Handbook for Sound Engineers

Grounding and Interfacing 1189

conductivity. The magnetic equivalent of electric
current and conductivity are flux density and permea-
bility. Hig-permeability materials have the ability to
concentrate the magnetic force lines or flux. The perme-
ability of air and other nonmagnetic materials such as
aluminum, plastic, or wood is 1.00. The permeability of
common ferromagnetic materials is about 400 for
machine steel, up to 7000 for common 4% silicon trans-
former steel, and up to 100,000 for special nickel alloys.
The permeability of magnetic materials varies with flux
density. When magnetic fields become very intense, the
material can become saturated, essentially losing its
ability to offer an easy path for any additional flux lines.
Higher permeability materials also tend to saturate at a
lower flux density and to permanently lose their
magnetic properties if mechanically stressed.
The basic strategy in magnetic shielding is to give
the flux lines a much easier path to divert them around a
sensitive conductor, circuit, or device. In general, this
means that the shield must be a complete enclosure with
a high magnetic permeability. The choice of the most
effective shielding material depends on frequency. At
low frequencies, below say 100 kHz, high-permeability
magnetic materials are most effective. We can calculate
how effective a conduit or cable shield will be at low
frequencies:

(32-8)

where,
SE is shielding effect in dB,
μ is permeability of shield material,
t and d are the thickness and diameter (in the same
units) of the conduit or shield.^7

Thus, standard 1 inch EMT, made of mild steel with a
low-frequency permeability of 300, will provide about
24 dB of magnetic shielding at low frequencies, but this
will diminish to zero around 100 kHz. Fortunately, only
low-frequency magnetic fields are generally a problem.
In severe cases, nesting one magnetic shield inside
another may be necessary.

Typical copper braid or aluminum foil cable

shielding has little effect on magnetic fields at audio

frequencies. If a shield is grounded at both ends, it

behaves somewhat like a shorted turn to shield the inner

conductors from magnetic fields.^8 Depending on the

external impedance between the grounded ends of a

cable shield, it may begin to become effective against

magnetic fields somewhere in the 10 kHz to 100 kHz

range. Box shields of aluminum or copper are widely

used to enclose RF circuits because they impede

magnetic fields through this eddy current action and are

excellent shielding for electric fields as well. There is an

excellent explanation of this high-frequency shielding

in reference 9. However, copper or aluminum shielding

is rarely an effective way to prevent noise coupling

from audio-frequency magnetic fields.

32.4 Grounding

Historically, grounding became necessary for protec-

tion from lightning strokes and industrially-generated

static electricity—i.e., belts in a flour mill. As utility

power systems developed, grounding became standard

practice to protect people and equipment. As electronics

developed, the common return paths of various circuits

were referred to as ground, regardless of whether or not

they were eventually connected to earth. Thus, the very

term ground has become vague, ambiguous, and often

fanciful. Broadly, the purpose of grounding is to electri-

cally interconnect conductive objects, such as equip-

ment, in order to minimize voltage differences between

them. An excellent general definition is that a ground is

simply a return path for current, which will always

return to its source. The path may be intentional or acci-

dental—electrons don’t care and don’t read

schematics!^10

Grounding-related noise can be the most serious

problem in any audio system. Common symptoms

include hum, buzz, pops, clicks, and other noises.

Because equipment manufacturers so often try to

explain away these problems with the nebulous term

bad grounding, most system installers and technicians

feel that the entire subject is an incomprehensible black

art. Adding to the confusion are contradictory rules

proposed by various experts. Ironically, most universi-

ties teach very little about the real-world aspects of

grounding. Graduates take with them the grounding

fantasy that all grounds are equipotential—that is, have

the same voltage. The fantasy certainly allows them to

avoid complicated real-world interpretation of all those

ground symbols on a schematic diagram, but the same

Figure 32-13. Zero coupling at right angles.

SE 20 1 Pt

d

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