Room Acoustics in the Architectural Design Process A. Benjamin Spaeth Stuttgart University, casino IT, Germany http://www.casino.uni-stuttgart.de [email protected] Abstract. The basic principles, methods, and digital tools for the analysis of room acoustics are the topic of this paper. Students have been using these analysis and simulation tools during a seminar at Stuttgart University to systematically investigate acoustical situations. Additionally to lecturing on the underlying physics of room acoustics we were trying to convey the impact of geometry on room acoustics and therefore the impact of room acoustics on shapes in architecture. This paper describes the implementation of a real time simulation module for room acoustics within our Virtual Reality System. Keywords: Room Acoustics; Virtual Reality; Design Tools; Education. Introduction it is of importance to include room acoustics in the early stages of the planning and the subject of The perception of space is realized by a combination room acoustics should be a part of the curriculum at of all human senses. Aristoteles said that the most university. important human sense that helps us to experience In this paper the basic principles, methods, and physical objects is the sense of touch. But he had also some digital tools for analyzing room acoustics are realised that the perception of space could not exist introduced and discussed. Throughout a number of without the sense of vision and the sense of hearing. seminars students investigated acoustical situations Nowadays, we know that even scent and taste can and halls using these digital tools. The aim was to not take part in our perceiving space. In this paper we only convey the underlying physics of room acous- will focus on the relevance of hearing to the percep- tics, but also to study whether the actual tools for tion of space. analysis are useful planning tools for architects. With The designing and shaping of the physical pres- this seminar we wanted to hone the students’ senses ence of space is the main field of architecture. Archi- to the coherence of geometry and room acoustics, tecture joins materials, shape and utilization into a and to the implicit responsibility of architects to this concluded object. The architect is responsible for subject. the overall approach to geometry and the materials. “Room acoustics like light is a very important ele- Room acoustics, playing a very important role in the ment in design, which helps to guarantee the well- perception of space, is mainly influenced by these being of each individual, in the sense of physical, two aspects of geometry and material. Conclusively, psychological and social aspects.” (Schricker, 2001) Section 09: Virtual Reality - eCAADe 26 367 Influencing characteristics temporal resolution of about 5-10 msec. A signifi- cant energetic event after 50 ms is recognized as an First, the basic relationships between sound, echo. Haas used sound at the same energy level dur- space, geometry, and their perception have to be ing his investigations about the temporal diversifica- introduced. tion. Due to the longer propagation route the energy At a temperature of 20°C, sound travels through intensity of the second signal is usually lower than air at a speed of 343 m/s homogenously in all direc- that of the first signal. This leads to time shifts of up tions (Barron, 1993). The unit m/s itself already de- to 80 ms which are not perceived as echoes. picts that the phenomena consists of a spatial and In the evaluation of room acoustics is not only a temporal component. The propagation of sound the direct propagation of sound, but also the reflec- only occurs in a medium such as in air, in liquids or tions in their temporal progression and their inten- in solids, and the sonic energy decreases the farther sity that are crucial. Room acoustics depends on the the sound propagates through the medium. When room geometry. This seems to be a trivial observa- a sound wave hits an object, it is normally reflected tion, but it is the main focus of the following system- following the rule that the angle of the reflection is atical evaluation because this is fundamental to the equal to the angle of incidence. The quality of the re- architect’s planning efforts. flective surface defines the ratio of absorption and The reflections of sonic energy are unique for diffusion of the incoming sonic waves. The behavior each room and their sum creates the acoustical im- of sound depends on its frequency. This means that pression of the room. For a definition of the acousti- the propagation of sound varies at different frequen- cal characteristics of a room the following character- cies. This appears if we look at the dimensions of per- istics are considered. ceivable sound waves. Based on the equation cf = f * λ (cf: speed of sound, f: frequency, λ: wavelength), Reverberation Time (RT) the wavelength of a tone at 1000 Hz equals 0,343 m. According to the equation of Sabine RT60[s] = 0.16 * The length of the sound wave can thus be similar V[m3] / A[m2] the reverberation time describes the to the dimensions of objects or room elements. In time that has elapsed while the level of an emitted this case the waves are no longer reflected but bent sound has decreased about 60 dB. The room volume around the objects. This can be observed in the case is represented by V, and the equivalent absorption of the lower frequency ranges, while in the high fre- area by A. The RT is the best known acoustic char- quency range the waves behave more like light and acteristic and a value used to measure the hall- are simply reflected. The sound absorption rate of a effect. Rooms intended for speeches are designed surface also depends on the frequency and has to be to have reverberation times of about 0.5 to 1.0 s. taken into account, too. Concert halls have ideal RTs of around 1.4 to 2.0 s, Additionally to the physics of sound the func- and churches often have RTs of around 3-4 s (Mehta, tionality of the human sense of hearing is important 1999). The RT is a statistical dimension and is valid for in order to fully understand room acoustics. Humans the whole room. are able to perceive sound differing in intensity, space and time. The spatial localization occurs ac- Impulse Response (IR), Impulse Diagram cording to the law of Haas. The direction from where The impulse diagram is the record of the sound pres- the first wave front is arriving is identified as the ori- sure level over time, after the stimulating a room gin of the sound. The successive sonic energy which with an acoustic impulse. The impulse response has the same content is perceived either as echo or and its corresponding diagram are characteristic for as room volume. The human hearing is capable of a each position in the room. The IR diagram allows for 368 eCAADe 26 - Section 09: Virtual Reality Figure 1 Mozartsaal, RT 1.84 s (after Sabine), impulse response diagram, simulation in CATT a differentiated overview on the incoming sonic en- time has to be considered separately for each posi- ergy at a specific position and time. tion. The EDT is comparable to the reverberation time, but it is comprised only from identifiable re- Early decay time (EDT) flections in the first phase of sound propagation. The EDT consists of a few isolated early reflections. EDT EDT is usually defined at a difference in the sound is sensitive to room geometry since early reflections pressure level of (-)10 or (-)15 dB. are coming from identifiable surfaces in the room. The clarity of a room is determined by these early Therefore like the impulse response the early decay reflections. Reflections during the first 80 ms raise Figure 2 Mozartsaal, sound roses, simulation in CATT Section 09: Virtual Reality - eCAADe 26 369 the spatially differentiated perception of the field of can be emitted from a virtual source. Modified by the sound and thereby its transparency. Subsequent re- acoustically relevant surface properties like absorp- flections convey the impression of a spatial volume, tion or diffusion, the rays are traced and calculated. because the signals overlap themselves and a dis- The result of this simulation shows each ray with its tinguished perception is not possible any more. The corresponding energy level. The sum of these rays sound field evolves and and the signals add up. creates the acoustical impression. Soundfield, soundroses The number of reflections increases with an increas- Figure 3 ray tracing method, CATT ing travel time, whereas the energy level of the single signal decreases. The single rays blur to a ho- mogenous sound field. The impression of the sound becomes more voluminous and pleasant the faster and the more homogenous the sound field is. The sound roses in Fig. 2 show the spatial distribution of incoming signals at a single position in a specified time period. The density of single sound events can thereby be estimated. Methods of Analysis and Representation In Fig. 4 different ray paths can be observed, There are different methods to systematically exam- whereby the color represents the energy level of the ine room acoustics. Apart from the individual hear- signal. The calculation of a large number of such rays ing experience and its evaluation, objective physical creates a more and more detailed impression of the parameters exist that can be measured or calculated. acoustical situation. The present paper is limited to these objective pa- The output of these simulations are diagrams rameters and ignores the evaluation of their psycho- containing all relevant data as described in the chap- acoustic impacts on human sound perception.