We live in a vibration-filled world, surrounded by sound that imparts information and emotion. The essential component of music and language, sound gives clues as to where we are and what is happening around us.
Sound is a wave created by vibration, a ripple in air and water, or through the steel beams of a skyscraper’s skeleton. It bounces off walls and bends around corners. It can be absorbed like water into a sponge or reflected from a hard surface like light off a mirror. The physical properties of sound allow it to be manipulated and shaped.
Acoustical engineers use the physics of sound to enhance a room used for performing arts so that music sounds lively and speech is intelligible. Applications for acoustical engineering range from preventing sound from entering or leaving a structure to ensuring the proper functioning of a sound system in an outdoor stadium. Acoustical engineers may be called upon to make sure that restaurants or casinos are appropriately boisterous or that a classroom is quiet.
The proper management of sound can be the difference between success and failure in symphony halls, churches, airports, auditoriums, office buildings and entertainment complexes. Every building and outdoor facility can benefit from proper attention to acoustics. Considering the acoustical aspects as early as possible, even before preliminary design and site selection, can prevent costly or irreparable mistakes. Once the seating capacity has been determined, the quality of an auditorium or theater’s acoustical environment is a major if not prime consideration in determining the building’s layout and shape.
Acoustical considerations profoundly affect the aspects of design. Design factors that determine the acoustics of a space include size, background noise, geometry of the walls, floor and ceiling, and the density of the room surfaces. The control of acoustics in a building or room is further complicated by how sound is transmitted, reflected, diffused and absorbed.
Sophisticated monitoring instruments and advanced computers have expanded the capabilities of acoustical engineering, but the discipline still demands the skills and insight of an artist. Dealing with the human perception of sound requires consideration of factors that often cannot be easily quantified.
The physics of sound enable acoustical engineers to configure a building or room for the desirable acoustical properties. In effect, they “fine tune” a space by shaping the walls, adding acoustical elements and reinforcing natural sound with electronic systems.
The Physics Of Sound
Sound occurs whenever something vibrates. The oscillation of the vibrating object, whether a person’s vocal cords, the cone of a stereo speaker or the string of a guitar, creates disturbances, or waves, in a medium. Anything that vibrates will create a sound, even turbulent airflow in a heating-ventilation-air conditioning system.
Unlike light, sound requires a carrier medium. The carrier medium is most commonly air or water but it can be wood or steel, or even the ground. The ability of the medium to conduct sound depends on its density and elasticity. Because sound travels through the disturbance of interacting molecules, it cannot be transmitted in a vacuum.
As the wave or disturbance travels through the air, it transports energy by displacing air molecules from a state of rest, or equilibrium. These in turn disturb other molecules, pushing or pulling them, and so on until the vibration stops and the energy dissipates. The disturbance creates tiny differences in the air pressure. Our ears detect sound through these small fluctuations in air pressure that move the eardrum.
These fluctuations consist of small high-pressure areas, where the molecules are compressed and energetic, and small low-pressure areas, where the particles are thinner and less active. Together, these areas constitute a wave. If the difference between the high and low states – known as amplitude – is small, the air molecules strike the eardrum with little power, so the sound is soft or weak. If there is a large variation between the two, the molecules hit the eardrum harder and the sound is loud.
Wavelength. The distance between two adjacent wave peaks or two adjacent wave valleys is the wavelength of the sound. The wavelength depends on the velocity of the sound in its medium and the frequency of vibration. Low-frequency sounds have long wavelengths, and high-frequency tones have short wavelengths. The velocity of sound in air is approximately 1,130 feet per second, so a tone with a frequency of 256 vibrations per second has a wavelength of 4 feet, 3 inches.
Frequency. The number of sound waves that pass a fixed point per second is the frequency of the sound. One wave each second is 1 Hertz. The normal human hearing range is between 20 Hz and approximately 20,000 Hz (20 KHz), but this span shrinks as we age. Frequencies above 20 KHz are considered ultrasonic, and are useful in medical imaging systems.
Loudness & Pitch. Humans hear amplitude as loudness, and frequency as pitch. Trained musicians are capable of detecting the difference in frequency of two tones separated by only 2 Hz. However, pure tones are rare. For example, middle C is 256 Hz, but it includes components, known as harmonics, that vibrate at 512 Hz, 768 Hz, and more.
The distance at which a sound can be heard is determined by its intensity, or loudness. Sounds spread in all directions from the source, so a sound is nine times as intense at a distance of 10 feet as it is at 30 feet from the source. Tones of the same frequency but with great differences in intensity will sound to us as different pitches.
Like A Pebble In A Pond
A common way to describe the propagation of a sound wave in air is to compare it to the ripples in a pond caused by dropping a pebble into it. Like the sound wave, the water wave spreads in all directions from the point of disturbance. Both have peaks and troughs, and transmit the wave not by displacing the medium but by a series of interactive particles colliding with their neighbors. The wave intensity lessens the further it moves from the source. Both waves continue until they hit a boundary surface, where most are reflected away at angles determined by the geometry of the surface and the angle at which the wave meets it. These new waves interact with the original wave, enhancing it at times and dampening it at other points. Some of the wave energy is lost through transmission to the structure containing the medium.
But unlike the two-dimensional water wave in the pond, the sound wave in the air expands in three dimensions. There may be many sources of sound, some of them unintended.
In a room, sound waves hit walls, ceiling, floors, and even the furniture and people in the room. Some of the sound energy may be absorbed, but much of it will be deflected. A complex system of overlapping waves of different frequencies is often present. Outside noise – traffic, low-flying airplanes or even the building’s HVAC system – can intrude into the space. Sound behaves differently in a small, enclosed space, such as a classroom, than it does in the open space of a church or auditorium.
The work of the acoustical engineer consists of controlling and shaping sound waves to fit the purpose of a room or building. Sometimes the job is keeping the sound inside the room; sometimes it is keeping sound out. The acoustical engineer may be called in to ensure proper acoustics in a symphony hall or opera house, or to provide the best sound possible in a multiple-use building such as a church or school auditorium, where music clarity and speech intelligibility are equally important.
In addition to dealing with the physics of sound, which can be reduced to numbers and formulas, the acoustical engineer must cope with something even more complex: how our brains interpret sound to give us clues about our environment.
The Perception Of Sound
Humans have an incredible ability to process the variations in sound pressure into concrete information about their surroundings from the sounds that are captured by their ears, converted into electro-magnetic signals in the inner ear, and analyzed by the brain.
We use sound to help determine depth and distance in our environment. When a sound is generated, we hear it through a direct path from the source, and then we hear the reflections of the sound off walls. Each reflection is further reflected again and again, as the sound continues to bounce off the walls until all the energy is lost. Reflected sound is heard as either an echo or reverberation, depending on the dimensions of the space, room shaping and relative intensity of sound.
Our brains contain a catalogue of sounds heard during our lifetime. New sounds are constantly being compared to these archives and categorized as nearby, far away, normal, unusual or threatening.
We determine the location of a sound by clues contained in the pressure waves that reach our ears. If the sound is louder in our left ear than our right one, we know the source is located to the left. Through the slight degradation in frequency caused by the back of the ear, and by using head and torso correlation, we can tell if a sound comes from ahead or behind.
We know if we are in a large room or a small one if echoes are produced by sound waves bouncing back after hitting walls, and by how long it takes a reverberation to fade away. If a sound is soft, we know it is produced farther away than is a sharp, loud sound. Unexpected sounds awaken us from a deep sleep, while familiar noises are filtered out.
We can determine the distance, direction, loudness, pitch and tone of several different sounds at the same time. The ear can distinguish different subjective aspects of a sound by detecting and analyzing different physical characteristics of the waves. All of this is accomplished by the brain in a millionth of a second, with no conscious thought on our part.
The brain keeps a sound in memory for approximately 70 milliseconds (one millisecond is a thousandth of a second, or 0.001 second). When a reflected sound wave arrives at our ears within this span, the brain is incapable of distinguishing it from the direct sound. If this reflection persists, we hear a prolonged sound, or reverberation.
If the reflected sound arrives later than 70 milliseconds after the direct sound and is strong enough, we hear it as a separate sound, or echo, because the original sound is perceived as having died out.
Acoustical engineers rely on the physical properties of sound as well as our perceptions, to shape the acoustics in a room or other space to the intended application.
When sound waves hit a wall or other obstacle, some of the energy is reflected and some is transmitted into the new medium. How much of this energy is reflected depends on the relative densities of the media. Smooth concrete and stone are much denser than air, so most of the sound is reflected off those materials. On the other hand, a fabric-wrapped fiberglass panel will absorb much of the sound energy that hits it.
The most important factor for the acoustic suitability of a room is the reverberation time, or the time it takes for a sound wave to die out. Known technically as RT60, reverberation time is how long it takes the reflections of an impulsive sound, such as a sharp handclap or a bursting balloon, to decay to one-millionth of its original intensity (a decay of 60 dB on the decibel scale).
If the reverberation time is long (more than 1.5 seconds), the room sounds “lively” because the sound persists longer. If a space’s reverberation time is short (less than 1 second), it seems “dead” because the sound decay is rapid. Reverberation time varies with the space’s volume and shape, and with the amount of sound-absorbing or reflecting material in the room. The reverberation time must be long enough to blend sounds but short enough so that successive sounds are separated enough for comprehension.
The need for clarity in understanding speech means that rooms used for talking must have a relatively short reverberation time. Rooms used for music need a longer reverberation time than would be unacceptable for the spoken word. For example, an opera house’s optimal reverberation time is 1.4 to 1.5 seconds while an elementary classroom ideally should have a reverberation time of no more than 0.55 seconds.
Music sounds better with some reverberation time, but extended reverberation degrades our ability to hear speech clearly and coherently. This smearing of speech makes it especially difficult for the hearing impaired to understand spoken words in reverberant rooms. Reverberation time in a performing arts space used for music can be 1.8 seconds, but in a large conference room an RT60 of more than half a second can cause problems. Short reverberation times are also desirable in motion picture theaters.
Musicians appreciate a space with good reverberation, which makes the audience feel the sound is enveloping them. Reverberation blends the voices of a choir and the instruments in an ensemble, and adds depth to a pipe organ’s output. Pipe organs sound most magisterial with a RT60 of up to 3 seconds.
Even different types of classical music can benefit from varying reverberation time lengths. Music by composers such as Wagner and Mahler sound better with longer reverberation times, while the music of Bach and Mozart is enhanced by intermediate reverberation. Rock music rocks best with short reverberation.
The ideal solution is to design performance spaces for either music or speech, but not both. Single-use allows the architect and the acoustical engineer to seek acoustical excellence. Of course, single-use is not often financially feasible, but the large, multi-purpose performance space is the most difficult to control acoustically. A multi-function performance space requires variable acoustic systems such as drapes and reverberation chambers.
Because reverberations are repeated reflections of sound, they can be reduced by avoiding hard, parallel wall surfaces of stone or concrete, and by adding draperies, upholsteries, acoustic panels and other sound-absorbing materials. Rooms with the same volume and shape will have different reverberation times if, for example, one is treated with acoustical panels rather than hard and parallel surfaces.
The shape of a room will affect its acoustical performance. Smooth, concave reflecting surfaces will focus sound waves, creating large echoes. Parallel walls reflect sound back and forth. This creates a rapid, pulsing echo effect known as “flutter” that degrades the sound.
Large rooms allow sound to reflect more, so reducing the size of the space can help reduce reverberation. A good rule of thumb is 3.5 cubic meters per person for speech, and 6 cubic meters or more per person for music. People do serve as efficient sound absorbers, so an auditorium with every seat filled will be less reverberant than a half-empty one.
Sound wavelengths can be very long, so structures designed for sound reflection and diffusion must be large. Massive flying reflecting “clouds” dangling from the ceiling have been used since Roman times to reflect sound from the stage into the audience. Clouds are effective but bulky, and must be carefully placed to avoid ruining the audience’s view of the stage. These reflectors can be sized to reflect specific frequency ranges but allow others to be reflected unaltered.
In a multi-use space, large adjustable curtains are often the best method of altering reverberation times. These sound-absorbing curtains are cumbersome and expensive, but usually more effective in attenuating reflection than are static sound-absorbing panels. They are brought into the room to soak up the reverberations for speech and dramatic presentations. When the space is used for music, the curtains are retracted to expose the walls for increased reverberation and sound diffusion. These systems provide the ability to increase room volume, thus affecting reverberation time. The world-famous Morton Myerson Symphony Hall in Dallas uses curtains to increase or decrease room volume.
A properly designed and installed sound system with directional speakers can increase the speech intelligibility of a multi-use space without degrading desirable reverberation. However, a suitable acoustic environment for electronic sound reinforcement must be attained before a sound system can be effective. This usually involves quieting the background noise level and installing sound-absorbing treatments.
A silent background allows actors and musicians to use the full dynamic range of voices and instruments. In a silenced classroom, teachers can make themselves heard without shouting. A quiet office with fewer distractions is more productive.
Reducing the background noise, whether from highway traffic or a structure’s HVAC system, is essential to a good acoustical environment. Our hearing allows us to filter out some levels of background noise, but extraneous noise introduces a haze that hinders understanding by obscuring clear sounds and covering up fainter ones.
Sound is absorbed when a wave hits a material that converts all or part of the acoustical energy into heat, or lets it pass through and not return.
Sound insulation prevents unwanted outside noise (such as traffic noise) from entering a space. Dense materials such as concrete make effective insulators, but the same effect can be accomplished at a lower weight by installing double walls that are structurally separate, providing an air cavity that helps dampen sound.
In most structures, acoustical engineers often find that the biggest culprit in background noise is the HVAC system. Fans, pumps, compressors and other rotating equipment vibrate, so they are noisy, and air moving through the duct system adds an omnipresent “whoosh.”
The design of a movie theater or any performing space should, if possible, separate the HVAC units in a structure removed from the critical area. Air distribution systems can be designed with oversized, lined ducts, sound attenuators and low airflow velocities to avoid ducting noise into the performance space.
In dealing with inherently noisy spaces such as gymnasiums and school cafeterias, acoustical engineers cannot remove the noise source, because the people using the facilities are the primary noisemakers. The engineers’ best solution is usually to change the surface densities by placing soft sound-absorbing materials on suitable wall surfaces and ceilings to cut the reflected sound and deaden the room.
The management of sound is essential to the effective use of any space, place or building. Proper acoustics enable sound decisions.