Here is a simple illustration, showing the direct sound and a reflection off the front wall:
The reflected sound wave causes cancellation, resulting in a dip in the response where the difference is equal to a half wavelength. That dip can be calculated:
f = 172 / x where x is the distance (reflected - direct) in metres
For the example above with a 1m offset from the front wall:
Direct = 2.6m
Reflected = 4.6m
x = 2m
f = 172 / 2
f = 86 Hz
Hence with 1m offset from the wall, a dip will be seen at 86 Hz.
If you know the frequency
If you are looking at an in-room plot, and want to know if a particular dip is related to a boundary issue, use the formula to find the distance.
Wall offset = f / 86 (metres)
Avoid similar offsets
If the offset from each adjacent boundary is the same, the dip will be worse. Hence it's best to avoid having similar offsets. The worst case scenario is one in which the offset from the floor, front and side walls is the same.
In looking at the frequency response, it's easy to misinterpret. Where you see a dip, is it caused by SBIR, room modes or phase shift between a sub and mains? The phase issue can be eliminated by measuring a woofer only so there is no crossover with a sub included. Then you can calculate the points where you should see SBIR dips. It is possible to combine a room mode peak with a boundary interference dip and get them to cancel, although often this won't be possible.
Dealing with SBIR
In recording studios where SBIR gets the most attention, speakers are often soffit mounted to avoid this issue. The entire speaker is flush mounted into an angled baffle. Woofers can also be placed near the floor to avoid floor bounce. Room modes should also be considered at the same time because the best position for SBIR considerations may not be better regarding modes.
One way to tame SBIR is with acoustic treatment, where the reflection is damped with some absorption that is adequate to work down to that frequency. Another strategy which might be useful in small rooms is to mount the speaker against the front wall and use some treatment to tame early reflections. It won't work well without treatment.
It's also important to balance imaging considerations. The challenge is that the best location for imaging might not be the best for bass. This is where 3 way systems using active woofers come into their own. The midrange driver can be placed for best imaging, while the woofer can be close to the floor. There is more flexibility to achieve the best of imaging and bass response.
Here a problem with many stand mounts and floorstanders becomes clear. When placed in a real room without paying attention to boundary issues, the lower midrange will often become recessed while room modes introduce peaks in the bass. This means the bass will boom, but the lower midrange will lack fullness. It's something that many audiophiles have adjusted to, but those who seek accuracy in their systems will often not know what they are missing.