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Acoustic Design Principles
Transcript of Acoustic Design Principles
Chapter 17: Acoustic Design Principles
What are Acoustics?
Acoustics is defined as the science that deals with the production, control, transmission, reception, and effects of sound. Many people mistakenly think that acoustics is strictly musical or architectural in nature. (In this case, we will focus on the architectural aspect.) While acoustics does include the study of musical instruments and architectural spaces, it also covers a vast range of topics, including: noise control, SONAR for submarine navigation, ultrasounds for medical imaging, thermoacoustic refigeration, seismology, bioacoustics, and electroacoustic communication.
Although interior designers' experience the world in a strongly visual way, they are often deeply affected by messages received by their other senses as well. Perhaps the most critical of these is the sense of hearing. Sound in a well-designed space reinforces the function of the space and supports the occupants' experience. A poorly designed acoustic environment hinders both the function and the enjoyment of the space, and it can even damage the health of the user.
High-pitched sounds have higher frequencies than low-pitched sounds. High frequency corresponds to a short wavelength. Bass notes have lower frequencies; low frequency and long wavelengths go together. Frequency is an important variable in how a sound is transmitted or absorbed and must be taken into account when designing the acoustics of a building.
The vibration of people's vocal cords produces human speech. These vibrations are modified through the throat, nose, and mouth. Each sound that makes up speech lasts only one-fiftieth to one-thirtieth of a second.
The human ear is sensitive to a very large range of sound power, which is measured in acoustical watts. A source's acoustical magnitude is a measure of the power of sound and may be called sound power, sound pressure, and sound intensity, depending on the type of power that is being measured. Although there are technical differences between these terms, the term sound power will be used here.
The way people experience a change in loudness is subjective; it is not related in a linear way to sound power. A sound that people perceive as twice as loud as another sound is actually much more than twice as powerful.
The loudness of sounds is measured in a way that relates actual sound intensity to the way humans experience sound. Loudness is measured according to a mathematical logarithmic scale of decibels (db). A decibel is a unit for expressing the relative pressure or intensity of sounds on a uniform scale from zero decibels for the least perceptible sound, to around 130 decibels for the average threshold of pain. People hear a doubling of sound pressure and intensity not as twice as loud, but as a barely perceptible change.
Sound waves travel at different velocities depending on the medium they are traveling through. Sound travels through air at around 0.3 kilometers (km) (1087 ft.) per second at sea level. Sound travels through water more rapidly than it does through air, at around 1.4 kilometers (4500 ft.) per second. Through wood, sound moves at about 3.6 kilometers (11,700 ft.) per second; sound moves through steel at around 5.5 kilometers (18,000 ft.) per second.
Sound waves radiate spherically from a point source. A point source, like a tuning fork, is relatively small compared with the wavelength produced. A line source, such as the string of a violin, creates cylindrical waves. Large, vibrating surfaces, such as the heads of large drums, create plane waves. When sound originates from a source like the human voice, it radiates more strongly in some directions than in others.
Sound energy, like heat energy, can be absorbed or reflected by objects. The ground, building surfaces, and other objects interact with sound waves with which they come in contact. Because of the complexity of these interactions, sound fields in the real world cannot be described by simple mathematical expressions.
How much sound energy is absorbed or reflected by a surface has a significant effect on what a person hears within a space. When little sound is absorbed and a great deal is reflected, sounds are mixed together. When steady sounds are mixed together, they accumulate in a reverberant field and produce a noisy space. In a reverberant space, speech becomes less intelligible, but music may sound better. Conversely, when much of the sound energy is absorbed and little is reflected, the room sounds quiet for speech but may sound dead to music.
Perceiving a sound as high or low depends on its frequency. The peaks in sound waves will pass a stationary point at different rates. A high-pitched sound has peaks that pass at a higher frequency (more frequently), while the peaks of a lower-pitched sound pass at a lower frequency (less frequently). The frequency with which these peaks pass a given point is measured as the number of cycles completed per second, in Hertz (Hz). One Hertz equals one cycle per second, so a wave whose peaks pass at 50 cycles per second has a frequency of 50 Hertz.
The human ear can pick out specific sounds to which a person wants to pay attention, but more frequently, it combines sounds that are distinct from each other in frequency and phase, such as musical chords. Most sounds are actually complex combinations of frequencies. Musical tones combine fundamental frequencies with harmonics (overtones). A trained conductor can distinguish a single instrument in a 120-piece orchestra. Amazingly, people have the ability to pick out one voice in background noise much louder than that voice, a phenomenon that is known as the cocktail party effect.
The lengths of sound waves for audible frequencies vary from 15 meters (50 ft.) for very low pitches to less than a few millimeters (less than 1 in.) for very high pitches. When a sound wave strikes a surface that is large in comparison to the wavelength, a portion of the sound energy is reflected (like light from a mirror) and a portion is absorbed. The harder and more rigid a surface the sound wave strikes, the more sound is reflected. Reflected sound leaves a surface at an angle equal to the angle at which it strikes it.
QUESTIONS! YOUR FAVORITE!
Reverberation is the persistence of sound after the source of the sound has ceased; this is the result of repeated reflections. Reverberation affects both the intelligibility of speech and the quality of music. The reverberation time of a space is the amount of time that sound bounces around a room before dying out to an inaudible level. It is also defined as the time required for sound energy to decay 60 decibels, or to one millionth of its initial intensity.
The reverberation time of a room should be appropriate to the use of the space. For speech in offices and small rooms, a reverberation time of between 0.3 and 0.6 seconds is desirable. The reverberation time for auditoriums ranges from 1.5 to 1.8 seconds. The quality of the sound can be controlled by modifying the amount of absorptive or reflective finishes in a space.
The reverberation of sounds in lecture halls, theaters, houses of worship, and concert halls sustains and blends sounds, making them much smoother and richer than they would be in open air. Short reverberation times are best for speech because they allow clarity for consonant sounds. However, some reverberation enriches a speaker's voice and gives the speaker some sense of how well his or her voice is carrying to the audience.
Music benefits from longer reverberation times that extend and blend the sounds of instruments and voices. Music sounds dead and brittle with too short a reverberation time, but it loses clarity and definition when the reverberation time is too long.
Natural (as opposed to electronic) sound reinforcement is the amplification of a sound being heard from various reflections as well as directly from the source. Covering the ceilings of meeting rooms, classrooms, and auditoriums completely with sound-absorbing material eliminates the potential for sound-reinforcing reflections off the ceiling and may result in inadequate sound levels in the rear of the room. Having to install an electronic sound-reinforcing system can be avoided by leaving the center of the room as a reflecting surface.
Wavelengths and Frequencies
A vibrating object radiates sound waves outward from the source equally in all directions until they hit a surface that either reflects or absorbs them. Sound waves have peaks and valleys, similar to ocean waves. The distance between the peak of one sound wave and the peak of the next is called the wavelength.
Building & Mechanical Systems
The history of modern acoustics began with the design of the Fogg Art Museum Lecture Hall at Harvard University in Cambridge, Massachusetts. When the building was erected in 1895, the acoustics of the main lecture hall space were a disaster, and the space could not be used for lectures. Wallace Clement Sabine, a 27-year-old new assistant professor in the physics department, was asked to find a solution. He started by considering the age-old problem of why the acoustics of some rooms were good, while others were mediocre or impossible.
Sabine isolated himself from his colleagues in the physics department and worked with two lab assistants late in the evening and early in the morning to avoid the impact of street noise and the vibrations from the newly constructed Harvard Square subway line. He had promised the university authorities to return everything to normal each morning by class time, so he and his assistants dragged hundreds of upholstered seat cushions from the nearby Sanders Theater to the lecture hall after midnight each night and back again at dawn. Sabine studied and measured the sound quality of similar spaces. He used his ears and a stopwatch to measure the length of reverberations from organ pipes.
Through his efforts, Sabine was able to develop reverberation equations and absorption coefficients for many common building materials. He discovered that the reverberation time of a room is directly proportional to the cubic volume of the room and inversely proportional to the sound absorption provided at the room's boundary surfaces and by the room's furnishings. His equation uses the simple dimensions of the room and absorption coefficients of materials to determine the acoustic effect of the space, offering a simple method for architects to determine favorable room proportions and treatments.
If you have ever tried to draw someone's ear, you will have noticed that people's ears are as individual as their fingerprints. They are small and large, simple and convoluted, smooth and fuzzy—and even hairy, but all healthy ears have the same parts.
The structures of the human ear enable people to collect sound waves, which are then converted into nerve impulses. The outer ear is an efficient sound-gathering funnel but lacks the collecting and focusing capacity of a cat's ear.
The middle ear is an air-filled space surrounded by bone and bounded by two membranes: the eardrum on the outer side and a flexible membrane that separates it from the inner ear on the inner side. The primary function of the middle ear is amplification. The vibrations from the eardrum are transmitted across the middle-ear space by three tiny levers of bone to the extraordinarily sensitive inner-ear mechanism (cochlea). The three bones, called the ossicles, are the malleus (hammer), incus (anvil), and stapes (stirrup). The movement of each bone increases the amplification of sound. The footplate of the stirrup bone is attached to a flexible membrane that covers an opening into the inner ear called the oval window. Moving back and forth like a piston, the stirrup bone sets in motion the fluids of the inner ear. In the short but intricate journey from the eardrum to the inner ear, the sound wave is amplified as much as 25 times.
When sound waves are rapidly reflected back and forth between two parallel flat or concave surfaces, an effect called flutter can result. Flutter is a rapid succession of echoes with sufficient time between each reflection for a listener to be aware of separate, discrete signals. People perceive flutter as a buzzing or clicking sound.
Standing waves operate on the same principle and are caused the same way that flutters are, but they are heard differently. Standing waves are perceived as points of quiet and of maximum sound in a room. Certain frequencies of voice or music are exaggerated as they bounce back and forth repeatedly between opposite parallel walls. When the walls are exactly half a wavelength apart, the tone is very loud near the walls and very quiet halfway between them because the waves cancel each other out in the center of the space.
Standing waves must be avoided in rooms for music performance, but the problem presents only an annoyance for speech. Standing-wave problems in rooms with parallel walls are improved by slightly tilting or skewing two of the walls or by adding acoustic absorptive material to one wall. Rooms for music rehearsals and broadcast studios often have nonparallel walls, and undulating ceilings are also used. The proportions of a room can minimize the effect, which is especially noticeable for bass frequencies.
THE DESIGNERS ROLE
When designing a building, an architect and interior designer must recognize potential noise problems and take steps to solve them. The acoustic design of the building should be integrated with other architectural requirements. By carefully planning the building's siting and structure, the architect can reduce noise penetration into the building. The overall building design and function should be reviewed in terms of desirable acoustic qualities. Noise sources should be placed as far as possible from quiet areas. The internal acoustics of individual rooms must be reviewed. For special acoustic issues, an acoustic consultant should be brought into the process as early as possible.
Addressing Interior Acoustic Design Issues
“Acoustic attenuation” is the term used for the reduction of the magnitude of a sound signal by a variety of means. This reduction may be the result of separating a sound source from a listener, enclosing the source to isolate the sound, absorbing the sound with materials that change the sound energy to heat, or canceling sound waves by electronic means.
What is the range of wavelengths we can hear? Descirbe the difference!
People can hear wavelengths of sounds that vary from more than 15.25 meters (50ft.) for very low pitches to less than 25 millimeters for very high pitches. Perceiving a sound as high or low depends on its frequency. The peaks in sound waves will pass at a higher frequency (more frequenly), while the peaks of a lower pitched sound pass at a lower frequency.
What is a unit of sound called, and what does it stand for?
A decibel is a unit for expressing the relative pressure or intensity of sounds on a uniform scale from zero decibels for the least perceptible sound, to around 130 decibels for the avergage threshold of pain.
What are three qualitites of sound waves that affect how we perceive sound?
Sound waves travel at different velocities depending on the medium they are traveling though. Sound travels through air at around 0.3 kilometers (km) (1087ft.) per second at sea level. Sound travels through water more rapidly than it does through air, at around 1.4 kilometers per second. Through wood, sound moves at about 5.5 kilometers per second.
What is meant by sound diffusion, and how is it used for the design of spaces?
Diffusion occurs when sound is reflected from a convex surface. Flat horizontal and inclined reflectors produce some diffusion as well. Diffusion produces a sound level that remains fairly constand throughout a space, which is a very desirable quality for music performance.
What is meant by reverberation time, and why is it important for the design of a space?
Reverberation is the persistence of sound after the source of the sound has ceased; this is the resulf of repeated reflections. Reverberation affects both the intelligibility of speech and the quality of music. The reverberation time of a space is the amount of time that sound bounces around a room before dying out to an inaudible level. It is also defined as the time required for sound energy to declay 60 decibels, or to one millionth of its inital intensity.
What is meant by acoustic attenuation, and what are four methods to accomplish it?
This term is used for the reduction of the magnitude of a sound signal by a variety of means. This reduction may be the result of separating a sound source from a listener, enclosing the source to isolate the sound, absorbing the sound with materials that change the sound energy to heat, or canceling sound waves by eletronic means.
What are three methods for reducing noise in a building?
Most rooms have a variety of acoustic fields. The area within one wavelengh of the lowest fequency of sound produced in the room is called the near field. For the male human voice, that distance would be about 3.36 meters. The reverberant field is the area close to large obstuctions such as walls, where conditions approcah a diffuse acoustic field. The free, or far, field falls between the near and reverberant fields.
What are three materials that are highly acoustically reflective?
Glass is massive but thin, so it's ability to attenuate sound is marginal. Concrete is a massive material that reflects sound. Resilent flooring: cork, asphal.
Give two examples of acoustically transparent materials.
Soft, porous, acoustially absorbent materials are often covered with perforated metal or other materials to provide protection and stiffness. These coverings are designed to be acoustically transparent, except at higher frequencies. With even smaller holes, the high frequencies can also pass through.