The results below are derived from the measured and the simulated B-format RIR, through the observation of the spectral and spatial behaviour of a series of acoustic parameters and the inspection of the reflection patterns found at each reception point. Results has been analysed and discussed in terms of perceived reverberation, clarity of sound and spatial impression, considering the different purposes this space has served through history.
4.1. Acoustics of the York Minster’s Chapter House Today
The most relevant descriptor of the acoustic environment of a room is its reverberation time (T30
). The analysis of the decay curves of the measured RIR shows an average value of T30m
of 5.4 s. As can be seen in Table 4
, similar values are obtained in the lower frequency bands, in which the greatest values of standard deviation are observed, and T30
values significantly decrease at the higher frequency bands, where usually the sound absorption of materials is greater and the air absorption is more pronounced. The Chapter House has a large volume considering its total surface area and it lacks acoustically absorbent finishing materials, which leads to this high level of reverberation more typically found in churches or cathedrals with considerably greater volumes [54
]. Its T30
is extremely high for a “meeting room”, since, typically, the recommended reverberation time to achieve adequate speech intelligibility in a conference or a meeting room of a comparable volume is around 1.1 seconds [55
]. Its reverberation time is also above the values considered suitable for music reproduction (of around 2 s depending on the type of music), being even slightly above the limits of preferred values for organ music and medieval plainchant (set about 2–4 s) [56
Looking at the Early decay time (EDT), which better assesses the subjective impression of reverberation [58
], spatially averaged values are only slightly lower than those obtained for the T30
). Nevertheless, SD values obtained for EDT are considerably higher than those obtained for T30
in all frequency bands, which indicates that the perceived reverberation depends on the relative position of the receiver in the room, meaning that the reduced source-receiver distance that certain source-receiver combinations have may emphasise the role of direct sound and the proximity to the surrounding walls may emphasise the contribution of early reflections arriving at certain receiving points. In any case, the room has an averaged EDTm
of 5.2 s, which is again above the optimal range suggested for speech (below 1.2 s) and for vocal and organ music in highly reverberant spaces, being 2.1 s and 4.2 s, respectively [57
The analysis of the energy parameters derived from the measured RIR gives us complementary information about the balance between early and late reflections, or in other words, the clarity of sound in the space [58
]. The definition, D50
(-), has been used to assess the clarity of the spoken word; whereas the clarity parameter, C80
(dB), was chosen to analyse the clarity of music. The central time, TS
(ms), which is strongly correlated with the decay time and therefore less sensitive to spatial variations [59
], is useful to ascertain the clarity of sound in general terms. In Figure 8
, the left column shows the spatially averaged values of the acoustic parameters measured for all the source-receiver combinations as a function of frequency. The high TS
mean values are indicators of a poor clarity of sound at low-mid frequency bands in the entire audience area. D50
mean values bellow 0.3 up to the 2 kHz band denote a poor clarity of the speech transmission in those bands, and the C80
values of less than 4 dB below 500 Hz are indicative of poor musical clarity especially at low frequencies. The error bars show the spatial dispersion in terms of the standard deviation (SD). It must be noted that they are relatively small, especially for the amphitheatre-like arrangement (S2), remaining mostly below 1 JND. Values of SD significantly exceeding this threshold are only found for the front-stage arrangement (S1) at the 500 Hz frequency band (max variation of 1.57 JND in the case of D50
at 500 Hz). The spectrally averaged values are also included in the figure as a function of the source-receiver distance. It can be seen that, when the source is located in the centre (S2) greater values of D50m
are achieved, since the S-R distance remained below 7 m. As expected, there is a tendency for sound clarity indicators to decrease as the relative distance from the receiver to the sound source increases. However, it must be highlighted that the receivers closest to the side walls have higher C80m
values than those that are at a similar relative S-R distance, but located in a more central area of the room. For instance, D50m
values obtained at R4 when the source is located in S1 (S1-R4 dist = 6.7 m) are 1 JND above of those obtained in R3 (S1-R3 dist = 6.4 m), which means that such difference is perceptible by listeners. This fact may be due to the presence of early reflections nearer to the walls. In any case, the values of the energy parameters denote poor clarity of the sound (D50m
< 0.3 and C80m
< −2 dB), which indicates that how the sound energy behaves in the space is unfavourable to musical definition, and especially worrying for effective speech transmission, even at the receiver points closest to the source, for both source positions.
Additionally, the Early Lateral Energy Fraction (JLF
) parameter is used to assess the spatial impression perceived by listeners in the Chapter House, since it has been demonstrated that it is related to the apparent source width (ASW) [60
]. Furthermore, the JLF
parameter has been proven to play an important role in the way that music is experienced in this type of building together with the T30
and other factors depending on the music motif [57
]. As shown in Figure 9
, looking at the spatially averaged results, lower values of JLF
are found at low-mid frequencies when the source is located in the centre of the room (S2) than in S1 due to the geometry of the space, although the SD values indicate that it depends on the receiver position. This is relevant since the spatial impression is strongly correlated with the low frequency early-arriving sound [61
]. Looking at the results obtained at each S-R combination, it is noted that considerably high values of JLFm
(around 0.5, while a typical value for a suitable spatial impression is between 0.2–0.3 [62
]) are obtained in those positions located closer to the walls (R4 to R9) with S1. Nevertheless, when considering S2, only R1 and R6 reach comparable values.
Further analysis of the direction of arrival of sound reflections and their relative intensity was conducted with the aim of understanding the specific role of the polygonal shape and specific architectural elements, such as the vault, in the interaction with the sonic events that take place in the Chapter House. For this purpose, 3D sound intensity vectors derived from the measured B-format RIR were inspected by using Iris 1.4 acoustic software. A resolution of 2 ms was set, which constrains the analysis to 500 Hz and above [63
]. The time interval window named as “Speech” was used for the representation in Figure 10
and Figure 11
, in which the red lines represent the direct sound (arriving at 0–2 ms), and the green and dark blue lines (arriving at 2–50 ms and 50–80 ms, respectively) represent the contribution of the early reflections. The reflections arriving after 80 ms or late reflections are coloured in sky blue.
shows the XY view of the 3D plots in the measured S-R combinations and allow us to observe the direction from which the early reflections, contributing to the clarity of sound, come from at each reception point depending of the position of the sound source. In the plots, the length of each ray represents their level in relation to the intensity of the direct arrival. Note that late reflections have been omitted here for the sake of clarity. It can be observed that, in the front-stage arrangement (S1), there are a considerable number of early rays arriving at those receivers that are at a greater distance from the source, with an attenuation of 5–10 dB relative to the intensity of the direct arrival which likely contribute towards a clearer perception of sound in those positions. Such rays are mainly coming from the closer lateral walls (in the case of R5, R6 and R9) and from the entrance (in the case of R10). Nevertheless, when the source is located in the centre (S2, amphitheatre-like arrangement), S-R distances are shorter and the intensity of the direct sound is higher, so the early reflections in general arrive with a greater attenuation, with the exception of the first reflection coming from the floor. It is only in those receivers closer to the walls (R5, R8 and R10) that a small number of rays approximately 5 dB lower than the direct arrival coming from the back, are found.
Focusing only on the early lateral rays we get information related to the spatial impression. With S1, a significant number of early lateral rays are observed in all the listener positions except in those receiver points located along the symmetry axe of the room (R2, R10), which is in good correlation with the high value of the JLFm
parameter obtained for those S-R combinations. Conversely, the intensity plots for S2 show limited lateral energy arriving at those points located in the central part of the space, which corresponds in general to lower values of the early lateral fraction parameter (Figure 9
). As an example of this, the full Iris plot calculated for the receiver position R8 with both source positions is shown in Figure 11
, which also provides insight into the general distribution of the energy, by including the late arrivals.
It can also be seen in Figure 11
that virtually no early reflections are coming from the upper part of the room (stained glasses and the vault), not even in a significant way when the receiver is located in the centre of the space immediately below the centre of the vault (R2). This lack of early reflections coming from above was expected given the vault’s height (see geometry details in Table 1
and Figure 7
). In general, the late energy arriving at all the receiver positions is significant and is coming from all around the space, whilst early energy is mainly coming from below and the horizontal plane, but not from above.
Acoustic simulations were then used to assess in more detail the influence of the architectural features of the space in its function as a meeting place, including its original configuration where the speaker and/or the listeners where sitting in the limestone stalls (Figure 3
c). Simulated mappings were generated to analyse the spatial distribution patterns of the acoustic parameters. The horizontal grid for the colour mappings was defined at approximate ear height (1.20 m from the floor in the centre area and 0.6 m from the sitting area of the stalls) were set. Figure 12
shows the simulated mappings of the speech transmission index (STI) and D50
at 1 kHz obtained for the different source positions in the current state. These parameters have been selected in this case since they are indicators of the clarity and intelligibility of the spoken word. When looking at the simulation mappings, no significant improvement in terms of speech intelligibility is observed in the stalls in comparison with the open floor area. In general, values denote “poor” speech intelligibility throughout the audience area with the three source positions analysed. Despite this, it can be seen that some STI and D50
values obtained in a number of receivers located in the stalls are higher than those obtained at points that are at a closer relative distance from the sound source, and this effect can be perceptible when these differences are greater than 1 JND. Furthermore, when the source is located in the stalls (S3), there is a potential lack of direct sound in those stalls located in the adjacent walls to which the sound source is located due to the geometry of the space, and also because of the marble columns, and this is detrimental to intelligibility in that section of the sitting area.
For more details, Figure 13
and STI values obtained at each S-R combination, including the receiver positions in the stalls. The results reveal that when the source is in the centre, higher values of D50m
and STI are obtained in the stalls, while with S1 and S3 no significant improvement can be seen, and even lower values are obtained. These results demonstrate that, although the early reflection pattern inspection for those receivers located in the stalls shows a group of strong early reflections of 1st and 2nd order arriving before 50 ms (even arriving before 10 ms in the case of those coming from the walls immediately behind the receivers) that reinforce the clarity of the speech in those positions, this reinforcement does not have a significant impact in terms of intelligibility or sound clarity since the reverberant field predominates at that distance from the sound source.