Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 161

Spatial impression measures below 5dB up to
10 dB are referred to as being less spatial, others
between +1 dB up to +7 dB as very spatial.

7.2.2.13 Lateral Efficiency (LE) for Music (Jordan), (LF)
(Barron) and (LFC) (Kleiner)

For the subjective assessment of the apparent extension
of a musical sound source—e.g., on stage—the early
sound reflections arriving at a listener’s seat from the side
are of eminent importance as compared with all other
directions. Therefore the ratio between the laterally
arriving sound energy components and those arriving
from all sides, each within a time of up to 80 ms, is deter-
mined and its tenfold logarithm calculated therefrom.
If one multiplies the arriving sound reflections with
cos^2 V, being the angle between the direction of the
sound source and that of the arriving sound wave, one
achieves the more important evaluation of the lateral
reflections. With measurements this angle-dependent
evaluation is achieved by employing a microphone with
bi-directional characteristics.
Lateral Efficiency, LE, is

(7-41)

where,

EBi is the sound energy component measured with a

bidirectional microphone (gradient microphone).

The higher the lateral efficiency, the acoustically
broader the sound source appears. It is of advantage if the
lateral efficiency is within the range of 0.3dLEd0.8.
For obtaining a uniform representation of the energy
measures in room acoustics, these can also be defined as
lateral efficiency measure 10log LE. Then the favorable
range is between –5 dBd10 log LEd–1 dB.
According to Barron it is the sound reflections
arriving from the side at a listener’s position within a
time window from 5 ms to 80 ms that are responsible
for the acoustically perceived extension of the musical
sound source (contrary to Jordan who considers a time
window from 25 ms to 80 ms). This is caused by a
different evaluation of the effect of the lateral reflec-
tions between 5 ms and 25 ms.
The ratio between these sound energy components is
then a measure for the lateral fraction LF:

(7-42)

where,

EBi is the sound energy component, measured with a

bidirectional microphone (gradient microphone).

It is an advantage if LF is within the range of

0.10dLFd0.25, or, with the logarithmic representa-

tion of the lateral fraction measure 10log LF, within

–10 dB d 10log LF d–6 dB.

Both lateral efficiencies LE and LF have in common

that, thanks to using a gradient microphone, the resulting

contribution of a single sound reflection to the lateral

sound energy behaves like the square of the cosine of the

reflection incidence angle, referred to the axis of the

highest microphone sensibility.^27 Kleiner defines, there-

fore, the lateral efficiency coefficient LFC in better

accordance with the subjective evaluation, whereby the

contributions of the sound reflections vary like the

cosine of the angle.

(7-43)

7.2.2.14 Reverberance Measure (H) (Beranek)

The reverberance measure describes the reverberance

and the spatial impression of musical performances. It is

calculated for the octave of 1000 Hz from the tenfold

logarithm of the ratio between the sound energy compo-

nent arriving at the reception measuring position as

from 50 ms after the arrival of the direct sound and the

energy component that arrives at the reception position

within 50 ms.

(7-44)

In contrast to the definition measure C 50 an

omnisource is used during the measurements of the

reverberance measure H.

Under the prerequisite that the clarity measure is

within the optimum range, one can define a guide value

range of 0 dBdHd+4 dB for concert halls, and of

2dBdHd+4 dB for musical theaters with optional

use for concerts. A mean spatial impression is achieved

if the reverberation factor H is within a range of

5dBd Hd+2 dB.

Schmidt^2 examined the correlation between the

reverberance measure H and the subjectively perceived

reverberation time RTsub, Fig. 7-18. For a reverberance

measure H= 0 dB, the subjectively perceived reverbera-

LE

E 80 Bi–E 25 Bi

E 80

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

LF

E 80 Bi–E 5 Bi

E 80

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

LFC

pBi t upt td

5

80

³

E 80

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

H 10

Ef–E 50

E 50

= log©¹§·--------------------- dB