Acoustics

Branch of physics involving mechanical waves

Goto Deco Sound to know more.

Lindsay's Wheel of Acoustics, which shows fields within acoustics

Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.

Hearing is one of the most crucial means of survival in the animal world and speech is one of the most distinctive characteristics of human development and culture. Accordingly, the science of acoustics spreads across many facets of human society&#;music, medicine, architecture, industrial production, warfare and more. Likewise, animal species such as songbirds and frogs use sound and hearing as a key element of mating rituals or for marking territories. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge. Robert Bruce Lindsay's "Wheel of Acoustics" is a well accepted overview of the various fields in acoustics.[1]

History

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Etymology

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The word "acoustic" is derived from the Greek word &#;κουστικός (akoustikos), meaning "of or for hearing, ready to hear"[2] and that from &#;κουστός (akoustos), "heard, audible",[3] which in turn derives from the verb &#;κούω(akouo), "I hear".[4]

The Latin synonym is "sonic", after which the term sonics used to be a synonym for acoustics[5] and later a branch of acoustics.[5] Frequencies above and below the audible range are called "ultrasonic" and "infrasonic", respectively.

Early research in acoustics

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The fundamental and the first 6 overtones of a vibrating string. The earliest records of the study of this phenomenon are attributed to the philosopher Pythagoras in the 6th century BC.

In the 6th century BC, the ancient Greek philosopher Pythagoras wanted to know why some combinations of musical sounds seemed more beautiful than others, and he found answers in terms of numerical ratios representing the harmonic overtone series on a string. He is reputed to have observed that when the lengths of vibrating strings are expressible as ratios of integers (e.g. 2 to 3, 3 to 4), the tones produced will be harmonious, and the smaller the integers the more harmonious the sounds. For example, a string of a certain length would sound particularly harmonious with a string of twice the length (other factors being equal). In modern parlance, if a string sounds the note C when plucked, a string twice as long will sound a C an octave lower. In one system of musical tuning, the tones in between are then given by 16:9 for D, 8:5 for E, 3:2 for F, 4:3 for G, 6:5 for A, and 16:15 for B, in ascending order.[6]

Aristotle (384&#;322 BC) understood that sound consisted of compressions and rarefactions of air which "falls upon and strikes the air which is next to it...",[7][8] a very good expression of the nature of wave motion. On Things Heard, generally ascribed to Strato of Lampsacus, states that the pitch is related to the frequency of vibrations of the air and to the speed of sound.[9]

In about 20 BC, the Roman architect and engineer Vitruvius wrote a treatise on the acoustic properties of theaters including discussion of interference, echoes, and reverberation&#;the beginnings of architectural acoustics.[10] In Book V of his De architectura (The Ten Books of Architecture) Vitruvius describes sound as a wave comparable to a water wave extended to three dimensions, which, when interrupted by obstructions, would flow back and break up following waves. He described the ascending seats in ancient theaters as designed to prevent this deterioration of sound and also recommended bronze vessels (echea) of appropriate sizes be placed in theaters to resonate with the fourth, fifth and so on, up to the double octave, in order to resonate with the more desirable, harmonious notes.[11][12][13]

During the Islamic golden age, Abū Rayhān al-Bīrūnī (973&#;) is believed to have postulated that the speed of sound was much slower than the speed of light.[14][15]

Principles of acoustics have been applied since ancient times: a Roman theatre in the city of Amman

The physical understanding of acoustical processes advanced rapidly during and after the Scientific Revolution. Mainly Galileo Galilei (&#;) but also Marin Mersenne (&#;), independently, discovered the complete laws of vibrating strings (completing what Pythagoras and Pythagoreans had started years earlier). Galileo wrote "Waves are produced by the vibrations of a sonorous body, which spread through the air, bringing to the tympanum of the ear a stimulus which the mind interprets as sound", a remarkable statement that points to the beginnings of physiological and psychological acoustics. Experimental measurements of the speed of sound in air were carried out successfully between and by a number of investigators, prominently Mersenne. Meanwhile, Newton (&#;) derived the relationship for wave velocity in solids, a cornerstone of physical acoustics (Principia, ).

Age of Enlightenment and onward

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Substantial progress in acoustics, resting on firmer mathematical and physical concepts, was made during the eighteenth century by Euler (&#;), Lagrange (&#;), and d'Alembert (&#;). During this era, continuum physics, or field theory, began to receive a definite mathematical structure. The wave equation emerged in a number of contexts, including the propagation of sound in air.[16]

In the nineteenth century the major figures of mathematical acoustics were Helmholtz in Germany, who consolidated the field of physiological acoustics, and Lord Rayleigh in England, who combined the previous knowledge with his own copious contributions to the field in his monumental work The Theory of Sound (). Also in the 19th century, Wheatstone, Ohm, and Henry developed the analogy between electricity and acoustics.

The twentieth century saw a burgeoning of technological applications of the large body of scientific knowledge that was by then in place. The first such application was Sabine's groundbreaking work in architectural acoustics, and many others followed. Underwater acoustics was used for detecting submarines in the first World War. Sound recording and the played important roles in a global transformation of society. Sound measurement and analysis reached new levels of accuracy and sophistication through the use of electronics and computing. The ultrasonic frequency range enabled wholly new kinds of application in medicine and industry. New kinds of transducers (generators and receivers of acoustic energy) were invented and put to use.

Definition

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Acoustics is defined by ANSI/ASA S1.1- as "(a) Science of sound, including its production, transmission, and effects, including biological and psychological effects. (b) Those qualities of a room that, together, determine its character with respect to auditory effects."

The study of acoustics revolves around the generation, propagation and reception of mechanical waves and vibrations.

The steps shown in the above diagram can be found in any acoustical event or process. There are many kinds of cause, both natural and volitional. There are many kinds of transduction process that convert energy from some other form into sonic energy, producing a sound wave. There is one fundamental equation that describes sound wave propagation, the acoustic wave equation, but the phenomena that emerge from it are varied and often complex. The wave carries energy throughout the propagating medium. Eventually this energy is transduced again into other forms, in ways that again may be natural and/or volitionally contrived. The final effect may be purely physical or it may reach far into the biological or volitional domains. The five basic steps are found equally well whether we are talking about an earthquake, a submarine using sonar to locate its foe, or a band playing in a rock concert.

The central stage in the acoustical process is wave propagation. This falls within the domain of physical acoustics. In fluids, sound propagates primarily as a pressure wave. In solids, mechanical waves can take many forms including longitudinal waves, transverse waves and surface waves.

Acoustics looks first at the pressure levels and frequencies in the sound wave and how the wave interacts with the environment. This interaction can be described as either a diffraction, interference or a reflection or a mix of the three. If several media are present, a refraction can also occur. Transduction processes are also of special importance to acoustics.

Fundamental concepts

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Wave propagation: pressure levels

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Spectrogram of a young girl saying "oh, no"

In fluids such as air and water, sound waves propagate as disturbances in the ambient pressure level. While this disturbance is usually small, it is still noticeable to the human ear. The smallest sound that a person can hear, known as the threshold of hearing, is nine orders of magnitude smaller than the ambient pressure. The loudness of these disturbances is related to the sound pressure level (SPL) which is measured on a logarithmic scale in decibels.

Wave propagation: frequency

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Physicists and acoustic engineers tend to discuss sound pressure levels in terms of frequencies, partly because this is how our ears interpret sound. What we experience as "higher pitched" or "lower pitched" sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time, and then presented in more meaningful forms such as octave bands or time frequency plots. Both of these popular methods are used to analyze sound and better understand the acoustic phenomenon.

The entire spectrum can be divided into three sections: audio, ultrasonic, and infrasonic. The audio range falls between 20 Hz and 20,000 Hz. This range is important because its frequencies can be detected by the human ear. This range has a number of applications, including speech communication and music. The ultrasonic range refers to the very high frequencies: 20,000 Hz and higher. This range has shorter wavelengths which allow better resolution in imaging technologies. Medical applications such as ultrasonography and elastography rely on the ultrasonic frequency range. On the other end of the spectrum, the lowest frequencies are known as the infrasonic range. These frequencies can be used to study geological phenomena such as earthquakes.

Analytic instruments such as the spectrum analyzer facilitate visualization and measurement of acoustic signals and their properties. The spectrogram produced by such an instrument is a graphical display of the time varying pressure level and frequency profiles which give a specific acoustic signal its defining character.

Transduction in acoustics

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An inexpensive low fidelity 3.5 inch driver, typically found in small radios

A transducer is a device for converting one form of energy into another. In an electroacoustic context, this means converting sound energy into electrical energy (or vice versa). Electroacoustic transducers include loudspeakers, microphones, particle velocity sensors, hydrophones and sonar projectors. These devices convert a sound wave to or from an electric signal. The most widely used transduction principles are electromagnetism, electrostatics and piezoelectricity.

The transducers in most common loudspeakers (e.g. woofers and tweeters), are electromagnetic devices that generate waves using a suspended diaphragm driven by an electromagnetic voice coil, sending off pressure waves. Electret microphones and condenser microphones employ electrostatics&#;as the sound wave strikes the microphone's diaphragm, it moves and induces a voltage change. The ultrasonic systems used in medical ultrasonography employ piezoelectric transducers. These are made from special ceramics in which mechanical vibrations and electrical fields are interlinked through a property of the material itself.

Acoustician

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An acoustician is an expert in the science of sound.[17]

Education

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There are many types of acoustician, but they usually have a Bachelor's degree or higher qualification. Some possess a degree in acoustics, while others enter the discipline via studies in fields such as physics or engineering. Much work in acoustics requires a good grounding in Mathematics and science. Many acoustic scientists work in research and development. Some conduct basic research to advance our knowledge of the perception (e.g. hearing, psychoacoustics or neurophysiology) of speech, music and noise. Other acoustic scientists advance understanding of how sound is affected as it moves through environments, e.g. underwater acoustics, architectural acoustics or structural acoustics. Other areas of work are listed under subdisciplines below. Acoustic scientists work in government, university and private industry laboratories. Many go on to work in Acoustical Engineering. Some positions, such as Faculty (academic staff) require a Doctor of Philosophy.

Subdisciplines

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Archaeoacoustics

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St. Michael's Cave

Archaeoacoustics, also known as the archaeology of sound, is one of the only ways to experience the past with senses other than our eyes.[18] Archaeoacoustics is studied by testing the acoustic properties of prehistoric sites, including caves. Iegor Rezkinoff, a sound archaeologist, studies the acoustic properties of caves through natural sounds like humming and whistling.[19] Archaeological theories of acoustics are focused around ritualistic purposes as well as a way of echolocation in the caves. In archaeology, acoustic sounds and rituals directly correlate as specific sounds were meant to bring ritual participants closer to a spiritual awakening.[18] Parallels can also be drawn between cave wall paintings and the acoustic properties of the cave; they are both dynamic.[19] Because archaeoacoustics is a fairly new archaeological subject, acoustic sound is still being tested in these prehistoric sites today.

Aeroacoustics

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Aeroacoustics is the study of noise generated by air movement, for instance via turbulence, and the movement of sound through the fluid air. This knowledge was applied in the s and '30s to detect aircraft before radar was invented and is applied in acoustical engineering to study how to quieten aircraft. Aeroacoustics is important for understanding how wind musical instruments work.[20]

Acoustic signal processing

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Acoustic signal processing is the electronic manipulation of acoustic signals. Applications include: active noise control; design for hearing aids or cochlear implants; echo cancellation; music information retrieval, and perceptual coding (e.g. MP3 or Opus).[21]

Architectural acoustics

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Architectural acoustics (also known as building acoustics) involves the scientific understanding of how to achieve good sound within a building.[22] It typically involves the study of speech intelligibility, speech privacy, music quality, and vibration reduction in the built environment.[23] Commonly studied environments are hospitals, classrooms, dwellings, performance venues, recording and broadcasting studios. Focus considerations include room acoustics, airborne and impact transmission in building structures, airborne and structure-borne noise control, noise control of building systems and electroacoustic systems.[24]

Bioacoustics

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Bioacoustics is the scientific study of the hearing and calls of animal calls, as well as how animals are affected by the acoustic and sounds of their habitat.[25]

Electroacoustics

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This subdiscipline is concerned with the recording, manipulation and reproduction of audio using electronics.[26] This might include products such as mobile phones, large scale public address systems or virtual reality systems in research laboratories.

Environmental noise and soundscapes

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Environmental acoustics is concerned with noise and vibration caused by railways,[27] road traffic, aircraft, industrial equipment and recreational activities.[28] The main aim of these studies is to reduce levels of environmental noise and vibration. Research work now also has a focus on the positive use of sound in urban environments: soundscapes and tranquility.[29]

Musical acoustics

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The primary auditory cortex, one of the main areas associated with superior pitch resolution

Musical acoustics is the study of the physics of acoustic instruments; the audio signal processing used in electronic music; the computer analysis of music and composition, and the perception and cognitive neuroscience of music.[30]

Noise

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The goal this acoustics sub-discipline is to reduce the impact of unwanted sound. Scope of noise studies includes the generation, propagation, and impact on structures, objects, and people.

• Innovative model development
• Measurement techniques
• Mitigation strategies
• Input to the establishment of standards and regulations

Noise research investigates the impact of noise on humans and animals to include work in definitions, abatement, transportation noise, hearing protection, Jet and rocket noise, building system noise and vibration, atmospheric sound propagation, soundscapes, and low-frequency sound.

Psychoacoustics

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Many studies have been conducted to identify the relationship between acoustics and cognition, or more commonly known as psychoacoustics, in which what one hears is a combination of perception and biological aspects.[31] The information intercepted by the passage of sound waves through the ear is understood and interpreted through the brain, emphasizing the connection between the mind and acoustics. Psychological changes have been seen as brain waves slow down or speed up as a result of varying auditory stimulus which can in turn affect the way one thinks, feels, or even behaves.[32] This correlation can be viewed in normal, everyday situations in which listening to an upbeat or uptempo song can cause one's foot to start tapping or a slower song can leave one feeling calm and serene. In a deeper biological look at the phenomenon of psychoacoustics, it was discovered that the central nervous system is activated by basic acoustical characteristics of music.[33] By observing how the central nervous system, which includes the brain and spine, is influenced by acoustics, the pathway in which acoustic affects the mind, and essentially the body, is evident.[33]

Speech

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Acousticians study the production, processing and perception of speech. Speech recognition and Speech synthesis are two important areas of speech processing using computers. The subject also overlaps with the disciplines of physics, physiology, psychology, and linguistics.[34]

Structural Vibration and Dynamics

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Structural acoustics is the study of motions and interactions of mechanical systems with their environments and the methods of their measurement, analysis, and control. There are several sub-disciplines found within this regime:

Applications might include: ground vibrations from railways; vibration isolation to reduce vibration in operating theatres; studying how vibration can damage health (vibration white finger); vibration control to protect a building from earthquakes, or measuring how structure-borne sound moves through buildings.[35]

Ultrasonics

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Ultrasound image of a fetus in the womb, viewed at 12 weeks of pregnancy (bidimensional-scan)

Ultrasonics deals with sounds at frequencies too high to be heard by humans. Specialisms include medical ultrasonics (including medical ultrasonography), sonochemistry, ultrasonic testing, material characterisation and underwater acoustics (sonar).[36]

Contact us to discuss your requirements of Acoustic Panels. Our experienced sales team can help you identify the options that best suit your needs.

Underwater acoustics

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Underwater acoustics is the scientific study of natural and man-made sounds underwater. Applications include sonar to locate submarines, underwater communication by whales, climate change monitoring by measuring sea temperatures acoustically, sonic weapons,[37] and marine bioacoustics.[38]

Research

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Professional societies

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Academic journals

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Conferences

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See also

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References

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Further reading

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Introduction to Acoustics

© Fred Tepfer
Bailey Avenue Eugene, OR
non-commercial use freely granted

Contents

1. Importance of Acoustics
2. Notable Failures
3. The Nature of Sound
4. Absorption and Reflection of Sound
5. Transmission of Sound Between Rooms
6. Amplification of Sound
7. Myths and Truths
8. A Pattern for Classroom Acoustics
9. Glossary
10. Other Sources

1. Importance of Acoustics

Acoustics are fundamentally important to learning environments. Learning is intrinsically linked with communication, and aural (sound) communication is acoustics. Similarly, learning is about concentration, and external noise is a major distracting factor in education. This article is about typical classroom environments, up to about 1,000 square feet. Large specialized rooms like auditoriums, gyms, and cafeterias needs careful acoustical engineering and should not be designed using the rules of thumb described below.

The importance of acoustics is not limited to classrooms. Noise in corridors and public spaces can soar if they are too reverberant (too much echo), with voices raised louder and louder to overcome the background echo, just like shouting conversations at a noisy cocktail party or restaurant. In addition, sound is an important navigational tool for people who are blind or low vision, and either end of the reverberation scale (too "live" or reverberant, or too "dead" or absorptive) can prevent them from finding their way.

2. Notable Failures

Most of us are familiar with the "schools without walls" of the 's. They were based on assumptions about teaching and acoustics that in most cases were not valid. The school without walls is based on the open office environment ["Dilbert" cubicles]. These are moderately successful because of the major acoustical difference between individuals working alone in the offices. However, in the school, groups of people are expected to communicate nearly constantly in the same acoustical environment, which is quite a different acoustical challenge. Solutions appropriate to creating privacy are often inappropriate in learning environments. Beware especially of architects and vendors who think in terms of an office environment while talking about classrooms. The "School Zone" part of the Armstrong Ceilings web site is one example of this. That approach can result in adequate but not excellent learning environments.

[picture of kid moving an accordian-type partition]
What's wrong with this picture? If a wall is light enough to be moved by a child, it is not going to be an effective barrier for sound.

3. The Nature of Sound

Sound is created by vibrations of air or other materials. When someone speaks, their vocal cords vibrate, which creates vibrations in the air that travel to the ears of the listeners much as waves travel across a pond when you throw in a stone. When the sound is higher, the waves are closer together (higher frequency sound) and when the sound is lower, the waves are farther apart. Longer waves (lower sound) pass through thinner materials and curve more easily around barriers. Shorter waves (higher sound) are reflected by relatively thin materials and don't bend much around barriers. Nearly all spoken sound is in the range of 125 Hz (cycles per second) to 4,000 Hz, although people can hear from about 20 Hz to 20,000 Hz. All sound waves carry well through open air, or even through small holes and cracks in walls and ceilings. Because of the logarithmic nature of sound, a small hole will let through a lot of sound.

"Noise" is just unwanted sound, and "signal" is what you are trying to hear. In every sonic envirnment there is background noise, and if the signal isn't much louder than the noise, you will have trouble hearing. In a large room with several groups of students all in conversation, unless enough sound is absorbed instead of reflected off of walls and passed on to the next student, the overall effect is a lot of noise. Typical school cafeterias are built without much (expensive) sound absorbing material, so they are very noisy.

4. Absorption and Reflection of Sound

Sound waves can be reflected or absorbed, and the science of acoustics is largely about what to reflect (send back into the room, what to transmit (sent to the next room), and what to absorb (turn into heat energy). Environments for music want more reverberation, enough to "warm" the sound with reflections. If too much is absorbed, less sound reaches the audience and it sounds "dry" or "dead" , the musicians need to work harder, and the lack of reverberation makes the slightest error more apparent. By contrast, environments for speech want less reverberation, although moderate amounts of reflection are useful to reinforce the sound as long as the overall time that it takes a sound to decay (or die away) isn't too long. A desirable reverberation time for classrooms is about .75 seconds for interactive (discussion-based) spaces and 1.0 seconds for lecture halls. By contrast, a symphony hall might have a two second reverb time. My personal preference for classrooms is toward the reverberant end of what is considered acceptable.

Education is speech-based, whether solely from presenter to listener or a discussion among a whole classroom of students, so the nature of speech informs acoustical design for classrooms. Speech is made up of vowels, which are sounds near the lower end of speech frequencies ("oo", "uh", "ah", etc.), and consonants, most of which are in the upper part of the speech range ("t", "s", "k", etc.).

When we abbreviate written language we remove vowels yet retain meaning, but if we remove only consonants, the sense is usually lost. Take the word baseball, for example, whose consonants are bsbll, still recognizable, and whose vowells are aea, which we don't recognize as being related to the word "baseball". Similarly, if you turn up the treble and turn off the bass in an audio system, speech remains intelligible (try it!). However, if you turn off the treble and turn up the bass, speech becomes a muddy mess.

This suggests that classroom acoustics needs to absorb more in lower ranges of the speech window than in the higher ranges . So how do some common classroom materials perform? NRC, or noise reduction coefficient, is the average the absorption at certain frequencies and is the rating touted by interior materials manufacturers. NRC is a very imperfect indicator acoustical performance. As an average rating, it tells you nothing about which parts of the sound spectrum are being absorbed. Here is a more detailed look broken out by frequency, with higher numbers absorbing more sound, lower numbers absorbing less.

Coefficients of absorption

(table still under construction)         Material

500 Hz

Hz

Hz

Hz

NRC

carpet (glued down)

.14

.37

.60

.65

concrete block (coarse, bare)

.31

.29

.39

.25

concrete block (painted)

.06

.07

.09

.08

gypsum wall board

.05

.04

.07

.09

wood fiber tile glued to ceiling

5/8" mineral fiber ceiling tile

.55

3/4" mineral fiber ceiling tile

.70

1"  6 lb/cubic ft. fiberglass ceiling tile

.70

.93

.98

1.03

.90

 

Many acoustical materials are optimized for office environments, such as glued-down carpet, acoustical panels, and relatively inexpensive mineral ceiling tile. This environment is designed to provide speech privacy by muddying consonants (high frequency sound) and not absorbing vowes (low frequencies), making speech unintelligible a short distance away. This is the REVERSE of good classroom design where you want to reinforce speech intelligibility. Keep reading for a pattern for excellent classroom acoustics for learning

Absorption of sound is particularly difficult in special environments like cafeterias, kitchens, gymnasiums, and swimming pools. Conventional materials may be subject to damage, or absorb odors, or be incompatible in other ways. However, materials do exist that work well. For example, for a gym, walls can be built of a special slotted concrete block. Because the absorption is inside the block's core, no amount of ball impact can compromise its integrity.

5. Transmission of Sound Between Rooms

The control of noise from one room to another is the other major challenge in acoustics. As with absorption, different materials transmit more or less sound at different frequencies. In transmission, blocking the entire speech range is important, and this factor is reflected in the STC rating (Sound Transmission Class) of a wall. Unlike the NRC, the STC takes into account the performance of the wall at its worst frequency. The STC value of a wall is a relatively reliable relative indicator of the number of decibels of attenuation that can be expected from a wall system. STC values are a very useful tool in acoustical design but should be downrated for actual field conditions, as even the best installation will never match the lab rating.

Critical to sound transmission issues is the background noise in the receiving room. If the background noise is higher than the amount of sound passing through (and around) the wall, then users won't hear the sound from the adjacent room. If background is lower than what's transmitted, then room occupants will hear sounds from the adjoining space. This tells us that some acoustical problems can be overcome by increasing background sound, but that can cause problems as people need to raise their voices to be heard.

Transmission tips:

• In general, materials with more mass block sound more than lighter materials, especially for low frequencies (which are very hard to block)
• Sound likes to travel around barriers. To be fully effective, walls should go from floor to the structure above, and holes should be carefully caulked or filled. Even small holes make a big difference. Most transmission failures are caused by cracks and holes.
• Batt insulation is useful, but it has to be installed very carefully or the effectiveness is compromised. It's best to have it inspected by someone who understands this before walls are covered up. Unlike thermal insulation, even minor gaps, cracks, and other installation problems result in a major reduction in effectiveness.
• Making the wall finishes of different thicknesses/masses helps stop sound waves, perhaps by adding an extra layer of gypsum board to one side of the wall.
• More effective than batt insulation is decoupling of the two faces of the wall by having two sets of studs, one for each wall surface (staggered studs). Resilient channels can be used with similar effects

Wall with staggered studs

6. Amplification of Sound

Sound amplification goes hand in hand with education in this day and age. Most media now include sound (videos, CD/DVD, Internet content, etc.), so a room needs to provide for and be friendly to a loudspeaker system. On the other hand, except for large rooms not commonly encountered in K-12 education, voice amplification is not normally used, with two significant exceptions:

• If room users include one or more people who are hard of hearing, they may use an assistive listening system that amplifies the voice of the speaker, and can be coordinated with sound output from multimedia sources. These systems can be built in or may be portable. They are effective for one-way communication but can be challenging in a discussion or group project setting.
• Some schools have experimented with amplifying the teacher's voice through ceiling-mounted loudspeakers. While quite effective as a presentation tool, this does nothing for discussions, and runs contrary to much of the current direction of education toward more interaction and involvement of the students. Furthermore, many of these systems use low quality microphones and loudspeakers that distort the sound. My personal opinion is that this is not a generally effective solution.

7. Myths and Truths

Myth: control noise just by installing carpet. Carpet is not an acoustical panacea. It is effective for reducing noise from feet and movement of furniture. However, for absorbing noise, the carpet system commonly used in schools (glued down) absorbs mostly in the high frequencies of speech, i.e. the consanants. Padding the feet of furniture can help with furniture noise, but doesn't help with foot noise.

Myth: Ceiling tile is enough. Acoustical ceiling tile, either glued to the ceiling (usually in 12" squares) or sitting loosely in the suspended steel grid (usually 2 foot by 2 foot or 2 foot by 4 foot), can be a very effective means of absorbing sound. However,there are many types of tile with different absorption characteristics (wood fiber, mineral fiber, glass fiber, etc.). To ensure effectiveness, someone has to match the acoustical problem to the solution. Furthermore, nearly all ceiling tile is ruined by painting unless very special paints are used with great care. Many older schools develop acoustical problems when acoustical tile is painted after it gets dirty or stained or just looks old. "Best Practices" classroom acoustical design for classroom interaction usually makes the center of the ceiling reflective, and provides high-performance acoustical tile near the perimeter. Misplaced or poorly selected acoustical material can require louder voices and ultimately vocal stress.

Truth: Heating and cooling systems can be a major acoustical problem. HVAC systems can contribute to noise three ways.

1. The heating/ventilating equipment, the fans and terminal units, make noise. This noise can be transferred directly, usually to the spaces under, over, or next to the unit.
2. HVAC systemscan also created noise through the distribution system because of excess air or water velocity (pipes or ducts too small) or through poor workmanship (such as small holes that create hisses and whistles).
3. Air ducts can also create a path for sound to travel between rooms, solved only with expensive sound traps (which also increase fan energy consumption). For this reason, ducts should generally not go from room to room but should be branched off from a main in the corridor.

Truth: Outdoor noise can be a significant problem. The location of a school can be a significant determinant of acoustical noise problems, especially if buildling users are expected to open windows. Locations near major roads, airports, or any other source of noise can be a major problem, in some cases even with the windows closed. Measurement of ambient noise should be a part of site selection for a school.

8. Pattern

For a typical classroom, keep the head wall (front) reflective with hard surfaces. Provide absorptive materials around the sides and back of the ceiling and the upper part of the wall, using a high NC material (.8 or higher) such as one inch thick compressed fiberglass. Provide a relatively non-absorbent material in the middle of the ceiling and on the rest of the wall surfaces. Consider using carpet to reduce foot and furniture noise (or use other methods). Also consider a sloping ceiling at the front of the classroom to reflect more sound out into the room.

[diagram and link to nonoise.org article]

Other sources to consult:

The best classroom acoustics source: http://www.nonoise.org/library/classroom/
A good source: http://www.mbiproducts.com/roomacoustics.html
Dictionary of terms: http://www.armstrong.com/commceilingsna/glossary.html

Glossary

Selected glossary items:

Absorption In acoustics, the energy of sound waves being taken in and trapped within a material rather than being bounced off or reflected. Materials are rated in terms of their ability to absorb sounds.

Articulation Class (AC) Rates the listener's ability to understand the spoken work within a space, expressed as a decimal with 1.0 being perfectly understandable. The privacy index is derived from the A1 calculation. Lower A1 ratings (less than 0.2) indicate that adjacent spoken words are less intelligible, therefore less distracting. The sum of the weighted sound attenuations in a series of 15 test bands. Note: AC has replaced Noise Isolation Class (NIC) as the accepted industry standard performance value. NIC is based on hearing sensitivity rather than discernment of actual speech, which is the primary concern in open office layouts prevalent in acoustical design work. Verify the rating methodolgy with manufacturer's published data.

Ceiling Attenuation Class (CAC) Rates a ceiling's efficiency as a barrier to airborne sound transmission between adjacent closed offices. Shown as a minimum value, previously expressed as CSTC (Ceiling Sound Transmission Class). A single-figure rating derived from the normalized ceiling attenuation values in accordance with classification ASTM E 413, except that the resultant rating shall be designated ceiling attenuation class. (Defined in ASTM E .) An acoustical unit with a high CAC may have a low NRC.

DBA (A-weighted decibel) A single-number measurement based on the decibel but weighted to approximate the response of the human ear with respect to frequencies.

Decibel (dB) - A unit to express differences in power. In acoustics, equal to ten times the logarithm of the ratio of one sound and lower-intensity reference sound. One decibel indicates a difference of about 26% and is about the smallest change the ear can detect. The dB level is a logarithm quantity; the maximum normal level is approximately 120dB

Fiberglass Panels - Glass strands laid in mats and formed into a rigid or semi-rigid board, sometimes requiring a separate stable material laminated to the fiberglass.

Mineral Wool - A man-made wool-like material of fine inorganic fibers made from slag, used as loose fill or formed into blanket, batt, clock, board, or slab shapes for thermal and acoustical insulation.

Noise Reduction Coefficient (NRC) - Average sound absorption coefficient measured at four frequencies: 250, 500, 1,000 and 2,000 Hz which rates how well a ceiling or wall panel absorbs sound. NRC is the fraction of sound energy, averaged over all angles of direction and from low to high sound frequencies that is absorbed and not reflected.

Reverberation Time - Time required for a sound to decay to a value one millionth of its original intensity or to drop 60 decibels.

Sound Transmission Class (STC) - A single-number rating of a wall or ceiling's efficiency as a barrier to airborne sound at 16 speech frequencies from 125 to Hz. STC is a decibel measure of the difference between the sound energy striking the panel or construction on one side and the sound energy transmitted from the other side.

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