Bob Gold's Room Mode Calculator

[Sixstringer]

Hi folks!!

I just ran my control room dimensions through Bob Gold's calculator and I must say, I don't really understand the results.

http://www.bobgolds.com/Mode/RoomModes.htm

I have mostly green so I am assuming that is good.

Here are my room dimensions in inches.......

220" L x 150" W x 101.25" H

Is anyone able to decipher the results for me?

Thank you!

[Sixstringer]

Just trying to get some love for my post here.

[Soundman2020]

I just ran my control room dimensions through Bob Gold's calculator and I must say, I don't really understand the results. ... 220" L x 150" W x 101.25" H ... Is anyone able to decipher the results for me?

Those are actually pretty good dimensions! Very nice. Smooth distribution of modes in the low end, and overall size is nice too. That should work out nicely for a control room.

Just trying to get some love for my post here.

- Stuart -

[Sixstringer]

I just ran my control room dimensions through Bob Gold's calculator and I must say, I don't really understand the results. ... 220" L x 150" W x 101.25" H ... Is anyone able to decipher the results for me?

Those are actually pretty good dimensions! Very nice. Smooth distribution of modes in the low end, and overall size is nice too. That should work out nicely for a control room.

Just trying to get some love for my post here.

- Stuart -

Thanks for the reply, Stuart.

I framed/built the control room dimensions based on the ratios that are recommended on this site. I was actually surprised that once finished, the results on the mode calculator turned out to be positive.

I am pretty sure I know your answer to the next question, but, do you have any idea how much acoustic absorption or trapping I am going to need in this room?

Thanks again!

[Soundman2020]

I framed/built the control room dimensions based on the ratios that are recommended on this site. I was actually surprised that once finished, the results on the mode calculator turned out to be positive.

Don't you just love it when reality matches theory!

I am pretty sure I know your answer to the next question, but, do you have any idea how much acoustic absorption or trapping I am going to need in this room?

According to Bob, you need exactly 434 sabins of absorption. That's the same as 434 square feet of "perfect absorption", which of course does not exist in the real world.

Plus, it's not just the total amount that matters: it's where you place it, and how thick you make it at each point... That's where the art of studio design comes in....


- Stuart -

[Sixstringer]

How would you calculate the amount of absorption in a "Superchunk"? I am making them for four corners (each wall to wall connection).

[Soundman2020]

How would you calculate the amount of absorption in a "Superchunk"?

Use the surface area of the diagonal surface that faces the room. So for example, if the front face is 2 feet wide and 8 feet high, then that's 16 square feet, which would be 16 sabins of absorption, if it were "perfect". Of course, there is no such thing as a "perfect" absorber, but a superchunk comes pretty close. It's a bit more complicated than just "how many square feet of surface area", but that will give you a rough estimate. So if you had four such superchunks, then you'd have about 64 sabins of absorption.

- Stuart -

[Sixstringer]

Gad Zooks!!

434 sabins would basically cover the entire room. Sheesh!

[Soundman2020]

Gad Zooks!!

434 sabins would basically cover the entire room. Sheesh!

Yup! Nobody ever said that small rooms are easy to treat, acoustically...

Actually, it's not as bad as it sounds. If you look a bit further down the information column on Bob's calculator, it tells you how much area you have on each surface, plus also some combinations of surfaces. The ceiling pretty much has to be 100% covered, and so does the rear wall, so you can just make up the difference with the side walls and the front.

This is another reason why John's "inside-out" construction is such a good idea for small rooms...


- Stuart -

[Sixstringer]

Down at the bottom of all that info, on Bob's site, it says....

"Frequency Regions:
- No modal boost: 1hz to 30hz
- Room Modes dominate: 30hz to 115hz
- Diffraction and Diffusion dominate: 115hz to 460hz
- Specular reflections and ray accoustics prevail: 460hz to 20000hz
Count (30.8-205hz) : Axials=13, Tangentials=54, Obliques=72
Count (30.8-100hz) : Axials=6, Tangentials=6, Obliques=1
Critical Distance (direct = reverberant field): 13.70ft"

Is this telling me that I shouldn't have issues at 1Hz to 30Hz, but will have problems from 30Hz to 115Hz (meaning I need a lot of bass trapping)?

What does the "diffraction and diffusion dominate" mean exactly?


Thanks so much!!

[Soundman2020]

Is this telling me that I shouldn't have issues at 1Hz to 30Hz, but will have problems from 30Hz to 115Hz (meaning I need a lot of bass trapping)?

Wellll.... sort of! Your conclusion is correct, though: you do need lots of bass trapping!

But what that actually means is that the room does not support any modes below 30 Hz: that's the lowest modal frequency, so from there on down there is nothing that modes can do. that doesn't mean there aren't any problems down lower: just that there are no MODAL problems.

Now, the fact that your very lowest mode is at 30 Hz. tells you something else: it is a small room, and there are likely very big gaps between the first few modes, which could be a problem. Bigger rooms have many more modes, and the go down much lower, so there are several modes for each possible note, which is good. But smaller rooms only have a very few modes in the low end, so they are like honest politicians "very few, and very far between". That's not such a good thing.

So basically that line means that you will have modal issues between 30 Hz and 115 Hz, and possibly other issues to, but mostly modal, since there just aren't enough of them to go around, and spread evenly across the spectrum.

What does the "diffraction and diffusion dominate" mean exactly?

In that frequency range (115hz to 460hz in your case), the modes are now close enough together and numerous enough that you have good modal support for each note, without favoring one over another. So modes are not an issue in this region. The biggest acoustic characteristics in this range are now involving not modes, but diffraction (waves "bending" around edges) and diffusion (waves "scattering" as they hit surfaces). Those are the dominant or most prominent acoustic effects in this range. Above 460 Hz, those two are no longer the biggest issue, and neither are modes: now it is purely reflection issues that you will be dealing with, as everything above that basically just bounces off hard surfaces, much like light rays reflecting off a mirror.

This also gives you a hint at what you need to concentrate in terms of treatment for each region: bass trapping needs to be "tuned" to deal with the 30-115 Hz range, surfaces need to be angled and absorption provided to deal with the 115-460 range, and above that, thinner absorption and more careful reflection control, possibly even diffusion at very high frequencies. In a larger room, diffusion devices would be more of an option for you, but they don't work well in small rooms because of the lobing issue.

So that's basically what it is telling you: what problems are dominant for each frequency range, which shows you the right path to take for treating that.

The final one is interesting: "Critical Distance (direct = reverberant field): 13.70ft". You didn't ask about that, but it is useful to know. That's the distance that separates the "near" field from the "far" field, or the "direct" field form the "reverberant" field. Way too often people buy into the hype put out by speaker manufacturers about their "near-field" and "far-field" and even the curious "mid-field" monitors, and why theirs are so much better for "creating" a near-field than anyone else's speakers. All garbage: The speaker manufacturer can do nothing at all about the near field and far field, since those are characteristics defined by the room itself, not by the speakers! The "near field" is the area within the room where the direct sound from the speakers is dominant, meaning louder than the reverberant field, and it occurs at a specific distance, which is set by the room dimensions. If you are closer than that to the speakers, then you are in the near field, regardless of the marketing hype that is printed on your speaker boxes, and if you are more distant than that, then you are in the far field (once again, regardless of what the speaker manual says). The "mid field" is, therefore, non-existent as a concept in room acoustics. It would have to be the exact distance at which near-field and far field meet, which is called the "critical distance". You can actually measure where that point is in your room, with a sound level meter. Close to the speaker, the level drops off fast, at 6 dB per distance doubling: that's the near field. Far away, the level drops at 3 dB per distance doubling, or even less: that's the far field. Walk around your room with a meter in your hand while playing pink noise, and you can find that point. Theoretically for your room, it happens at 13.7 feet from your speakers.... Anyone further than that is only hearing mostly the reverberant field of the room, not so much the direct sound. Anyone closer is hearing mostly direct sound, less reverberant field. And yes, it is possible that the critical distance can be beyond the walls of the room, in which case there is no reverberant field in the room... And that is also a problem!


- Stuart -

[Sixstringer]

After spending a moment to bask in your genius, I believe I am in understanding of your explanation.

Thanks a lot.

Will heavy bass trapping compensate for the low frequency modal issues in my room ( < 130) or is it something that I will just have to live with?

I am gathering from your description that my only major concern is the really low end modes, is this correct?

Is being within the near-field (13'6") a positive thing for mixing? Does a listener beyond that 13'6" threshold that you mention hear a mix incorrectly? Since the room is about 18-19 feet long, it seems that only the very back wall will be out of the near field zone?

[Soundman2020]

Will heavy bass trapping compensate for the low frequency modal issues in my room ( < 130) or is it something that I will just have to live with?

Heavy bass trapping cannot get rid of modes: what it does is to "damp" them, acoustically, in much the same way as the shock absorbers your car "damp" the bumps in the road. The don't get rid of them, but they do make the ride a lot smoother. "Bass trap" is actually a bit of a misnomer: in reality, the absorb all frequencies. But since modes are "louder" they get absorbed more, which brings them down in level, and smears the energy around a bit too. The smaller a room is, the more bass trapping it needs to get those modes damped well.

So yes, bass trapping will tighten up the low end in your room, suppressing the modes as much as possible, and evening out the frequency response a bit as well. The best bass traps are called "Superchunks", and you make them yourself by cutting panels of ordinary OC-703 insulation into triangles, and stacking those in the corners, floor to ceiling. Simple, cheap, effective.

I am gathering from your description that my only major concern is the really low end modes, is this correct?

Well, the modal response is your BIGGEST issue, without doubt, but not the only one by any means. Next comes first reflections: you need to kill those as much as possible, so that they can never get to your ears. Or at least, they can't get there until at least 20ms has passed since the corresponding direct wave got there, and even then they can only get there 20 dB quieter. That's the ideal, but even 10 ms / -10dB is reasonably good.

Is being within the near-field (13'6") a positive thing for mixing?

Yes! absolutely! That's where you DO want to be. In the place where all your ears can hear is the pure, clean, clear direct sound from the speakers, uncolored by any room effects. As I mentioned above, the ideal situation is that the direct wave for every sound hits your ears, then you don't hear that same wave again until 20 ms has passed. After that you SHOULD hear it, but much quieter: -20 dB softer. Some specs are a little more relaxed, and say 15ms and .10dB, or even 10ms and -10dB, but the point is that there has to be an Initial Time Delay Gap (ITDG) where all that your brain is dealing with is the direct sound, followed a short time later by the softer reverberant echos and reflections of that sound bouncing around the room. That's the best thing you can feed your brain to help it get an accurate picture of what the music sounds like.

Does a listener beyond that 13'6" threshold that you mention hear a mix incorrectly?

They still hear it correctly, just not as cleanly. If the room is well designed, then the frequency balance of what they hear will still be accurate, but the sound will be more diffuse, not so clean. They won't be in the sweet spot either, so it doesn't matter so much.

Since the room is about 18-19 feet long, it seems that only the very back wall will be out of the near field zone?

Right. But since the front face of the speaker (which is what actually produces the sound) is a foot or two away from the front wall, and the rear wall has a foot or two of treatment on it, it is unlikely that they would actually be in the far field. The direct distance from speaker face to ear is what matters.

- Stuart -