Before we begin, if you’re unfamiliar with the concept of a “room mode”, it might be useful to review three other postings:
At the end of that last posting, I said:
“P.S. As I mentioned briefly in this posting, Active Room Compensation has one additional feature – multichannel processing. That will be explained later in Part 2.”
So, that’s the theme for this article – the multichannel aspect of Active Room Compensation.
A monophonic view
Let’s start by looking at Figure 1, below.
This is a basic representation of the fundamental room mode in one dimension of a room. If a loudspeaker is placed at the location of the red circle and it produces energy at the same frequency of the room mode, then the room will resonate, just like a closed pipe, as can be seen in the animation.
One important thing to notice in the figure above is that when the pressure on one side of the room goes positive, the opposite side goes negative. In other words, the two sides of the room are in opposite polarity. This information will come in handy later.
Now let’s look at the second harmonic – this is a resonance that has a frequency that is two times that of the fundamental mode. Its behaviour in one dimension of the room is shown below. Again, the loudspeaker is in the position of the red circle.
Now, you’ll notice, the two sides of the room have the same polarity – when one goes positive, so does the other. It’s the centre of the room that is in opposite polarity to the sides.
If you measure the natural response of a loudspeaker that is otherwise flat (let’s not split hairs over whether we’re talking about on-axis magnitude response or the power response – for the purposes of this discussion, it’s irrelevant) in a one-dimensional room like the one shown above, you’ll see that there is a natural peak in the response at each frequency where you have a room mode. The result will look something like Figure 3, below
As you can see there, every mode is excited by the loudspeaker (we’re assuming that the loudspeaker is not sitting on a “node” and therefore not coupling to the room mode at all).
So, if you build a room compensation system that only takes one loudspeaker into account, then it will measure a response similar to the one in Figure 3, and it will create a compensation filter that looks something like the one shown in Figure 4.
Assuming that you’re careful about your measurements, and you consider things like phase response in your creation of the filter, this system will work very well with just one small problem: most people don’t use only one loudspeaker – they use at least two.
So, how does this change things?
Now, in stereophonic sound*!
Let’s go back and consider our room modes again, this time with two loudspeakers.
Figure 5, above, shows the same room as in Figure 1, but now I’ve plotted the locations of two loudspeakers, one on the left (atypically, in red) and one on the right (in black). Notice that these two points in the room, when the room mode is ringing, are opposite in polarity (or “out of phase” as many people say…). However, consider that, in most recordings, the bass (which is, in most cases, a good estimation of the frequency band of the fundamental room mode) is panned to the centre, and therefore is “in phase” in the two channels.
In other words, in almost all cases, the two loudspeakers are producing the same signal at the same level, in phase (and therefore with the same polarity). However, the room is ringing in opposite polarity at the two loudspeakers.
What’s the result of this conflict? It’s simple – the room mode is naturally cancelled by the signals in the loudspeakers. In other words, there is no need to apply room correction for a recording like this, with the loudspeakers in the locations that they’re shown in, for this room mode.
So, if you measure each loudspeaker individually, you’ll put a dip in their responses that should not be there to compensate for a room mode that is not ringing. You must consider both loudspeakers playing a correlated signal, and how that will interact with the room mode.
Now let’s look at the next harmonic, shown below in Figure 6.
Now you can see that the mode is ringing in the same phase at the two loudspeaker positions. So, if the bass (still a good guess…) is in phase in the two channels (also a good guess…) then this resonance will be twice as bad as it would be with only one loudspeaker. Again, we need to consider the behaviour of the room mode with a correlated signal in the two loudspeakers – but this time things are worse instead of non-existent.
So, this means that we have to re-consider our room compensation filter. Instead of measuring each loudspeaker independently and building a filter for each one and ignoring that people rarely listen in mono, we have to measure the two (or more) loudspeakers and analyse the way that different signals will interact with the room modes.
For example, in the simple case shown above, we might wind up with the two filters shown in Figure 7 and 8.
Now, you might be saying “I understand Figure 7 – lots of signals have the same polarity in the two loudspeakers (like the vocals, the bass, the kick drum – anything panned to the centre). But what signals are out-of-phase if I’ve connected by loudspeakers correctly?”
The answer to this comes mainly from classical recordings where it is normal to use microphones (usually omnidirectional) that are spaced apart. In this case, signals enter the two microphones are different times (depending on the angle to the sound source) – and a time difference results in a phase difference.
This processing is done in Beolab 90‘s Active Room Compensation to ensure that the loudspeakers are best optimised, not only to the room they’re in, but their locations within it, and its interaction with the recordings you’re playing. The end result is that each loudspeaker “knows” that the other one is in the room – each not only considers the other’s effect on the room’s response, but they “help each other” to control the room modes.
Of course, I’ve left out a lot of details in this description – for example, the actual responses of the correlated and negatively correlated signals will not really look like the ones I’ve shown here; most rooms contain more than one dimension; and I haven’t talked about boundary effects. In addition, everything I’ve said here is just an example using a very simplified view of the universe. The measurements of the loudspeakers at the microphone positions will result in very different responses than the ones shown here, which will, in turn, result in very different compensation filters.
*Most people don’t seem to know it, but “stereophonic sound” (or “stereo” if you’re into the whole brevity thing) means that you have two or more audio channels. Just thought I’d be explicit here – in case anyone was wondering.