EQ Decisions When Audio Mastering

Looking at the first EQ band – the low cut...

Why might you want to cut everything below a certain frequency? In mixing, you would low cut most channels except the kick and bass, but what about EQ in audio mastering? We know the final master may be played on a crummy kitchen radio which won't produce the extreme lows anyway, but it may also be played on a very expensive hi-fi with a frequency response as wide as you need.

One reason for this is that during the mixing stages the monitors used may only extend down to about 60Hz. Anything happening below that may have gone un-noticed. That's not so bad if the only thing down there is the extended bottom of the kick. But having reached the audio mastering stages, some unwanted rumble may be happening right down in the 30 - 40 Hz area. Only full range speakers, or a sub woofer will pick this up.

The speakers here at TGM Audio extend down to a useable 25Hz so we can spot anything that may have been missed. If your speakers will not produce frequencies so low, you can use headphones for this. A £150 pair of headphones will probably extend down as low as you need. If you have no way of telling what is happening down there, it might be a good idea to apply at least some kind of low cut. Most commercial pop records have little going on below 30Hz anyway. You may get some idea by the use of your spectrograph or frequency analyser. A cut at 30Hz would give you peace of mind.

Audio Mastering EQ Low Cut

view of audio mastering eq low cut

Low cutting some bottom end is also a way to concentrate more energy into the important midrange areas by use of compression and limiting (how this happens will become apparent soon). That's not to say you should just cut the low off everything - as I mentioned before, the extended bottom end of the kick might be down there, which on a quality set of speakers will sound very nice.

You will learn about how this energy is concentrated into the mids when you get to the dynamics processing chapters of the mastering tutorial. You will also discover how having a kick with an extended bottom end can actually help it translate to a little kitchen radio/laptop speaker even though these extreme low frequencies will be lost.

The lows play an important role. What we perceive as 'warmth' is mostly the work of the lows and low mids so a low cut too far may take the warmth away. The following chart illustrates how certain bands of frequencies are perceived to the ear.

Audio Frequency Perception Chart

audio mastering frequency perception chart

If the track in question appears to have low or excessive amounts of any of the above, it may be possible to correct the balance by reducing or lifting the specified area of the spectrum, or the surrounding areas instead.

Looking at the last EQ band - the high cut...

Why would we want to cut away any of the highs? As said before about the balancing act, a reduction in the highs gives the perception of a gain elsewhere. In some cases this might add to the warmth, or it could give a more rounded sound. It may also allow for a little more energy to be concentrated into the mids (this will be explained soon). On the other hand, the extreme highs may play an important part by providing air or sparkle.

The following three paragraphs are of utmost importance - please read carefully.

I mentioned earlier about the frequency response of a crummy kitchen radio or a set of laptop speakers. However I only stated where the frequencies roll off at the bottom, and where they roll off at the top – somewhere around 100Hz to 14kHz. But there is something to be said about the frequencies in between. Rarely are they reproduced evenly on any music playing system. The room acoustics can also contribute to an un-even frequency response at the final listening stage.

Some hi-fi's have a built in EQ allowing the user to tweak the tone to his or her taste. But you need to be aware that the consumer's hi-fi may well be about to boost, or reduce a group of frequencies and there is nothing you can do about it. I hope you can now see that the best fighting chance a master has to sound correct on different systems, and in different environments, from a tonal point of view, is to have an even spread of frequencies across the whole spectrum from the beginning.

An artist may love bass, a lot of people do, and the track may sound like it should contain a fair amount (think about reggae). But to produce a master with a heavy bottom end can be a risky business, knowing that the consumer's hi-fi, and room acoustics, by chance, might already be boosting the low frequencies.

You will soon discover how creating a fat bottom end can be achieved by use of compression and limiting without actually raising the volume of the lows compared to the rest of the mix.

David Eley - TGM Audio Mastering

Next Page... Fixing Problem Frequencies in Audio Mastering