Finding Instrument frequencies using Notch filtering in Audio Mastering

I would like to share these tips to beginners or those struggling to master their own projects. This can help you find out which fundamental/dominant frequencies are used by different instruments using Notch filtering technique. Since a mastering engineer does not know what frequencies are adjusted/boosted or assigned by the audio mixing engineer; the mastering engineer could still determine what frequencies are to be adjusted during the audio mastering stage.

There are however a lot of techniques in spotting the instrument “sweet spot”. One method is performing musical instrument frequency analysis; a graphical plot that show where in the frequency spectrum does a specific instrument dominates. However, one drawback is that the audio wave you are working is already a summation of different musical instrument signals and the results are not conclusive.

Thus, what seems to be reliable based on my experience is notch filtering. To define, a notch filter is simply an operating mode of parametric equalizer to radically cut/stop a band of signals at an extremely narrow bandwidth range. My favorite setting for a notch filtering (high precision adjustment) is a cut of -48dB with a Q of 100.

The purpose is to detect where the instrument signals “significantly” attenuated or faded. I can do this by sweeping through the frequency range and when this point is found; this is the instruments most dominant frequency (also called “sweet spot” or fundamental).

Different musical instruments have different dominant frequencies which are assigned by the audio mixing engineer. For example, vocals and the presence of the music tends to occupy at a very wide range of 500Hz to 5000Hz, cymbals at 8000Hz to 15 KHz, kick drums at 50Hz to 200Hz, bass guitar at 30Hz to 200Hz.

But take note these are very wide frequency ranges. If it’s very wide, the mastering engineer cannot effectively cut or boost problematic frequencies if they occur in the mastering. In another way, the mastering engineer could also not emphasize important frequencies that the instrument should be boosted to improve its presence in the track. A good example is the kick drum and bass.

Some songs have bass guitar mixed at a lower frequency than the kick. But other tracks may have kick drums occupying at the subwoofer frequencies. Using notch filtering, the mastering engineer can isolate these two effectively. The mastering engineer will simply do notch filtering adjustment then sweep through this frequency range. Let’s have a demonstration:

Example #1: Find what specific frequency the kick drum needs to be adjusted in the provided audio sample.

Trial 1: Sweep to 200Hz

Audio result (observed the kick drum spikes):

Trial 2: Sweep to 90Hz

Audio result:

Trial 3: Sweep to 50Hz

Audio Result:

Observations: Which do you think is the kick drum “sweet spot” or dominant frequency? For the 200Hz, you can still hear the kick drum spikes as well as the 50Hz. The correct frequency to be adjusted is 90Hz. It is because at 90Hz, there is a substantial loss of kick drum sound. This implies that 90Hz is the location of the kick drum center frequency. During mastering session, if there are any EQ related adjustments on kick drum (example using a multi-band compressor or any EQ tweaks); you can simply adjust at 90Hz then apply the desired cut and boost in dB. This will have the most profound effect on kick drum sound.

Example #2: Find what frequency do you need to adjust “presence” elements (vocals, etc,) if it’s somewhat buried in the mix and your mix sounds a bit muddy.

Trial 1: Sweep at 3000Hz
(Since these types of frequencies are wide-band type of signal, you can decrease the Q from 100 to 25 to spot it effectively)

Audio result:

Trial 2: Sweep at 1500Hz

Audio result:

Trial 3: Sweep at 800Hz

Audio results:

Observation: My choice is at 3 KHz since the vocal sounds so weak at this point (try listening with low volume) and there is a substantial loss of audio clarity and musical presence. Therefore, in any presence adjustment during mastering for this specific track, you can apply a wide band boost center at 3 KHz, for example: 3 KHz, Q=0.8 +1.5 to +3dB, this will have the most profound effect on improving clarity and presence.

Content last updated on August 15, 2012