Handbook for Sound Engineers

160 Chapter 7

(7-36)

The value for a good clarity measure C 80 depends
strongly on the musical genre. For romantic music, a
range of approximately 3dBdC80 d+4 dB is
regarded as being good, whereas classic and modern
music will allow values up to + 6 to +8 dB.
According to Höhne and Schroth,^14 the perception
limit of clarity measure differences is about
'C 80 |±3.0 dB.
According to Reichardt et al.,^22 there is an analytical
correlation between the clarity measure C 80 and the
center time tS, as given by

(7-37)

where,
C 80 is in dB,
tS is in ms.

This correlation is graphically depicted in Fig. 7-17.

7.2.2.11 Sound Coloration Measures (KT) and (KH) for
Music (Schmidt)

The sound coloration measures^23 evaluate the
volume-equivalent energy fractions of the room impulse
response of low- and high-frequency components (KT,
octave around 100 Hz and KH, octave around 3150 Hz,
respectively) related to a medium-frequency range in an
octave bandwidth of 500 Hz.

(7-38)

(7-39)

The measures correlate with the subjective impres-

sion of the spectral sound coloration conditioned by the

acoustical room characteristics. Optimum values are

KT;H=3 to +3 dB.

7.2.2.12 Spatial Impression Measure (R) for Music (U.

Lehmann)

The spatial impression measure R24,25 consists of the

two components spaciousness and reverberance. The

spaciousness is based on the ability of the listener to

ascertain through more or less defined localization that

a part of the arriving direct sound reaches him not only

as direct sound from the sound source, but also as

reflected sound from the room’s boundary surfaces (the

perception of envelopment in music). The reverberance

is generated by the nonstationary character of the music

that constantly generates build-up and decaying

processes in the room. As regards auditory perception, it

is mainly the decaying process that becomes effective as

reverberation. Both components are not consciously

perceived separately, their mutual influencing of the

room is very differentiated.^26 Among the energy frac-

tions of the sound field that increase the spatial impres-

sion are the sound reflections arriving after 80 ms from

all directions of the room as well as sound reflections

between 25 ms and 80 ms, that are geometrically situ-

ated outside a conical window of ±40°, whose axis is

formed between the location of the listener and the

center of the sound source. Thus all sound reflections up

to 25 ms and the ones from the front of the

above-mentioned conical window have a diminishing

effect on the spatial impression of the room. The tenfold

logarithm of this relation is then defined as the spatial

impression measure R in dB.

(7-40)

where,

ER is the sound energy fraction measured with a direc-

tional microphone (beaming angle ±40° at 500 Hz to

1000 Hz, aimed at the sound source).

One achieves a mean (favorable) room impression if

the spatial impression measure R is within a range of

approximately 5dBdRd+1 dB.

Figure 7-17. Center time tS as a function of the clarity
measure C 80.

C 80 10

E 80

Ef–E 80

= log©¹§·---------------------- dB

C 80 10.83 0.95–= tS

tS 114 10.53–= C 80

C

KT 10

Ef 100 Hz

Ef 500 Hz

= log©¹§·--------------------- dB

KH 10

Ef3150 H z

Ef500 H z

= log©¹§·----------------------- dB

R 10

Ef–E 25 – E 80 R–E 25 R

E 25 + E 80 R–E 25 R

= log ----------------------------------------------------------------- dB