Measuring and Treating Room Modes 

This video outlines room modes and gives an overview of basic treatment methods for dealing with room modes and standing waves.
Bass Traps
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Amcoustics Room Mode Calculator
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"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."