Handbook for Sound Engineers

1660 Chapter 48

'Do, 'D 1 , 'Ds, and 'D 2 are found as in NAG.

Q. The directivity factor (Q) of a transducer used for
sound emission is the ratio of sound pressure squared, at
some fixed distance and specified direction, to the mean
sound pressure squared at the same distance averaged
over all directions from the transducer. The distance
must be great enough so that the sound appears to
diverge spherically from the effective acoustic center of
the source. Unless otherwise specified, the reference
direction is understood to be that of maximum response.

Geometric Q can be found by using the following
equations:

1. For rectangular coverage between 0 degrees and 180
   degrees,

(48-54)

2. For angles between 180 degrees and 360 degrees
   when one angle is 180 degrees, and the other angle
   is some value between 180 degrees and 360 degrees

(48-55)

3. For conical coverage,

(48-56)

C‘. C‘ is the included angle of the coverage pattern.
Normally C‘ is expressed as an angle between the
6 dB points in the coverage pattern.

EPR. EPR is the electrical power required to produce
the desired SPL at a specific point in the coverage area.
It is found by the equation

(48-57)

a. The absorption coefficient (a) of a material or surface
is the ratio of absorbed sound to reflected sound or inci-
dent sound

(48-58)

If all sound was reflected, a would be 0. If all sound

were absorbed, a would be 1.

. The average absorption coefficient ( ) for all the
surfaces together and is found by

(48-59)

where,

S1,2 ... n are individual surface areas,

a1,2 ... n are the individual absorption coefficients of the

areas,

S is the total surface area.

MFP. The mean-free path (MFP) is the average

distance between reflections in a space. MFP is found

by

(48-60)

where,

V is the space volume,

S is the space surface area.

'Dx. 'Dx is an arbitrary level change associated with

the specific distance from the Hopkins-Stryker equa-

tion so that

(48-61)

In semireverberant rooms, Peutz describes ' Dx as

(48-62)

where,

h is the ceiling height.

SNR. SNR is the acoustical signal-to-noise ratio. The

signal-to-noise ratio required for intelligibility is

(48-63)

SPL. SPL is the sound pressure level in dB-SPL re

0.00002 N/m^2. SPL is also called Lp.

QGeom^180

arc sin sinT

2

-----------

©¹

§·sinI

2

-----------

©¹

§·

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

QGeom^360

angle

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

QGeom^2

1 T

2

• cos---

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

EPRwatts 10

SPLdes + 10 d Bcrest+'D 2 – 'Dref–Lsens

= --------------------------------------------------------------------------------------------------------------^10

a

IA

IR

=----

a a

a

S 1 a 1 ++S 2 a 2 } Snan

S

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

MFP^4 V

S

------ -=

'Dx 10– Q

4 SDx^2

---------------^4 N

Sa

= log ------ -+

'Dx 10– Q

4 SDx

--------------- 2

4 N

Sa

= log ------ -+

0.734** V

hRT 60

----------------------------

Dx!Dc

Dc

log-------------------

+

** 200 for SI units

SNR 35 2%–log Alcons

29 – log RT 60

= ©¹§·----------------------------------------