Remove Low-Level Hum

[abbaker2]

I used a Radio Shack “Super Cardioid Vocal Microphone” to record background narratives for some silent home movies. I am using Windows XP and the narration files are all .wav format.

I discovered that I have some low-level hum on the narratives.
Can I use Audacity to remove the hum? If so, what version should I use?

Thanks in advance.

abbaker2

[steve]

Use Audacity 1.3.8
If you are on Vista, read this before installing: http://wiki.audacityteam.org/index.php? ... s_Vista_OS

Hums can come in various flavours. Usually they are caused by interference from the mains electricity supply. Of course it is best to try and avoid them in the first place by using good quality microphone leads, keeping away from electrically noisy equipment such as motors, TVs, and so on, and ensuring that everything that should be earthed is earthed correctly. However we often have to work with less than perfect recordings and there are several tools in Audacity that can help.

In the US, mains hum is at 60Hz, in Europe at 50Hz. There are often also harmonics of these frequencies, so in the US you are likely to get unwanted frequencies at 60, 120, 180, 240Hz.... and in Europe 50, 100, 150, 200, 250 Hz.... If you select a section of "silent" audio (hum only) and get up the "Plot Spectrum" tool from the "Analysis" menu you should be able to see them (you may need to amplify the silence to make the noise clearly visible if it is very quiet).

Notch filters are good for removing the main part of the hum - I can't remember if one is included as part of the standard Audacity install, but if not you can use the "Nyquist Prompt" from the Effects menu and enter a little Nyquist script to do the job. You will need to select the audio first, then select "Nyquist Prompt" from the Effects menu, then enter the code below and apply it.

For the US (60Hz hum)

Code: Select all

(notch2 
(notch2 
(notch2 
(notch2 
(notch2 
s 
300 10)
240 8)
180 4)
120 2)
60 1)

For Europe (50Hz hum) use this:

Code: Select all

(notch2 
(notch2 
(notch2 
(notch2 
(notch2 
s 
250 10)
200 8)
150 4)
100 2)
50 1)

That should take care of the worst of it.
You can then use the "Noise Removal" effect to clean up any remaining hum. You will see instructions for this effect here: http://wiki.audacityteam.org/index.php? ... se_Removal

(the Nyquist code given above is for Audacity 1.3.8 and later - it will not work on stereo track in earlier versions.)

[kozikowski]

My personal preference is to go with the noise removal first. There are some very good controls (in Audacity 1.3) that allow a delicate touch so you can remove the trash and not much else.

Once you use the notch filters, those sounds are gone forever, performance or not, and there's no guarantee you're going to hit the right notches or enough of them. For example, the last hum job I did had strong energy at 60, 180 and 360 (strong buzz) but almost no energy at 120. Notching out 120 may have been complete overkill.

184 is F# below middle C and 369 is F# above. Low B an octave down is 123Hz. All assuming concert tuning of a perfect piano and all dangerously close.

Koz

[steve]

he last hum job I did had strong energy at 60, 180 and 360 (strong buzz) but almost no energy at 120. Notching out 120 may have been complete overkill.

It seems quite common for mains hum to be a lot more pronounced on odd harmonics than on even harmonics.
Have you EVER seen hum that is strong on 60, 120,240 Hz but low at 180 and 300Hz?
(UK/Europe equivalent would be 50,100,200Hz)

184 is F# below middle C and 369 is F# above. Low B an octave down is 123Hz. All assuming concert tuning of a perfect piano and all dangerously close.

This is where my "New Improved" hum remover comes in Hopefully finish and post it on the forum before I go on holiday.