Room Acoustics

Room acoustics concerns the behaviour of sound in enclosed spaces, and is critical for spaces ranging from recording studios to theatres and concert halls.

In open stage theatres the audience surrounds the stage which means that a large section of the audience is faced with the back view of a speaking actor. Human speech is directional in nature which results in the intelligibility of speech in sections to the rear of an actor being poor. The use of virtual reflectors as a means of overcoming this problem has been proposed and computer simulations are presented which demonstrate the effectiveness of this technique [1,2,3].

A computer algorithm has been investigated for the generation of three-dimensional acoustical forms [4]. The geometric forms of auditoria are analysed and it is shown that the majority can be categorized in terms of a limited number of basic plan forms and cross-sections. Most of these basic categories can be collapsed to yield a single generic plan form and cross-section that can be simply defined in terms of a number of node co-ordinates. An algorithm is developed for the rapid generation of 3D auditoria models from a limited number of dimensions related to generic plans and sections. A number of models generated using this technique have been employed to investigate the effectiveness of stepped walls in general purpose auditoria with a wide fan shaped plan. The results obtained suggest that although the use of stepped walls is very effective in ensuring strong lateral reflections they tend to negate the beneficial effects that the use of the simple plan form brings to rooms intended for speech.

Research has been carried out into the acoustical characteristics of the four patterns that represent the architectural development of the historic Masjids (Mosques) in Cairo: the semi-closed Rewaq pattern; the semi-closed Iwan pattern; the closed Iwan pattern and the closed simple geometrical pattern [5]. Detailed architectural and acoustical analyses were performed on samples collected through a comprehensive historical survey of Masjids built between the 9th and 18th centuries. The acoustical analysis involved the application of geometrical acoustics using RAYNOISE software. Simulations showed that the semi-closed Rewaq pattern has the most problematic acoustic environment due to its enormous volume and the use of massive pillars that tend to confine the sound rays in a limited area around the source. The semi-closed Iwan pattern yields the best acoustical qualities relative to the other three patterns with high values of speech SPL (65-70dBA) and a D50 value that equates to speech intelligibility of 70% or higher. In the closed Iwan pattern, only the worshippers in the Qibla Iwan experience a good acoustic environment whereas other worshipers may experience some difficulties in hearing and understanding the voice of the Imam. The closed simple pattern has fair acoustical qualities with most of the worshipers receiving a speech SPL equal to or higher than 50dBA. This is due to its excessive height and the resulting long distance travelled by the rays reflected from the ceiling (about 42m) before reaching the plane of the worshipers. The three patterns all have long reverberation times in the low- and mid-frequency range. A long reverberation time is often considered appropriate for the spiritual nature of these places. Additional research has sought to quantify the absorption characteristics of people performing prayers according to the Islamic rules [6]. The absorption characteristics of Muslim worshippers have been investigated in the reverberation chamber according to ISO 354. The four positions of the group prayer according to the Islamic rules (standing, bowing, prostration and sitting in rows on carpet) were simulated with thirteen male persons in the reverberation chamber in addition to a fifth position where the audience were sitting down in a random way on carpet to simulate the position of listening to the Friday sermon. Results were calculated for the sound absorption per person and showed that at mid- and high-frequencies the maximum absorption occurs for the standing position followed by the bowing position, the random sitting position, sitting in rows and finally the prostrate position.

Research has been carried out on the material characteristics of diffusers for acoustical renovation of two performing halls in Korea: the Chamber Hall (450 seats) and the M-Theater (630 seats) in the Sejong Performing Arts Center, Seoul [7]. The Chamber Hall was completely refurbished into a recital hall from its previous shape of a rectangular conference space. The saw-tooth shaped wall diffuser profile in the Chamber Hall was designed using glass fibre reinforced concrete for mid-frequency sound diffusion. The M-Theater was renovated as a live and intimate space for dramatic performances with a design that included more seats on the upper floors and additional spaces above ceiling reflectors. The vertically-patterned diffuser profile with protruded cubic surfaces in the M-Theater was designed using glass fibre reinforced gypsum for sound diffusion in the major speech frequency range. Designed diffuser profiles were evaluated for both halls by measurements of scattering and diffusion coefficients of the 1:10 scale model diffusers. The effects of diffusers in both halls were also investigated by covering the lateral walls close to the stages with reflective materials to control diffusive surfaces. As a result, spatial uniformity increased with diffusive wall profiles in both halls.

Field measurements were conducted to validate the acoustic design of The Little Theater in Sejong Performing Arts Center after refurbishment with sidewall diffusers to improve the acoustic quality [8]. The results show that unoccupied reverberation time varied from 0.9s to 1.1s, and 1-IACC (Interaural Cross Correlation Coefficient) was more than 0.6. It was also found that sidewall diffusers resulted in decrease of Coefficient of Variation of early decay time and 1-IACC.

For room acoustics modelling, FDTD models have been used to predict low-frequency sound fields in small volumes with and without a limp panel formed from a porous material which partially or completely subdivides the volume [9]. The porous panel is incorporated into FDTD using a Rayleigh model as proposed by Suzuki et al. However, to accurately reproduce the low-frequency sound field it is found necessary to introduce an additional Moving Frame Model (MFM) to account for motion of the porous panel. For spaces that are completely subdivided by a porous panel, the MFM accounts for a spring-mass-spring resonance that can occur below the lowest acoustic cavity mode. The MFM assumes lumped mass behaviour of the porous panel which is coupled to the FDTD update equations that incorporate the Rayleigh model. FDTD is compared against measurements using transient excitation with a pulse input to a loudspeaker in a small reverberant room under three different conditions: (1) empty room, (2) with a mineral fibre panel partially dividing the room, and (3) with a mineral fibre panel completely dividing the room. Close agreement is obtained between experimental results and FDTD incorporating the MFM; this validates the models as well as implementation of the loudspeaker as a hard velocity source.

Selected publications

[1] Oldham DJ and El-Zeky L (1999) Modification of the acoustical properties of an auditorium by means of virtual reflectors. Architectural Science Review vol 42 pp 63-68.

[2] El-Zeky L and Oldham DJ (2000) A computer investigation of the performance of virtual reflectors: Part 1 - Development of the method. Building Acoustics vol 7 issue 1 pp 57-73.

[3] El-Zeky L and Oldham DJ (2000) A computer investigation of the performance of virtual reflectors: Part 2 - The effects of room conditions. Building Acoustics vol 7 issue 2 pp 151-160.

[4] Hetherington RE and Oldham DJ (2007) A computer algorithm for the generation of three dimensional acoustical forms: application to an investigation of the acoustics of a fan shaped auditorium. Building Acoustics vol 14 issue 4 pp 311-327.

[5] Oldham DJ and Elkhateeb A (2008) The architectural development of the historic Masjids in Cairo and its acoustical impact. Building Acoustics vol 15 issue 3 pp 197-229.

[6] Oldham DJ and Elkhateeb A (2008) The absorption characteristics of Muslim worshippers. Building Acoustics vol 15 issue 4 pp 335-348.

[7] Jeon JY, Seo CK, Kim YH, Lee PJ (2012) Wall diffuser designs for acoustical renovation of small performing spaces. Applied Acoustics vol 73 pp 828-835.

[8] Jeon JY, Seo CK, Lee PJ (2011) Acoustical design for remodelling the Little Theater in Sejong Performing Arts Center. Journal of Architectural Institute of Korea vol 27 pp 313-320.

 [9] Ferreira N and Hopkins C (2013) Using finite-difference time-domain methods with a Rayleigh approach to model low-frequency sound fields in small spaces subdivided by porous materials.