Measuring and Treating Room Modes
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Published 2020-12-28
Bass Traps
www.gikacoustics.com/product-category/bass-traps/
Amcoustics Room Mode Calculator
amcoustics.com/
"Room modes are one of the most talked-about aspects of room acoustics, but there’s a lot of confusion out there about what room modes are and why they are important.
“Modes” are resonant frequencies inside the room that correspond to certain dimensions within the room. Sound waves are usually thought of in terms of frequency, but we can also use “wavelength” to identify sound waves by their size. For instance, a 1kHz sine wave has a wavelength of 13.56”. This means the physical distance between two peaks of the sine wave is 13.56”.
Lower frequencies have much longer wavelengths, often in the bass range. If you work with sound or play an instrument then you’re probably familiar with the way the physical volume of an area affects sound. Instruments use strings or chambers of varying sizes to produce different ranges of frequencies. We see this illustrated clearly when we see the size of a bass instrument compared to a treble instrument.
It’s exactly the same for a room -- both instruments and rooms can be thought of as resonant chambers.
So if your room is 20’ long, that will correspond to a wavelength of about 56Hz. So in this case, 56Hz will be one of your room modes. If you have a rectangular room you can get a feel for what the room modes do by playing a tone with the same wavelength as your room’s dimensions(56Hz for a 20’ long room) and observing its effects in different parts of the room.
With the tone playing, go to the back wall with your back up against the wall. The tone will likely be very loud at this point in the room. As you slowly walk forward, the tone will get softer until you hit the middle of the room where it can often disappear almost entirely. Then as you keep going toward the front wall it will get louder again.
This behavior corresponds to the wavelength graph above. The middle of the room is at the trough of the wavelength, which corresponds with the minimum volume of that frequency in that location.
This exercise shows the “axial mode”, the simplest of the three types of room modes. Axial modes, in a rectangular room, are simply these frequencies associated with the wavelengths of the three dimensions, where sound travels in a straight line across one dimension, between the two parallel surfaces. Tangential modes travel along two dimensions and Oblique modes travel along all three dimensions and are the weakest of the three room modes.
In a simple rectangular room, it’s easy to calculate your Room Modes based on your room’s dimensions. There’s even some pretty useful tools online that let you enter your room measurements and get a breakdown of what frequencies will cause standing waves and where in the room they’ll be localized.
So with that -- what do we do with room modes? The answer is, these calculators are most useful when designing a new room from scratch where there is some flexibility in the room’s size. These calculations are invaluable to plan your room’s dimensions for optimal acoustic performance. But you can also use this information to predict where some of the sound issues will pop up.
Keep in mind these calculators only work if the room in question has a rectangular shape. Differences in building materials as well as window and door placement can cause two rooms with the exact same dimensions to produce different room modes. Also modes & standing waves are not the only acoustics problem you’ll have in a room, so mode calculators are no substitute for measuring your room.
In terms of treatment, modal problems are one of many things addressed by a comprehensive bass trapping strategy. Unless you are in a tiny room, room modes occur at deep bass frequencies, and improving deep bass response requires a lot of thick panels in the room.
The thicker the panel the better it will do in the bottom 2 octaves of bass (20-80Hz), but coverage area is the most important thing, as always. Monsters & Soffits are the go-to here, possibly with Range Limiters installed on them for maximum effect under 80Hz.
Good bass trap placement can also help, particularly when you are starting with a few bass traps. Bass hotspots -- often corners -- are usually related to modal issues, so target those locations where you can.
Sometimes it can help the modal issues if you can disrupt the reflections that are contributing to the standing wave. For an Axial room mode this means placing treatment on one of the two surfaces that the wave is acting between, such as the rear & front walls in the above example. Remember, these modes aren’t just between your parallel walls, but also between your floor and ceiling as well. And again, these are just the axial modes."
All Comments (15)
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Thanks for sharing! Greetings from Perú!.
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Great explanation 👍👍👍👍
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Bruh, aren't room modes measured at half wavelengths not full? So, a 20' room would give 28Hz not 59Hz? The pressure map you show also suggests a half wavelength so...
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how to learn room treatment and sound proofing ?
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I need to trap an F#4 369.99hz 36". What material will catch it best and what percentage of it could be caught?
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What a pity that typical room sizes of 20 feet correspond to typical alternating current frequencies of 50-60 Hz ... I have a low frequency sound in my room, no it is not my ears, no it is not the hum I go out I don't hear it I come back I do. I also measured it with a good microphone and a bit of Labview code ... my best guess is a neighbor's household appliance is mounted on a wall somewhere. I need to investigate it. Thanks for the info.
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Hi. I love your videos, I've learned a lot from them. Regarding the room mode calculation. I have read that the calculation for room modes is: Speed of sound / room size x 2 = How come did you get a 59hz room mode with a 6m room?
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Thanks for sharing, what happens when it’s not a square , do these still apply ?
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I have bass traps on the front wall, but still have the problem where bass sounds better when I stand in front of my sofa and softens when I sit.
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Correct me if I'm wrong, but won't the room modes in a given room vary depending on the location of the source of sound?
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cam here thinking he'd teach the formula to calculate lmao
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I'm liking it, but at 1:05 - 20 feet long, wouldn't that be 28Hz? Cheers!
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The background noise in your vids is irritating
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There is a myth about corner modes. If you use a mode calculator, it is VERY rare that you'll see corner Axial modes. In fact, of all of the room dimensions I'm punching into a room mode calculator, I have yet to see an Axial mode that only effects the corners, so putting corner bass traps without dealing with the rest of the surface isn't really going after the Axial modes. I would treat Axial modes first, as they are the biggest problems. But you have to have what I've been told at least 50% of the surfaces covered for a sizable difference. Also, how much energy at those frequencies is also important. If you are listening to music at 70dB, there's not going to be as much of a problem in the low frequencies as there would be if you are listening to at 100dB, as an example. It's also important to match the size/volume of the room to the speakers and the energy you plan on putting in the room. For treating the lower frequencies under 125hz, some would rather build thicker walls with more mass so that you don't have big bass traps all around the room, otherwise you may have to build a pseudo wall using a lot of bass traps all around the room floor to ceiling. But first you have to choose the right room dimensions for the usage. Bedrooms, even though they are becoming popular for setting up a DAW for mixing, are typically too small, especially if you have things like 8 ft ceilings, square dimensions and rooms like 10ftx10x8ft. That's just a big mess that requires too much treatment since you'll have a lot of treatment needed and you'll end up with much less usable space. Plus, 99.99% of the homes have thin walls, and are NOT designed to be used for music listening/producing applications. 2x4 studs with sheet rock and building insulation isn't that great of an absorption in the lower frequencies, especially when you get below 100hz. They also have a relatively low STC rating. Adding more sheet rock and green glue isn't going to help all that much either.
