An important element in a properly functioning building is correct building acoustics. Achieving a low level of background noise in a classroom, for example, will ensure that the teacher’s voice is audible; the sounds of an orchestra will be optimal in a concert hall with proper acoustics. The systematic study of room acoustics began at the end of the nineteenth century, and consequently a scientific understanding of building acoustic design is almost entirely a twentieth century phenomenon. The means to achieve low noise levels in buildings were developed during the twentieth century. One of the greatest differences between old and new auditoriums is the low noise levels achieved in those built from the mid-twentieth century onward. Noise from external sources can enter a room through vibration paths (structureborne transmission) or can pass directly into the building through adjacent walls (airborne transmission).
Where very low noise or vibration levels are needed in auditoriums, recording studios, and operating theaters, vibration isolation (springs and resilient materials) are used, as are physical breaks in vibration paths. Airborne noise is reduced by the use of constructions such as double partitions separated by air gaps containing absorbent materials. The failure to achieve the desired backgroundnoise levels is often due simply to poor workmanship. Building service equipment such as boilers, etc., should be mounted on vibration isolators, and the structure and airborne paths should be considered in the design. Ventilation outlets may require silencers; low-velocity air conditioning is favored because it is quieter. Noise generated within rooms can also be a problem. For example the twentieth century saw a great increase in the use of atria, which can be very noisy, reverberant spaces due to footfall noise and the sounds of noisy items such as escalators. Perversely, a few rooms require the application of noise in order to achieve confidentiality. Masking noise from a radio is often added to hospital waiting rooms to provide privacy for consultations.
By the end of twentieth century the most critical design requirements for correct acoustics in rooms were well established. For speech, the requirement is often for intelligibility rather than fidelity. This requires the speech to be louder than the background noise and limits the room size that can be used without electronic enhancement. The direct path between the speaker and listener should not be obstructed, and the audience should be as close as possible to the speaker; placing seats on an incline is useful. Speakers should always face the audience, however, the audience can surround the speaker in small theaters. There should be hard surfaces close to the speaker and listeners to create beneficial reinforcing sound reflections. Where the stage area of a theater is very absorbent due to the scenery, reinforcing reflections from the ceiling and proscenium arch are vital. Surfaces that generate late-arriving reflections are usually treated with absorption to prevent echoes. Many of these principles were exploited in ancient amphitheaters, but it is only in the twentieth century that the scientific reasons for good or bad acoustics were understood. These requirements are easier to achieve when the speakers are located in one place, such as the stage of a theater and difficult in courts and debating chambers where there are many different speaker positions. The room sound should not reverberate excessively, otherwise the sound of one syllable will run into the next syllable. Acoustic absorption is used to reduce reverberance. During the twentieth century, various technologies for testing speech intelligibility were developed. In the early 1970s, T. Houtgast and J.J.M. Steeneken reported on their studies showing a direct correlation between modulation reduction factors and speech intelligibility. Their studies are the basis for the Speech Transmission Index (STI) program, an objective measure used in performance specifications. Perceptual tests may also be carried out, but these tests are rather slow to do. Developments in electronics have influenced building acoustics. Speech reinforcement and public address systems are used for emergency evacuation and day-to-day messaging. Electronic reinforcement will sound unnatural if the speech appears to come from the loudspeakers rather than the speaker. This is solved by applying delays to the sound coming from the loudspeaker. The Haas (precedence) effect states that the sound that arrives first—the first sound heard—will usually determine the perceived location of the sound. For a room with a high ceiling such as an auditorium, a single large cluster of loudspeakers may be used. For a room with a low ceiling, a series of loudspeakers placed along the length of the room may be used. Each loudspeaker covers a different area and has different delays. To prevent feedback, sound from the loudspeakers should not be directly picked up by the microphone. Loudspeakers can be sited forward of the speaker and directional microphones used. If a space is overly reverberant, only the frequency ranges important for speech intelligibility are reproduced. Operators need to be trained to use these sound systems correctly, otherwise the speech produced may be of poor quality.
Sound reproduction rooms such as control rooms in studios tend to be small. Low-frequency resonances of the room cause coloration, but the audible effects are reduced by appropriate choice of room dimensions and techniques for the application of resonant acoustic absorption. These consist of vibrating membranes over a cavity, with resistive material such as mineral wool in the cavity. At higher frequencies, the coloration caused by early-arriving loud reflections must be minimized. Absorbers (which remove sound energy) or diffusers (which spatially and temporally disperse sound energy) can be used. Absorber technology was developed throughout the twentieth century, while diffuser designs date only to the mid-1970s. In the 1970s, revolutionary new diffuser designs used phase gratings based on mathematical sequences; designs even exploited the emerging mathematical discipline of fractals. More modern diffusers use numerical optimization algorithms to provide curved diffusers that complement contemporary architecture.
A great concert hall acts as an extension to the musical instruments, embellishing and improving the sound produced by the musicians. In 1895–97, physicist Wallace C. Sabine, in tests aimed at correcting the poor acoustics in the lecture hall at Harvard University’s Fogg Art Museum, established the need to obtain the correct reverberance. Sabine has been called the father of architectural acoustics, and was the first to apply quantitative measures to the design of a concert hall (the first auditorium that was designed by Sabine was the new Boston Music Hall, opened October 15, 1900). In the last 30 years of the twentieth century, additional parameters to reverberance have been identified as important. For example, acousticians now understand how the hall shape influences sound quality. In previous centuries, trial and error had established the ‘shoebox’ shape (long, high, and narrow) as providing a good sound; it is now understood that this shape worked because it provided beneficial side reflections. Circular and elliptical shapes risk focusing sound-creating ‘‘hot spots.’’ The influence of amphitheaters, theaters, and early cinema can be seen in fan-shaped halls built in the mid twentieth century, but this shape can lead to a lack of side reflections. New shapes created for halls over the last half of the twentieth century include the Vineyard Terrace, which subdivides the audience area so the dividing walls produce beneficial early reflection. In the last decades of the century, acousticians developed a better understanding of the needs of musicians. Unless musicians receive reflections from surfaces around and above the stage, they cannot hear themselves or others, and so cannot create a good tone and blend or play in time.
Multipurpose halls create problems given the different acoustic demands of different events. The general principle is to design for a primary purpose, and then adjust the acoustics for other uses. Absorption is used to deaden a concert hall ready for electronic music, and electronic means are used to make a speech theater more reverberant for music. Electronic enhancement began with the assisted resonance system in the 1960s, and there was slow and steady growth in the use of enhancement systems in theaters from that time. Sound is picked up from the stage using microphones. The signals are then delayed and played from loudspeakers to create extra reflections and so increase reverberance.
The biggest influence that electronics has had on building acoustics has been the computer. Sophisticated computer-based instrumentation has allowed accurate measurement of building acoustics. Computer-based prediction models have enabled the improved understanding and design of acoustic technologies, from building elements to the whole rooms. Much of the mathematics used by acoustic engineers was developed in the nineteenth century, but this has only been exploitable at the end of the twentieth century using computers. There was also increased interest in virtual acoustic prototypes, which would allow building acoustics to be listened to in virtual environments, allowing nonacoustic experts to more readily understand the principles of good acoustic design.
