OK, one more. A not-so-usefull-but-rather-cool one. The animated graph below is the complete walk from the back wall to the front wall of the same studio as all the other graphs on this page. OK, so its not ALL the way from back wall to the front wall, because REW only allows up to 99 measurements in one MDAT file and the room is longer than 198 inches. So this animation runs from 102 inches (2.6m) behind the mix position, to 90 inches (2.3m) in front of it. The rest of the room isn't that interesting anyway, but this gives you nearly 6 meters (16 feet) of "walk". In this case, you are starting at the BACK of the room, and walking FORWARDS, towards the speakers.
(As before, the first and last frame are held for a couple of seconds, and the mix position is paused.)
You can clearly see the modal stuff here: any peak (or null) that rings and stays in the same position on the graph, while just changing in amplitude, is a mode. Because modes are standing waves, they will always occur at the same frequency
everywhere in the room: they will only change in intensity as you move from place to place. in other words, you are placing yourself at different points on the standing wave, but because the wave is at a fixed frequency, and forms a fixed pattern in the room, you can move to different spots on that wave as you walk around. This is an easy way of determining if a problem in your room is modal, or not modal. Peaks and nulls that stay in the same place are modal. Modes also "ring": they decay slowly. In waterfall plots, they appear as sharp peaks with long tails.
Peaks and nulls that seem to move along the above graph (left to right, or right to left), are not modes. They are phase summations and cancellations, due to some form of reflection or other. If the peak or null moves UP the graph to a higher frequency as the mic moves, then it is getting closer to the surface that is causing the problem. If it moves DOWN to a lower frequency, you are getting further away from the surface. Phase summations and cancellations do NOT ring: there is no decay time for them. They appear as peaks with no tails at all, or valleys.
For an example of a mode, take a close look at 24 Hz. That's the the first axial mode for this room (1.0.0). You can see when the mic is that close to the front and back of the room, there is a peak that rings, but in the middle of the room, it is a null. Clearly, as the mic moves forward it is "walking" along this axial mode, and the pressure peaks are near the front and back walls, with the pressure null in the middle of the room. And if you look closely at 48 Hz, you can see the first harmonic of that mode (2.0.0), which does the opposite: it peaks
in the middle of the room, but drops down to a null close to the walls.
For an example of a phase issue, look at the graph around 45 Hz at the beginning of the animation, and you'll see a "hole" or steep dip in the waterfall that slowly moves over to the left, down to lower an lower frequencies, until ir goes completely off the bottom edge by the time the mic gets to the mix position: that is SBIR from the rear wall of the room. There's no ringing associated with it, and because it goes down in frequency as we move towards the front wall, it must be coming from the back wall. There's another similar one that starts at about 180 Hz and also moves down: That's probably part of the same SBIR issue. Since SBIR is a phase cancellation, it also causes comb filtering that continues all the way up the spectrum, right to the top, at multiples of the lowest dip frequency. The 180 Hz hole is the 4th multiple of 45Hz... which implies that there should be another related dip that starts at 90 Hz and also descends... Take a close look, to see if you can find it....
(Don't get confused by the 60 Hz issue that pops up and disappears at random! That's not modal: it is the faulty mic cable that I mentioned before... It looks like it it is modal, because it is a peak that rings and is always at the same frequency, but if you look carefully it does not decay at all: it stays at a constant level.)
Learning to decipher these patterns and clues is the key to understanding the acoustic response of your room, so that you can then treat it properly, and tune it properly. Click here for an example of a high-end room that has been fully treated and tuned:
Of course, because the mic is only moving from back to front along the center-line of the room, and always at the same height, this animated graphs does not tell you much about reflection issues that are occurring between the side walls, or between the floor and ceiling. To get a better idea of what those are doing, you could conduct a similar "walking mic" test at the mix position by moving the mic right from left to right across the room, and also from floor to ceiling going up the room. For precision tuning of the room, I sometimes do that, when the client really wants things to be as good as possible, and to find out where the limits of the "sweet spot" are. It can be interesting to that, but not really necessary, for most home studios.
By doing a test like this in your room, you'll find it easier to figure out what is what. Just looking at the REW results for one single point in the room (the mix position) like we normally do when testing and tuning a room can tell you a lot about the room, but mostly only about that one spot. It still leaves you with some mysteries in identifying many of the issues. Moving the mic around the room in carefully planned patterns with fixed intervals, as in these graphs here, can tell you a LOT more about the room, especially when you step through them, or even animate them, as I have done here.
- Stuart -