Help in understanding REW

[Rudy81]

I have finally made my first few full range plots of my system. First, thanks to all who did such a great job with the software and the help files. This is a great boost to improving our audio experience.

I am using an ECM 8000 on my Home Theater PC. I calibrated the audio card and have attempted to follow the directions as best I could. Following is a plot, actually it is 4 plots, made while I made changes to the mid range crossover settings on my system. Pretty neat to see the effect of just changing the mid range.

However, I have two main questions. When I set the system up, the volume calibration was set to 75 dB. However, I notice the plot rarely reaches 75 dB. Only a few bass peaks reach that decibel level. Why is the plot not 'centered' around 75 dB, Is that something I am doing wrong? Second questions relates to the high frequency roll off starting around 6 kHz. This is where my tweeter crosses over. Is that a measurement problem, a room problem or another mistake in setting up the software on my part? In my search of the forum, I noticed many of the full range plots exhibit the same high frequency roll off. I am trying to understand the process better. Thank you for your help.

[Wayne A. Pflughaupt]

Only a few bass peaks reach that decibel level. Why is the plot not 'centered' around 75 dB...

Well, the SPL meter is only going to register the loudest frequency. It's not an "averaging" device.

Regards,
Wayne

[Rudy81]

Wayne, thank you . I guess this is a rather normal plot then, other than the mid range variations I induced.

[brucek]

Only a few bass peaks reach that decibel level.

The 75dB at the listening position is set (when testing full range) with band limited pink noise using a low cut of 500Hz and a high cut of 2000Hz. You have measured from 0-20000Hz. The system has no way of knowing what the level is outside its bandpass, so there can often be a difference with the actual measurement. It isn't an issue.

Second questions relates to the high frequency roll off starting around 6 kHz. This is where my tweeter crosses over. Is that a measurement problem,

If you're using a generic cal file, this may cause a difference.
You should also measure your tweeter near field (<12") with a horizontal on-axis setup to establish whether the room is having an effect.

brucek

[Wayne A. Pflughaupt]

Second questions relates to the high frequency roll off starting around 6 kHz. This is where my tweeter crosses over. Is that a measurement problem, a room problem or another mistake in setting up the software on my part? In my search of the forum, I noticed many of the full range plots exhibit the same high frequency roll off.

Are you using our calibration file for the mic? Also, what mic orientation are you using when taking the measurements (vertical or horizontal)?

Regards,
Wayne

[Rudy81]

I have taken measurements both with and without the generic mic calibration file. With the calibration, the dropoff is even worse. I will take some nearfield tests to determine if my reflection point absorbers are having too great an effect and attenuating the high frequencies excessively.

I am now pointing the mic directly at the speaker being measured. Pointing the mic vertically again shows a bigger rolloff in the high frequency range.

Thank you guys for taking the time to entertain my questions. I find I sometimes need to understand how something works rathen than just follow directions without knowing the basis for the instructions.