Influence of Instrument Cables

In the realm of electric guitars, numerous factors significantly contribute to sound shaping. However, amidst the pickups, amplifiers, and effect pedals, an inconspicuous element also plays a significant role: the cable (seriously!). As part of a project at Düsseldorf University of Applied Sciences, different cable lengths were compared to examine their influence on the electrical signals and sound, in particular the sound generated by an electromagnetic pickup.

One main element of an electromagnetic pickup as used on most electric guitars is the coils (wound wire), which in the electrical sense represents an inductance. When a guitar is connected to an amplifier via an instrument cable, calculations show that a low-pass behavior with a resonance around the cutoff frequency occurs. Low frequencies are allowed to pass through, while high frequencies are attenuated. In the transition range (at the resonance frequency), a distinct peak forms in the transmission frequency response as long as the resonance is not dampened too much by ohmic resistances (such as the guitar controls or the input impedance of the amplifier or other devices connected to the guitar), and eddy currents occurring in the metal elements of the pickup. The resonance frequency is determined mainly by the inductance of the pickup coil and the capacitance of the guitar cable. ^[Lemme]

The most important cable property here is its electrical capacitance. This is described with a so-called "capacitance rating," which is measured in picofarads per meter (pF/m). A typical value is 100 pF/m. This means that a 1-meter-long cable has a capacitance of 100 pF, a 2-meter-long cable has 200 pF, and so on. Therefore the resonance frequency (resulting from the coil inductance and cable capacitance) changes when the cable length is varied. A longer cable (higher capacitance) results in a lower resonance frequency.^[Zollner]

In the case of, for example, Stratocaster guitars and medium cable lengths (5 meters), the resonance frequency is around 3 kilohertz, a frequency range in which the human ear is particularly sensitive and which contributes to the bright sound of the guitar.

Note: This is not a cable topic, but it belongs here nonetheless: The height, or intensity, of the resonance depends on the input impedance of the connected amplifier (or in fact of any connected device such a effect pedal). It is generally required that this input impedance that the guitar "sees" is 1 megohm (1 MΩ) or more to avoid significantly damping the resonance. The occurrence of the resonance is indeed desireable to what is commonly understood as a "good" guitar sound! If an electric guitar is connected to a typical line input (e.g., a mixing console with typically around 50 kΩ input impedance), no resonance of any significance and thus no peak in the frequency response occurs.

With the help of a studio guitarist (Philipp Wisser), short samples were recorded using both a 3-meter cable and a 7.5-meter cable. Both cables are from the same manufacturer and series. 
The aim was to cover a broad frequency spectrum to analyze the impact of cable length as accurately as possible. The signal was captured using a DI box.

Guitar used: Fender Mexico Stratocaster (Neck Pickup active)

The following audio examples and frequency spectra aim to illustrate the impact of cable lengths on the sound of electric guitars. In the following audio example, two DI signals are compared. Upon closer listening, it becomes evident that the shorter cable has more high-frequency content.

In the following figure, the frequency spectrum of the recording with the 3m cable is overlaid transparently on the frequency spectrum of the 7.5m cable. A drop in high frequencies is visible.