ACOUSTIC MODELLING OF PERFORMANCE SPACES: HENRY WOOD HALL (LONDON)
This work describes the analysis of the acoustic quality of Henry Wood Hall (Trinity church square, London), comparing real measurements with predictions carried out with CATT V9 and according to standard ISO 3382-1 which specifies methods for the measurement of room acoustical parameters in performance spaces.
The Henry Wood Hall, formerly the Holy Trinity Church, is an orchestral rehearsal and recording studio, named after the conductor Sir Henry Wood The hall. It is set in the beautiful conservation area of Trinity Church Square, and it is the city’s premier rehearsal and recording venue. Large enough for a full symphony orchestra and choir, yet intimate enough for a single soloist, the Hall has been the scene of constant musical activity for over thirty years. Since 1975, the Henry Wood Hall has been the scene of constant musical activity. Besides rehearsals for all the leading London orchestras and chamber groups, there has been constant recording activity covering all types of music in the classical repertoire, from solo pianists to full scale grand opera.
The room has different materials, such us wooden floor, wooden stage, thick curtains on the walls and other separating areas, plaster columns and walls. There are two different levels: the main level comprises the stage and the second level has an organ, diferent levels for the choir , with a wooden finish as well as carpet in some areas. The volume of the hall is 6096m3 ( 31.23x 18.7x 10.44m)
Sources and receiver positions during a rehearsal in Henry Wood Hall
The first measurements were taken with a hemidirectional sound source in two different positions near the podium, and the second measurements were taken with a subwoofer at the same positions.
The source was placed on the floor, although the requirement from the standard is that a height of the acoustic centre of the source should be 1,5 m above the floor. However, this wont have an effect for the results because the values that are going to be analysed are T30 and C80.
Microphone positions were at positions representative of positions where listeners would normally be located.For reverberation time measurements, it is important that the measurement positions sample the entire space, so the microphones were placed in a circle at 1.5m from the floor.
Some assumptions were made for the prediction, i.e assuming there are no receivers under the balcony
METHODOLOGY
ANALYSIS OF MEASURED ACOUSTIC PARAMETERS
The optimum reverberation time for any space depends on its volume and the purpose. In the case of concert halls we can see that at 1KHz the optimum reverberation time range goes from 1-2s in a range of a volume of 1000-5000m3.
In the case of Henry Wood Hall, a rehearsal space, there is no need for speech, so we would be aiming to achieve values for music ( for 6000 m3 around 1.8s)
According to the volume of the hall (6097 m3= 18,7 x 31,6 x 10,4m ) an optimum reverberation time of 1.8 s should be achieved assuming the hall IS used for music rehearsals and not for opera or other purposes that require good STI.
The measurements of reverberation time (T30) carried out with two sound sources and 6 microphone positions in the hall, according to the methodology show the following reverberation time results.
The curves in both positions show that ,in both cases, the standard deviation of all the receiver from the average is very small, hence the energy is similar in every point of the space. Thus, we could consider the space as diffuse.
Instrumentation used and status of the hall during measurements with the curtains closed
In terms of clarity, C80, different values were obtained for all receivers and sources but only the best and worst results among the receivers are shown.
As can be appreciated in both positions of the source the best results are achieved for R2 (the position the closest to the sources) , whereas the worst results are for R5 and R6, (the positions the furtherst from the sources), in a range of -10+5 dB at 1kHz.
Mapping of different parameters predicted with the calibrated model: T30, C80, SPL and STI
MODEL COMPARISON AND CONCLUSIONS
Modelling basic, medium and complex geometry of Henry Wood Hall
After analysing the models with different degree of complexity in geometry, a comparison of the different results is described below:
First, regarding reverberation time it is noticeable how the medium calibrated model is the most similar to reality. The different levels of complexity modelling the geometry lead to the conclusion that it is more efficient to have a medium accuracy of the model, in terms of time for modelling, rendering, and difficulty of calculations. The more detail in the drawing the more time and problems CATT V9 can give (number of planes, number of rays depending on Algorithm used...) and always , of course, taking into account the geometrical acoustics theory, where any distance below 34cm (1KHz) is not going to be processed by CATT V9. Although computer modelling can be precise, it is not perfect, and at error from 0-6% could be achieved for middle and high frequencies, whereas an error of 10-20% is achieved for low frequencies.
In terms of clarity the results of Receiver2 are shown in Fig 32, because this is the receiver with best values .It can be seen how the most accurate results are at 2000 Hz, and there is more difference at 500 Hz. The good matching of results at 250 Hz are not reliable for being too low frequencies for prediction in computer modelling . It can be seen in Table 4 that the percentage of error for clarity is higher than for reverberation time. An error from 8-50% can be achieved for middle and high frequencies, whereas an error of 8-40% is achieved for low frequencies.
Assessing the fact of adding extra absorption with the curtain behind the conductor, it can be seen in Fig 33 that the reverberation time drops significantly half a second at mid frequencies from the initial values, but it is not as effective at low and high frequencies. In terms of clarity the results don’t change a lot, keeping the initial range of 3-10 dB at mid frequencies.