Title: Understanding Cable Shielding for Audio Frequencies
In the world of audio transmission, cables play a critical role in ensuring high-quality sound. However, they are vulnerable to electromagnetic interference (EMI), which can adversely affect the audio signal. Cable shielding is essential to prevent EMI and protect audio data from external interference. This article explores different types of cable shielding and examines their effectiveness specifically for audio frequencies. At the end of this article is the reference document used in writing this article.
Types of Shielded Cables:
- Coaxial Cables:
Coaxial cables, commonly used for audio applications, operate within a frequency range suitable for audio signals, but may be useful up to 50GHz. They consist of an inner conductor surrounded by an insulating layer, a conductive layer (woven braid or metallic foil), and an outer insulating layer. The shield is grounded at multiple points for high frequencies and a single point for low frequencies.
- Triaxial Cables:
Triaxial cables, similar to coaxial cables, have an additional shield isolated from the signal return shield. This second shield provides enhanced rejection of interference, making them suitable for audio applications where signal quality is crucial.
- Twinaxial Cables:
Twinaxial cables, designed for audio transmission are useful up to 10 MHz, feature two-wire twisted balanced lines enclosed in a ground shielding braid. The twisting mechanism cancels out induced noise voltage pickup caused by low-frequency magnetic fields, ensuring cleaner audio signals. Twinaxial cables find applications up to 10 MHz. See Furutech video RCA Interconnect with balanced cable build instructions on the Furutech video page.
- Quadraxial Cables:
Quadraxial cables, composed of double-shielded twinaxial cables, offer even better protection against EMI for audio signals. The outer shield is grounded to the earth, while the inner shield is connected to the system ground or earth if no system ground is available. This is my personal favorite because you can start with a Twinaxial cable and add another layer of custom shielding. More on this in the future.
Cable Shield Grounding:
Grounding the cable shield is crucial to prevent electromagnetic energy from penetrating the shield. The grounding configuration can be asymmetric (grounding at one end), symmetric (grounding at both ends), or intermittent along the cable’s length. The effectiveness of different grounding methods depends on the electromagnetic coupling mode and the length of the cable used for interconnection.
Understanding Transfer Impedance of Cable Shield:
To assess cable shielding effectiveness for audio frequencies, it is essential to consider the transfer impedance. The transfer impedance quantifies the relationship between the shield current and the longitudinal induced voltage on the outer side of the shield.
Transfer impedance is influenced by factors such as shield resistance, coupling inductance, and skin inductance. Lower transfer impedance values indicate better cable shielding and reduced susceptibility to EMI.
Effectiveness of Cable Shielding:
For audio frequencies, where signal integrity is critical, the choice of cable shielding is crucial. Among the various types discussed, solid semi-rigid coaxial cables deliver the highest effectiveness, followed by braided triaxial, braided coaxial, shielded quadraxial, and shielded twinaxial cables.
In audio transmission, cable shielding is vital to prevent electromagnetic interference and maintain the integrity of the audio signal. Selecting the appropriate type of cable shielding depends on the specific audio application’s requirements. Proper grounding techniques and understanding transfer impedance are key factors in achieving effective shielding for audio frequencies. Staying informed about the latest advancements and best practices in cable shielding ensures optimal audio performance and reliable sound transmission for the custom cable industry and the DIY .
1 V.Prasad Kodali, Engineering electromagnetic compatibility: principles, measurements, and technologies, vol. I.
2 C.Sankran, Power Quality, vol. I Florida: CRC Press LLC.
3 G. Vijayaraghavan, Mark Brown, and Malcom Barnes, Practical Grounding, Bonding, Shielding and Surge Protection.
More information: below is a download of the reference article this article is based on.