Room acoustics as a discipline involves the study and analysis of direct and reflected sound. Appropriate room acoustics are essential in all spaces where sound is to be transmitted to a listener; this includes both speech and music. Room acoustics design criteria are determined according to the room's intended use. Acoustic, unamplified music, for example, is best appreciated in spaces that are "warm" and reverberant. Speech, by contrast, is more intelligible in rooms that are less reverberant and more absorptive. It is possible to create suitable acoustics for both speech and music in the same space, although this is rarely accomplished without some degree of compromise.

The term "room acoustics" typically brings to mind spaces where music is performed and recorded: concert halls, recording studios and scoring stages, for example. While acoustics are especially important to the success of these spaces, a much wider variety of facilities benefits from well-designed acoustics. Lecture and convention halls, classrooms, board rooms, council chambers, courtrooms, places of worship, theaters, cinemas and broadcast studios all depend on their acoustical quality. Speech intelligibility is essential in all of these spaces. Different acoustic design criteria are required for rooms where music is to be played, where "natural" acoustics help support unamplified musical instruments.

Design Criteria

The design for any room should be based on its estimated percentage of use for a particular function. This is particularly important for multipurpose spaces that may need to serve, for example, both as a lecture facility as well as for music recitals. Often, such different requirements pose a design conflict that is difficult to resolve, especially if the room is large. As a general rule, speech is intelligible in rooms having a reverberation time of one second or less. Conversely, music is composed of a wide variety of repertoires and genres, each of which has its own desirable range of reverberation or "liveness" provided by the room.

In addition to criteria for reverberation time, spaces used for critical listening should be designed with concern for the audio signal's imaging and echoes. Imaging includes the apparent size and location of sounds that are part of audio reproduction. Ongoing psychoacoustic research has attempted to define the early sound field thresholds for perception of reflections, changes to the audio image, and echoes based on the sound level of reflections and their delay after the direct sound.

If a critical listening space is designed to be "neutral," that is, without added "coloration," then the early reflection levels should lie at or below the threshold for image shift. The threshold for image shift is the level at which a sonic image appears to move from its actual location. Achieving these relatively low reflection levels in a studio control room requires treatment of all surfaces involved in providing first-order sound reflections to the listener. One surface that cannot be treated by the studio design consultant is the upper surface of the mixing console. Future mixing console design should consider using control surfaces made of porous material, such as sintered aluminum.

Another consideration in the design of critical listening spaces is eliminating "rattles and resonances" often associated with metal fixtures, such as lighting, ducts, diffusers, and furniture. Difficulties are often resolved by applying visco-elastic damping material. Damping is normally available as sheet material with a self-adhesive backing or in liquid form. The sound intensity produced by a vibrating surface is normally proportional to the velocity of the panel vibration. Damping reduces the panel velocity and, hence, the sound.

Basic Principles of Room Acoustics

The main difference between indoor and outdoor sound propagation is in the level of reflected sound. Indoor environments naturally create more reflected sound than do outdoor environments. Reflected sound can be divided into three distinct categories: early and middle-reflected sound, reverberation (late-reflected sound) and standing waves.

Early reflections contribute more to the subjective perception of reverberance, or "liveness" of a space. Early and middle reflections occur within the first quarter of a second after arrival of the direct sound. Early sound is considered to be 40 ms after arrival of the direct sound for speech while for music 80 ms is more appropriate. Once sound reflections have built up to a point where they are not discernible as discrete events, the late reverberation process takes over. In most well-designed spaces, reverberation is a statistical phenomenon, no longer relying on specific room shape and sound propagation paths. For this reason, the statistical study of room acoustics, which ignores the path of specific reflections but considers reflected sound as an aggregate probability, is employed with respect to reverberation. Statistical analysis methods are applicable to rooms with relatively uniform sound absorbing material distribution and reasonable aspect ratios.

In spaces having a diffuse sound field where sound is uniformly distributed throughout the space, reverberation decays logarithmically, although the decay sounds even and consistent to the human listener. The reverberation time is defined as the time for reflected sound to decay 60 dB. Generally, it is necessary to avoid assessing early sound reflections as part of reverberation since the reflections contribute to sound build-up, rather than to sound decay. The first 5 to 10 dB of decaying sound reflections are generally not used to determine the reverberation time, which is determined from the remaining decay.

Specular Reflection

The manner in which sound reflects depends on the shape, texture and material of the room boundary. Specular reflections, those reflections conforming to Lambert's law of reflection, where the angle of incidence equals the angle of reflection, typically occur at smooth and relatively flat surfaces. For a surface to be a good reflector of sound, its dimensions should be at least one wavelength or larger than the lowest frequency being reflected. For instance, the wavelength of the musical note middle C (256 Hz) is approximately 1.35 meters (4.5 feet) long. Two octaves higher, a little above 1 kHz, the wavelength measures just over 0.345 meters (13 inches). In order to adequately reflect low-frequency sounds which have larger wavelengths, the reflectors must be relatively large.

Diffusion

Sound can also reflect in a diffuse manner. The reflection is fragmented into many reflections having less intensity, which are scattered over a wide angle, creating a uniform sound field. Diffusion can be created in a variety of ways, most often by introducing surfaces having irregularities in the form of angled planes or convex surfaces sized at least as large as the wavelength being diffused. Three-dimensional surfaces such as ornamentations, columns and statuary serve as diffusing elements and were integral to the acoustics of 17th, 18th, and 19th century performance spaces. The depth of the diffusing undulations must be at least one-tenth the wavelength being diffused. However, it is possible, if attempting to create a relatively low-frequency diffuser (for example, the octave below middle C, which has a wavelength of 2.7 meters [9 feet] ), to have specular reflections at higher frequencies. For this reason, in some concert halls, there are macro as well as micro diffusive elements to accomodate diffusion in different frequency (and therefore wavelength) ranges. Most common diffusers work well between 800 Hz and 4 kHz.

Echoes

Echoes are reflections that can be heard distinctly and separately from the early reflected and reverberant sound. For most general purposes involving speech communication, echoes are normally heard due to intense reflections arriving 40 ms and later after the direct sound signal has reached the listener. In other words, the difference in path length between the direct sound and the reflected sound is at least 13.8 meters (46 feet) corresponding to a propagation time of 40 ms or greater. Ironically, echoes are most commonly detected in the front rows of an auditorium and onstage. This results from the front row being farthest from the rear wall, thus generating the largest path length difference between the direct sound and the sound radiating directly from the rear wall or the combination of the ceiling and the rear wall. Sometimes, only a performer or lecturer is able to perceive an echo! Typically, using sound absorbing or diffusing materials. Even surfaces as small as 10m_ (100 feet_) can require treatment to suppress an echo. Generally, very absorptive rooms must be designed with extreme care in regard to the placement of reflective materials.

Flutter Echoes

A flutter echo results when sound travels back and forth between two parallel surfaces and is attenuated much more slowly than reflections from other surfaces. Flutter echoes, which are usually perceivable at frequencies of 250 Hz and greater, largely rely on parallel room boundaries to be sustained. Angling room boundaries, therefore, can help eliminate high-frequency flutter echoes.

Focusing

The cardinal rule in the design of rooms is to avoid sound reflectors that focus in the plane of listening. A focusing surface concentrates sound energy, which may then be intense enough to be perceived as an echo. Surfaces such as domes, barrel-vaulted ceilings and concave rear walls can cause sound focusing and are notorious for generating strong echoes. Such architectural elements should be designed with extreme care to avoid acoustical defects.

Reverberation

Reverberation is directly proportional to room volume, inversely proportional to the surface area and inversely proportional to the amount of sound absorbing material. It is possible to reduce reverberation by the following means: adding sound absorbing material, reducing room volume or increasing surface area.

Reverberation time is the measure used to quantify reverberation and is the time required for sound reflections to decay 60 dB, one-millionth of their original amplitude. The Sabine reverberation formula, named for the physicist who first recognized this relationship, applies to rooms that have a relatively diffuse (uniform) sound field: T=0.05V/S_ where T is the reverberation time; V is the room volume in ft_; S is the room surface area (ft_); and _ is the average absorption coefficient.

While there are other reverberation time equations, such as those described by Norris-Eyring and Fitzroy, for example, the Sabine equation was the first developed, and it remains valid in most cases. In order to determine the reverberation time in a diffuse room, it is necessary to sum up all of the room's sound absorption due to each material's contribution. This can be accomplished in each frequency range by multiplying the surface area by the sound absorption coefficient for a particular frequency range for all materials located within the space. Just as reflections are not entirely specular or diffuse, no material is entirely sound absorbing or reflecting.

As a general guide, it is not advisable to concentrate large amounts of sound absorbing material on one surface only, particularly where that surface is distant from a group of listeners. In order for a diffuse sound field to exist, sound absorbing material needs to be distributed over both the wall and ceiling surfaces. In a rectangular space, for example, it is not good design practice to concentrate sound absorbing on two parallel surfaces or on two pairs of parallel surfaces. This simply reduces reflections coming from the absorptive surfaces and may result in an echo by enhancing the audibility of the reflected sound from the remaining pair (or pairs) of room surfaces. The reflections from the absorptive surfaces are decreased in amplitude, resulting in a relative increase in the amplitude of the remaining reflections.

Standing Waves

Standing waves are also known as room modes. Room modes are most easily perceived when listening to low-frequency tones in small rooms having hard surfaces. Standing waves usually occur between hard parallel wall surfaces and are of particular concern in relatively small rooms, such as music practice rooms, voice recording booths, small audio control rooms and other spaces used for recording or for monitoring recordings. In an ideal case, it can be assumed that walls are infinitely rigid and stiff, so that minimum sound absorption occurs and there is little phase difference between the incident sound and the reflected sound at the point of reflection. Rooms in which two or more major dimensions (for example, length, width and height) are equivalent to multiples of half-wavelengths are notorious for causing additive standing waves and undesirable resonances. The frequency of resonance is higher in small rooms due to the smaller dimensions and shorter wavelengths. For this reason, standing waves are a much more important consideration in small rooms where the frequency of interest lies within the normal speech range of 100 Hz to 5 kHz. It is noteworthy that standards require acoustical laboratories to have the lowest useful 1/3-octave frequency band contain at least ten modes (standing waves) to assure reasonably accurate measurements. This requirement results in a smoother frequency response (i.e., less amplification of a single frequency), due to overlapping modes. The lower limiting frequency is usually 100 Hz. For this reason, laboratories do not usually measure below this frequency, in spite of the fact that there is a growing need for data below 100 Hz.

In studios used for the production or reproduction of audio material, sufficient low-frequency absorption is important. The sound absorption in this case acts as damping, reducing the amplitude and broadening the frequency range of the resonance.

Sound Absorbing Materials

All materials have some sound absorbing properties. Incident sound energy which is not absorbed must be reflected, transmitted or dissipated. A material's sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. The coefficient can be viewed as a percentage of sound being absorbed, where 1.00 is complete absorption (100%) and 0.01 is minimal (1%).

Incident sound striking a room surface yields sound energy comprising reflected sound, absorbed sound and transmitted sound. Most good sound reflectors prevent sound transmission by forming a solid, impervious barrier. Conversely, most good sound absorbers readily transmit sound. Sound reflectors tend to be impervious and massive, while sound absorbers are generally porous, lightweight material. It is for this reason that sound transmitted between rooms is little affected by adding sound absorption to the wall surface.

There are three basic categories of sound absorbers: porous materials commonly formed of matted or spun fibers; panel (membrane) absorbers having an impervious surface mounted over an airspace; and resonators created by holes or slots connected to an enclosed volume of trapped air. The absorptivity of each type of sound absorber is dramatically (in some cases) influenced by the mounting method employed.

1. Porous absorbers: Common porous absorbers include carpet, draperies, spray-applied cellulose, aerated plaster, fibrous mineral wool and glass fiber, open-cell foam, and felted or cast porous ceiling tile. Generally, all of these materials allow air to flow into a cellular structure where sound energy is converted to heat. Porous absorbers are the most commonly used sound absorbing materials. Thickness plays an important role in sound absorption by porous materials. Fabric applied directly to a hard, massive substrate such as plaster or gypsum board does not make an efficient sound absorber due to the very thin layer of fiber. Thicker materials generally provide more bass sound absorption or damping.
   
   
2. Panel Absorbers: Typically, panel absorbers are non-rigid, non-porous materials which are placed over an airspace that vibrates in a flexural mode in response to sound pressure exerted by adjacent air molecules. Common panel (membrane) absorbers include thin wood paneling over framing, lightweight impervious ceilings and floors, glazing and other large surfaces capable of resonating in response to sound. Panel absorbers are usually most efficient at absorbing low frequencies. This fact has been learned repeatedly on orchestra platforms where thin wood paneling traps most of the bass sound, robbing the room of "warmth."
   
   
3. Resonators: Resonators typically act to absorb sound in a narrow frequency range. Resonators include some perforated materials and materials that have openings (holes and slots). The classic example of a resonator is the Helmholtz resonator, which has the shape of a bottle. The resonant frequency is governed by the size of the opening, the length of the neck and the volume of air trapped in the chamber. Typically, perforated materials only absorb the mid-frequency range unless special care is taken in designing the facing to be as acoustically transparent as possible. Slots usually have a similar acoustic response. Long narrow slots can be used to absorb low frequencies. For this reason, long narrow air distribution slots in rooms for acoustic music production should be viewed with suspicion since the slots may absorb valuable low-frequency energy.

Is room acoustics an art or a science? Recent technology has refined the acoustician's ability to predict a room's acoustical requirements. It is now possible, for example, to provide active acoustical enhancement by introducing synthesized sound reflections through an array of loudspeakers, thus improving the quality of the transmitted sound dramatically. More specific design criteria are also evolving to suit different uses. Acknowledging the uniqueness of the design criteria required for each space is vital to the success of the facility, especially if it is multipurpose.

Art implies intuition and mastery. Science can aid in the development of both. But what role does luck play? Were the grand masters simply lucky? Is it luck or skill that allows an artist to appeal to a broad audience? It is in fact a combination of both. Today's room acoustics, like many arts, is an opinion-dominated field, one that is influenced as much by history as it is by technology.

DUBBING STAGE: SKYWALKER RANCH, NICASIO, CALIFORNIA

The dubbing stage, along with the various recording, mixing and editing rooms in this facility, was engineered to meet the owner's acoustic criteria for sound isolation, room acoustics and background noise level. Floating floors, double-glazed windows and masonry walls combined with furred drywall construction achieved the sound isolation requirements. The background noise level in the dubbing room was controlled to a maximum of NC 15 using in-duct silencers, plenums, oversized ventilation ducts and a plaque air diffuser supply system. The reverberation time was controlled to 0.4 seconds. A portion of the Technical Building was constructed over a parking garage. Acoustical tests were conducted and construction designed so as to control the noise intrusion of car engines.

It was desired that arches be part of the room's design. Cost studies conducted during the value engineering phase of the project dictated that the arches be constructed of glass fiber reinforced gypsum rather than plaster. A 1-to-10 scale model was built as both an aesthetic study model as well as an acoustical testing model. A 3mm (1/8-inch)-diameter microphone was used to receive the test signal in the model, and the sound reflection patterns in the model were displayed on an oscilloscope screen. The test indicated that the arches as designed would diffuse the sound, not create echoes. These test results were confirmed after the room was built.

During the construction phase, onsite field visits were conducted every two weeks to review the various sound-rated constructions and the installations of the ventilation system. Post-construction measurements of background noise were made in all noise-critical spaces to verify that the design criteria had been met.

The acoustical design of this building received an Honor Award from the American Consulting Engineers Council in 1988, in part because some of the recording spaces in the complex are among the quietest in the world.

SCREENING ROOM: DOLBY LABORATORIES, SAN FRANCISCO, CALIFORNIA

This room is used for film screening, presentation, audio recording and for training. New products for improving motion picture sound are demonstrated, evaluated and developed here. It is located on the third floor of a building originally constructed in 1910. The size and shape of the room were optimized for motion picture presentations. The coffered ceiling creates a desirable aesthetic and helps to diffuse sound evenly throughout the room. To achieve Dolby's reverberation time criterion of approximately 0.3 seconds in the mid-frequencies, about 70% of the wall and ceiling areas were made sound absorptive, using 25 mm (1 inch)-thick sound absorbing material over deep air spaces. The acoustical quality in the room can be varied using retracting sound absorbing quilts in the side walls.

To develop structure-borne vibration control design standards for the 35mm and 70 mm projectors in the projection room, vibration measurements were made on similar projectors at a nearby theater. The screening room projectors were mounted on a floating concrete slab, isolated from the surrounding floating floor.

Double stud walls, a sound-isolating gypsum board ceiling and 75 mm (3-inch)-thick acoustically gasketed doors control noise intrusion from the outside as well as from the theater to adjoining areas. Double glazing with a 200mm (8-inch) air space was used to control projector room noise transfer into the screening room. The background noise in the room varies between NC 15 and NC 20 depending on the ventilation fan speed and thermal load. The office space below the screening room has an exposed ceiling for aesthetic reasons. The entire screening room is isolated from the office space by a concrete floating floor.