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20th August 2007
#31
Gear Nut

Quote:
Originally Posted by jayfrigo
Don't confuse the depth of the diffusor itself with the distance required for the scattered sound to combine to create a coherent wavefront. The Cox/D'Antonio book gives the example that since most diffusors used in listening rooms are effective above 300-500 Hz, a minimum distance of 3 meters is recommended.

You are right that this is based on three wavelengths, which at the lower frequency limit of 300 Hz would be 11.3 ft or 3.39 meters. The most effective range of the typical QRD that we see most often does not quite reach down to 300 Hz, so you can get by with my original suggestion of about 10.5 feet.

It is frequency based, so you could be closer to RPGs flutter-free than to the typical QRD we are used to seeing on control rooms. Converseley, a large diffractal such as the one in the back of Gateway Mastering requires additional distance for the lower frequency components to be most effective. For custom designs, you need to do the math.

Also, your post suggests, whether intended or not, that the smaller commercial diffusors need less distance since they are less deep, but this is not quite true. It is not three times the depth of the diffusor that is needed, rather three times the wavelength of the lowest frequency that is effectively diffused. This can be calculated by dividing the speed of sound by the frequency.
Diffusers are typically effective at the frequency that is 4 times it's depth. If you need to be 3 times the wavelength away, that would be 12 times the depth of the diffuser.

So for a 9" RPG diffractal, you would need to be (12*9" = 108" = 9") 9 feet away. If we go to a "super diffractal" which is made from a prime 7 QRD of diffractal units, we get up to 18" deep. This means that we need to be (12*1.5 ft = 18 ft) 18 feet away!

Their statement of three times the wavelength is just an educated guess of the ideal. But, if we restrict ourselved to "ideal" lots of people would simply not be able to use their rooms at all!

If you look at their designs, they clearly do not respect this distance in practice.

Yes, distance is needed from diffusers...but 10 feet is not essential doctrine for salvation.

Jason
20th August 2007
#32
Gear Nut

Quote:
Originally Posted by Ethan Winer
Bob,

I have to disagree with David's basic premise stated at the outset:

"early reflections provided by the room are 'good' data for our hearing, and they contribute mightily toward the perceived timbre and spatial details of the music coming from the speakers."

--Ethan
To really understand these issues we need to change how we refer to reflections. Early reflections and the "normal" way they are discussed are definitely a convenient shorthand, but no altogether accurate.

Reflections have:
1) Direction.
2) Timing.
3) Intensity.
4) Spectral content.

Whether reflections are audible or not is a function of timing and intensity. The earlier the reflection the more intense it has to be to be heard, the further on after the initial event and it can be lower in db and still distinguished.

Reflections that just cross the threshold of being audible widen the soundstage and increase the size of instruments without any timbral change. As they continue to be "more audible", then you hear timbral changes and eventually echo.

It is important to understand that this effect is not a binary early/late, audible/not audible. The effects of reflections are a function of time and intensity.

Direction (sidewalls and back are the best) and spectral content (the same as the initial sound is best) also matter, but I am starting to hear the yawns.

Jason
20th August 2007
#33
Lives for gear

Verified Member
Quote:
Originally Posted by Bob Olhsson
Dave Moulton did a simple demo for me in a bare room that turns most of what we thought we knew about acoustic treatment and imaging right on its ear.
Bob,

Can you invite Dave to this discussion?

Greg Reierson
Rare Form Mastering
20th August 2007
#34
Moderator

Verified Member
Quote:
Originally Posted by Jason Jones
Yes, distance is needed from diffusers...but 10 feet is not essential doctrine for salvation.
You can't give a hard, fast number as it changes with frequency, and indeed with the design of the diffussor; but for diffusors on side walls at first reflection points, it's a more important spot than behind your head, so I'd be more concerned with short distances there.

The bigger point is that you can simply choose better and more appropriate treatment for the application at hand. Use diffusors calculated for higher frequencies that need less distance and couple that with geometry changes and/or absorption to accomplish your goals. Just because some people ignore the distance doesn't mean you should; and just because one particular size of QRD has often been seen in studios does not mean it's the right tool for the job in this case. Maybe 2D instead of 1D... maybe abfusor... maybe a smaller custom QRD, maybe flutter free on top of an angled reflector... It's a complex system, not a simple "one size fits all" problem.
20th August 2007
#35
Moderator

Verified Member
Quote:
Originally Posted by Lupo
The point is still the same: there are designs that works in the closer field and designs that needs more than 10.5 feet. I think that rule is a bit outdated since it only works on one design frequency.
Definitely true; this is why I kept stating "for the typical QRD we most often see in studios" or something like that. 10.5 feet works best for that one particular RPG that we see all the time. And sure, if you cheat a little and get 9.5 ft., you're probably OK, but 6.5 ft. would make me look at a different product. I hope my point is clear that this may not be the appropriate choice of diffusor for all cases; and I think you are essentially making the same point. Choosing a 2D diffusor aimed at higher frequencies may work better in the application of side walls (Russ Berger designed Space Array maybe? Don't know - haven't used it yet or run the numbers), though you still have to fit it into a whole-room plan. You can't ignore where the reflections are going, and can't ignore what happens below or outside the effective range of the diffusor.
20th August 2007
#36
Gear Nut

Quote:
Originally Posted by jayfrigo
You can't give a hard, fast number as it changes with frequency, and indeed with the design of the diffussor; but for diffusors on side walls at first reflection points, it's a more important spot than behind your head, so I'd be more concerned with short distances there.

The bigger point is that you can simply choose better and more appropriate treatment for the application at hand. Use diffusors calculated for higher frequencies that need less distance and couple that with geometry changes and/or absorption to accomplish your goals. Just because some people ignore the distance doesn't mean you should; and just because one particular size of QRD has often been seen in studios does not mean it's the right tool for the job in this case. Maybe 2D instead of 1D... maybe abfusor... maybe a smaller custom QRD, maybe flutter free on top of an angled reflector... It's a complex system, not a simple "one size fits all" problem.
I agree that early sidewall reflections are easier to screw up with an improper diffuser choice. In fact for most rooms, a "normal" QRD is a poor choice for early sidewall reflections.

It is not just "some people" that ignore the 3 x freq "rule". It is the people that came up with the rule and they do it on a regular basis. I am not saying forget it, but, particularly in the back of the room it is a good choice to be flexible here. Though, if you are talking a prime 7 QRD at an early sidewall reflection point at 6.5', I would agree this is not good.

Though, at reflection points, if you are using a high frequency only diffuser, you will still get a strong specular reflection at lower frequencies. The Space Arrays and the flutter free only start to work around 2 khz. You can mount on an angled panel, but that will give you reflections that are different in spectrum from the original signal.....another compromise.

Jason
20th August 2007
#37
Gear Nut

Quote:
Originally Posted by jayfrigo
......though you still have to fit it into a whole-room plan. You can't ignore where the reflections are going, and can't ignore what happens below or outside the effective range of the diffusor.
Exactly!

Jason
20th August 2007
#38
Gear Guru

Quote:
Originally Posted by Jason Jones
Reflections have:
1) Direction.
2) Timing.
3) Intensity.
4) Spectral content.
Okay.

Quote:
Whether reflections are audible or not is a function of timing and intensity.
How do you define audible? As distinct echoes or as having any effect on the sound?

Quote:
The earlier the reflection the more intense it has to be to be heard, the further on after the initial event and it can be lower in db and still distinguished.
Okay, that sounds like you mean echoes. To my way of thinking, the main - heck, the only - impact of early reflections is the skewed response due to comb filtering. That is, if they're early you don't hear an echo, so all that's left is the skewed response. The stronger the reflection, the more severe the peaks and nulls are.

Quote:
I am starting to hear the yawns.
LOL, not from me! Good discussion.

--Ethan
21st August 2007
#39
Gear Nut

Quote:
Originally Posted by Ethan Winer
Okay.

How do you define audible? As distinct echoes or as having any effect on the sound?.......................

LOL, not from me! Good discussion.

--Ethan
I am defining audible as having any effect on the sound. Though, I can see how my post might have been confusing.

My point is that this whole process is a continuum. A barely audible reflection will give increased soundstage and size to instruments. If we turn up the reflection more, eventually it will transition into timbral coloring. Turn it up some more and eventually, it will be an echo and then a separate source sound.

This is not unique to early reflections or late reflections. They all do the same thing. They all go through the same continuum. Though, they will have different thresholds (in absolute terms) to determine there audible effects.

The direction and spectral content also throw some curveballs - or in some cases even spitballs.

Jason
21st August 2007
#40
Lives for gear

Verified Member
Still very interested in the 50ms area to which "sir" bob is reffering ... but the discussions is making a few things clear ... thanks all ...

how far is 50Ms ... I never was very good in maths at school-time

340M/S 1Ms=340/1000=0.34M/ms

50ms = 50*0.34M=17Meters ... ??? is this correct ...

I agree and understand that placing the diffusors ( QRD/Skylines ) on the side-walls at the first reflection points is a bit tricky ...
but what about placing them on the wall behind the speakers .... which is about 3.8 meters from the mastering-console
21st August 2007
#41
Gear Nut

Quote:
Originally Posted by inlinenl
Still very interested in the 50ms area to which "sir" bob is reffering ... but the discussions is making a few things clear ... thanks all ...

how far is 50Ms ... I never was very good in maths at school-time

340M/S 1Ms=340/1000=0.34M/ms

50ms = 50*0.34M=17Meters ... ??? is this correct ...

I agree and understand that placing the diffusors ( QRD/Skylines ) on the side-walls at the first reflection points is a bit tricky ...
but what about placing them on the wall behind the speakers .... which is about 3.8 meters from the mastering-console
50 ms = .005*340M/S = 1.7 meters

This would be extra distance for the path of the reflection vs. the distance of the source sound.

I think with that distance you would be fine going with diffusors on the front wall. Are you planning on covering the wall, or just hitting a couple of spots? That will make a difference on diffusor choice and how you want to set them up.

But, I must add, there really is nothing special about the first 50 ms.

Jason
21st August 2007
#42
Lives for gear

Verified Member
>50 ms = .005*340M/S = 1.7 meters

One zero too much, it's one 20'th of a second, or 340/20 = 17 meters.

>But, I must add, there really is nothing special about the first 50 ms.

Hehe.. Well, I guess that's correct if you look hard enough at it - which I belive you've done a very long time ago. =) I see your point, but to answer Wims question:

20-50 ms is the classic spec of the ears integration window. From the first transient/start of an aural event, the ear/brain waits 20-50 millisec before deciding what's going on. (much simplified) Any echoes which are not too loud to be detected as a separate event, and too low to be detected at all, is integrated into the frequency response of the original sound. This is the cause of the flanger/chorus effect of short delays. In acoustics, the short reflections ofte cause the same flanger/chorus effect - comb filtering.

Andreas
21st August 2007
#43
Gear Nut

Quote:
Originally Posted by Lupo
>50 ms = .005*340M/S = 1.7 meters

One zero too much, it's one 20'th of a second, or 340/20 = 17 meters.

>But, I must add, there really is nothing special about the first 50 ms.

Hehe.. Well, I guess that's correct if you look hard enough at it - which I belive you've done a very long time ago. =) I see your point, but to answer Wims question:

20-50 ms is the classic spec of the ears integration window. From the first transient/start of an aural event, the ear/brain waits 20-50 millisec before deciding what's going on. (much simplified) Any echoes which are not too loud to be detected as a separate event, and too low to be detected at all, is integrated into the frequency response of the original sound. This is the cause of the flanger/chorus effect of short delays. In acoustics, the short reflections ofte cause the same flanger/chorus effect - comb filtering.

Andreas
Mea culpa on the math...I keyed on the 5 and the milli and blew it!

As to your second point, I will quote Floyd Toole from 2006:

“Unfortunately the audio engineering literature has several reinterpretations of this result, including the notion that there is masking or fusion in more respects than just localization, within an interval of about 20-30 ms, which has been called the Haas zone...This is not so....There is no range of delays or time zone wherein certain things happen that do not happen elsewhere.”

Jason
21st August 2007
#44
Lives for gear

Verified Member
some off you're answers are putting me on a roller-coaster ... well I asked for the ride anyway ....

is it 1.7 or 17 lupo/jason !!!!

thanks alll
21st August 2007
#45
Gear Guru

Quote:
Originally Posted by inlinenl
is it 1.7 or 17 lupo/jason !!!!
Sound travels at a rate of about one foot per millisecond. heh

We Americans sure do have that one easy!

--Ethan
22nd August 2007
#46
Lives for gear

Verified Member
Quote:
Originally Posted by Jason Jones
As to your second point, I will quote Floyd Toole from 2006:

“Unfortunately the audio engineering literature has several reinterpretations of this result, including the notion that there is masking or fusion in more respects than just localization, within an interval of about 20-30 ms, which has been called the Haas zone...This is not so....There is no range of delays or time zone wherein certain things happen that do not happen elsewhere.”

From Brian C. J. Moores Introduction to Psychology of Hearing(fifth edition), page 253 - Precedence Effect:

"1. Two brief sounds that reach the ears in close succesion are heard as a single sound if the interval between them is sufficiently short. The interval over which fusion takes place is not the same for all types of sounds, The upper limit of the interval is about 5ms for single ckicks, but may be as long 40 ms for sounds of a complex character, such as speech or music. It is as if the lagging sound, or echo, has ben suppressed from conscious perception. This effect has beeen called "echo suppresion." However, the percept does change if the echo is removed.

2. If two succesive sounds are heard as fused, the location of the total sound is determined largely by the location of the first sound. This is known as the "precedence effect", although it has also been called the "Haas effect, after Haas (1951), and the "law of the first wavefront" (Blauvert, 1997). .... "

(it continues elaborating the subject for several pages).

I believe this effect, along with the comb filtering of mixing complex signals, is what gives rise to the 20-50ms talk. Cleaning up the first 20ms of the room impulse response is suggested in the standard litterature (Everst, Cox / D'Antonio) and is reffered to as a common trait among great listening rooms. I refer once again on the illustration on page 19 in this .PDF. A standard for desirable reverb time was requested in this thread, but I feel it is at least as important to pay attention to the first few dozen millisecs of the impulse response. But I'm just an amateur with too many books, so take it for what it's worth.

Cheers,

Andreas Nordenstam
22nd August 2007
#47
Lives for gear

Verified Member
Hello Lupo, Andreas

really enjoy you're answers. I value you're input .. already printed out the PDF a few days ago .. gonna stick with it for a while ..

A standard for desirable reverb time was requested in this thread : well until now nobody is able to give me some hard-numbers .... why ...
no hard numbers on rev. times in the PDF .. or did I miss it ..
23rd August 2007
#48
Lives for gear

Verified Member
Quote:
Originally Posted by inlinenl
A standard for desirable reverb time was requested in this thread[/B] : well until now nobody is able to give me some hard-numbers .... why ...
no hard numbers on rev. times in the PDF .. or did I miss it ..
How about something in the 200-300mS range across the spectrum? Much easier said than done!

Greg Reierson
Rare Form Mastering
23rd August 2007
#49
Moderator

Verified Member
Quote:
Originally Posted by Greg Reierson
How about something in the 200-300mS range across the spectrum? Much easier said than done!
If you really would like more discussion on RT60 for mastering rooms, you'd be within reason even if you went up to 400ms (.4 sec), and while 200 (.2s) is OK too, it is perhaps a bit on the low side. So, there's a range for you to think about.

Unfortunately, reverb times in small rooms aren't as useful as one might think, considering the cutoff frequency - the frequency below which modal behavor is a big pain in the butt as opposed to a nice dense spacing as in larger rooms ( essentialy... wait for it... reverberation!). Reverb times are more telling in larger rooms. You must remember to look at the entire spectrum, and not one band only. The RT60 will be different for different frequencies, and you do indeed want to get the range across the spectrum pretty even.

The other important thing to consider is ringing at problem frequencies. Ringing can skew the results of your RT60, or conversely, octave band measurements may not indicate the severity of a ringing problem. Either way, we're back to reverb times not being as useful as you might think for small room acoustics.
23rd August 2007
#50
Lives for gear

Verified Member
Hi!

Am always happy to be able to help! Especially in this esteemed company.

Quote:
Originally Posted by inlinenl
A standard for desirable reverb time was requested in this thread : well until now nobody is able to give me some hard-numbers .... why ...
no hard numbers on rev. times in the PDF .. or did I miss it ..
It's a curve, not a single figure. It can be found in the pic posted earlier in this thread! :-) https://www.gearslutz.com/board/attac...agalmtijd2.gif

EDIT: Crossposting going on. What Jay said! )
23rd August 2007
#51
Lives for gear

Verified Member
Quote:
Originally Posted by Greg Reierson
How about something in the 200-300mS range across the spectrum? Much easier said than done!

Greg Reierson
Rare Form Mastering
Thanks Greg for giving some hard-numbers , so mine isn't that BAD ( see the pic I added before of my room ) ...
23rd August 2007
#52
Lives for gear

Verified Member
Quote:
Originally Posted by jayfrigo
If you really would like more discussion on RT60 for mastering rooms, you'd be within reason even if you went up to 400ms (.4 sec), and while 200 (.2s) is OK too, it is perhaps a bit on the low side. So, there's a range for you to think about.

Unfortunately, reverb times in small rooms aren't as useful as one might think, considering the cutoff frequency - the frequency below which modal behavor is a big pain in the butt as opposed to a nice dense spacing as in larger rooms ( essentialy... wait for it... reverberation!). Reverb times are more telling in larger rooms. You must remember to look at the entire spectrum, and not one band only. The RT60 will be different for different frequencies, and you do indeed want to get the range across the spectrum pretty even.

The other important thing to consider is ringing at problem frequencies. Ringing can skew the results of your RT60, or conversely, octave band measurements may not indicate the severity of a ringing problem. Either way, we're back to reverb times not being as useful as you might think for small room acoustics.

thanks Jay ... what is small ... what is big ..... I think I have a medium-room .... 8M x 4.8M .... so my rev times are pretty OK ( we will have a new messuring-session soon, because they were not 100% consistent ) ..... just wanted to have some feedback on the first values found in my room .. giving away my reberb-times in the picture I added before ... ( secret bussines info ??? ) as having the new room it gives a bit off confidence to know they're within limits .... thank's Wim

Q : anybody willing to give away their EQ responses .. would love to see the sterlingsound EQ responses .. Jay you should have a few off you're own ... old rooms will be okay too ... please .. any other current GS ME ????

soon we will have a "room-plugin" or sexy-hardware-box ( like that sony-conv-reverb ) with a meassure mike and auto-EQ-adjust you can choose an EQ response " some default's like .. sterling-chris/bob-ludwig/jay/wisseloord etc. etc. heh

...... I have all within 4 dB after 100Hz ... ( not telling the low-end .. I'm scared )

I'm off for a short holiday to the coast with my girl and three kids ( they're already there ) ... I should leave now ... !! shut it down now .. I can't let it go .... let it go ...
23rd August 2007
#53
Moderator

Verified Member
Quote:
Originally Posted by inlinenl
thanks Jay ... what is small ... what is big ..... I think I have a medium-room .... 8M x 4.8M ....
Acoustically speaking, that's still firmly in the "small room acoustics" category. To get out of small room acoustics, the cutoff frequency has to be low enough so that low frequency modal problems start to become less pronounced and the modal desity and spacing improves. This requires a prety large space, something like a recital hall, or perhaps a very large tracking room. Even the largest of control rooms are considered small rooms for acoustics purposes.
23rd August 2007
#54
Gear Nut

Quote:
Originally Posted by Lupo

From Brian C. J. Moores Introduction to Psychology of Hearing(fifth edition), page 253 - Precedence Effect:

"1. Two brief sounds that reach the ears in close succesion are heard as a single sound if the interval between them is sufficiently short. The interval over which fusion takes place is not the same for all types of sounds, The upper limit of the interval is about 5ms for single ckicks, but may be as long 40 ms for sounds of a complex character, such as speech or music. It is as if the lagging sound, or echo, has ben suppressed from conscious perception. This effect has beeen called "echo suppresion." However, the percept does change if the echo is removed.

2. If two succesive sounds are heard as fused, the location of the total sound is determined largely by the location of the first sound. This is known as the "precedence effect", although it has also been called the "Haas effect, after Haas (1951), and the "law of the first wavefront" (Blauvert, 1997). .... "

(it continues elaborating the subject for several pages).

I believe this effect, along with the comb filtering of mixing complex signals, is what gives rise to the 20-50ms talk. Cleaning up the first 20ms of the room impulse response is suggested in the standard litterature (Everst, Cox / D'Antonio) and is reffered to as a common trait among great listening rooms. I refer once again on the illustration on page 19 in this .PDF. A standard for desirable reverb time was requested in this thread, but I feel it is at least as important to pay attention to the first few dozen millisecs of the impulse response. But I'm just an amateur with too many books, so take it for what it's worth.

Cheers,

Andreas Nordenstam
The fusion zone is a function of time and relative db. A graph of the data shows the level of sound, for any time delay, where the reflection will not be heard as a "separate spatial event". The threshold is higher for shorter times and decays relatively linearly, but it is still there nonethless. Maybe if I can figure out my scanner I could post some graphs, if anyone is interested.

Comb filtering can be bad...no beefs with that. The typical listening room needs control of the "normal" early reflection points. But you can get more out of your room with a more detailed analysis of reflection control and diffusor use....this is where you get into the location of the reflections, spectral content, etc. etc.

BTW, Andreas, there is no such thing as too many books.

Jason
24th August 2007
#55
Motown legend

Verified Member
Quote:
Originally Posted by lucey
Sounds interesting Bob, what did he do and what did you think?
He had designed some speakers that deliver a flat response across 180 degrees. The imaging in the bare room was holographic, among the best I've ever heard. His conclusion is that early reflections aren't any problem at all but early reflections that don't have a flat frequency response are a big problem because they change the perceived tonality of the speaker.

Something that led him to this was the fact that many of the best translating control rooms a lot of us "old timers" ever encountered did not have any early reflection treatment while some of the worst have been certified LEDE rooms.
24th August 2007
#56
Gear Guru

Verified Member
Quote:
Originally Posted by Bob Olhsson
He had designed some speakers that deliver a flat response across 180 degrees. The imaging in the bare room was holographic, among the best I've ever heard. His conclusion is that early reflections aren't any problem at all but early reflections that don't have a flat frequency response are a big problem because they change the perceived tonality of the speaker.

Something that led him to this was the fact that many of the best translating control rooms a lot of us "old timers" ever encountered did not have any early reflection treatment while some of the worst have been certified LEDE rooms.
Interesting. Seems similar to the Bose live PA speaker design.

Room tuning is definitely more art/science than hard science IMO, so many interactive variables.
26th August 2007
#57
Motown legend

Verified Member
Bose uses reflected sound which is something very different.

The problem with room tuning and early reflection absorption is that they are oversimplifications that have little to do with how we actually hear. Dave actually has serious academic credentials in addition to having been an engineer at Columbia Records. He also, last I heard, wasn't in the room design business.
26th August 2007
#58
Moderator

Verified Member
Quote:
Originally Posted by Bob Olhsson
He had designed some speakers that deliver a flat response across 180 degrees. The imaging in the bare room was holographic, among the best I've ever heard.
And herein lies one problem in trying to impliment this in a DIY situation with typical speakers. As recalled in one of my earlier posts in the thread, wide dispersion monitors are a part of what makes this approach work.
14th September 2007
#59
Lives for gear

Quote:
Originally Posted by Bob Olhsson
He had designed some speakers that deliver a flat response across 180 degrees. The imaging in the bare room was holographic, among the best I've ever heard. His conclusion is that early reflections aren't any problem at all but early reflections that don't have a flat frequency response are a big problem because they change the perceived tonality of the speaker.

Something that led him to this was the fact that many of the best translating control rooms a lot of us "old timers" ever encountered did not have any early reflection treatment while some of the worst have been certified LEDE rooms.
Came across this on David's website. Looks interesting!
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