In response to our recent tech tip for removing hum in a recording (see inSync 4/14/99) a few readers wrote in to say they thought a simple approach would have been to use a good EQ to notch out the 60 Hz hum. In this case since the recording was of a church choir a notch at 60 Hz may not damage the tonal quality of the recording. Obviously notching out 60 Hz would ruin most pop ensemble recordings.
The only problem with the EQ approach, however, is that the hum was an air conditioner unit. These rarely produce a simple hum at 60 Hz like an electrical ground loop (in that case the EQ trick would most likely work fine). They produce all sorts of low (and some mid) frequency garbage that is generally too complex to be removed by a simple EQ. Consequently the trick of removing it by some sort of phase cancellation or a noise removal system probably going to be the best bet. However, its fairly easy to try EQ first on the off chance it will do the job.