Once the tone has been fully processed with compression, distortion and any pre-distortion effects (such as a phaser) the last main link in the chain is the final EQ and tone shaping (though time-based effects are still to come).The final EQ shaping is reliant on the speaker response, the speaker configuration, and whether the speaker cabinet is open or closed back, and of course the setting of the tone stack.
Equal-Loudness Contours
The equal loudness curves represent the way our ears are most sensitive to the high-end and bottom-end at high volumes; while at low volumes, our ears are most sensitive to the midrange (circa 4kHz). Effectively, this means that at high volumes our perception colours the sound with a wide mid-scoop.
You may be familiar with the ‘loudness’ button on old cassette players and even some CD players or music software? These are special tone controls which replicate this mid scoop but at lower levels, creating an impression of loudness without actually raising the SPL.
Similarly, a guitar tone with scooped mids will seem louder than a more middy guitar tone at the same level.
Mid-Scooping
Since scooped tones seem louder, tighter and ‘punchier’, many guitarists like to scoop the tone especially when practising at (low) bedroom levels. If you do this, remember that the guitar is an inherently mid-dominant instrument, so by scooping the mids we cut the most essential frequencies to a guitars sound.
Although mid-scooped tones sound acceptable at bedroom levels, they don’t translate well to stage levels or rehearsal levels. In these (louder) scenarios, the equal-loudness contour comes into play by scooping the mids out even more! Guitars rely on their mid frequencies to project and cut through mixes, but with such an extreme mid scoop these guitar tones quickly lose their tight bedroom sound, and instead become lost in the mix of the drums and bass.
Consider that by scooping the mids, you are now relying on the bass and treble of your tone to cut through – which is impossible since you’re competing with a bass guitar, and cymbals.
In band situations, play to the guitars’ tonal strengths. Keeping a healthy level of mids will help you cut through and be heard without needing to raise your level and risk ruining the balance of the band.
Amp Design
Also, a feature of guitar amplifiers is that they are also inherently mid-scooped. The mid scoop is so extreme that the only way to approximate a flat response is to turn the mid knob all the way up, and back off the high and bass knobs almost to zero. This means that even with the tone knobs set to 12 o’clock the tone is already being scooped considerably. Bear this in mind when ever you are adjusting your tone, and try avoid over-scooping the tone.
The Tone Stack
The tone stacks on most amps are generally broken into low, mid, high, and occasionally for even higher frequencies, ‘presence’. The more distortion that you have after the tone stack (such as strong power amp distortion) the less affect the tone stack will have. This is because distortion is a form of non-linear compression – the most prominent frequencies coming from the EQ section will be hit the hardest, so any peaks and dips in the EQ curve before the distortion will be ‘flattened’ out to some extent. So for tones with plenty of power-amp distortion the tone stack also serves as a pre-dist EQ of sorts. For lower power-amp gain, the effects of the tone stack will have more influence over the final tone. With higher power-amp gain the tone stack settings may need to be slightly exaggerated to achieve a similar effect.
Remember though that Fender style amps have the tone stack before the pre-amp distortion, so in this case, high pre-amp gain will also reduce the effectiveness of the tone controls. However since Fender amps are typically played at lower gain levels this rarely presents a problem. Also, you’ll notice that Fender tone stacks are capable of much more extreme EQing than most other amps – for instance few players will rarely set a bass knob on a Fender above three or four because this quickly becomes ‘too much’. As such Fender tone stacks have plenty of range even at full gain.
Top End Roll-Off
It is essential for the top end of an electric guitars tone to be rolled off, particularly if the tone is overdriven. The electric guitar itself has very little important high frequency information, (important content goes only high as 8-10kHz) but overdrive and distortion by their very nature, create overtones far above this range. These harmonics sit above the typical frequency spectrum of the main guitar sound. They sound separate from the main guitar tone, and they make the tone overly trebley.
So the solution is to have a low-pass filter (high-cut). Having a low-pass filter is not the same as simply turning down the high knob on the tone stack. A tone knob has a ‘bell’ or ‘peaking’ EQ whereas a low-pass filter is a shelving EQ.
The traditional way of getting the required low-pass filter is to simply use a speaker with a poor treble response. Larger speakers have a reduced treble response so electric guitar speakers are generally 10″-12″ in diameter.
In the modern ‘direct’ recording approach of home studios it has become common to bypass the power-amp and speaker entirely by running the output of the pre-amp straight into the desk or recorder. In this instance a low-pass filter can be applied during editing or mixing, or alternatively some guitarists use so-called ‘speaker simulators’ before the desk which contains low-pass filter circuit. In dense mixes, or mixes where the guitar is not a focal point, these approaches can be quite effective however they can not accurately replicate a real guitar speaker. Guitar speakers are a far from ideal low-pass filter, and they tend to add a peaky response to the mids. Also, when a guitar speaker is being driven hard (and, hey, it should always be loud shouldn’t it! 🙂 ) a guitar speaker will also introduce fairly audible distortion of its own. Multi-speaker cabinets are even more difficult to emulate – since all sorts of phase cancellation and lobing will occur whenever multiple speakers are used.
Cabinet Design and Bass Response
While the design of the speaker probably has the most effect on the top end roll-off, the cabinet is probably the most significant factor influencing the bass end of a guitars tone. Bass is non-directional by nature and fills a room, whereas treble tends to ‘beam’ in a straight path.
Open backed cabinets disperse sound equally from the front and back of the speaker, however the sound coming from the front of the speaker is 180 degrees out-of-phase with the sound from the back. Since, bass is non-directional the bass from the front and back will combine and cancel each other. The very lowest frequencies are the least-directional, and become more directional as frequency increases, so the most cancellation occurs at the lowest frequencies with higher frequencies being canceled
progressively less. So open-backed cabinets tend to have a controlled but de-emphasised low end. Open-backed cabinets are typical of Fender combo amps, where the open-back serves to complement the bright sound which Fender amps are known for.
Another feature of open-backed cabinets is that they create a more open room-filling reverb sound. Because there is equal sound energy emanating from the back (away from the listener) as from the front, at least half of the final sound that a listener hears has followed an indirect path by bouncing of walls and objects before finally reaching the listener. Since closed-back cabinets only output sound from the front, the ratio of direct to reflected sound is much higher, so there is less room reverb in the tone.
Closed back cabinets tend to have a more prominent bass-end since the bass from the back of the speaker cannot combine with the bass from the front. They also have a more focused and directed sound, since they do not induce nearly as much room reverb as there is no sound emanating from the back. Since the sound from these cabinets is more ‘directed’ there is significant tonal variation depending on whether you stand right in front of the cabinet, or away and of to the side. Since treble frequencies beam more than lower frequencies, the further ‘of axis’ (to the side) you stand, the less treble will reach your ears, creating a darker sound.