Don't miss the advanced techniques at the end of the mastering tutorial :)

Problem Frequencies in Audio Mastering Continued

The resonating tones you have uncovered may well be problem frequencies when you consider the uncertainty of the consumer's player and environment. As mentioned, you should consider applying a small, notch like reduction at these areas using mastering EQ - a linear phase EQ preferably as demonstrated earlier.

A pure tone resonance found at the audio mastering stage could be caused by a bass amp reproducing one frequency louder than the rest. Perhaps the room the bass amp is in might be causing resonance. Or it could be down to microphone response or placement, picking up a certain frequency better than another. In a perfect world, all these instances would have been dealt with in the recording and mixing stages using channel EQ and compression (or better still, prevented in the first place with good recording techniques). But you must be prepared to deal with such nuisances during the mastering stages.

That aside, these 'pure' tone resonances are not always considered the most problematic. Covered next are the kinds of resonance we find more displeasing to the ear. It is these areas that require more attention.

Some resonant areas will appear to be an un-defined mixture of sounds.

In the true sense of the word these areas are not really resonance as they can be made up of more than one tone, but they take on the same resonant like characteristics when discovered using the sweeping EQ technique. They are a mixture of sounds all fighting for space in the mix. You will probably agree that this is more displeasing to the ear than pure tone resonance. This situation is caused by the overlapping effect of all the different playing instruments, reverbs and FX all happening simultaneously. There will be a muddy like texture to the area in question, which will most probably sound worse the further down the spectrum you find it.

Why does it sound worse the further down the spectrum you go? To answer this I'll refer to a mixing technique - low cutting the mixer channels.

One of the reasons why you apply low cuts to most mixer channels is because low frequency audio doesn't mix too good. In most mixing situations, all the mixer channels will have low cuts except the kick and bass. This keeps everything out of the kick and bass-line's area preventing it from becoming muddy. Here's a way to visualise this muddiness:

Imagine playing two notes simultaneously on a bass guitar all the way down at the bottom of the bass guitar's range. Will they sound nice? They simply won't sit right together. Play the same two notes but much further up, as high up the bass as they can go. They will sound much more pleasant to the ear than when they were played lower down. This is why muddiness can be more of a problem in the lows and low mids. Low frequencies simply don't blend together very easily.

Why can the kick and bass be together without sounding bad?

In most cases the kick is not a 'tuned' sound. It doesn't have one particular frequency, so it doesn't clash with the note the bass is playing.

A good mix with appropriate low cuts on instruments, vocals and FX should reveal little or no muddiness in the lows or lower mids. Unfortunately, this is not always the case. The fact is, your consumer's player, or environment, may be about to boost a group of frequencies. What if the final master already contains a boost in the same area - in the form of a muddy like texture? The combined result will not sound too pleasant. This is where your attention should be focused. A reduction will certainly help with translation to the consumer's player. It will also help mask the unpleasant sound of a muddy like texture. But sometimes, the only solution for a problematic clump of overlapping sounds is to go back to the mixing stages and create better definition of the area in question using channel EQ.

When you're reducing a pure tone, a relatively high Q setting will suffice as you're homing in on a narrow banded area. When reducing an area of muddiness caused by many different sounds occupying the same space, the banded area may possibly be wider requiring a lower Q setting. When sweeping across the spectrum looking for problem frequencies, try to detect the width of the problem area. The further down the frequency spectrum, the wider/lower the Q setting will most likely be.

This mastering technique is commonly used. Over time, your ears will learn to distinguish resonances, or other problem frequencies without the aid of a boosted EQ. Practicing this technique does more than find the areas that need attention, it also helps to build a solid relationship between you and the frequency spectrum. On the next page you will find a video mastering tutorial demonstrating this mastering technique using the downloadable track found here:

Maria Sins-Changes.wav 

David Eley - TGM Audio Mastering

Next page... Linear Phase EQ Audio Mastering Demonstration

book on audio mastering techniques, tips and tricks