Trace arithmetic unexpected 3dB offset

[Clayton Williamson]

Team,

Using REW 5.31.3, the division trace arithmetic is 3dB different from what I expect. For example, here is a measured curve (from the RTA) divided by itself. Rather than a flat line at 0dB, it is flat at 3dB for both A/B and |A| / |B|. Did I miss a setting?

 

[John Mulcahy]

Set the axis to SPL.

 

[Clayton Williamson]

Ahh, thanks!

Dang, I tried dBFS, dBV, and dBV, but didn't think to try dBSPL. That's odd to me though, since the measurements were made in dBFS. Why does x dBFS / x dBFS not equal 0dBFS?

 

[greatadrian]

Preferences -> View -> Full scale sine rms is 0 dBFS (untick it). (I hate this stupid AES convention that full-scale sine wave is 0 dBFS )

Note that, in dBFS view, for |A| / |B| you gonna see 0 dB across all frequencies. However, for A/B (complex division) it will be -3 dBFS RMS.