Handbook for Sound Engineers

1242 Chapter 34

In a high-noise system, a 10 dB head room factor
may not be achievable. By using a limiter, the head
room factor can be reduced to as low as 6 dB while
maintaining reasonable voice intelligibility. For
music-reinforcement systems, on the other hand, as
much as 20 dB head room may be desirable to avoid
clipping important musical peaks. For the simplified
(outdoor) system, however, a 10 dB head room factor
will be assumed.
How large an amplifier is needed to achieve the
desired LP? And, what information is needed about the
loudspeaker? The answers are contained in the electrical
power required (EPR) equation

(34-6)

where:
LP is the average LP required at distance D 2 ,
H is the head room in dB,
Ls is the sensitivity of the loudspeaker (1 W/1 m),
D 2 is the distance to the farthest listener.

Example:
Let
LP= 90 dB,
H=10dB,
Ls= 113 dB (1 W/1 m),
D 2 = 128 ft.

The term Ls is the rated sensitivity of the loud-
speaker. This important specification represents the LP
that the loudspeaker will produce at one meter from its
mouth with a one watt input power level and must be
obtained from the manufacturer’s specifications or from
measurements performed in the field. Thus, this sensi-

tivity is usually referred to as the “one-watt/one-meter”

(1 W/1 m) sensitivity. In the past, some manufacturers

used a one-watt/four-feet (1 W/4 ft) sensitivity rating.

The value of H (head room), of course, may be

changed for a particular system, and a different D 2

could be used to find the EPR at some other distance.

34.2.2.9 Equivalent Acoustic Distance (EAD)

In the simplified system, if the talker were to stand rela-

tively close to the listener, the talker could be heard and

understood easily without the need for a sound system.

One way of stating the goal of the sound system, then, is

to say that it should create the illusion that the talker is

close to the listener.

A simple experiment can determine just how close a

talker needs to be to a listener for comfortable commu-

nication. Simply talk in a normal voice and walk back-

wards (away from the listener) until communication

becomes difficult. Then walk toward the listener again

until communication is comfortable. At this point, the

equivalent acoustic distance (EAD) has been estab-

lished. The idea is to use the sound system to create the

illusion that the talker is this EAD away from the

listener, Fig. 34-4).

In the simplified system, a 25 dB SNR is the goal.

That means the LP at the farthest listener’s ears should

reach 70 dB SPL for an assumed noise level of 45 dB

SPL. Looking at the chart in Fig. 34-4 for a normal

voice talker, and a 70 dB SPL (noise plus 25 dB SNR)

level, the normal voice talker would have to be about

2 ft from the listener to achieve this desired LP level. A

raised voice talker would only have to be about 4 ft

from the talker. Depending on the talker, one of these

distances would be the required EAD. If the actual

voice level from a talker (at some reference distance

like one meter or four feet) is known, the EAD for the

simplified system (outdoors) is as follows

(34-7)

where,

Ds is the reference distance from the talker,

LPt is the average LP from the talker at distance Ds,

LPd is the desired LP at the listener.

Example:

Let,

Ds = 2 ft,

LPt = 71 dB (at 2 ft),

LPd = 65 dB.

Figure 34-3. Dynamic range and head room.

Dynamic

range

Peak level

Nominal (average) level

Noise floor (electronic noise or

acoustic room noise)

Headroom

10 dB for speech

20 dB for music

EPR 10

Lp HLS 20

D 2

+–+ log3.28----------

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

For metric distances, replace the constant

3.28 with the constant 1.00

EPR 10

90 10 113+–+ 20 log3.28----------^128

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

=76.3 W

EAD DS 10

LPt–LPd

= -----------------------^20 -