Difference in input levels when "Check Levels in settings" and "Check Levels in Measure"

[sergioTOOL]

Hi

During the settings procedure I´m setting my input level to -18dB FS as you recommend (is that the value reading in the dB Meter?). But after when checking levels, prior to carrying out the measurement, I´m getting a very low input signal, around -34dB. The latter is checked with a high pitch noise and the former with pink noise.
When I increased the input level in the settings to reach -9dB, I got a good input level at the "measurment-check levels" and then I was able to carry out a measurement. Although when I got a high good input signal level during "measurment-check levels", the actual measurement clipped every single time, so I had to set the level to low "good signal level".

What do you reckon is the reason for the difference in input levels? what procedure you suggest I use? and what headroom you recommend when taking a measurement, is 0.5dB below clipping alright?

Another question. When I change my measurement position (which I will when carrying out RT60 measurements soon) sound levels will vary. So at the 2nd or 3rd position I will not get 75dB anymore but say 71dB. I suppose in this case I have to recalibrate the system by increasing the output level. Input levels should not need re-calibrating as the microphone will still be receiving 75dB. Am I right?

Many thanks for your help!

[brucek]

What do you reckon is the reason for the difference in input levels? what procedure you suggest I use? and what headroom you recommend when taking a measurement, is 0.5dB below clipping alright?

The Check Levels routine in the Measurement screen and the Check Levels routine in the Settings screen use different values to generate the pink noise level test signal.

The Check Levels routine in the the Settings screen uses the fact that you have the Check/Set levels with Subwoofer pull-down selected to generate its test noise (this a band limited signal with a low and high cut of 30Hz - 80Hz).

REW double checks and verifys that the end frequency of the sweep that you have selected in the Measurement panel roughly matches the test signal setting for sanity, and so its Check Level routine uses the end frequency as the test pink noise hi-limit cutoff. So, if you had 20KHz as the end frequency to measure for your sweep, you can see how the pink noise would be different (between the two Check Levels).

You're revealing that your end frequency in the Measurement panel is incorrect by your statement of The latter is checked with a high pitch noise and the former with pink noise.

Often, even if you have the levels set correctly, a peak or dip in the response of the speaker being tested will have energy at a frequency that trips the sanity check and announces that levels are incorrect.

Simply return to the Settings screen and adjust the input level of the soundcard a bit lower and then run Calibrate SPL and carry on.

As long as the measure is below clipping, you're fine.

So at the 2nd or 3rd position I will not get 75dB anymore but say 71dB.

The absolute level when using RTA is not important. The relevant data is the shape of the plot. If the level clips, then turn down the REW input level a bit, and vise-versa. There is a clip indicator on the RTA page.

brucek

[sergioTOOL]

Thanks

One more thing. Since Im only interested on a limited freq range 125-4000Hz. Should I limit my measurement sweep to that range or is it good practice to have a broader range, or at least start the sweep from 20Hz...

[brucek]

imited freq range 125-4000Hz. Should I limit my measurement sweep

If you choose that range, REW will automatically sweep a wider range than that, but will display the range you chose. So, simply choose the 125-4000.

brucek