Room Measurement Troubleshooting

[damonrpayne]

I am having some oddness with my measurements, I'm wondering if anyone can point me to literature regarding how to read ETC curves. I am clearly doing something wrong, and on top of that I may have some insane issue in my room or equipment though my gut says the problem is between keyboard and chair.

1) When measured, my ETC graphs are never time-aligned to zero ms, as Doug Plumb indicates they should be. My left and center are about -2ms, and Doug indicates any kind of delay anywhere in the signal path could cause this, OK no problem. My Right speaker on the other hand starts at t=-6ms! My surround speakers are nearly perfect at t=0ms. I am assuming this is not a deal breaker and I manually time-align my impulse response to t=0.

2) I am clearly missing something regarding what the impulse response and ETC time domain actually means. For example, if the microphone is about 13feet away from the left speaker, shouldn't there be NO sound reaching the mic until t=12ms or so? I read that reflections below 10ms and especially 5ms are very important to fix, but I don't know many people who are sitting 4 feet from their speakers.

3) I just want to clarify this: if a tool like RPlusD shows what appears to be a reflection at t=18ms, that means: a) the reflection occurred on a surface 18ms of time travel from the driver in question or the reflection occurred 9ms of time travel from the driver in question and traveled 9ms more to reach the microphone at t=18ms. I'm obviously reading something incorrectly since putting a 4'x2' OC703 trap in 7 different positions and re-measuring had almost no effect on the graph.

Right now I have the mic taped to the top of my chair, which I understand can introduce all sorts of reflections, so the one thing I know I can and will do different is to move my chairs and get a microphone stand.

Right speaker:

Center:

All thoughts are welcome.

[mas]

A few quick comments.

There is propogation delay and equipment latency.

As much as we might like, it is not practical for the direct ARRIVAL time to happen in zero time.

Much equipment allows you to normalize the response so that the initial arrival time is made "0"...but each piece of gear is different.

Your concern is the dif between the initial direct arrival time (which is not necessarily one point in a multidriver system.

Various reflections can be generally isolated by placing a piece of absorptive material between the source and the mic at various positions where it will intercept the various paths, thus isolating the various reflective paths. It is a quick way to determine the gross orientations of specular reflections, as the appropriate response peak will be damped. Then you can do a bit more work to id the specific reflective point on the surface...

Give me a holler, your micing technique will have a great deal to do with the response as well. You can minimize much of the floor bounce by laying the mic on the floor (assuming that it is non-resonant!) and it will act as a poor man's PZM.

[damonrpayne]

Thank's mas, I don't want to be annoying but at your invitation I will give you another call on my drive home this week.