Room modes are a collection of resonances that are unique to each room. The are based primarily on the size and shape of the enclosing boundaries seen by sound waves. They present the greatest obstacle to accurate bass reproduction, and they cause problems in even the best sounding rooms.
Types of modes
There are three types of modes - axial, tangential and oblique.
Axial modes tend to dominate the modal response. They are formed between opposite wall surfaces.
Oblique modes are less significant due to their longer path which causes their energy to dissipate.
Getting back to basics
To understand modes, it helps to consider a simpler situation than a normal room. First, let's consider a sine wave in a free field environment with no boundaries (not even the ground).
Now let's consider what happens when we add a boundary. We'll add in the ground.
Now we have a secondary source with a longer path length. That path difference means that the reflected sound wave will interfere with the direct one. We can expect a dip caused by the difference in phase of the two sound waves. The higher the level of the reflected wave, the greater the impact on the response. This effect is referred to as SBIR (Speaker Boundary Interference Response). The first three reflections are covered there - those off the side and rear wall and the floor.
In a rectangular room there are 6 enclosing boundaries. Now consider the first three diagrams. In the midrange, those nodes and anti-nodes become closely spaced together so that they are reasonably smooth. It is in the lower midrange that there is a transition point where nodes and anti-nodes are spaced far enough apart that they no longer can offer a smooth response. This transition point is called the Shroeder frequency, and it will often be around 200 - 300 Hz. Below that point we see major peaks and dips in the response until we reach the lowest mode. As wavelengths approach and exceed the largest room dimension, we no longer encounter modes and the room becomes uniformly pressurised. The room provides acoustic boost below that point depending on the extent to which bass is either absorbed or transmitted through the envelope.
The importance of position
Room modes create a complex pattern of peaks and dips in a room. The existence of modes is determined by the room shape and its acoustic properties. The way in which those modes are excited depends on the location of the speaker and listening position. As a result, shifting the location of the speakers and listening position will change the response. Placing a subwoofer in the corner will tend to excite the most room modes and achieve the maximum output. This may or may not be a good thing. If you then sit against the back wall you are likely to experience strong peaks and a boomy sound. Shift your seat forward and the bass level and boom will be tamed. This is the simplest form of adjustment to suit the room, but you can certainly improve on that with more advanced techniques.
Here is a section through a room with a depth of 3.9m and a sub in the corner. Position 1 is with a couch against the wall, 2 is moving out and 3 is in the middle of the room. So you can see that if you sit against the wall in position 1, you will see a strong peak at 44 Hz and also one at 88 Hz. Moving out from the wall, both will decrease but in position 2 there will be a dip at the second order resonance. In the middle of the room (position 3), there will be a null at the first order resonance and a peak at the second. All this assumes a sub in the corner as shown to excite all these modes. This is in fact my current listening room. The first order axial mode for this wall pair is at 44 Hz. This can be calculated by this formula:
Frequency = 0.5 x 344 / Distance between walls in metres
The second order mode is a multiple as is the third and so on. In this particular room, in position 1 the 44 Hz mode is dominant with most sub and speaker positions. The result is a 15 dB peak which causes the room to boom.
A decay plot can show up the presence of modes in both frequency and time domain.
The top of the red plot shows the response of a full range speaker, while the purple plot shows the response after 150 ms. Where the gap between the two plots is small, that indicates modal ringing.
- 44 Hz - first order depth mode
- 61 Hz - first order height mode
- 84 Hz - second order depth mode
- 130 Hz - third order depth mode
Note that two of the four easiest to pick modes are depth modes. The position is against the wall (position 1 on the previous diagram). So this is not surprising. Harman Audio has a useful tool for quickly working out the modes that could possibly be excited in a room.
It is a handy tool to use in making sense of measurements.
So what does all this mean?
Room modes do two things. Firstly, they introduce peaks and dips to the response. Secondly, they cause ringing in the time domain so some frequencies stay in the room longer. This is a serious impediment to accurate bass reproduction. Understanding room modes is just the beginning. Dealing with them ideally involves a mix of acoustic treatment, placement, multiple bass sources and EQ.