REW Full-Range Features?

[Longfellowe]

All,

I’m trying to tame a very live room and I’m having a little difficulty understanding all the information I’ve got with Room EQ Wizard. I understand the frequency vs. SPL plots, and the waterfall is very helpful, but I don’t understand the impulse display screens. In particular it doesn’t seem that there is any way to determine which frequencies are contributing to the signal decay which, in my case, goes on for 400+ mS before fully descending into the noise floor. A full-range waterfall display would tell me that, but the one in REW only seems to go to 760 Hz. Is there a function or feature I’ve overlooked? Overall the program is excellent, very easy to use, and very useful. Any help or comments would be appreciated.

Longfellowe

[JohnM]

The impulse response is in essence a recording of what it would sound like in the room if you played an extremely loud, extremely short click - something like the crack of a pistol shot, but shorter. The reason for measuring the impulse response (by more subtle means than firing a gun in the room) is that it completely characterises the behaviour of the system consisting of the speaker(s) that were measured and the room they are in. An important property of an impulse, not intuitively obvious, is that it if you break it up into individual sine waves you find that it contains all frequencies at the same amplitude. Strange but true. This means that you can work out a system's frequency response by working out the frequency components that make up its impulse response. REW does this by Fourier Transforming the impulse reponse, which in essence breaks it up into its individual frequency components. The plot of the magnitude of each of those frequency components is the system's frequency response.

When an impulse response is measured by means of a logarithmically swept sine wave, the room's linear response is conveniently separated from its non-linear response. The portion of the response you see before time 0 is actually due to the system's distortion - if you look closely, you may see that there are small, horizontally compressed copies of the main impulse response there - each of those copies is due to a distortion harmonic, first the 2nd harmonic, then the third, then the fourth etc. as time gets more negative. The period after time 0 is the system's response without the distortion.

In a perfect system of infinite bandwidth with totally absorbent boundaries, the impulse response would look like a single spike at time 0 and nothing anywhere else - the closest you get to that is measuring the soundcard's loopback response. In a real system, finite bandwidth spreads out the response (dramatically so when measuring a subwoofer as its bandwidth is very limited). Reflections from the room's boundaries add to the initial response at times that correspond to how much further they had to travel to reach the microphone - for example, if the microphone were 10 feet from the speaker and a sound reflection from a wall had to travel 15 feet to reach the microphone, that reflection would contribute a spike (smeared out depending on the nature of the reflection) about 5ms after the initial peak, because sound takes about 5ms to travel that extra 5 feet.

When measuring full range responses from loudspeakers (rather than subwoofer responses) the reflections are easier to spot as the higher bandwidth of the full range system keeps the spike of the impulse (and the reflections) quite narrow, but you need to zoom in on the time axis to see them. They are easier to spot with a linear Y axis (set to %FS instead of dBFS) and also show up more readily if you select the "Show Full Range Energy-Time Curve" option.

You can "clean up" the impulse response by placing absorption at the primary reflection points, which you can find using the mirror trick - have someone hold a mirror against the wall (or floor or ceiling) while you sit in your listening position and move it until you can see each of your speakers, the positions where your speakers are visible are the primary reflection points. Absorbers at these points reduces the spikes after the main peak which are due to the reflections, using larger areas of absorber will also provide a general reduction in the "liveliness" of your room.

The main contributors to the slow decay of the impulse response in full range measurements are low frequency modal resonances, addressing these (by EQ and/or room treatments) will correspondingly speed up the decay of the impulse response.

HTH,

John

[Longfellowe]

John,

Thanks very much for taking time to provide such a detailed description about some of the inner workings of REW. I’m still working to understand some of your points, but that’s a good thing. You’ve already figured out my motive in all these questions. I’m trying to get my room to sound as good as possible with acoustic treatment before I crank up the equalizer. I’m concerned about the frequency of the reflections because 4 inch sound damping panels are a lot more expensive than 1 inch panels and take up more space in the room as well. This room is really hard, with very clear flutter echoes when you stand in the middle and clap your hands.

I had not before made the mental leap of using a short impulse sound to generate frequency response curves, but I understand your explanation of how it could be done. When running my sweep, however, I didn’t hear anything like an impulse, just a sine tone steadily increasing in frequency. I know nothing about how FFTs work, but I’m guessing that it may take the sweep and consider it an “infinite” number of impulse sounds. In any event, after your explanation I can now see how the frequency vs. SPL plot would represent the interference pattern of the room’s reflective surfaces at each individual frequency. One thing does trouble me though: if, at the microphone position, I get the direct sound at 0 degrees phase shift, a reflective sound at 180 degrees phase shift and another reflective sound of the same amplitude at 360 degrees would not the 180 and 360 sounds cancel each other out and leave me with only the amplitude of the 0 degree direct sound? This would cause me to believe that I have no reflections at that frequency when, in fact, I have both a peak and null that I can’t see. Maybe I don’t care since they are canceling? Obviously the situation of a 180 and 360 arriving at exactly the same amplitude is not very “real world”, I’m using it only as an extreme example.

I did notice the peaks before time 0 on the impulse graphs and was sorely puzzled by them. Eventually I decided to ignore them since (I reasoned) anything before time 0 could not be part of my test run except in the Twilight Zone. I mean, negative time!! what a concept!! Since these blips represent harmonics, it would seem that the amount of their attenuation as I refine my acoustic treatment would be a good indication of the effectiveness of that treatment. Is that a valid assumption?

I took your advice and changed to %FS and turned on “Full Range E/T Curve”. Zooming in showed the highest spike at 9mS after 0 time, with lesser spikes at 5, 8, 21, 30, and 37mS. Would a reduction in these spikes also be a good (better?) measure of acoustic treatment effectiveness than the harmonics noted in the previous paragraph?

Thanks again…

Longfellowe

[JohnM]

The levels of harmonics are indictors of the overall linearity of the reproduction and capture chain and will not be affected by acoustic treatments. Reducing the spikes after the initial peak is certainly desirable and the extent to which they are reduced is an indicator of how effective the treatments are and how well they have been positioned.

Reflections can cancel each other out, which can cause some unexpected results - e.g. if treatment attenuates one of the reflections more than the other then they no longer cancel and the response might seem to have got slightly worse rather than better. However, such exact cancellations are rare and would typically be very localised, not really worth worrying about.

For an explanation of how the swept tone allows derivation of the impulse response read the "Transfer Function Measurement With Sweeps" paper which is linked from the REW home page, or just accept it that it works and press on with treating the room and measuring the effects

[Longfellowe]

John,

Thanks again for a most informative reply. I will press on, keeping an eye on the impluse spikes as I treat the room.

Longfellowe