The Frequency Thread

[mojo321]

Hi all, I am starting this thread in a attempt to gain understanding of how roll-offs and boosts are done with a certain goal in mind.  I am tired, quite honestly, of randomly trying different value caps and resistors to achieve some change in tone, by blind luck.  So here is a first entry, something that I snipped off of the web somewhere:  ( I left the signoff in fairness to the original poster, though I am not affiliated with them)
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Description of sound frequencies. Please read our guide on  "Using Guitar Equalizers" before reading this.

This would be a useful list for guitarists who uses the equalizer often:

40-60 Hz Boomy - a sound over abundant in low lows. These waves move a lot of air, hence Boomy.
60-150 Hz Fat - the octave above Boominess. Makes things sound big, but not earth- shaking.
125-250 Hz Woofy - a somewhat nebulous term for sounds that are sort of "covered" - masked by low-end energy.
250-500 Hz Puffy - is like an octave above Woofy. It's still sort of a cloud, but not as big.
200-400 Hz Warm - obviously a positive characteristic often found between 200 and 400 Hz. Could easily degenerate into Woofiness or Puffiness if overdone.
500-1kHz Boxy - seems to remind one of the sound in a small box-like room.
500-1kHz Low end of Voice
500-5kHz Power range - mid-range band which contains the 1st and 2nd harmonics of most important sounds.
1.5-2.5 kHz Telephony - accentuating the limited bandwidth characteristic commonly associated with telephones with a roll-off both above and below.
2.5-4 kHz Cutting - Here, "cut" means to put an incisive "point" on the sound.
2.5 kHz Punch - Accentuating this range punches through vocals.
3-6 kHz Presence - Anywhere from 3-6 kHz can be used to make a sound more present.
7-10 kHz Sibilance - Dangerous "s" sounds and lots of other trashiness can often be found at 7-10 kHz.
10-12 kHz Zizz - refers to a pleasantly biting high-end resonance (think of a "harpsichord"-type brightness found around 10-12 kHz.
12-15 kHz Glass - A very translucent, but palpable brilliance associated with 12-15 kHz.
15-20 kHz Sparkle - A real smooth stratospheric brilliance almost beyond hearing, but can certainly be sensed.
Above 10 kHz Brightness - Most generally achieved by a global (shelving) EQ of everything above 10 kHz.
Below 10 kHz Darkness - The opposite of brightness (a general lack of highs at 10 kHz and beyond).
125-500 Hz Muddiness - Actually a compound problem: Woofiness plus Puffiness (excess low end and also low mids).
125-500 Hz Thinness - The opposite of Muddiness (a deficiency of lows and low mid frequencies).

Our favourite frequencies for electric playing:

Low end
120-150hz
This is for low end thump.

Low mid
400hz
Ever get a great sound that disappears in a track?
This is the frequeny to reach for, adding a couple of db's around 400hz will fatten up the tone.

High mid
1-2khz
Cutting this frequency will take away some boxiness, especially if you're using a small cab w/an SM57

High end
5khz
Boosting this frequency will give you more presence without the scratchiness.
Guitar amps produce very little above 5khz but boosting above with a wide bandwith (Q) can add some "air" to the tone.

Thats all in a nutshell. Have fun and I hope you've found this guide useful. If you've learned something or you have further clarifications, feel free to drop us an email. We'll love to hear from you!

Do also visit our store for specialized guitar backing tracks. At DollarBackings,  we've a wide range of unique and original guitar backing tracks covering all genre and all playing styles! See you soon.

Regards,
DollarBackings Admin
Your #1 source for Professional Guitar Backing Tracks
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So next would be a discussion on how to isolate these bands within a circuit using different cap values or something?  Or am I way off base?  I have lined up all of my stomps and tried to get them all dialed in about the same, and it is amazing what subtle differences there are from one to the next.  I like some aspects of one, but other aspects of another.  I want to know why, what, and how!

[IvIark]

The easy way to do that is with high or low pass filters.  

A low pass filter cuts highs and in a simple form would be a resistor in series with the signal followed by a cap to ground.
A high pass filter cuts lows and in a simple form would be a cap in series with the signal followed by a resistor to ground.

The formula for calculating the frequency is 1 / 2Pi R C and so assuming you had a 10K resistor and 0.1uF cap that would work out at 159hz.  

If you use a first order low pass filer all frequencies above it will be rolled off at 6db per octave, which essentially means the signal level is approximately halved everytime the frequency doubles.  
If you use a first order high pass filter all frequencies below it will be rolled off at 6db per octave.

Doubling up on these filters (for instance resistor, cap to ground, resistor, cap to ground) means they become second order and that will increases the rolloff to 12db per octave.  That will give you a quicker frequency roll off for when 6db isn't enough.

In a lot of these effects, high pass filters are often used before gain stages to cut some bass frequencies to stop the drive getting to boomy, and low pass filters are used at the end (traditional tone control) to cut back any spikey top end.

This is a good calculator to work out the frequencies for you.

http://www.muzique.com/schem/filter.htm

[mojo321]

Excellent Mark, thanks for that.  So, if I use a low pass filter, can I send the highs, instead of to ground, to a split off circuit for processing(like a boost), and then later blend it back in?  Or does it not work like that, because the ground an essential part of how a roll off functions?

[IvIark]

No, the higher frequencies would be removed using this kind of filter.  If you wanted to separate frequencies and deal with them independently, you could split the signal into maybe 3 separate gain stages and deal with them each as you see fit.

You could take them to 3 separate opamp channels, have a different filter at each opamp, and then even alter the gain of each opamp individually so that maybe the lowest frequencies were amplified less than higher frequencies.

Rather than using high and low pass filters you could use bandpass filters which attenuate low and high frequencies so you can specify quite accurately what frequencies each opamp will be amplifying.  But it is getting quite complex there when you take the amplifier and bandpass filter components that will be required into account.  But something like that may be good if you just wanted to add distortion to the very highest frequencies and leave bass alone for example.

[mojo321]

Got it.  OK, so I looked up a bandpass filter.  It looks like signal goes in series through R1 then C1, and then parallel C2 and R2 to ground.    But the equation looks a little more complicated than just considering it a low-pass filter plus a high-pass filter.  You're right.  

But I am excited to start looking at these circuits with a better idea of what is going on.  Thanks for that.

I am brainstorming that Tychobrahe Octavia pedal.  It looks pretty loaded with filters, what with so much of it going to ground like that.   But exactly what is going on is gonna take some time.

[mojo321]

Handy link:
http://www.electronics-tutorials.ws/filter/filter_4.html