applying high pass filter eq to acoustic guitar

B

banjo71

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I've been working on EQ'ing some acoustic guitar. I was wondering if someone might help me understand something. I was making a high pass filter for a rhythm guitar in standard tuning, capoed up two frets, playing the G position, to clear up the muddiness of the sound. The lowest note ever hit was the low A on the real A chord, (6th string at 5th fret) at 220 Hz. I just got a spectral frequency response plugin that runs on RTAS, so I looked at the response and there is a lot of activity between 47 - 220 Hz. I just adjusted the filter until it sounded best. So this is more of an acoustic guitar question.. If the lowest note is 220 Hz, where does the extra low end come from on the guitar? If the lowest note is 220Hz in this case, why do lower frequencies show up significantly? Are they maybe octave harmonics or something along those lines?
 
Boom from the body, room noise, any number of places. High pass filtering is one of the biggest keys to clean, balanced mixes, and probably and of the most underused tools by beginners.

EDIT: Good for you for figuring out the discrepency! Use your ears when deciding how aggressive to HPF, not just your eyes. Some instruments get much higher filters than you think. Its not uncommon to HPF keys and electric guitars up to 200-250hz in busy rock mixes, or vocals up to 200hz in pop and rock. Bass can often be HPF up to 50 or 60HZ if you don't need sub, which tightens bass tracks wonders.
 
Like Brian said, use your ears, but for the highpass cutoff frequency you're usually going to start somewhere around the lowest note you play because you don't want or need anything lower than that, and if you go much higher than that the guitar will start losing body and sound thin. The usual approach is to sweep the filter towards higher frequencies until you can j-u-s-t hear a change, and then back off a bit. The important thing is to use the highpass to get rid of all that stuff that accumulates in the lows. It makes everything dark and eats up headroom. Keep using that frequency analyser you got. You'll be amazed how much junk is down there.
 
banjo71 said:
(6th string at 5th fret) at 220 Hz
Click to expand...
The fundamental of the A note on 6th string, 5th fret on a regular tuned guitar is 110Hz, not 220Hz. The capo you used on the 2th fret didn't change that.

But, as already said above, there's lots of info below that. One more example: when you touch the strings to mute them, you just moved them a little in a very, very low frequency. If your signal path is any good (starting at your fingers and going all the way to the ADC), that low amplitude, very low frequency information will show up on the recorded track.

When mixing rock/pop songs, I rarely set my HPF below 160Hz on guitar and keys (and sometimes as high as 630Hz!). I like to let the bass and the kick alone on the first 3 octaves.
 
It also helps to tweak "in the mix". Meaning, you can solo to get close, but you'll not get it right if you don't hear it in context. I don't even bother trying to solo when eq'ing anymore. I just play the mix and fix pieces. Others may disagree, but it's easier for me.
 
For an acoustic guitar - a lot depends on the guitar itself and how it is miked. An acoustic with a fairly high frequency set for hi pass can sound tinny with no body (the 'wood' of the guitar). I'll scoop some of those lows out if the mix is getting muddy - typically this only happens when there are multiple guitars.
 
I usually find 75Hz HPF is a good starting point for acoustic guitars. I usually like 60-80Hz, it's almost always somewhere down there, but it varies with different guitars, some are definitely a lot boomier than others so I will move the microphone back instead of trying to EQ it later, 200Hz is a good frequency for cutting, the mud is usually between 150-400Hz, but you have to be careful or it will remove the warmth, knowing when it is 'just right' is what requires the most skill, and only experience can teach you.
 
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