METHODS OF ACOUSTICAL TREATMENT OF SPACES REVERBERATION TIME, ECHO ETC. STUDY OF ACOUSTIC MATERIALS

AR-461: BUILDING SCIENCE

By:

RAVINDAR KUMAR

Assistant Professor

Department of Architecture and Planning

NED University of Engineering and Technology

Karachi

LECTURE NO. 16

TOPIC:  METHODS OF ACOUSTICAL TREATMENT OF SPACES REVERBERATION TIME, ECHO ETC. STUDY OF ACOUSTIC MATERIALS

INTRODUCTION:

Acoustic Treatment[1] is applying sound or noise reducing products to floors, walls, and ceilings to control sound and create a more pleasant environment.  Many large spaces such as gyms, pools, cafeterias, and multi-purpose rooms may have hard surfaces that may be easy to clean, but reflect noise and create problems acoustically.  Garbled speech, echo, slap back and other issues arise in these types of spaces when acoustics are not addressed.  An acoustical treatment for these spaces would be to add the correct amount of acoustical absorption to absorb unwanted, reverberated noise.

Flat, hard surfaces such as concrete block, drywall gypsum board, and metal decking reflect sound.  When a large space is constructed of these materials, sound will reflect off of multiple surfaces and reverberate throughout the space.  The amount of time it takes for the sound to reduce by 30 dB is called the reverberation time.  This time is used as a gauge to provide an idea of how much reverberation is in the space.  For any room other than performance spaces, above 3 second reverberation time is high. 

All building materials have properties of acoustical absorption measured in sabins at third octave frequencies.  Using the length x width x height of a space to determine the room volume and calculating the total acoustical absorption in the room using the total square footage of each building material multiplied by the acoustical absorption value in sabins provides you with the current reverberation time of the space.  Next you calculate how much acoustical absorption needs to be added to the room to bring the reverberation time down to acceptable levels.  We use the square footage number as a base line for design of the acoustic treatment.

SOUNDPROOFING:

Soundproofing[2] is any means of reducing the sound pressure with respect to a specified sound source and receptor (noise control). There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.

Two distinct soundproofing problems may need to be considered when designing acoustic treatments - to improve the sound within a room, and reduce sound leakage to/from adjacent rooms or outdoors. Soundproofing can suppress unwanted indirect sound waves such as reflections that cause echoes and resonances that cause reverberation. Soundproofing can reduce the transmission of unwanted direct sound waves from the source to an involuntary listener through the use of distance and intervening objects in the sound path.

DISTANCE:

The energy density of sound waves decreases as they spread out, so that increasing the distance between the receiver and source results in a progressively lesser intensity of sound at the receiver. In a normal three dimensional setting, with a point source and point receptor, the intensity of sound waves will be attenuated according to the inverse square of the distance from the source.

DAMPING:

Damping means to reduce resonance in the room, by absorption or redirection (reflection or diffusion). Absorption will reduce the overall sound level, whereas redirection makes unwanted sound harmless or even beneficial by reducing coherence. Damping can reduce the acoustic resonance in the air, or mechanical resonance in the structure of the room itself or things in the room.

ABSORPTION:

Absorbing sound spontaneously converts part of the sound energy to a very small amount of heat in the intervening object (the absorbing material), rather than sound being transmitted or reflected. There are several ways in which a material can absorb sound. The choice of sound absorbing material will be determined by the frequency distribution of noise to be absorbed and the acoustic absorption profile required.

POROUS ABSORBERS:

Porous absorbers, typically open cell rubber foams or melamine sponges, absorb noise by friction within the cell structure. Porous open cell foams are highly effective noise absorbers across a broad range of medium-high frequencies. Performance is less impressive at low frequencies. The exact absorption profile of porous open cell foam will be determined by a number of factors including the following:

• Cell size
• Torosity
• Porosity
• Material thickness
• Material density
• Resonant absorbers
• Resonant panels, Helmholtz resonators and other resonant absorbers work by damping a sound wave as they reflect it.

Unlike porous absorbers, resonant absorbers are most effective at low-medium frequencies and the absorption of resonant absorbers is always matched to a narrow frequency range.

REFLECTION:

In an outdoor environment such as highway engineering, embankments or panelling are often used to reflect sound upwards into the sky.

DIFFUSION:

If a specular reflection from a hard flat surface is giving a problematic echo then an acoustic diffuser may be applied to the surface. It will scatter sound in all directions.

ROOM WITHIN A ROOM:

A room within a room (RWAR) is one method of isolating sound and stopping it from transmitting to the outside world where it may be undesirable.

Most vibration / sound transfer from a room to the outside occurs through mechanical means. The vibration passes directly through the brick, woodwork and other solid structural elements. When it meets with an element such as a wall, ceiling, floor or window, which acts as a sounding board, the vibration is amplified and heard in the second space. A mechanical transmission is much faster, more efficient and may be more readily amplified than an airborne transmission of the same initial strength.

The use of acoustic foam and other absorbent means is less effective against this transmitted vibration. The user is advised to break the connection between the room that contains the noise source and the outside world. This is called acoustic de-coupling. Ideal de-coupling involves eliminating vibration transfer in both solid materials and in the air, so air-flow into the room is often controlled. This has safety implications, for example proper ventilation must be assured and gas heaters cannot be used inside de-coupled space.

NOISE CANCELLATION:

Noise cancellation generators for active noise control are a relatively modern innovation. A microphone is used to pick up the sound that is then analyzed by a computer; then, sound waves with opposite polarity (180° phase at all frequencies) are output through a speaker, causing destructive interference and cancelling much of the noise.

RESIDENTIAL SOUNDPROOFING:

Residential soundproofing aims to decrease or eliminate the effects of exterior noise. The main focus of residential soundproofing in existing structures is the windows. Curtains can be used to damp sound either through use of heavy materials or through the use of air chambers known as honeycombs. Single-, double- and triple-honeycomb designs achieve relatively greater degrees of sound damping. The primary soundproofing limit of curtains is the lack of a seal at the edge of the curtain. Double-pane windows achieve somewhat greater sound damping than single-pane windows. Significant noise reduction can be achieved by installing a second interior window. In this case the exterior window remains in place while a slider or hung window is installed within the same wall openings.[citation needed]

NOISE BARRIERS AS EXTERIOR SOUNDPROOFING:

Since the early 1970s, it has become common practice in the United States (followed later by many other industrialized countries) to engineer noise barriers along major highways to protect adjacent residents from intruding roadway noise. The technology exists to predict accurately the optimum geometry for the noise barrier design. Noise barriers may be constructed of wood, masonry, earth or a combination thereof. One of the earliest noise barrier designs was in Arlington, Virginia adjacent to Interstate 66, stemming from interests expressed by the Arlington Coalition on Transportation. Possibly the earliest scientifically designed and published noise barrier construction was in Los Altos, California in 1970.

SOUND DIFFUSORS:[3]

Diffusors (or diffusers) are used to treat sound aberrations in rooms such as echoes. They are an excellent alternative or complement to sound adsorption because they do not remove sound energy, but can be used to effectively reduce distinct echoes and reflections while still leaving a live sounding space. Compared to a reflective surface, which will cause most of the energy to be reflected off at an angle equal to the angle of incidence, a diffusor will cause the sound energy to be radiated in many directions, hence leading to a more diffusive acoustic space. It is also important that a diffusor spreads reflections in time as well as spatially. Diffusors can aid sound diffusion, but this is not why they are used in many cases; they are more often used to remove coloration and echoes.

SOUNDING BOARD:[4] 

A sounding board is a structure placed above or behind a pulpit or other speaking platform which helps to project the sound of the speaker. The structure may be specially shaped to assist the projection, for example, being formed as a parabolic reflector. In the typical setting of a church, the sounding board may be ornately carved or constructed. It is also called a tester. The term may also be used figuratively to describe a person who listens to a speech or proposal in order that the speaker may rehearse or explore the proposition more fully. The term is also used inter-personally to reference one person listening to another person, especially their ideas. When a person listens intently and provides feedback, they provide perspective that otherwise would not be available through introspection or thought alone.

ACTIVE NOISE CONTROL:[5]

Active noise control (ANC) (also known as noise cancellation, active noise reduction (ANR) or antinoise) is a method for reducing unwanted sound. Sound is a pressure wave, which consists of a compression phase and a rarefaction phase. A noise-cancellation speaker emits a sound wave with the same amplitude but with inverted phase (also known as anti-phase) to the original sound. The waves combine to form a new wave, in a process called interference, and effectively cancel each other out - an effect which is called phase cancellation. Depending on the circumstances and the method used, the resulting sound wave may be so faint as to be inaudible to human ears.

A noise-cancellation speaker may be co-located with the sound source to be attenuated. In this case it must have the same audio power level as the source of the unwanted sound. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. the user's ear). This requires a much lower power level for cancellation but is effective only for a single user. Noise cancellation at other locations is more difficult as the three dimensional wave fronts of the unwanted sound and the cancellation signal could match and create alternating zones of constructive and destructive interference. In small enclosed spaces (e.g. the passenger compartment of a car) such global cancellation can be achieved via multiple speakers and feedback microphones, and measurement of the modal responses of the enclosure.

Modern active noise control is achieved through the use of a computer, which analyzes the waveform of the background aural or non-aural noise, then generates a signal reversed waveform to cancel it out by interference. This waveform has identical or directly proportional amplitude to the waveform of the original noise, but its signal is inverted. This creates the destructive interference that reduces the amplitude of the perceived noise. The active methods (this) differ from passive noise control methods (soundproofing) in that a powered system is involved, rather than unpowered methods such as insulation, sound-absorbing ceiling tiles or muffler. The advantages of active noise control methods compared to passive ones are that they are generally:

• More effective at low frequencies.
• Less bulky.
• Able to block noise selectively.

ACOUSTIC FOAM:

Acoustic foam is foam used for acoustic treatment. It is typically open-celled. It attenuates airborne sound waves by increasing air resistance, thus reducing the amplitude of the waves. The energy is dissipated as heat. Many acoustic foam products are treated with dyes and/or fire retardants. Acoustic foam is often cut into the form of wedges to facilitate the breaking down of the sound waves.

SOUND TRAP:

A sound trap is a special acoustical treatment of Heating Ventilating and Air-Conditioning (HVAC) ductwork designed to reduce transmission of noise through the ductwork, either from equipment into occupied spaces in a building, or between occupied spaces. In its simplest form, a sound trap consists of an offset in the ductwork to reflect the sound back to its source. This configuration is often combined with the use of sound-absorbing material inside the trap. The physical dimensions of the sound trap may be selected to tune the trap to specific frequencies of sound. As such, it is then essentially a Helmholtz resonator used as a passive noise-control device.

REVERBERATION:

Reverberation is the persistence of sound in a particular space after the original sound is removed.[6] A reverberation, or reverb, is created when a sound is produced in an enclosed space causing a large number of echoes to build up and then slowly decay as the sound is absorbed by the walls and air.[7] This is most noticeable when the sound source stops but the reflections continue, decreasing in amplitude, until they can no longer be heard. The length of this sound decay, or reverberation time, receives special consideration in the architectural design of large chambers, which need to have specific reverberation times to achieve optimum performance for their intended activity.[8] In comparison to a distinct echo that is 50 to 100 ms after the initial sound, reverberation is many thousands of echoes that arrive in very quick succession (.01 – 1 ms between echoes). As time passes, the volume of the many echoes is reduced until the echoes cannot be heard at all.

REVERBERATION TIME:

RT₆₀ is the time required for reflections of a direct sound to decay by 60 dB below the level of the direct sound. Reverberation time is frequently stated as a single value however it can be measured as a wide band signal (20 Hz to 20kHz) or more precisely in narrow bands (one octave, 1/3 octave, 1/6 octave, etc.). Typically, the reverb time measured in narrow bands will differ depending on the frequency band being measured. It is usually helpful to know what range of frequencies is being described by a reverberation time measurement. In the late 19th century, Wallace Clement Sabine started experiments at Harvard University to investigate the impact of absorption on the reverberation time. Using a portable wind chest and organ pipes as a sound source, a stopwatch and his ears, he measured the time from interruption of the source to inaudibility (roughly 60 dB). He found that the reverberation time is proportional to the dimensions of room and inversely proportional to the amount of absorption present.

The optimum reverberation time for a space in which music is played depends on the type of music that is to be played in the space. Rooms used for speech typically need a shorter reverberation time so that speech can be understood more clearly. If the reflected sound from one syllable is still heard when the next syllable is spoken, it may be difficult to understand what was said.[9] "Cat", "Cab", and "Cap" may all sound very similar. If on the other hand the reverberation time is too short, tonal balance and loudness may suffer. Reverberation effects are often used in studios to add depth to sounds. Reverberation changes the perceived harmonic structure of a note, but does not alter the pitch. Basic factors that affect a room's reverberation time include the size and shape of the enclosure as well as the materials used in the construction of the room. Every object placed within the enclosure can also affect this reverberation time, including people and their belongings.

ABSORPTION:

The absorption coefficient of a material is a number between 0 and 1 which indicates the proportion of sound which is absorbed by the surface compared to the proportion which is reflected back into the room. A large, fully open window would offer no reflection as any sound reaching it would pass straight out and no sound would be reflected. This would have an absorption coefficient of 1. Conversely, a thick, smooth painted concrete ceiling would be the acoustic equivalent of a mirror, and would have an absorption coefficient very close to 0.

MEASUREMENT OF REVERBERATION TIME:

Historically reverberation time could only be measured using a level recorder (a plotting device which graphs the noise level against time on a ribbon of moving paper). A loud noise is produced, and as the sound dies away the trace on the level recorder will show a distinct slope. Analysis of this slope reveals the measured reverberation time. Some modern digital sound level meters can carry out this analysis automatically. Currently several methods exist for measuring reverb time. An impulse can be measured by creating a sufficiently loud noise (which must have a defined cut off point). Impulse noise sources such as a blank pistol shot or balloon burst may be used to measure the impulse response of a room.

Alternatively, a random noise signal such as pink noise or white noise may be generated through a loudspeaker, and then turned off. This is known as the interrupted method, and the measured result is known as the interrupted response. A two port measurement system can also be used to measure noise introduced into a space and compare it to what is subsequently measured in the space. Consider sound reproduced by a loudspeaker into a room. A recording of the sound in the room can be made and compared to what was sent to the loudspeaker. The two signals can be compared mathematically. This two port measurement system utilizes a Fourier transform to mathematically derive the impulse response of the room. From the impulse response, the reverberation time can be calculated. Using a two port system, allows reverberation time to be measured with signals other than loud impulses.

Music or recordings of other sound can be used. This allows measurements to be taken in a room after the audience is present. Reverberation time is usually stated as a decay time and is measured in seconds. There may or may not be any statement of the frequency band used in the measurement. Decay time is the time it takes the signal to diminish 60 dB below the original sound.

CREATING REVERBERATION EFFECTS:

It is often desirable to create a reverberation effect for recorded or live music. A number of systems have been developed to facilitate or simulate reverberation.

CHAMBER REVERBERATORS:

The first reverb effects created for recordings used a real physical space as a natural echo chamber. A loudspeaker would play the sound, and then a microphone would pick it up again, including the effects of reverb. Although this is still a common technique, it requires a dedicated soundproofed room, and varying the reverb time is difficult.

PLATE REVERBERATORS:

A plate reverb system uses an electromechanical transducer, similar to the driver in a loudspeaker, to create vibration in a large plate of sheet metal. A pickup captures the vibrations as they bounce across the plate, and the result is output as an audio signal. Early units had one pickup for mono output, later models featured two pickups for stereo use. The reverb time can be adjusted by a damping pad, made from framed acoustic tiles. The closer the damping pad, the shorter the reverb time. However, the pad never touches the plate. Some units also featured a remote control.

ECHO:[10]

In audio signal processing and acoustics, an echo (plural echoes) is a reflection of sound, arriving at the listener some time after the direct sound. Typical examples are the echo produced by the bottom of a well, by a building, or by the walls of an enclosed room. A true echo is a single reflection of the sound source. The time delay is the extra distance divided by the speed of sound.

ECHO AS ACOUSTIC PHENOMENON:

If so many reflections arrive at a listener that they are unable to distinguish between them, the proper term is reverberation. An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived. Echoes are reflected off walls or hard surfaces like mountains and privacy fences.

This illustration depicts the principle of sediment echo sounding, which uses a narrow beam of high energy and low frequency. When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10 of a second.

Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 20°C, the reflecting object must be more than 17.15 m from the sound source at this temperature for an echo to be heard by a person at the source. Sound travels approximately 343 meters/s (1100 ft/s). If a sound produces an echo in 2 seconds, the object producing the echo would be half that distance away (the sound takes half the time to get to the object and half the time to return). The distance for an object with a 2-second echo return would be 1 sec X 343 meters/s or 343 meters (1100 ft).

In most situations with human hearing, echoes are about one-half second or about half this distance, since sounds grow fainter with distance. In nature, canyon walls or rock cliffs facing water are the most common natural settings for hearing echoes. The strength of an echo is frequently measured in dB sound pressure level SPL relative to the directly transmitted wave. Echoes may be desirable (as in sonar) or undesirable (as in telephone systems).

ECHO SOUNDING:[11]

Echo sounding is the technique of using sound pulses directed from the surface or from a submarine vertically down to measure the distance to the bottom by means of sound waves. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes. Echo sounding can also refer to hydro acoustic "echo sounders" defined as active sound in water (sonar) used to study fish. Hydro acoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish. The word sounding is used for all types of depth measurements, including those that don't use sound, and is unrelated in origin to the word sound in the sense of noise or tones.

TECHNIQUE OF ECHO SOUNDING:

Distance is measured by multiplying half the time from the signal's outgoing pulse to its return by the speed of sound in the water, which is approximately 1.5 kilometers per second. For precise applications of echo sounding, such as Hydrography, the speed of sound must also be measured typically by deploying a sound velocity probe into the water. Echo sounding is effectively a special purpose application of sonar used to locate the bottom.

SOUND ABSORBERS:

These materials are used to eliminate sound reflections to improve speech intelligibility, reduce standing waves and prevent comb filtering. Typical materials are open cell polyurethane foam, cellular melamine, fiberglass, fluffy fabrics and other porous materials. A wide variety of materials can be applied to walls and ceilings depending on your application and environment. These materials vary in thickness and in shape to achieve different absorption ratings depending on the specific sound requirements

ACOUSTICAL FOAM PANELS:

These acoustical foams are used in a wide variety of applications ranging from Recording and Broadcast Studios to Commercial and Industrial Facilities. It is available in Polyurethane or in Class 1 Fire Rated foam.  These products can be applied directly to walls, hung as baffles or used as freestanding absorbers.

FABRIC WRAPPED PANELS:

The Glass Fiber Absorbers includes acoustic Panels, Clouds, Ceiling Tiles, Corner Bass traps and Baffles. In a variety of colors, sizes, shapes and applications, is available in the market for sound control and noise elimination.

SOUND CHANNELS ACOUSTICAL WALL COVERING:

Sound Channels is a dimensional fabric that offers excellent acoustical properties, unmatched fade resistance, and more.

CLOUDSCAPE CEILING TILES:

These Ceiling Tiles absorb noise and block sound transmission.  Designed to fit standard 24" x 24" grid systems or direct mount to wall or ceiling. These sound control Baffles and Banners are used when free-flow of air is crucial along with noise suppression. It is perfect for large areas such as gyms, auditoriums and outdoor venues.

FIBERGLASS BLANKETS AND ROLLS:

Quilted fiberglass material combines noise barrier performance and efficient sound absorption. This lightweight, semi-flexible, easy-to-handle material is a vinyl coated; grey colored fiberglass facing cloth, quilted to a supporting 2 lb. /cu. ft. density fiberglass. It is available in Rolls or Panels.

SPECIALTY SOUND ABSORBERS:

In addition to standard fiberglass absorbers, there are custom products to address specific acoustical needs. These include materials that both absorb and diffuse or reflect sound, to have custom facings and custom graphics.  

SOUND DIFFUSERS/DIFFUSORS:

These devices reduce the intensity of sound by scattering it over an expanded area, rather than eliminating the sound reflections as an absorber would. Traditional spatial diffusers, such as the poly-cylindrical (barrel) shapes also double as low frequency traps. Temporal diffusers, such as binary arrays and quadratics, scatter sound in a manner similar to diffraction of light, where the timing of reflections from an uneven surface of varying depths causes interference which spreads the sound.

ART DIFFUSERS:

The Art Diffuser is a patented, two dimensional, binary array sound diffuser for use in many applications and critical listening environments.

QUADRAPYRAMID DIFFUSER:

This diffuser generates a uniform polar response over a broad frequency range using a pre-rotated pattern to create 16 angles of reflection.

QUADRATIC RESIDUE DIFFUSER Q:

A true quadratic residue diffuser designed for uniform broadband scattering and reducing High-Q reflections.

DOUBLE DUTY DIFFUSERS:

Poly-cylindrical "Double Duty" Diffusers will act to scatter sound in any location. Bass absorption will vary with size.  A 2' X 4' has maximum absorption at 125 Hz.

PYRAMIDAL DIFFUSERS:

Molded in a one-piece offset inverted pyramidal shape, the internal cavity can be lined with 1½" thick absorber layer.  Reduces flutter echo.

             

HIPER PANEL, FLAT PANEL DIFFUSER:

The HiPer Panel combines absorption and diffusion into one sound panel, similar in appearance to standard fabric-covered acoustical wall panels. This is used when a flat diffuser panel is required.

BARRIERS & VIBRATION CONTROL:

These materials range from dense materials to block the transmission of airborne sound to devices and compounds used to isolate structures from one another and reduce impact noise.

BARRIERS:

Sound barrier materials are used to reduce the transmission of airborne sound. There are products such as the standard one pound per square foot non reinforced barrier, transparent material when observation or supervision is required, reinforced vinyl to create a hanging barrier partition. 

COMPOSITES:

Composite materials are manufactured from combinations of various materials from open and closed celled foams to quilted fiberglass and barrier. These products are used to block and absorb sound for machine enclosures as well as blocking airborne sound and impact noise. Some of these products include Composite Foams, Strati-Quilt Blankets and Floor Underlayment. 

VIBRATION CONTROL:

Vibration control products are used to absorb vibration energy and prevent structural noise transmission. These include vibration damping compounds and vibration pads, isolation hangers, and resilient clips.

REFERENCES: 

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[1] From: http://www.enoisecontrol.com/Acoustic_Treatment.html (Retrieved April 14, 2011)

[2] From: http://en.wikipedia.org/wiki/Acoustical_treatment (Retrieved April 14, 2011)

[3] From: http://en.wikipedia.org/wiki/Diffusion_(acoustics) (Retrieved April 14, 2011)

[4] From: http://en.wikipedia.org/wiki/Sounding_board (Retrieved April 14, 2011)

[5] From: http://en.wikipedia.org/wiki/Active_noise_control (Retrieved April 14, 2011)

[6] Valente, Michael; Holly Hosford-Dunn, Ross J. Roeser (2008) Audiology Thieme. pp. 425–426. ISBN 9781588905208

[7] Lloyd, Llewelyn Southworth (1970) Music and Sound by Ayer Publishing pp. 169 ISBN 9780836951882

[8] Roth, Leland M. (2007) Understanding Architecture by West view Press. pp. 104–105. ISBN 9780813390451

[9] "So why does reverberation affect speech intelligibility?" MC Squared System Design Group, Inc. http://www.mcsquared.com/y-reverb.htm (Retrieved April 18, 2011)

[10]ECHO; From: http://en.wikipedia.org/wiki/Echo_(phenomenon) (Retrieved April 18, 2011)

[11]ECHO SOUNDING; From: http://en.wikipedia.org/wiki/Echo_sounding (Retrieved April 18, 2011)