Calculating room modes in a weird room

[Walter]

I am trying to determine my room modes using https://amcoustics.com/tools/amroc. However, I have no idea what value to use for the ceiling height. My ceiling is nothing but a piece of foam board less than 1 inch thick at about 3 meters height. Above that is the sloping roof that I have no way of measuring--somewhere in the range of another 2.5-3 meters, I'd estimate--but it is only made of corrugated metal so I'm not sure even it would have much effect. Any ideas? Should i Use 3 meters? 6 meters? Infinity?

Also, what effect will the fact that 60-70% of one short wall is made up of the glass in my balcony doors have?

 

[abdo123]

Have a look at this : https://www.audiosciencereview.com/...ds/arbitrary-shape-room-mode-simulator.20805/

and this: https://www.audiosciencereview.com/...ency-response-simulation-low-frequency.24850/

the technical level is incredibly high, but once you figure it out it should work out well.

I would not take the low frequency simulation (second link) as a bible unless all 6 of your boundaries are made out of concrete. but it should tell you with good accuracy what the room modes are so you can use that information with your own in-room measurements to figure out the most offending room modes and then use placement to mitigate them as much as possible.

 

[Walter]

Have a look at this : https://www.audiosciencereview.com/...ds/arbitrary-shape-room-mode-simulator.20805/

and this: https://www.audiosciencereview.com/...ency-response-simulation-low-frequency.24850/

the technical level is incredibly high, but once you figure it out it should work out well.

I would not take the low frequency simulation (second link) as a bible unless all 6 of your boundaries are made out of concrete. but it should tell you with good accuracy what the room modes are so you can use that information with your own in-room measurements to figure out the most offending room modes and then use placement to mitigate them as much as possible.

Thanks. Until the travel situation and my finances further improve, I can do nothing at the moment but plan. I can't even find a calibrated USB microphone here in Cambodia, and the unreliability of the mail service means I don't order anything online unless I am willing to pay EMS or international DHL/FedEx shipping rates.

And even my "real" walls are only made of plaster cover (cheap) brick that lets lots of sound in from the neighbors. Luckily, my office is one floor higher than the top floor on either side so I don't have to worry much about bothering them, except maybe with deep bass.

Edit: Actually, I realized that neither of those links help answer my main question, which is what measurement(s) should I use for the ceiling height?

 

[kiwifi]

You could always try measuring your room modes...

https://www.jochenschulz.me/en/blog/how-to-measure-room-modes-with-rew-freeware

 

[Walter]

You could always try measuring your room modes...

https://www.jochenschulz.me/en/blog/how-to-measure-room-modes-with-rew-freeware

That would require a microphone, which as I mentioned, I can't buy at the moment.

 

[GalZohar]

If you only want room modes up to a certain frequency (say 80Hz) you can just play a test tone at each frequency 20-80Hz 1Hz at a time and check for peaks and nulls around the room. Takes a bit of time, but you might want to test it anyway even if you measure the room with a mic (the mic won't tell you how each frequency would sound different if you move to a different position). You could just do it without a mic. This can at least help you optimize your listening position for best response up to 80Hz (or whatever frequency you choose as your max). As you go higher in frequencies, the density of the room modes increases, so they're more difficult to distinguish by ear (or even my measurement).

In a room that isn't a 100% match to the theory (no room is, really) calculating is practically impossible. Even in rooms that do match the theory, the calculations are just an initial estimation.

 

[Walter]

If I currently had a speaker with significant output below 80Hz, that would help to determine the location of the modes, but not their frequencies, which is what I am interested in. To do that, I would need a speaker that was flat down to 20Hz, or one for which I already had an accurate anaechoic FR curve.

This is an office first and foremost, so practical aspects determine the furniture layout. There is little option to move the speakers. I've already done that within the small range possible, and found no audible difference, but I have very little output in the bottom two octaves. This is all about planning for the next system.

 

[GalZohar]

Your speaker doesn't have to be flat down to 20Hz, but does need reasonable SPL that you can hear at the frequencies you wish to test. As long as there is some SPL, you can play a test tone at that frequency and move around the room to hear the peaks/dips.

What information about room modes do you need to plan the next system? Usually you just have to get some kind of system in and then test out how it sounds to find the optimal positions. Theoretical planning in advance can only work reasonably well for a rather rectangular room, otherwise it's just guesswork (and even for rectangular rooms, the guesswork might be far from perfect). If you can get the system first and then plan the room, that would give you an ability to make a better plan. Location of room modes isn't incredibly important for planning which gear to get, but more where to place it. Sure you might know how badly you need multiple subwoofers, but if your budget allows for them you should get them anyway.

 

[Walter]

Thank you to everyone has replied. There is no information here that I was not already aware of, which is an indication that I did not frame my question well--or maybe used a misleading title. I am specifically looking for information on what effect the foam board ceiling and corrugated metal roof will most likely have on the room modes, not on practical aspects related to the subject. Think of it as a thought exercise, if that helps. Maybe I need to repost this question with a title that makes that more clear.

 

[GalZohar]

The answer is that it is almost impossible to predict. Especially if there is no symmetry. If they don't have appropriate absorption properties, though, expect it to make the room modes a bit different than they would be with something like concrete, but not necessarily better. If the absorption properties are appropriate, the you may get weaker room modes in the frequencies that use that surface to resonate.

 

[Walter]

The answer is that it is almost impossible to predict. Especially if there is no symmetry. If they don't have appropriate absorption properties, though, expect it to make the room modes a bit different than they would be with something like concrete, but not necessarily better. If the absorption properties are appropriate, the you may get weaker room modes in the frequencies that use that surface to resonate.

Correct. So I am hoping someone can tell me what those absorption properties are, as far as can be estimated from my description, and how they will be likely to affect the room modes. I've done some research already but have not found an answer. For example, something like, "the ceiling will be transparent under 1000Hz, while the roof will reflect all frequencies, so for the purpose of the calculator you should probably try a ramge of..."

 

[GalZohar]

If the specific material composition wasn't measured by a professional and not commonly used for acoustic room treatments, you probably won't be able to know how well it absorbs/transfers any frequencies, especially ones that are relevant for room modes. Every material is different and guessing is probably not going to work very well.

 

[Walter]

If the specific material composition wasn't measured by a professional and not commonly used for acoustic room treatments, you probably won't be able to know how well it absorbs/transfers any frequencies, especially ones that are relevant for room modes. Every material is different and guessing is probably not going to work very well.

True. I'm hoping someone actually knows the absorption details or can link to a good source. I know at one point I found a really comprehensive list online, although I believe it was only from a sound insulation perspective with no data an affected frequencies.

 

[Hipper]

The room modes you hear at your listening position depends not just on your room - size, shape, materials - but also the positioning of your ears and speakers. Further as you seem to be restricted in these placements you have little choice but to accept what you hear and adjust with whatever you've got to hand - room treatment perhaps but most likely EQ/DSP.

I think some of the suggestions above are worth exploring. You say you can't use a calibrated mic (and presumably some software like REW). Some mics are not specifically calibrated but have general calibrating files that are good enough to be used, like the Behringer ECM 8000. The alternative as mentioned is to get a set of test tones and listen with your ears. This would be the most suitable:

http://realtraps.com/test-cd.htm

Even if you can at the moment only play sounds above 80Hz it's still worth controlling the rogue frequencies. I'd check it up to 300Hz. It will improve your current listening experience.

 

[Walter]

Sigh... I'm clearly still not being understood. I appreciate all the advice on practical methods of determining the best solutions as far as EQ and room treatment, but at the moment, that is not what I am asking about. That falls on me, since I'm the one requesting the info, but I'm just not sure how else to phrase this. I'll try one more time: I want to know if anyone knows the effects of the ceiling and roof materials mentioned on the room modes--which presumably means having an idea of the absorption coefficients at various frequencies. It may well be that nobody knows, either because my description is too vague (it is a rental so I had nothing to do with purchasing them) or because, as Galz says, if these materials are not typically used, nobody has bothered to calculate them. You can think of this as a mental exercise, or a math problem, or whatever, if that makes it more clear.