Equalisation or 'EQing' is the process of boosting or cutting a particular frequency range of a sound in order to subtly alter its sonic properties. For instance, it may be desirable to boost the low end, or remove some of the mid range 'clank' of the bass. Equalisation can be used in many ways to create different kinds of bass sound. There are several kinds of equaliser, the simplest consisting of just a bass and treble control. 'Graphic' equalisers, as found on some pedals, have fixed frequency bands which can be cut or boosted, and 'parametric' equalisers have more sophisticated controls for frequency, 'Q' which refers to the bandwidth of the frequency range to be adjusted, and cut/boost. Many equalisers also have low pass and high pass filters. A low pass filter cuts all frequencies above a certain point, effectively letting the low frequencies pass, and a high pass filter cuts all frequencies below a point, letting the high frequencies pass unchanged. It's worth bearing in mind that the lowest frequency produced by a bass guitar is around 40Hz, so a high pass filter can be very useful for reducing any subsonic rumblings.
Most bass amplifiers have equalisation built in, but it can be useful to have an external device for more detailed control and ease of bypassing. There are many pedals available featuring either graphic or parametric controls, and some combine EQ with distortion, which allows you to shape your sound more radically.
Compression circuits alter the dynamic range of an audio signal. Most often, a bass compressor is used to lower the peaks created when you play. This is very useful for 'smoothing out' the sound of your bass and giving you more sustain, and can also help your amplifier to perform more effectively. Typically, a simple compressor will have controls for 'threshold' (the point above which the signal is attenuated), 'ratio' (the amount the signal is actually lowered by), and also settings for 'attack' and 'release' which control the behaviour of the compressor. 'Limiters' provide more extreme compression, actually completely preventing peaks in your playing from exceeding a specified threshold.
Digitech Bass Squeeze
Mutli-band compression is the term for a device that splits an audio signal into separate frequency bands and compresses them individually, before combining them back together at the output stage.
Distortion is the process of changing the sound by allowing the signal to overload or 'clip'. This creates varying degrees of a more crunchy and aggressive sound, depending on the settings employed. Used widely in rock music, the effect can be achieved by turning up an amplifier too loud, or by using a distortion pedal or 'fuzz box', for more controlled results.
EBS valve drive
There are now many kinds of distortion pedals designed specifically for bass guitar, from simple distortion, fuzz and overdrive boxes, to much more complex digital modeling pedals, which digitally recreate the sounds of classic amplifiers and speakers. These can be very useful for recording, when it's not always convenient to mic up a bass amp due to noise and space considerations.
Chorus effects are produced by using a delay line to double the original signal, and then modulate its pitch, creating a kind of subtle but pleasing 'wobble'. This has the effect of making the original sound seem richer, almost as if played by several bass guitars at once. A famous use of chorused bass can be heard on Paul Young's version of 'Wherever I Lay My Hat, That's My Home', as played by Pino Palladino. Stereo versions are very popular, but obviously you'll only get the benefit of this if you have two amps!
Ashdown Chorus Plus
Audio filtering devices cut or boost specific frequency ranges in a sound, but in a much more extreme fashion than EQ. As described previously, the most common types are low pass and high pass, which are sometimes used for removing sub-sonic and ultra-sonic frequencies. However there are many other ways to use filters - 'resonant' filters produce some very interesting results by adding a resonant peak at the filter cut-off point. Classic examples of this are found in 'wah wah' pedals and envelope filters such as the Mooger Fooger Low Pass Filter.
Electro Harmonix Nano Bassballs
Envelope filters respond to the intensity of the note played. Playing harder opens the filter wider, allowing more of the original sound through and producing a squelchy, funky sound, often associated with 70s cop shows.
Morley Bass Wah
Wah wah pedals use a continuously variable pedal to control a resonant band-pass filter. While you can use a regular guitar wah wah pedal such as the Jim Dunlop CryBaby with a bass guitar, it's often more effective to use one that has been specifically designed for bass guitar, as regular guitar wah pedals do not allow any low frequencies through their filters.
Phasers use delay technology to modulate the signal, creating a series of peaks and troughs in the frequency spectrum which can be made to cycle up and down, creating a swirling effect. Phasing was famously used on the bass guitar by Anthony Jackson on the O Jays hit, 'For the Love of Money'.
TC XII Phaser
Flanging is another delay-based effect that is the result of mixing two identical signals together, one delayed by a small, but gradually changing amount. Flanging can be very effective on bass guitar, especially when combined with a pick.
MXR M-117 Flanger
Delay or 'echo' can be used in many different ways, and not just for bass guitar. Most delay units are flexible enough to be used to create chorus and flanging effects - check to see whether the unit has controls for modulation and feedback. If you're not planning on using many effects at once, this can be very useful, and may save you buying unnecessary extra pedals. Many modern delays are programmable, which allows you to save your favourite settings - great in a gigging situation, making it possible to change effects with a single 'stomp' of a footswitch.
TC Nova Delay
The most common implementation of pitch shifting for bass guitar is the 'octave divider' pedal. This doubles the pitch of the bass guitar at one and sometimes two octaves down, giving you a very weighty bass sound, and providing extended low range to a standard 4-string bass. If you're using an analog pedal such as the famous Boss OC2, remember that it will only respond to one note at a time - playing two notes together (even if by accident) will confuse the unit and make the sound very unpredictable as it tries to work out which note to process! Some more advanced units do now work polyphonically.
Boss Octaver OC-3
Digital pitch shifters may allow you to shift the pitch a fifth, octave, or even more, transposing the bass line into a different key altogether. Adding feedback to this can also be interesting, creating extremely unusual effects as the pitch shifter feeds back into itself. Bear in mind that there will always be some small delay or 'latency' to the shifted signal, as the processing circuitry performs its maths on your playing. Some digital pitch shifters can be used with an expression pedal to give a portamento or fretless effect as the pedal is moved.
Ring modulators create very unusual and often unmusical, dissonant sounds - they are therefore not very widely used. A ring modulator takes two signals and creates a new one, containing the sum and difference of the original frequencies. More often than not, the result will be a rather robotic, unearthly sound.
Loopers are delay lines with extremely long delay times, allowing you to record part of a performance and create a loop from it. One of the first people to use this technique was Jaco Pastorius, during his time with the band Weather Report. Its use has become much more common in recent years, with bassists using loopers to record a bass line and then play solo parts over the top. Many current devices have memory locations - these allow the player to pre-record various parts or riffs, and then quickly select them for use via a footswitch.
Boss RC-20 XL
Bass synth pedals usually comprise envelope filters, pitch shifters and digital trackers to create unusual sounds, sometimes reminiscent of vintage synth-bass sounds. The combination of a low pass filter and an octave divider for example, can sound very similar to a Moog synthesizer. Some models are able to track your bass line with a variety of different synthesizer tones.