Handbook for Sound Engineers

1356 Chapter 35

(35-25)

in V/Pa or its 20-fold common logarithm, the sensitivity
level

(35-26)

The reference sensitivity T 0 is normally specified for
1V/Pa.
Depending on the test conditions one distinguishes
the following sensitivities:

•The pressure open-circuit sensitivity TE (^) p as the ratio
between the effective output voltage at a certain
frequency and the effective sound pressure of a verti-
cally incident sound wave.
•The free-field open-circuit sensitivity TEf which
considers by special measuring conditions the pres-
sure increase conditioned by the cross-section dimen-
sions of the microphone.
•The diffuse-field sensitivity TE (^) r, which reflects the
diffuse sound incident on the microphone.
Directivity Behavior. The dependence of the micro-
phone voltage on the direction of incidence of the excit-
ing sound is called directional effect. The following
quantities are used for describing this effect:
•The angular directivity ratio *(-) is the ratio
between the free-field sensitivity TEd for a plane
sound wave arriving under the angle - to the main
microphone axis and the value ascertained in the
reference level (incidence angle 0°).
(35-27)
•The angular directivity gain D is the twenty fold
common logarithm of the angular directivity ratio.
•The coverage angle is the angular range within which
the directivity gain does not drop by more than 3 dB,
6 dB, or 9 dB against the reference axis.
Apart from the quantities describing the ratio
between the sensitivities of the microphone with sound
incidence from various directions deviating from the
main axis, it is also necessary to describe the relation-
ship between the sensitivities with reception of a plane
wave and those with diffuse excitation. With these
quantities it is then possible to ascertain the suppres-
sion of the room-sound components against the direct
sound of a source to be transmitted. This energy ratio is
described by the following parameters:

• The front to random factor is the ratio between the
  electric power rendered by the microphone when
  excited by a plane wave from the direction of the
  main axis, and the power rendered by the microphone
  excited in a diffuse field with the same sound level
  and same exciting signal. If the sensitivity was
  measured in the direct field as TEd and in the diffuse

field as TE (^) r, the front to random factor results as

. (35-28)

• The front to random index is the ten fold common
  logarithm of the front to random factor.

While the front to random factor of an ideal omni-

directional microphone is 1, that of an ideal cardioid

microphone is 3. This means that a cardioid microphone

picks up only of the sound power of a room picked

up by a comparable omnidirectional microphone at the

same distance from the source. This implies, for

instance, that with identical proportion of the sound

power, the speaking distance for a cardioid microphone

may be three times greater than that of an omnidirec-

tional microphone.

35.2 Transducer Data for Acoustic Simulation

To simulate an entire acoustic system, all parts must be

taken into account. Besides the room, loudspeakers and

natural sound sources as well as microphones and the

human hearing system have to be considered. In this

section, our main goal is to review existing practice and

outline advantages and disadvantages that the user of a

software program should be aware of when applying

performance data for a particular sound transducer. In

this regard, our intention is to talk about the simulation

of transducers with respect to the electroacoustic and

room-acoustic prediction of the acoustic system as a

whole. We will not be concerned with mathematical

methods applied in the design process of loudspeakers

or microphones. These usually provide a much higher

degree of accuracy in some regards, but at the same

time often provide insufficient data for other simulation

purposes. Specifically, for transducer design utilizing

BEM/FEM-based prediction methods we refer the

reader to available textbooks and publications.

35.2.1 Simulation of Loudspeakers

In computer aided acoustic design and especially for

sound reinforcement applications, the level of accuracy

TE u

̃

p ̃

---=

Gs 20

TE

T 0

= log----- - dB

*-

TEd -

TEd 0

=-----------------

JM

TEd^2

TEr^2

-----------=

(^1) » 3