Filter weirdness

[astanway]

• Generating a 50hz tone with harmonics in REW
• Recording with a u-mik1 on Smaart
• In MiniDSP, I set a 10hz high pass Butterworth 36db on my mains. I expect it does nothing, since my mains don't go that low
• Instead, when the filter is enabled, I see a 5db attenuation at 50hz.

The system is running two mains and two subwoofers. When this was generated, the mains were running full range, and the subs had a high pass at 25hz and low pass at 155hz. The mains have a 3db drop off at 40hz.

This is a MiniDSP Flex Eight HTX. Do I not understand how filters work or is MiniDSP doing something wrong?

 

[astanway]

Is it possible that the filter is introducing a phase shift to the signal that is then constructively interfering with the signal from subwoofers? Both boxes are playing at 50hz here.

 

[princetemi]

Try to simulate this in vituix cad. You will quickly see that everything changes phase. Even resistors can add a degree or more. It's pretty self explanatory and educational when you adjust components values on fly.

 

[tomchr]

I'd measure the frequency response of the MiniDSP directly to make sure it works as you expect it to.

Tom

 

[DSP_Geek]

At 10 Hz you might be running into finite wordlength effects: generally speaking, for IIRs precision decreases as the inverse square of filter corner frequency. Try setting the filter to 20 Hz and see what the response looks like.