SPL vs dbFS delta question

[G29]

Novice question. What is the difference between SPL and dbFS ?

When I externally multiply 2 signals and then compare the product against the Arthmetic multiply function in REW, everything (excluding noise floor) appears to be identical except for the SPL display.

In the first image, the brown REW multiplication is much higher than the externally generated green trace.

Using the dBFS (and other) displays, the 2 multiplication traces match each other (2nd image, green and brown).

When I switch between SPL and dbFS, the 2 imported signals that are used in the multiplication shift relative to the externally generated product. They shift in lock-step.

The REW generated product does not stay relative. The product jumps significantly high in the SPL display.

Any pointers would be helpful.

Thanks much.

P.S. Using V5.40 Beta 69

• Blue and Red traces are the source for the multiplications.
• Green trace is externally generated product.
• Brown trace is REW internally generated product.

SPL Display:

dbFS Display:

 

[G29]

Attached mdat file. Products only.

 

[G29]

Attached mdat file. Source signals only.

 

[John Mulcahy]

What is the difference between SPL and dbFS ?

SPL has a fixed dB offset from dBFS. REW's trace arithmetic keeps SPL figures correct, multiplying the SPL traces adds their dB values. If you want SPL to match dBFS you can go into the settings (gear icon) of the Cal files preferences for the input you are using and set the dBFS @ 94 dB SPL figure to 94.

 

[G29]

For my clarification.

The SPL appears to be kept in the MDAT file and is not stored in an exported .WAV file. Is that correct ?

I just ran a test by exporting the REW generated product to .WAV file in both SPL and dbFS modes.

Bringing the 2 .WAV files back into REW, loses the extra SPL level. They now behave like my externally generated products in that regard. No more dual amplitude states.

I wasn't clear if these 2 states were somehow stored in the .WAV file or something else in the .MDAT file. It appears to be the latter.

Thanks much.

 

[John Mulcahy]

The SPL offset is obtained from the SPL calibration of the input and stored in the mdat file. You can see the value in the Info panel.