Architectural Acoustics

or the acoustics of interiors, the branch of acoustics concerned with the diffusion of sound waves in an interior, their reflection and absorption by surfaces, and the influence of reflected waves upon the audibility of speech and music. The aim of architectural acoustics is the creation of designs for halls (such as theater, concert, and lecture halls) and radio studios with good sound conditions.

In closed interiors of more or less substantial size, the listener hears not only a direct sound but a series of its delayed repetitions, rapidly following one another, that bounce off the confining surfaces. Because the energy of sound waves is absorbed at every bounce and during their travels, these repetitions become weaker over time. When the source of sound is turned off, the amount of reflected energy in the room decreases until it is entirely absorbed. This gradual dying-away of sound is called reverberation. The duration of reverberation is an extremely important factor in the acoustic quality of a hall. If sounds die away too slowly, speech and music will not be heard clearly enough; if sounds die away too quickly, speech sounds too hollow and musical sound loses its continuity and expressiveness. Even with optimal reverberation, the acoustic value of a hall is influenced by the distribution of times of arrival of the early, more intense echoes and the directions by which they reach the listener. The most favorable conditions vary not only for speech and for music but for musical compositions of different types (chamber, stage, and symphonic music). Therefore, the acoustic planning of concert halls (choice of form, distribution of listeners, and development of reflecting or absorptive surfaces) often requires compromise solutions. In large-capacity halls, acoustical conditions may be improved by applying electronic acoustical systems of amplification and artificial reverberation. The hall of the Palace of Congresses in the Moscow Kremlin (seating capacity, 6,000) is an outstanding example of an electronically enhanced multipurpose hall (for congresses, concerts, the opera, and sound films).

Formerly, architectural acoustics included the problem of isolating interiors from sounds penetrating from outside; now these problems occupy a separate field called construction acoustics. The methods of architectural acoustics are also used in combating inside noise.

Architectural acoustics uses both a strict wave theory, and a less strict, but technologically more convenient, geometrical theory in which straight lines are used to depict the direction of travel and the boundaries of the main part of the flow of sonar energy carried by sound waves as they encounter obstacles or are bounced off. Geometrical analyses become more correct as the length of the sound wave decreases in comparison to the size of the obstacle.

Contemporary architectural acoustics grew out of the work of the American scientist Sabine, who in the 1890’s showed that in a closed interior, consecutive, multiple, and gradually weakening sound reflections merge into a steadily dying hum accompanying every emitted sound (so-called reverberation) and that the rate of damping was a fundamental index of conditions of audibility. The open-air theaters of ancient Greece and Rome provide examples of the application of acoustical knowledge in construction.

The acoustical testing of interiors is based on electrical measurements of a sound signal picked up in the room by a microphone and consists of determining the regularity of the distribution of the sound in space and studying the damping of the echo in time. In addition to the actual testing of halls, testing small models is finding more and more application. This method allows errors to be corrected in the planning stage of new halls and develops methods to correct the defects of already existing halls.

Acoustical conditions in an interior are controlled by setting up reflective shields and regulating the amount of sound-absorptive materials attached to surfaces. The theory of sound absorption and the methods of measuring it also pertain to architectural acoustics. Electronic equipment to enhance amplification and reverberation is being used more and more widely. Electroacoustical methods of imitating the echo of an interior are also used in laboratory work.

Room acoustics

One essential component of room acoustics is an understanding of psychoacoustics and the qualitative evaluation of sounds heard by people in rooms. Psychoacoustics is the study of the psychology of sounds. It includes studies conducted in laboratories and in actual listening rooms of how people react to the level, frequency content, direction, and arrival time of sounds. These studies have established a set of relationships among the acoustical qualities that have been found to be important in the perception of sound, the room surfaces that contribute to these qualities, and the physical components of the sound field in a room that contribute to these properties.

Several important design concepts are used to provide good listening conditions in rooms for speech and music. First is to provide good access to the direct sound for all people in the room. This usually involves raising the source of sound on an elevated stage, altar, or podium at the front of the room and sloping the floor surface to elevate the ears of people above the heads of those seated in front of them. The width and depth of the room should also be limited so that the natural direct sound can project from the speaker or instruments at the front of the room to the listeners. Second is to limit the background noise level in the room so that people can hear the sound they want to hear above the level of the ambient sound. Third is to limit the reverberation time in the room so that sounds are heard clearly and fully, while providing enough reverberant sound energy that sounds are heard as “full” and “live.” If there is too much reverberation in a room, the persistence of an initial syllable will cover up or mask the one that follows it, making it difficult to understand what is being said.

Noise control

Acoustical planning concepts for buildings include placing noisy activities away from activities that require relative quiet and locating noise-sensitive activities away from major sources of noise. Buffer spaces such as corridors or storage spaces are often used to separate two rooms that require acoustical privacy such as music rehearsal rooms in a school. Intruding noises from the exterior or from adjoining rooms can be reduced by using walls, ceilings, windows, and doors with appropriate transmission losses. A compound or double wall assembly can be used to reach a relatively high transmission loss with low mass per unit wall area. The separation between the two leaves or surfaces of the wall must be maintained as completely as possible for this to occur.

It is essential to control noise from building services. The location of air-conditioning plants on a site should be chosen so as to reduce propagation of noise to neighbors. Mechanical rooms in buildings that house air handling units, pumps, and other equipment should be located away from noise-sensitive rooms. Noise control treatments in the air-conditioning system include providing vibration isolators for equipment; providing flexible connections between ducts, conduits, and pipes to equipment; designing air ducts to operate with air velocities that will not create turbulent flow noise; and installing silencers or attenuators in the ducts to reduce noise produced by fans from traveling through the duct work. See Mechanical vibration

Sound reinforcement

Sound reinforcement systems, electronic enhancement systems, and sound amplification systems are used in many buildings. A sound reinforcement system amplifies the natural acoustic sounds in a room that is too large for people to hear with just “natural” room acoustics. This type of system reinforces the natural sounds that come from the room, increasing their apparent loudness with a series of loudspeakers.

In an electronic enhancement system, loudspeakers act as virtual room surfaces to create the perception that sounds are reflected from these surfaces at the proper times and with the proper loudness. These systems usually have a network of loudspeakers located throughout a room and connected to a microprocessor. The microprocessor can delay the signals to arrive at times corresponding to reflected sounds from the virtual room surfaces. It can also add reverberation and other special acoustic effects to create a virtual acoustic space.

A sound amplification system makes all sounds played in a space louder. It is usually not designed to supplement the natural room acoustics or to provide subtle virtual room effects to the amplified sounds.