Handbook for Sound Engineers

32 Chapter 2

would not be concerned with the effects of such interac-
tions since they came for the food and not the music.
Concert-goers or church attendees would be more
concerned, because their seat might be in a dead spot,
and the interactions disrupt their listening experience,
possibly to the point of reducing the information
conveyed via the sound system. A venue owner may
make a significant investment in good quality loud-
speakers, only to have their response impaired by such
interactions with an adjacent loudspeaker or room
surface, Fig. 2-16.

Phase interactions are most disruptive in critical
listening environments, such as recording studio control
rooms or high quality home entertainment systems.
Users of these types of systems often make a large
investment to maintain sonic accuracy by purchasing
phase coherent loudspeakers and appropriate acoustical
treatments for the listening space. The tonal coloration
caused by wave interference may be unacceptable for a
recording studio control room but may be artistically
pleasing in a home audio system.
Loudspeaker designers can use wave interaction to
their advantage by choosing loudspeaker spacings that
form useful radiation patterns. Almost all pattern
control in the low frequency decade is achieved in this
manner. Uninformed system designers create undesir-
able radiation patterns by accident in the way that they
place and stack loudspeakers. The results are poor
coverage and reduced acoustic gain.
The proper way to view the loudspeaker and room
are as filters that the sound energy must pass through en
route to the listener. Some aspects of these filters can be

compensated with electronic filters—a process known

as equalization. Other aspects cannot, and electronic

equalization merely aggravates or masks the problem.

2.8 Ohm’s Law

In acoustics, the sound that we hear is nature restoring

an equilibrium condition after an atmospheric distur-

bance. The disturbance produces waves that cause the

atmospheric pressure to oscillate above and below

ambient pressure as they propagate past a point of

observation. The air always settles to its ambient state

upon cessation of the disturbance.

In an electrical circuit, a potential difference in elec-

trical pressure between two points causes current to

flow. Electrical current results from electrons flowing to

a point of lower potential. The electrical potential differ-

ence is called an electromotive force (EMF) and the unit

is the volt (V). The rate of electron flow is called

current and the unit is the ampere (A). The ratio

between voltage and current is called the resistance and

the unit is the ohm (:). The product of voltage and

current is the apparent power, W, that is produced by

the source and consumed by the load. Power is the rate

of doing work and power ratings must always include a

reference to time. A power source can produce a rated

voltage at a rated flow of current into a specified load

for a specified period of time. The ratio of voltage to

current can be manipulated to optimize a source for a

specific task. For instance, current flow can be sacri-

ficed to maximize voltage transfer. When a device is

called upon to deliver appreciable current, it is said to

be operating under load. The load on an automobile

increases when it must maintain speed on an uphill

grade, and greater power transfer between the engine

and drive train is required. Care must be taken when

loading audio components to prevent distortion or even

damage. Ohm’s Law describes the ratios that exist

between voltage, current, and resistance in an electrical

circuit.

(2-9)

(2-10)

(2-11)

where,

E is in volts,

I is in amperes,

R is in ohms.

Figure 2-15. Phase interference occurs when waves from
multiple sources arrive at different times. Courtesy
Syn-Aud-Con.

R E

I

= ---

EIR=

I E

R

=---