spatial impression for very different spaces. the ... filespatial impression for very different...
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Spatial impression for very different spaces. The influence of the space features on the acoustic parameters recommended by ISO 3382
Raffaele Pisani, Paolo Onali Studio di Ingegneria Acustica, Via Cavalieri di Vittorio Veneto, 8 – 10098 Rivoli (TO), Italy, [email protected]
Maria Giovannini, Arianna Astolfi, Marco Masoero Politecnico di Torino – DENER, C.so Duca Degli Abruzzi, 24 – 10124 Torino, Italy, [email protected], [email protected],
The standard ISO 3382 is the most important reference for concert hall acoustic characterization; nevertheless there are still different aspects to study in order to apply it for acoustic design. The ISO standard defines spatial impression parameters and the measurement methods; particularly it suggests the figure of eight microphone in order to detect lateral arrivals of the sound reflections. Many authors have measured the parameters of ISO 3382, but about them not definitive conclusions are achieved. In order to investigate these parameters and to find new interpretation of the results, further measurements in very different spaces are carried out. The authors define other spatial parameters as strength lateral parameters that are calculated by acoustical signals detected by dummy head. The different strength parameters of the both left and right ears are compared each other in order to quantify the effective differences between the arrivals of the sound by left and right direction. The authors want to investigate the differences between the parameters in terms of spatial features influence and frequency dependence. Three different spaces are investigated: two courtyards of different shape and dimensions and a regular shape closed space. The investigated parameters are strength indexes: late lateral relative sound level, LG (late lateral strength), as defined in the ISO 3382, and further parameters as lateral strengths and the new dummy ear strengths.
1 Introduction
The draft ISO 3382-1: 2004 [1] in order to describe in an objective and comparable way for different theatres the sound level that the listener perceives in different points of the hall, defines the sound strength parameter, G. Among other parameters, the standard includes the early lateral energy fraction, LF, and the late lateral strength, LG; the latter is referred to the lateral sound energy arriving at the listener, starting from 80 ms after the direct sound. These parameters are defined as spatial indexes and, in the opinion of some authors, are strongly correlated with the “Apparent Source Width”, ASW, due to the first reflections and to the “Listener Envelopment”, LEV, due to the late reverberated sound, i.e. the late sound energy, respectively [2, 3, 5, 6].
This paper reports the strength parameters values with reference to the different time fractions of the energy, obtained from experimental surveys carried out with particular attention to the measurement instrumentation setting criterion. This survey is carried out in simple spaces, in order to be representative and easy to analyse from the point of view of the lateral reflections and, in this way, useful to evaluate and quantify the parameters sensitivity to the different geometry of the spaces.
Among the spatial indexes described in the literature, the indexes derived from the G parameter and referred only to the lateral energy are considered.
They are the lateral strength, ∞0LG , the early lateral
strength, 800LG (defined in a 80 ms window and
comprehensive of the first reflections) and the late lateral strength, ∞
80LG (defined in a temporal window that origins at 80 ms from the arrival of the direct sound and that includes the sound tail). For these parameters it is adopted the notation used by J. S. Bradley [4].
The indexes are considered as spatial impression describers and the following relations define them respectively:
∫
∫∞
∞
∞ ⋅=
0
210
0
2L
0
dt)t(p
dt)t(plog10LG (1)
∫
∫∞⋅=
0
210
ms80
0
2L
800
dt)t(p
dt)t(plog10LG (2)
∫
∫∞
∞
∞ ⋅=
0
210
ms80
2L
80
dt)t(p
dt)t(plog10LG (3)
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Forum Acusticum 2005 Budapest Pisani, Onali, Giovannini, Astolfi, Masoero
where pL(t) is the sound pressure measured by a figure of eight microphone and therefore coming only from the lateral directions, and p10(t) is the pressure 10 m from the same source in free field measured with an omni-directional microphone [3].
Bradley and Soulodre suggest using ∞80LG and
800LG as descriptors of listener envelopment and
apparent source width respectively [3, 6]. It is interesting to establish the relative influence of
the level, the time of arriving and direction of the reflections on design, useful to guarantee the desired spatial impression.
In order to better evaluate the above aspects, measurements of other strength indexes, obtained from signals arriving separately both to the left and right dummy head ears, were performed. The following definitions are introduced: - the right ear strength as detected by the right ear of a dummy head, GHR
( )
( ) ⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛
⋅=
∫
∫∞
∞
0
210H
0
2right
dttp
dttp10GHR
,
log (4)
- the early right ear strength, GHREarly
( )
( ) ⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛
⋅=
∫
∫∞
0
210H
ms80
0
2right
Early
dttp
dttp10GHR
,
log (5)
- the late right ear strength of the late arriving sound as detected by the right ear of a dummy head, GHRLate
( )
( ) ⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜
⎝
⎛
⋅=
∫
∫∞
∞
0
210H
ms80
2right
Late
dttp
dttp10GHR
,
log (6)
where )t(p210,H is the squared impulse response
detected from the dummy head at 10 m in free field conditions; )t(p2
right is the squared impulse response as detected by the right dummy ear; similar expressions are defined for the left ear.
In order to quantify the squared impulse response detected by the dummy head at 10 m in free field conditions, respectively for the left and right ear, the squared impulse response is best measured in the reverberation room as the sound exposure level. It is preferable to perform this measurement with the microphone placed at the dummy head auditory canal entry, rather then out of the dummy head, since in this way the head transfer function is taken into account.
In Figure 1 the GHR and GHL parameters are compared with the G parameter detected in reverberant room with the left and right ear microphones and with the omni-directional one respectively; for all signals the impulse response is determined. There is not a substantial difference between the parameters, since the relative sound exposure level is used for each microphone.
20
22
24
26
28
30
32
34
36
38
40
63 125 250 500 1000 2000 4000Frequency [Hz]
G, G
HL,
GH
R
[dB
G GHL GHR
Figure 1 – Comparison between G, GHL, GHR mean parameters measured in reverberant room after
calibration.
The calibration carried out in reverberant room for each microphone leads to a matching of the right and left ear.
The in-field measurements are carried out using both a figure of eight and omnidirectional microphone and dummy head omnidirectional microphones put at the beginning of the auditory canal.
2 Experimental setup
The strength measurements are carried out in two courtyards with different shape and dimensions, both characterized by wide vertical walls, no ceiling and dirt floor (Figures 2 and 3) and in a closed space, with regular shape and one symmetry axis (Figure 4). In the Figures the investigated spaces plans and the measurement points chosen for the parameters survey are shown. The Figures 5, 6 and 7 show the pictures of the three spaces.
The two courtyards have different geometrical characteristics: one has a regular plan and ample dimensions with a 54 m diagonal; the other has an irregular shape and a maximum length of 42 m; the walls height is about the same for the two courtyards. The measurements have been carried out by placing the source near the perimeter, in correspondence with the shorter side, and the microphone at 10 m distance from the source.
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The third case is a closed space with an inclined ceiling and two lateral walls covered with sound absorbing material; the major length is 34 m.
The three spaces are characterised by different reverberation times; the closed space has a higher reverberation time than the open spaces.
The energy fractions involved in the parameters calculation are determined by the impulse response, generated through an exponential sweep signal emitted by a omnidirectional speaker, positioned in the marked sites shown on the plans. A Shure KSM44 microphone is used with commutable omni-directional and figure of eight irradiation diagram; the lowest sensibility axis is aimed towards the source.
The sound exposure level at 10 m from the source in free field has been determined in a reverberant room, employing the same measurement apparatus and same set-ups used for the in-field measurements [1].
Figure 2: Plan of number 1 courtyard.
Figure 3: Plan of number 2 courtyard.
Figure 4: Plan of the closed space.
Figure 5: Picture of number 1 courtyard.
Figure 6: Picture of number 2 courtyard.
Figure 7: Picture of the closed space.
3 Results
The graphs of Figures 8 to 11 show the frequency trends of strength G, early lateral strength 80
0LG and
late lateral strength ∞80LG . Figures 8 and 9 respectively
refer to the open courtyards number 1 and 2, while figures 10 and 11 are relative to the closed space. The strength is significant in the spatial impression, since some authors suggest that a higher spatial effect is perceived where the sound level G is higher [2, 3].
S P1
P3
P7
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The data of Figure 8 are obtained by placing the source on the first courtyard symmetry axis (Ssimm) and the microphone at 10 m (Rs,10m).
The graph shows that the three indexes have similar frequency trends. The late energy provides a higher contribution to the global strength than the early one. This result may be explained considering that in the bigger courtyard the majority of the lateral energy arrives to the listener after the first 80 ms.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [d
B]
G 0,+ LG 0,80 LG 80,+
Figure 8: Comparison between the strength G and the lateral early and late strengths for the symmetrical
position in the first courtyard at 10 m from the source.
The results of Figure 9 were obtained in the second courtyard at a source-microphone distance of 10 m. In this case the first lateral energy provides a greater contribution to the strength; this fact may be explained by the smaller courtyard dimensions and implies a greater spatial impression. The frequency trends of early and late strength cross at almost 1 kHz, the latter being dominant at low frequency. This means that the listener envelopment is higher at low frequency, and the apparent source width is more clearly perceived at high frequency.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000
Frequency [Hz]
Leve
l [d
B]
G 0,+ LG 0,80 LG 80,+
Figure 9: Comparison between the strength and the lateral early and late strengths, for the position in the
second courtyard at 10 m from the source.
Two experimental configurations were considered for the closed space. Results in Figure 10 refer to a symmetrical configuration, source S1 and microphone P1 being placed on the symmetry axis 10 m from each
other. The measurements in Figure 11 were obtained for the asymmetrical configuration S1-P3, at a smaller source – microphone distance.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [dB
]
G 0,+ LG 0,80 LG 80,+
Figure 10: Comparison between the strength and the lateral early and late strengths, for the point S1-P1, in
the third space at 10 m from the source.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [dB
]
G 0,+ LG 0,80 LG 80,+
Figure 11: Comparison between the strength and the lateral early and late strengths for the point S1-P3, in
the closed space.
The strengths, in a symmetrical configuration, at almost the same distance from the source, are higher compared to the open space (Figures 8 and 10). In the same space, the lateral early strength is higher in the asymmetrical configuration, closer to the walls, than in the symmetrical one (Figures 10 and 11).
These parameters quantify in a global way the lateral energy without differentiate between the energy arriving to the listener from left and right direction. This fact is justified by using a microphone with figure of eight irradiation diagram.
In order to quantify the sound energy arriving to the left and right ears, two signals detected by the microphones positioned in a B&K 4128 dummy head are considered separately. After a microphone calibration, the detected signals lead to the strength for both the ears, GHL and GHR respectively. In a similar way the indexes referred to the temporal fractions of the impulse response can be calculated, i.e the early left ear strength, GHLEarly, the early right ear strength GHREarly, the late left ear strength, GHLLate, and the late right ear strength, GHRLate.
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Figures 12 and 13 show the comparison between the strength and both the early right and left ear strengths, detected in the second courtyard, for a source-microphone (S-R10m) measurement configuration.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [d
B]
G 0,+ GHL 0,80 GHR 0,80
Figure 12: Comparison between the strength and both the early right and left ear strengths for the second
courtyard at 10 m from the source.
-25
-20
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [d
B]
G 0,+ GHL 80,+ GHR 80,+
Figure 13: Comparison between the strength and both the late right and left ear strengths for the second
courtyard at 10 m from the source.
In Figure 12, that concerns the early ear strengths, a level difference between the right and left ears, only for some frequency bands, is observed. This fact may be associated to variation in frequency of the apparent source location, ASL. Figures 12 and 13 comparison suggests that only the first reflections can give information about this effect.
In Figures 14 and 15 the graphs are referred to the symmetrical configuration, S1-P1, and the asymmetrical one, S1-P7, in the closed space. They show the comparison between the strength and both the early right and left strengths.
The comparison between the graphs confirms that the ASL effect is much more important in an asymmetrical configuration. The ASL effect may be associated to a shift impression of the source.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000
Frequency [Hz]
Leve
l [dB
]
GR 0, 80 GL 0, 80 G 0,+
Figure 14: Comparison between the strength and both the early right and left ear strengths for the S1-P1
configurations in the third space.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [dB
]G 0,+ GL 0, 80 GR 0, 80
Figure 15: Comparison between the strength and both the early right and left ear strength, for the S1-P7
configuration in the closed space.
Conclusions
The shift impression of the source may be correlated to the differences between the early lateral reflections for the left and right ear.
A virtual shift angle is defined in order to obtain source location information, by considering a phase relation between left and right impulse response. This information may also be given by IACF and surely is associated to IACC, but without considering the sound level.
In Figure 16 the IACC referred the early energy, IACCEarly is shown for the symmetrical configuration S1-P1, and for the asymmetrical configuration S1-P7 in the closed space.
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Forum Acusticum 2005 Budapest Pisani, Onali, Giovannini, Astolfi, Masoero
0,0
0,2
0,4
0,6
0,8
1,0
125 250 500 1000 2000 4000
Frequency [Hz]
[-]
S1_P1 (symmetrical)S1_P7 (asymmetrical)
Figure 16: Comparison between the IACCEarly for the S1-P1 and S1-P7 configurations in the closed space.
The source shift observed from the Figures 14 and 15 is not observed for IACCEarly parameters having a continuous decreasing trend in frequency. This is also observed in Figure 17 where the IACCEarly and the early strength for both the left and right ear for the asymmetrical configuration are shown.
-15
-10
-5
0
5
10
15
125 250 500 1000 2000 4000Frequency [Hz]
Leve
l [dB
]
0,00,10,20,30,40,50,60,70,80,91,0
[-]
GL 0, 80 GR 0, 80 IACC O,80
Figure 17: Comparison between the early right and left ear strengths and the IACCEarly for the S1-P7
configuration in the closed space.
It is the authors’ opinion that the angular source shift variation and the level information include in lateral ear strength parameters can provide useful design suggestions for obtaining a correct subjective source location.
References
[1] Committee Draft ISO/CD 3382-1 – Acoustics – Measurements of the reverberation time – Part 1: performance spaces, 30-06-2004.
[2] M. Barron, The current status of spatial impressure in concert halls, Proceedings of ICA2004-The 18th International Congress on Acoustics Kyoto International Conference Hall – 4-9 April 2004, Kyoto, Japan
[3] J.S. Bradley, G.A. Soulodre, Objective measures of listener envelopment, J.Acoust.Soc.Am., 98 (5), p.2590-2597, 1995
[4] A.H. Marshall, M. Barron, Spatial Responsiveness in concert halls and the origins of spatial impressure, Appl. Acoust. , 62, p. 91-108, 2001
[5] M. Barron, Late lateral energy fractions and envelopment question in concert halls, Applied Acoustics, 62, p. 185-202, 2000
[6] J.S. Bradley, G.A. Soulodre, The influence of late arriving energy on spatial impressure, J.Acoust.Soc.Am., 97 (4), p.2263-2271, 1995
[7] J.S. Bradley, Using ISO3382 measures to evaluate acoustical conditions in concert halls, Proceeding of Room Acoustics Design & Science, April 2004, Kyoto, Japan
[8] Y. Ando, ‘Architectural Acoustics – Blending Sound Sources, Sound Fields, an Listeners’, Ed.Modern Acoustics and Signal Processing, 1998.
[9] Raffaele Pisani, Paolo Onali, Maria Giovannini, Arianna Astolfi, Marco Masoero, ‘La misura degli indici di spazialità’, 32° Convegno Nazionale di Acustica – 15-17 june 2005.
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