Handbook for Sound Engineers

Loudspeakers 627

relatively absorptive above 1 kHz. Its effect on the

tweeter’s response is most evident between 1 kHz and

3 kHz. The graphs in Figs. 17-50 and 17-51 display

loudspeaker response differences that are due entirely to

the boundaries formed by the speaker’s baffle. The

same transducer was used in each measurement.

This brief examination of some acoustic effects due
to loudspeaker boundaries is intended to provide a
starting point for further study and investigation. A
number of implications for loudspeaker design should
be readily apparent.
Transitional points in a loudspeaker’s shape (e.g.,
edges, slots) behave as acoustic sources. Energy arrivals
from these features always follow the primary wave in
time. Additionally, they can be reversed in polarity.
Acoustic absorption is a useful diagnostic tool as well as
a powerful design element for the loudspeaker engineer.

17.8.7 Conclusion

Loudspeaker system performance is a function of
several elements, including transducer design, crossover
topology, component location and orientation, and the
acoustic boundaries formed by the loudspeaker’s
housing. Each of these elements has a major effect on
the final result, and the most effective loudspeaker
designs successfully address all of these areas.

17.9 Characterization of Loudspeaker

Performance

17.9.1 Motivation

In considering the behavior of a loudspeaker, it stands to

reason that we need performance parameters with which

we can evaluate the effectiveness of a specific device for

an envisioned use. There are many performance areas in

which loudspeakers differ in significant ways, including

on-axis response, bandwidth, directivity, distortion, and

maximum acoustic output. Unfortunately, there are a

number of different formats for presentation of loud-

speaker performance data. Before attempting to interpret

such data, it behooves us to develop some general

concepts of loudspeaker performance.

The picture is made much more complicated by the

fact that we hear not only the direct sound produced by

a loudspeaker, but also the reflections caused by interac-

tions between the loudspeaker and the acoustic environ-

ment in which we are listening. Different loudspeakers

interact in different ways with acoustic environments,

with certain types and degrees of interaction being pref-

erable to others. For this reason, it is useful to develop a

concept of loudspeaker performance that will provide a

means for understanding (and, hopefully, predicting)

these interactions.

17.9.2 Efficiency and Sensitivity

Since a loudspeaker converts electrical energy into

acoustic energy, the concept of efficiency is relevant. As

we will see, this conceptual construct, while it is a good

starting point for study, has limitations in the character-

ization of loudspeaker performance.

Efficiency is defined as the ratio of power provided

by the system output divided by the power applied to

the input. As a result of conservation of energy, the effi-

ciency of a loudspeaker (or any energy-conversion

device) is always less than one. Most often, efficiency is

expressed as a percentage. Typical loudspeaker efficien-

cies range from less than 1% in the case of some hi-fi

products to approximately 25% for limited-bandwidth

horn-loaded devices.

Since a loudspeaker’s efficiency varies with

frequency, a single number for efficiency does not

generally provide adequate information for discrimi-

nating one device from another. Also, since human

hearing responds to changes in acoustic pressure, the

total power radiated into an acoustic space may or may

not be a good indicator of what the human ear–brain

perceives. Furthermore, the devices that drive loud-

speakers—incorrectly called power amplifiers—are

designed to control the voltage applied to a loudspeaker.

For these reasons, the concept of a loudspeaker’s func-

tional efficiency needs to be expanded.

The parameter most often used to characterize a

loudspeaker’s ability to produce acoustic output is

called sensitivity. A loudspeaker’s sensitivity is the

sound pressure level (SPL) produced at a reference

Figure 17-51. Dome tweeter on axis, mounted on 19 inch

square baffle with absorption on its edge.

108

102

96

90

84

78

72

66

60

20 100 500 1k 2k 5k 10k

Frequency–Hz

Magnitude–dB SPL

TEF