I'm with Bert. I.e., I think the "20 Hz" thing is not about what is audible in terms of pitch. Rather, it's about what is perceptible in terms of being (subjectively) problematic. I interpret Gilford's statement to mean that spacings greater than 20 Hz give a distinct, unwanted character to the room - in his terms, the "coloration" of the sound. Spacings less than about 20 Hz, he seems to believe, are less inclined to cause these big colorations. From a modal bandwidth standpoint, this does make sense to a degree. Modal bandwidths are typically 3 to 10 Hz (according to Bonello - Everest gives a slightly wider range, I think). Therefore, if you have wide axial mode spacings, the 20 Hz criteria lends itself some creedence.
Hypothetically: Two modes, 60 and 75 Hz. Assume a bandwidth of 10 Hz. Generally, this means frequencies between 55 and 65 Hz will excite the former and frequencies between 70 and 80 Hz will excite the latter. What Gilford is saying is that these ranges are so close, their response curves tend to overlap enough to smooth out the colorations. Now consider two modes at 60 and 85 Hz, Gilford is saying that the gap between 65 Hz (the top of the first mode's bandwidth) and 80 Hz (the bottom of the second modes bandwidth) is not sufficiently small to smooth the colorations.
At least that's how I interpret it. If you listen to LF sound in a "cubic" room, you get a wide variation of build-up and (what many of my customers like to refer to as) "suck-out." A room with a better spread of room dimensions (ratios, if you like) yields smoother bass. As I said earlier, the science of this makes some sense. Whether Ethan chooses 20 Hz or 5% or 12.7 cubits for his "criterion" is largely moot - so long as he considers some sort of "criterion" as an alternative "solution." IMO.
And speaking of Bonello's "5% rule," someone else brought this up to me recently and I am at a loss. I have reread Bonello's paper three times and I am uncertain where this "5% rule" comes from. He mentions no such thing in his 1979 AES paper. What's your take on this? Is it in another paper I'm not familiar with? Is it an interpretation of something else he wrote? I'd love to get to the bottom of that one!
But, even if we assume Bonello did state such a thing, it would be applied to all the modes, not just the axials. Such was the subject of Bonello's studies. Hence, it would be a very different application than that of Gilford's "20 Hz rule."
Edited for typos - %'s and Hz's and cubits!
_________________ ---lovecow---
"You've got to be very careful if you don't know where you're going, because you might not get there." - Yogi Berra
Last edited by lovecow on Mon Mar 28, 2005 9:11 am; edited 1 time in total
eric_desart Recording Org Pro Audio Forums
Joined: May 23, 2003
Posts: 816
Location: Belgium Antwerp
With recording I mean any space that registers a signal via a signal in the room with the intent to replay it in other rooms. Whether control, tracking, mastering whatever.
A listening space is the audio end users space, where the signal is only meant to listen to. e.g; living, home theater whatever.
Lots of critical educated listeners, with a lot of background are very pleased by their listening environment, while those spaces shouldn't fulfil requirement in function of accuracy for recording.
One rarely will see a living room with huge bass traps even for people investing a lot in audio.
There our brain does amazing things.
_________________ Best Regards - Eric Desart
Also or Non Respected Past RO Acoustics & Design Moderator?
Paul Woodlock Recording Org Pro Audio Forums
Joined: Aug 7, 2001
Posts: 675
Location: Peterborough, UK
With recording I mean any space that registers a signal via a signal in the room with the intent to replay it in other rooms. Whether control, tracking, mastering whatever.
A listening space is the audio end users space, where the signal is only meant to listen to. e.g; living, home theater whatever.
Lots of critical educated listeners, with a lot of background are very pleased by their listening environment, while those spaces shouldn't fulfil requirement in function of accuracy for recording.
One rarely will see a living room with huge bass traps even for people investing a lot in audio.
There our brain does amazing things.
hehe
Yeah, smoke a couple of splifs.... Which will do the following....
Even out your frequency response
Improve stereo image
Improve detail
Improve the song
Give you the munchies
Which is just as well really, as you won't be bothered with making bass traps and reflection asborbers after toking
I'll have to think about your comments about 20hz. Seems odd. Sometimes that means that in about a week I'll have an epiphany of understanding. I thought my ear was built logarithmically.
Quote:
And speaking of Bonello's "5% rule," someone else brought this up to me recently and I am at a loss. I have reread Bonello's paper three times and I am uncertain where this "5% rule" comes from. He mentions no such thing in his 1979 AES paper. What's your take on this?
There are two things mentioned in the "Master Handbook Of Acoustics 4th" about Bonello (two references, whereas you mention you've read one reference three times).
The first is the next paragraph after the one about Gilford which I mostly quoted above, in a section called "Mode Spacing and Coloration". Perhaps I'm reading it wrong (it's happened before). I'll quote both paragraphs.
Quote:
How close together must adjacent modal frequencies be to avoid problems of coloration? Gilford [ref 4] states his opinon that an axial mode separated by more than 20 hz from the enxt axial mode will tend to be isolated acoustically. It will tend not to be excited through coupling due to overlapping skirts but will tend to act independantly. In this isolated state it can respond to a component of the signal near its own frequency and give its proportional resonant boost.
Another criterion for mode spacing has been suggested by Bonello [Refs 5 and 6] who considers all three types, not axial modes alone. He states that it is desireable to have all modal frequencies in a critical band at least 5% of their frequency apart. For example, one modal frequency at 20-hz and another at 21-hz would be barelyl acceptable. However a similar 1-hz spacing would not be acceptable at 40-hz (5% of 40hz is 2-hz). Thus we see that Gilford's concern was primarily how far apart axial modes must be spaced to avoid problems resulting from independant and uncoupled modal action. Bonello's concern has to do with separations to avoid degeneracy (coincident) effects.
If you'd like to post a few words on the last two sentences I'd appreciate it. I think it means that frequencies less than 5% apart are problems, and frequencies more than 20hz apart are unrelated (not a problem).
The second thing that's attributed to Bonello which I've seen several places is mentioned on page 348 and a graph on page 349 of MHoA-4th, is the rules that
Quote:
He divides the low end of the audible spectrum into bands 1/3 octave wide and consideres the number of modes in each band below 200hz. The 1/3 octave bands are chosen because they approximate the critical bands of the human ear.
To meet Bonello's [refs 5 and 6] criterion, each 1/3 octave should have more modes than the preceeding one, or at least the same number. Modal coincidences are not tolerated unless at least 5 modes are in that band.
Coincidences I believe are defined back on page 344 by Everest
Quote:
Zero spacings between modal frequencies are a common source of coloration. Zero spacing means that two modal frequencies are coincident (called a degeneracy by acousticians), and such degeneracies tend to overemphasize signal components at that frequency
Ref 4: Gilford, C.L.S. "The Acoustic Design of Talks Studios and Listening Rooms", Proc. Inst. Elect. Engs., 106, Part B, 27 (May 1959) 245-258. Reprinted in J. Audio Eng. Soc., 27, 1/2 (1979) 17-31
Ref 5: Bonello, O.J., "A New Computer-Aided Method for the Complete Acoustical Design of Broadcasting and Recording Studios", IEEE International Conference on Acoustics and Signal Processing, ICASSP 79, Washington 1979, p 326-329
Ref 6: Bonello, O.J., "A new Criterion for the Distribution of Normal Room Modes", J. Audio Eng. Soc. 29, 9 (Sept 1981) 597-606. Correction in 29, 12 (1981) 905.
With recording I mean any space that registers a signal via a signal in the room with the intent to replay it in other rooms. Whether control, tracking, mastering whatever.
A listening space is the audio end users space, where the signal is only meant to listen to. e.g; living, home theater whatever.
Oh Oh.
I've been using the words 'recording room' to be equivilent to the places where you'd put a microphone and an acoustic guitar and a flute and drums and where one does recording.
And 'listening room' to be equivilent to control room, mastering room, home theater, possibly a living room -- defined by the absense of a microphone where you do playback.
This discusiion of modes has got me thinking: since it is desirable in a small (< 2500 cu ft) listening room to use dimensions that are likely to distribute the modes well, i.e. evenly, wouldn't splaying at least one pair of walls actually help?
For example, between a pair of parallel walls 14 feet apart, you can predict 5 axial modes at five specific frequencies below 200 Hz beginning at about 38 Hz.
But between a pair of splayed walls that start 12 feet apart and end up 16 feet apart, what happens? I know it's more complex, but wouldn't you get many many more weaker modes (compared to the parallel walls) that are distributed infintely? Wouldn't you get an infinite number of axial modes beginning at 35 Hz (for the 16-ft distance) through 47 Hz (for the 12-ft distance) and multiples thereof? And isn't such a distribution infinitely better in the overall scheme of things for the purpose of hearing more accurately in the room?
Sufficiently splayed walls help with flutter. Rooms with splayed walls still have modes.
Everest (MHoA pg 281) "Splaying one or two walls of a sound-sensitive room does not eliminate modal problems, although it might shift them slightly and produce somewhat better diffusion" [ref 7]
Gilford [ref 10] "slanting the walls to avoid parallel surfaces ... does not remove colourations; it only makes them difficult to predict."
Everest "Massive trapezodal shaped spaces, commonly used as the outer shell of recording studio control rooms, guarantee asymmetrical low-frequency sound fields even though it is generally conceeded that symmetry with the control position is desirable."
There's a nice picture on page 283 Figure 13-11 of a non-rectangular room's modes.
ref 7: Nimura, Tadamoto and Kimo Shibayama, "Effect of Splayed Walls of a Room on Steady State Sound Transmission Characteristics", J. Accous. Soc. Am., 29, 1 (January 1957), p 85-93
ref 10: Gilford, Christopher, "Acoustics for Radio and Television Studios", (1972), London, Peter Peregrinus, Ltd.
proudtower Recording Org Pro Audio Forums
Joined: Mar 14, 2005
Posts: 145
Location: achterhoek, netherlands
I also did, next to the disco's, auditoria etc where electro acoustics are involved, a lot of control and tracking rooms:
Boast boast:
Listen to the music of Intwine, I'm myself a bit proud about how this band produced by Gordon Groothedde sounds.
And Edwin van Hoevelaak and Norus Padidar (behind Hollands biggest selling artist Frans Bauer) also work in studio's I was involved in.
(Of course I am indebted to all the people in these forums and the resources on the net for learing every day new stuff).
But I also do my best for guys with smaller budgets/rooms, and then you cannot always follow the book because the rooms are to small to do crazy acoustic stuff and you have to improvize. I'm not shure if calculating modes is here always so extremely important, although I know some acousticians who try to make customers believe that you have to do all kinds of (expensive!) calculations or else you cannot create a great room.
Understand me:
Don't say this because modes are bullshit, but some people might conclude from these discussions that recording/listening to music in a small space with not to good mode distribution is totally impossible. And that I consider exaggerated, if you catch my drift. Same goes for thse idea's that you have to design a bunch of absorbers or difussors and then, as you yourself called it so witty, wrap a room around 'm.
And don't tease me about this proudtower shit: , I ever have registered to this group but forgotten my password or something so I either could get a new password (I don't want that, I want the same pass word for all these forums) or choose another stupid name They shouldn't force me to register in all these groups on the internet, I'm far to sloppy to remember all these passwords and usernames.
proudtower Recording Org Pro Audio Forums
Joined: Mar 14, 2005
Posts: 145
Location: achterhoek, netherlands
Yeah, smoke a couple of splifs.... Which will do the following....
Never, ever when your at work or playing! Could influence your timing....
And how do you know this anyway, I thought in your country this stuff was highly illegal?
Paul Woodlock Recording Org Pro Audio Forums
Joined: Aug 7, 2001
Posts: 675
Location: Peterborough, UK
Yeah, smoke a couple of splifs.... Which will do the following....
Never, ever when your at work or playing! Could influence your timing....
And how do you know this anyway, I thought in your country this stuff was highly illegal?
hehe
Interestingly cannibis was downgraded from a Class B to Class C drug in the UK last year. A step in the right direction.
All rather silly as Alcohol is much worse for you, and isn't illegal at all.
If you'd like to post a few words on the last two sentences I'd appreciate it. I think it means that frequencies less than 5% apart are problems, and frequencies more than 20hz apart are unrelated (not a problem).
Well, I think they are somewhat unrelated. Gilford considers axials of greater importance. His "20 Hz" rule is applicable to axial mode spacing only. Bonello's "rule" (more on that below) would pertain to spacing for all the modes, not just the axials.
Thanks, btw, for posting the rest of Bonello's criteria. I think I may have miscommunicated - I don't take issue with Bonello's criteria. As I understand them:
1. Number of modes per 1/3-octave band should increase with increasing frequency, or at least be equal.
2. Overlapping modes should be avoided, unless 5 or more modes exist in the same 1/3-octave band.
What I don't get is where a "5% rule" comes into play based solely on the above? Some easy math I'm missing? Sorry.
Quote:
Ref 5: Bonello, O.J., "A New Computer-Aided Method for the Complete Acoustical Design of Broadcasting and Recording Studios", IEEE International Conference on Acoustics and Signal Processing, ICASSP 79, Washington 1979, p 326-329
Now this paper I was unaware of. I will try and track this one down to see if sheds light on the "5%" thing.
Thanks, Bob!
_________________ ---lovecow---
"You've got to be very careful if you don't know where you're going, because you might not get there." - Yogi Berra
Ref 5: Bonello, O.J., "A New Computer-Aided Method for the Complete Acoustical Design of Broadcasting and Recording Studios", IEEE International Conference on Acoustics and Signal Processing, ICASSP 79, Washington 1979, p 326-329
Now this paper I was unaware of. I will try and track this one down to see if sheds light on the "5%" thing.
I got a copy. I read it. And it's basically the same as the JAES paper. But let's try and calculate this. See if someone else came up with 5% based on something that's not so obvious:
*********
Criterion #1: The number of modes N in (1/3-octave) band x should at least be equal, if not increase. I.e.,
N(x) <= N(x+1)
If N(x+1) = 4, N(x) can equal any number from 0 to 4. How the modes comprising N(x+1) or N(x) are spaced can vary greatly, yet the criterion would still be met (or not met). E.g.,
{N(x+1) = 4} = {36, 37, 40, and 40 Hz} -note this is the 40 Hz 1/3-octave band
Spacings = 1, 3, and 0 Hz
%Spacings = 2.7%, 8.1%, and 0%
Even with "5%" exceeded in the case of the 37 to 40 Hz spacing, Criterion #1 can be met, provided:
N(x) is <= 4 (number of modes in the 31.5 Hz band is less than or equal to 4)
AND
N(x+2) >= 4 (number of modes in the 50 Hz band is greater than or equal to 4)
*********
So let's look at Criterion #2: No double modes should exist in any band with 5 or fewer modes.
In the case of this Criterion, our example set {N(x+1) = 4} from above would not pass. However, let us consider another, related set, M(x) = 6:
{M(x) = 6} = {36, 37, 40, 40, 43, and 44 Hz}
Spacings = 1, 3, 0, 3, and 1 Hz
%Spacings = 2.7%, 8.1%, 0%, 7.5%, and 2.3%
Even though there is one (1) double mode @ 40 Hz, there are 6 modes in the band. Hence, even with two (2) mode spacings that exceed "5%," Criterion #2 is met.
*********
Concluding thought: Bob, I don't know where this "5% rule" came from. Obviously, it cannot have come from the referenced Bonello papers. It is not presented by Bonello, nor is it implied by his criteria, even indirectly. My belief is that it was from some completely different source, yet it got erroneously attributed to Bonello. Once this happened - like a lot of things in this world - it became a "fact" that could forever be attributed to the wrong person.
But I do not wish to imply that the "5% rule" is a bad one. It's as good as any. (Just like the "Gilford 20 Hz rule" is as good as any.) Perhaps we should christen it the "Everest 5% rule" and see if it sticks...
_________________ ---lovecow---
"You've got to be very careful if you don't know where you're going, because you might not get there." - Yogi Berra
1. Splays of typical 5-10° for the purposes of minimizing flutter will not help with modes. This is a phenomenon Newell likes to refer to as "acoustically parallel." While 5-10° might be "non-parallel" for higher frequencies, the lows still "see" the walls as "parallel."
2. More severe splays - typically > 10° - can force the room into higher tangential and oblique modes. Since these modes are spaced closer together, they tend to yield a better sounding room. The drawback is that a large room volume is required to fully accomplish this. Newell's Recording Spaces is a good resource to learn more about this.
3. Many, many designers will say that non-parallel is non-economical in a lot of instances. To get the angles correct - e.g., if a high-quality RFZ for the control room is a goal - a good designer is a prerequisite. On the "DIY Express," though, rectangular can actually work out better than non-rectangular since there are some tools (see other thread) and experts (present company included) that can be utilized for not much moola.
_________________ ---lovecow---
"You've got to be very careful if you don't know where you're going, because you might not get there." - Yogi Berra
eric_desart Recording Org Pro Audio Forums
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A room with a better spread of room dimensions (ratios, if you like) yields smoother bass. As I said earlier, the science of this makes some sense. Whether Ethan chooses 20 Hz or 5% or 12.7 cubits for his "criterion" is largely moot - so long as he considers some sort of "criterion" as an alternative "solution." IMO. 
Also or Non Respected Past RO Acoustics & Design Moderator? 
They shouldn't force me to register in all these groups on the internet, I'm far to sloppy to remember all these passwords and usernames.
closer together, they tend to yield a better sounding room. The drawback is that a large room volume is required to fully accomplish this. Newell's 
