoided.

Diffractions / reflections arrived within the first 2ms are most harmful.

Diffractions / reflections arrived from 10ms to 50ms may be desirable as they artificially create hall effect that simulates live music experience.



Regards,
Bill [/B]

I understood things other way. Reflections in 50ms interval for musical signal are suppressed by precedence efect and contribute to loudness and timbre rather than hall effect. Sense of space is a matter of delays in the +80ms range with adequate level - probably stronger than you can reach in domestic conditions. Beside it is lower frequency phenomenon.
Reflections under 10ms for 1KHz+ range are:
- not necessarily harmful, probably pleasant because of contributing to ASW (according Toole)
- harmful because they do contribute nothing and can smear exact localisation. (Geddes)

[gedlee]

I agree with Bill's data for the most part.

And yes Floyd and I disagree on the first 10 ms. Floyd's data is based on subjective preference data and yes <10 ms. sound adds to spatiousness, but it also adds coloration (confirmed in discussions with Blauert). So it depends on what you are looking for. You can have spatiousness with VER, but you have to accept coloration and lower image resolution. On the other hand, my approach is to minimize the VER for good image and low coloration while maximizing the >10 ms. reflections for good spatiuosness. This, I claim, achieves the best of both worlds, but requires a more complex system (speaker and room) design to achieve.

As regards VER with frequency, it is clear that VER at VLF are benign. We basically hear in the steady state at LF. Below 500 Hz our perception of reflections falls until below 100 Hz in a small room its all steady state percpetion with no awarness of reflections at all.

VER below 10 ms and above 500 Hz are not desireble IMO, but above 20 ms are clearly positive. 10-20 ms. is unclear. < 2 ms. is certainly bad for imaging above 1 kHz. but it can also lead to nonlinear types of sound effects.

[Paul W]

Recommended reading before you commit to anything permanent: "Sound Reproduction - Loudspeakers and Rooms" by Floyd Toole. One personal observation though, Floyd is somewhat ambiguous on the topic of very early reflections...like the comment "not necessarily harmful". He is not ambiguous on many other subjects, like direction of the reflections, etc.

Excellent book, very easy read. You might even change your mind about stereo vs multi-channel.

[gedlee]

Floyd was ambiguous about VER in his book, but in long conversations with him I nailed him down on the subject. He does say that VER "appear" (from the limited data) to be "prefered" to not having them. He also found this surprising since it is not what he expected either. However, the tests were such that what I recommend was never tried. He looked at situations with and without VER and found a preference for them, but that preference was based on a preference between the various alternatives and not all the alternatives were represented. In Floyd's style of tests it's very hard to control all the variables. For example, his directivity tests were based on a narrow directivity Quad speaker versus other wider directivity sources. But that's as much a test between the Quad's over-all sound quality and the other speaker as it is about directional versus non-directional. It's hard on some of this stuff to draw conclusions from the tests because so many variables have been changed.

His book is excellent and the only place where Floyd and I disgree is in the first 10 ms. Other than that we are completely on the same page. And in this 10 ms region, I find his data and his conclusions to be somewhat weak, certainly weaker than all else in the book.

I also find his fixation on multi-channel disappointing because I don't think that two channel is going away any time soon.

[Paul W]

Agree two-channel isn't going away anytime soon, certainly not in my lifetime.

Floyd does have me pumped about trying a serious multi-channel system...more than just HT noisemakers in the rear.

[gedlee]

Yes, I'm doing multi-channel too, but the lack of source material is a problem. Almost nothing that I listen to is in multi-channel. And even some of that is poorly mixed. It will be a decade before a wide array of quality source is available. And if current trends to music distribution and playback continue maybe even longer than that. I see movis and live concert playback going multi fairly quickly, I mean they already have, but daily music for the mass market may always be 2 channel.

[planet10]

I would speculate that, like a waterfall, the use of time is not necessarily the best "axis". One based on cycles would probably yield more useful information.

dave

[gedlee]

Partially agreed, and certainly at LFs, but above some frequency, I would guess about 1 kHz, it should be based on time. The ear actually responds more this way, sensing things sychronously at LF, i.e. cycles related, but more based on spectral intensity above 1 kHz, i.e. more like a time related sense.