Please help with strange tone removal

We record church audio using a line in on an iMac from an out on the soundboard and record with the latest stable version of Audacity.

I have uploaded a small WAV sample of a few seconds of “silence” from our WAV file. The usual Noise Removal plugin in Audacity is not successfully removing this. I’d love to know (1) how could this be removed and, more importantly, (2) any idea what this noise is and how it could be introduced?

Spectrum of WAV sample:

Copy and paste the code below into the “Nyquist Prompt” which is in the “Effect” meun …

    (setq mysound s)
    (setq q 10)         ; set the base Q for the filter
    (setq iter 10)     ; set the number of iterations
    (setq freq 970)  ; set base frequency

    ; start the DO loop
    (dotimes (i iter mysound)   
    (setf mysound (notch2 mysound (* freq (1+ i)) (* q (1+ i))))
    )                     ;end of loop

code pasted into ''Nyquist Prompt''.gif

Applying the above code removes the main whine, (which is a 970Hz fundamental and 10 harmonics),
the above code does not remove hiss-noise or the weird bleat which comes in at 0.9 seconds.

[ code originally from here … ]

That code worked great! thank you so much.

Any idea what could be causing this noise? I would really like to not have to filter this out each week.


If the noise is only present when you plug a cable into the line-in socket, you could try a cable which has better screening to block the EMI.
[ could save money by just using this expensive shielded audio cable within ~3m of computer ]

This cable is allegedly “double shielded” …

If the noise is still present when you pull out the line-in plug then the interference is occurring within your computer, and only fixable by substituting your computer’s internal sound-card with a good-quality external sound card (which accepts line-in ).