You may have heard of the phrase ‘bedroom tone’ – usually applied as an insult: “Man, that guitarist had a sweet Tele Custom, but the worst bedroom tone I ever heard”. There can be a lot of misunderstanding as regards amp equalization & what constitutes good tone, and it’s my opinion that knowing how your amplifier works is as essential as knowing how to tune your guitar. For bass players, it is often difficult to be heard alongside even one good-sized guitar amplifier, let alone a pair of Marshall/Mesa/HiWatt stacks set to 11!

First of all, let me start by saying I was made aware of some of the following concepts in an interview with guitarist & producer DW Norton, of Australian band Superheist. DW Norton specialises in recording heavy bands, and it was one line in the interview that caught my attention. To paraphrase (since I can’t find the article anywhere): the typical recording process goes drums / bass / guitars / vocals, in that order. Since a lot of metal bands use detuning, what happens is that the bass ends up being over-written by a lot of the guitar tone. If you want to actually hear the bass, it should be recorded after the guitars, so you can find gaps in the audio spectrum to fill with bass guitar.

Hang on a minute, says I – there are gaps in the audio spectrum? Where are they, and why are they there?

The answer to this goes all the way back to the early days of electric guitar and a certain Mr Leo Fender. Without getting into the engineering details, the short version is this – the early open-backed guitar cabinets suffered from weak bass and treble response, and the cheapest fix was to compensate with a few capacitors and resistors in the amplifier to change its frequency response. The result was Fender’s passive tone stack, with cut-only mids and a boost to the bass and treble. The Fender stack was adapted by Marshall, Vox, Alembic amongst others, and is to many people’s ears the sound of the electric guitar revolution.

Equal loudness curves
Equal loudness curves

Speaking of ears, they don’t work the same at all frequencies. If you have a look at the equal-loudness curve graph to the left, it turns out we are most sensitive to frequencies in the high-mids, around 1k – 5k, and are decreasingly sensitive to both bass and treble. As an example, look at the 20 phon line – to our ears, 20 dB at  1 kHz sounds as loud as 50 dB at 80 Hz,and 35 dB at 10 kHz.

In order to hear a smooth response, we naturally turn up the treble and bass on our stereos and amps. Notice that the difference tends to flatten out a bit at higher volumes, ie: we are relatively less sensitive to mids at a typical live band level of 100 – 110 dB. This is why a heavily mid-scooped bass or guitar tone that sounds great for solo practice at typical television viewing volume (60dB)  can become both muddy and shrill at gig volumes (the dreaded ‘bedroom tone’ I opened with). Inversely, if you have a mid-heavy tone, it can sound dull or nasal when played in isolation.

All knobs at 5

Do yourself a big favour and download Duncan’s Amp Tools TSC 1.3, a brilliant little calculator that shows you the frequency response curves based on several tone stacks. You can even edit component values to see what happens when you modify the tone stack, handy for amp modders.

The first graph shows the frequency response of Fender, Vox and Marshall tone stacks with the knobs set to halfway. Note that the Fender & Vox tone stacks have a 12 dB notch centred around 500 and 800 Hz respectively, while the Marshall stack has both a higher gain and a milder 5 dB notch at 700 Hz (as well as rolling off the bass at a higher frequency).

Scooped mids
Scoopy

If you go all early Metallica on your amp, you get something a bit like the curves to the left – this is the classic ‘mid-scoop’ beloved by the palm-muting set. By some accounts, they actually used to cut the mids all the way to zero until they started working with Bob Rock (who also fixed their bass tone issues!). Notice how that not only does the mid-scoop become more pronounced, but the centre frequency shifts to a lower position due to the increased treble.

A quick note on wah pedals couldn’t hurt at this point. As almost everybody knows, wah pedals provide a strong boost at a narrow band of frequencies. The reason they can be so effective a tool in making solos come alive is that those frequencies typically range from 400 Hz up to 2 kHz. The lower half of the wah travel is right in the typical mid-cut zone of most amps, so the wah really leaps out in comparison to the rhythm guitarist. Fixed-wah soloing a la Michael Schenker takes advantage of this effect, by finding the most noticeable centre frequency for the wah and leaving it there.

Flattened EQ
Flatus

Finally, what if you want to flatten your amp eq? Well, it requires a pretty drastic positioning of the eq knobs that few would come up with by themselves. With the three-knob eqs, you get a reasonably flat response with the bass on 1, mids anywhere from 5 to 10 and treble at 0. As to why you’d buck the mid-scoop trend: you may have access to a good rack eq or floor pedal that you prefer, or maybe you’re the second or third guitarist in the band and need to differentiate your tone from the other guitars. Try it out next time you plug in and you’ll be hearing more of the natural guitar tone and speaker colouration.

Now, on to a subject close to home for me: bass. Not every amplifier has a variation of the passive tone stack, but the trick is that most guitarists have been trained to hear that scoop as ‘good tone’, and will replicate it anyway! It’s a safe assumption that almost every electric guitar you play alongside will have a fair amount of mid frequency content removed somewhere between 300 Hz to 1 kHz. This is further enhanced in the various metal genres, where mid-scooping helps accentuate palm muted riffing.

Some bass amplifiers have a Low Mid control; for example, GK amps centre their low-mids on 250Hz, MarkBass around 350ish. Some have a graphic EQ, such as my old BBE 383 Pre-amp with a 6-band graphic (mids at 250 Hz and 600 Hz),  and the Ibanez Promethean head (mids at 400 and 800 Hz); these are all good frequencies to boost to help add definition and punch to a bass sound. Even better is a semi-parametric Mid with a variable centre frequency – these can be used to dial in the most appropriate centre frequency for the band you’re with and the room you’re in.

How? Finding the right spot is a little like hunting feedback in a PA system – you apply a large boost to the EQ level, and sweep the dial until the feedback leaps out at you. That tells you what frequency you need to cut to avoid feedback. In this case, it’s a bit tougher, as you need to make adjustments while playing, but at least you know that you need to be somewhere between 200 Hz and 1 kHz at the extremes. I suggest starting low, apply a large boost to the parametric mids and gradually shifting the frequency up during a couple of songs at sound check. At some point, you may find that your bass tone is actually beginning to drown out the rest of the band – this is your centre. Dial the boost back to a more subtle setting, and you’ll be able to hear yourself and compliment the mix.

So, in summary:

  • most guitar amp tone controls are NOT flat when everything is set to the midpoint,
  • excessive mid scooping can rob your tone of definition and sound bad at gig levels,
  • both bass and lead guitar can benefit from filling the mid-cut of the other guitar amps.

Needless to say, this article only covers the basics. I recommend searching for articles on ‘recommended equalization frequencies’ and having a good read through some of the results – you’ll get lots of ideas for both recording and live sound. Combine that with doing some research on the various amps you and your band mates use (download every manual you can get your hands on!) and you’ll be able to create a cohesive blend of tones that will impress your audience, and perhaps more importantly, keep your soundguy happy!

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