But if the instrument sounds good at the source you may not need to do anything?
Alan.
That's almost never true, IMO, at least for an even remotely dense mix.
There are two reasons to EQ an instrument while mixing - one, correctively, to fix something that doesn't work, and to, complimentarily, to make it fit better with the other instruments in the mix.
Ideally, you want to do as little corrective EQ as possible - with the right bass, right pickups, right technique, right amp/pre, and right mic/DI, you should be able to get a fundamental bass tone you're happy with. However, once you do, you'll almost certainly want to apply a tweak here or there to make the bass fit in better with the kick or add a bit of clarity to the line behind the guitars by bringing out certain overtones a little more. I suppose it's theoretically possible to get everything 100% perfect with mic selection, amp selection, and whatnot, but even most well-respected pros can't engineer a session so perfectly that not a drop of EQ is needed to make everything mesh (especially when you consider that for a lot of rock music, the "sound" isn't very much like the instrument in the room - see rock and metal kick drums).
Octave Replacement EQ of the Bass
If you had a bass player play scales for you, you would discover that on the lower pitches the instrument sound quite boomy and on the upper pitches sound "weak." For the different instruments commonly used in pop music, the bass perhaps has the most uneven tone. Many engineers will almost automatically reach for a compressor on the bass when mixing.
The Motown Engineering Department, in the 1960's, ran tests on the bass and determined that the instrument generated harmonic energy an octave up from the fundamental pitch being played which was as loud as the fundamental. The 1960's Motown mastering division used this data to filter out the real low octave of the bass sound, allowing higher recording levels without losing bass line clarity.
Using this data in the 1970's Bob Dennis (?) developed a mixing technique for the bass, which he called "Octave Replacement EQ." The "boom frequency" of 100 Hz is reduced while the energy an octave up is boosted. It usually sounds better to have the boost slightly less than the reduction.
OCTAVE REPLACEMENT EQ - TYPICAL SETTINGS
Frequency 100 Hz 200 Hz
Level -6 dB +5 dB
Q 1.4 1.4
Use of Octave Replacement EQ in mixing tends to make the bass line more even and sound louder.
This is a pretty cool approach, and one I'd actually forgotten. However, I'm curious how much this has changed since the Motown era - CDs handle low end better than records (so I've been told, anyway, that could be an awful oversimplification), consumer sound systems have a lot more low end, and bass amps seem to have gotten better at producing low end - we're a long way from the days of the guitarist and bassist both plugging into a Marshall half stack. Additionally, 5-strings are a lot more prevalent. Is this still as true anymore today as it was back then, that the "boom" around 100hz just isn't that distinct so cutting it and boosting the next harmonic overtone helps? I'm asking, not disagreeing with you.
