Morimoto, dokumenty, Akustyka
[ Pobierz całość w formacie PDF ]
Applied Acoustics 62 (2001) 109±124
www.elsevier.com/locate/apacoust
The role of re¯ections from behind the listener in
spatial impression
$
Masayuki Morimoto
a,
*, Kazuhiro Iida
b
, Kimihiro Sakagami
a
a
Environmental Acoustics Laboratory, Faculty of Engineering, Kobe University, Rokko, Nada,
Kobe 657-8501, Japan
b
AVC Research Laboratory, Matsushita Communication Industry Co., Ltd., Saedo, Tsuzuki,
Yokohama 224-8539, Japan
Received 19 July 1999; received in revised form 11 October 1999; accepted 27 June 2000
Abstract
This paper describes the results of two subjective experiments to clarify the role of re¯ec-
tions arriving from behind the listener in the perception of spatial impression. The experi-
ments investigate the eects of re¯ections from behind the listener on both listener
envelopment (LEV) and auditory source width (ASW) and which is more eective for LEV,
the early or late re¯ections. The results of experiments clearly show that: (1) The listener can
perceive LEV and ASW as two distinct senses of a sound image. (2) The role of re¯ections
arriving from behind the listener is to increase LEV in spatial impression. Namely LEV
increases as the relative re¯ection energy of sound arriving from behind the listener increases.
(3) The early re¯ections also contributes to the perception of LEV, while (4) the late re¯ections
are more eective for LEV than the early ones. However, it cannot be de®nitely concluded
whether C
80
aects LEV or not.
#
2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The auditory sensations associated with the acoustics of a space can be divided
into three groups. The ®rst group concerns temporal attribute (rhythm, durability,
reverberance, etc.). The second group involves the spatial one (direction, distance,
Portions of this paper were presented at the 125th (Ottawa, 1993) and 135th (Seattle, 1998) meetings
of the Acoustical Society of America.
* Corresponding author. Tel.: +81-803-78-6035; fax: +81-78-881-2508.
E-mail address: mrmt@kobe- u.ac.jp (M. Morimoto).
0003-682X/01/$ - see front matter
#
2000 Elsevier Science Ltd. All rights reserved.
PII: S0003-682X(00)00051-7
$
110
M. Morimoto et al. / Applied Acoustics 62 (2001) 109±124
spatial impression, etc.), while the third relates to the quality one (loudness, pitch,
timbre, etc.) [1]. Among these sensations, it is well-known that spatial impression is
one of the most important in concert halls. In this paper, the present authors de®ne
the term ``spatial impression'' as the spatial extent of the sound image. Of course, it
is the more general overall concept, because they regard it as a multi-dimensional
sense and they suppose it to correspond to the term ``spatial impression'' which
Bradley and Soulodre use [2,3].
In 1989, Morimoto and Maekawa [4], demonstrated that spatial impression com-
prises at least two components by subjective experiments using a multidimensional
analysis. One is auditory source width (ASW) which is de®ned as the width of a
sound image fused temporally and spatially with the direct sound image and the
other is listener envelopment (LEV) which is the degree of fullness of sound images
around the listener, excluding a sound image composing ASW.
Before proceeding, the dierence between our previous work and the present one
in nomenclature for such spatial characteristics should be clari®ed. Morimoto and
Maekawa [4] used the terms ``broadening,'' ``auditory spaciousness'' and ``envelopment,''
in place of such terms being used in the present paper as ``spatial impression,''
``auditory source width'' and ``listener envelopment,'' respectively, though the de®-
nitions of each term used by Morimoto and Maekawa [4] are identical to those of
the corresponding terms used in the present paper. It should, however, be noted that
there were some reasons why Morimoto and Maekawa [4] did not use the same
terminology as being used in the present paper, i.e. why they did not use ``spatial
impression'' to address the general overall concept: Morimoto and Maekawa did
not use this term to avoid possible confusion Ð the term ``spatial impression'' had
already been used mainly to describe the source broadening produced by early lateral
re¯ections. Besides, the abbreviation SI for ``spatial impression'' for this meaning
had already been circulated: Barron [5] and Barron and Marshall [6] did not use the
term ``spatial impression'' for the general overall concept, nor did they suppose that it is
a multidimensional sense in their extensive and pioneering work. Barron used the term
``spatial impression'' for the sense of source broadening produced by early re¯ections.
He stressed that early lateral re¯ections produced a very dierent impression from that
produced by reverberation: reverberation was described as providing a certain degree of
envelopment in the sound and giving an impression of distance from the source.
In 1995, Bradley and Soulodre [2] also con®rmed that spatial impression in con-
cert halls is composed of at least two distinct senses. The present authors believe
that, generally speaking, the listener perceives not only one sound image fused tem-
porally and spatially with the direct sound image based on the law of the ®rst wave
front, but also the other ones caused by re¯ections not aected by the law. More-
over, both sound images appear regardless of the delay times of re¯ections after the
direct sound and each sound image has its own spatial extent.
Fig. 1 illustrates the concepts of the two types of spatial impression. Of course,
ASW and LEV vary in terms of size and shape, depending on the nature of the
sound ®eld. The ®gure shows only one combination of ASW and LEV.
An alternative view is that ASW and LEV are an identical sense and that the dif-
ference between them is simply a matter of degree depending on the size. In other
M. Morimoto et al. / Applied Acoustics 62 (2001) 109±124
111
Fig. 1. Concepts of auditory source width (ASW) and listener envelopment (LEV).
words, spatial impression is a one-dimensional sense. For instance, a small degree of
spatial impression could be termed as ASW and a large one as LEV. But the border
between them is fuzzy.
Meanwhile, many pieces of research on physical measures related to spatial
impression have been reported over the 20 years since Keet [7]. Among them, well-
known measures are the lateral energy fraction and the degree of interaural cross-
correlation. If spatial impression is a one-dimensional sense which can be evaluated
by these measures, the results of the past experiments could yield a strange and
interesting conclusion about the acoustical design of concert halls. Based on the
character of the lateral energy fraction by Barron and Marshall [6], it can be con-
cluded that the re¯ections with the same angle from the aural axis produce the same
amount of ASW (though Barron and Marshall [5,6] use the term spatial impression
as mentioned above, the present authors regard it as equivalent to ASW, from the
de®nition of spatial impression used by them as mentioned above), when the sound
pressure level of re¯ections are equal. Furthermore, Morimoto et al. [8,9] indicated
that ASW produced by any sound ®eld with the same degree of interaural cross-
correlation measured without arti®cial ear simulators and A-weighting, so-called
DICC [10], is identical, regardless of the number and the arriving direction of
re¯ections. From these results, it can be concluded that it is possible to control
spatial impression by re¯ections which arrive only from in front rather than behind
the listener. In other words, re¯ections from behind the listener are not always
necessary to produce spatial impression. However, there is evidence that sound from
behind the listener is important. There must be some sense which needs re¯ections
from behind the listener. Yamamoto [11] reported that one of the subjective mea-
sures for sound in rooms is correlated with front/back energy ratio that is the ratio
112
M. Morimoto et al. / Applied Acoustics 62 (2001) 109±124
of sound energy from in front to that behind the listener. But he did not make clear
what its subjective signi®cance was. The present authors suppose that it must be LEV.
The ®rst report on the physical measure of LEV by Morimoto and Maekawa [4] in
1989 indicated that the degree of interaural cross-correlation of the late re¯ections
relates to LEV. Furthermore, the recent papers by Bradley and Soulodre [2,3] in
1995 indicated the late lateral sound level LG best predicts LEV. From these last
three pieces of work, it appears that the re¯ections from behind are not necessary in
order to produce LEV. However, in each case the experiments were conducted with
sound only arriving from in front of the listener.
The purposes of this paper are to make clear the role of re¯ections from behind
the listener in the perception of spatial impression, to con®rm that the listener can
perceive LEV and ASW as two distinct senses of a sound image and to investigate
whether or not the energy in the early part of the impulse response of a sound ®eld
contributes to LEV, and which is more eective for LEV, front/back energy ratio
(FBR) in the early or late part of the impulse response of a sound ®eld.
2. Methodology
The authors are of opinion that there are two approaches for studying on concert
hall acoustics: one is to predict and evaluate physical and subjective characteristics
of existing concert halls, and explains physical and subjective phenomena in them.
From this standpoint a study will be made with parameters in a range actually
observed in the existing concert halls. The other is to investigate physical and sub-
jective phenomena, which could take place in realizable concert halls, regardless of
whether they actually take place in existing concert halls or not. From this stand-
point a study should sometimes include extreme cases, even if they are not observed
in the existing concert halls.
Therefore, how to select conditions of experiments and calculations is often a
subject of discussion in the ®eld of concert hall acoustics. Some architectural
acousticians cannot accept the results of experiments and calculations performed
under conditions which cannot be observed in existing concert halls and criticize
them as useless, because they sometimes confuse an existing concert hall with a
realizable one: one should know that some architectural and acoustic conditions,
which are dierent from those observed in all existing concert halls, can be realizable
in concert halls.
The experiments in this paper are carried out from the standpoint of clarifying
physical and subjective phenomena, which could take place in realizable concert
halls. The main purpose is not to investigate LEV perceived in the existing concert
halls, but to prove a hypothesis that re¯ections arriving from behind the listener
contribute to the perception of LEV in spatial hearing. Therefore, a simple sound
®eld is used and physical factors are changed extremely, whether they can be
observed in the existing concert halls or not.
However, LEV in the existing concert halls can be inferred, if the relationships
between LEV and the physical factors are clari®ed from the results of the experiments
M. Morimoto et al. / Applied Acoustics 62 (2001) 109±124
113
in this paper and if measured values of the physical factors in existing concert halls
are presented.
3. De®nition of front/back energy ratio
In this paper, front/back energy ratio (FBR) is introduced as a physical factor to
investigate the eect of re¯ections arriving from behind the listener, as de®ned by
Eq. (1):
FBR
10
log
E
f
=
E
b
d
1
where E
f
and E
b
are energies of re¯ections arriving from in front and behind the
listener, respectively. In this equation, the energy of the direct sound and re¯ections
in the transverse plane, which is the plane that intersects both the horizontal and the
median plane at right angles and contains the entrances of left and right ear canals,
are excluded.
4. Experiment 1
It is well known that most of subjective evaluations in concert halls is related to
the early part of the impulse response of a sound ®eld. On the other hand, some
authors have reported that the late part contributes to LEV as described in Section 5
[2±4,14,15]. However, there is no conclusive evidence of their ®ndings. In this
experiment, therefore, FBR in both of the early and late parts were identical to
concentrate on the contribution of the re¯ections from behind the listener to LEV,
setting a problem of which part contributes to LEV aside for the moment.
In this experiment, the eects of re¯ections from behind the listener on not only
LEV but also ASW were investigated by changing FBR and the ratio of early to late
sound energy, C
80
(clarity). Therefore, this experiment is capable to determining
whether the listener can perceive LEV and ASW independently [4] and for ASW if it
is independent of the arrival direction or re¯ections from in front or behind listener
[5,8,9].
4.1. Method
In this experiment, a violin solo performance of Saint-Saens' ``Introduction et
Rondo Capriccioso'' (14 s long, bars 7±12) recorded in an anechoic chamber was
used as a music motif. The parameters were FBR and C
80
. DICC, that is the degree of
interaural cross-correlation measured without arti®cial ear simulators and A-weighting
[10,12], of the whole re¯ections (early re¯ections + reverberation), was kept constant.
Fig. 2 shows the arrangement of loudspeakers. Six loudspeakers each of which is
installed in a cylindrical enclosure (diameter: 108 mm, length: 350 mm) were arran-
ged at azimuth angles of 0
and
45
from the median plane, that is, they were
[ Pobierz całość w formacie PDF ]