Noise Gates - Frequency control of the key

A common problem when using Gates is that the key signal might be picked up from another sound source and open the gate to the signal you are trying to control, basically this happens when a microphone is picking up something from more than one source unintentionally.
Imagine you are mic-ing up Toms on a drum kit, the chances are that the mic for Tom 1 can also 'hear' Tom 2. You setup a gate on Tom 1 to control the sound of it but when you hit Tom 2, the mic from Tom 1 hears it, opens the gate and allows all the ringing from Tom 1 to be heard unintentionally. You can EQ the channel but the Toms are usually so close in frequency that you can EQ out the unwanted sound of Tom 2.
On some gates, you have the ability to frequency control the key signal, either with a hi-pass and lo-pass filter or with a frequency selector and Q control.
It is important to realise, these settings do NOT alter the frequency of the audio signal, they simply filter the part of it being used to key the gate. By making this as narrow as possible, we can allow the key to respond only to the fundamental frequency of Tom 1 and not to anything overheard from Tom 2 (and likewise for any gates you might have on the other Toms).
You will need to either recognise the rough frequency of the Tom by ear and then use trial and error to set up the unit or you will need a frequency analyser to work it out. You should be able to keep the frequency very narrow since the fundamental frequency of a tom is narrow.