Sound reflection

AH materials, even those considered to be sound-reflecting, absorb some small fraction of the sound energy impinging on them. Table 1 provides sound-absorption coefficients for some common building materials. 

Uses. Sound-absorbing materials are frequendy used to reduce reverberation, or the persistence of sound in a space after generation of the sound ceases to reduce focused reflections from concave surfaces to prevent echoes, or delayed sound reflections from distant surfaces and to prevent the buildup of sound by multiple reflections within rooms and other enclosures. Sound-absorbing materials also are used to reduce the transmission of noise from one location to another by multiple reflections from sound-reflecting surfaces. 

Fig. 11. Fundamental and harmonic imaging In the fundamental imaging mode (a), a narrow-band pulse of ultrasound (US) centered at a given frequency (e.g., 2.5 MHz) is emitted the sound reflected by the organs is used to create the image, (b) Microbubbles, because they are extremely compressible in comparison to organ tissue, not only reflect sound more efficiently than tissues but also emit harmonics. In the harmonic mode, the signal from the tissues is filtered out, leaving only the harmonics, resulting in specific imaging of the bubbles [37].

Unfinished Products. Unfinished fiber glass products are available in the form of boards, blankets, and batts in various thicknesses and densities. These products are used by fabricators who apply finishes to make products suitable for ceilings, walls, open-plan office screens, etc. They also are used for sound absorption behind decorative and protective facings such as perforated or expanded metal and wood grilles. Thicker materials have better low frequency performance than thinner materials. Low frequency performance can be improved by spacing the material away from a sound-reflecting surface rather than applying the material directly to the surface. 

In navigation and target detection, a scanning active sonar can be used to provide a realistic visual picture of sound-reflecting objects around the source. The scanning beam is rotated, and the reflected beams show the position of objects in a given plane as echo blips . Another display is required to show the depth/altitude of the object observed... 

In this section we very briefly review the principles of sound reflection. Additional introductory material is presented in the preceeding papers in this publication, and a more complete analysis appears in standard acoustics textbooks such as that of Kinsler and Frey (1), or Pierce (2), or on a broader introductory level that of Crawford (3). 

If transmission measurements are impossible, another approach is to measure the amount of sound reflected at the interface between the sample and a known solid— often the container wall. The amount of sound reflected is a function of the impedance mismatch between sample and solid defined by a reflection coefficient, R 2. where z is the acoustic impedance of the material (= cp) (5). 

Acoustical board 9- ku-stik or -sti-kol-) n. A low-density, sound-absorbing structural insulating board having a factory-applied finish and a fissured, felted-fiber, slotted or perforated surface pattern provided to reduce sound reflection. Harris CM (2005) Dictionary of architecture and construction. McGraw-Hill Co., New York. 

A possible solution for barriers and shields is to place a small panel across the top of the shield or barrier at an angle perpendicular to the sound source to stop the vertical movement of sound. Another solution is to apply sound-absorbing material to the ceiling to reduce sound reflected downward. 

In the workshop situation, the sound reflectivity of surfaces such as metal roofs and walls can be reduced by fitting porous material, such as heat insulation, which serves a donble purpose. A workshop with an arched metal roof can focus noise along the centre of the floor. 

Restricting the number of doors, windows and ventilation openings in the buildings to the essential minimum also, the doors, etc. should if possible be located on the side facing away from the adjacent residents, taking due account of sound reflection from the walls of any other buildings at the rear. For the environmental comfort of plant operating personnel, the control rooms should adequately sound-insulated. 

Exhaust pipes and stacks Should be installed in acoustically screened parts of buildings, but with due regard to possible sound reflection from adjacent wall surfaces. Installing sound attenuators which are unaffected by dirt or can easily be cleaned (see Funke, 1973). Cowls on stacks and chimneys to be designed as acoustic deflector cowls, if possible. 

The Delay time is set in seconds. The greater the delay the larger the room size or further away the sound-reflecting circuit. 

Sound absorbers are usually employed for modilying the sound reverberation in a room, suppressing undesired sound reflections from remote walls (echoes), and reducing the acoustical energy density and, hence, the sound pressure level in noisy rooms. There are two standard methods of measuring absorption coefficients. One is for measuring normal incidence absorption coefficients in an impedance tube and the other is for measuring random incidence absorption coefficients in a reverberation room (Bies and Hansen, 2009). 

The fu-st part of ISO 17497 specifies a method of measuring the random incidence scattering coefficient of surfaces caused by surface roughness (ISO, 17497-1, 2004). The measurements are made in a reverberation room, either in full scale or on a scale model. The measurement results can be used to desalbe how much the sound reflection from a surface deviates from a specular reflection. The results obtained can be used for comparison purposes and for design calculations with respect to room acoustics and noise control. The method is not intended for characterizing the spatial uniformity of the scattering from a surface. 

The second part of ISO 17497 specifies a method of measuring the directional diffusion coefficient of surfaces in a free field (ISO, 17497-2, 2012). The diffusion coefficient characterizes the sound reflected from a surface in terms of the uniformity of the reflected polar distribution. The directional diffusion coefficient is a frequency dependent value derived from the polar distribution of the scattered sound (Cox and D Antonio, 2009). First the scattering from a surface is measured in terms of a polar distribution. Then the diffusion coefficient is evaluated at 1/3 octave bandwidth intervals, which has the advantage of smoothing out some of the local variations in the polar responses. 

Construction Low thermal conductivity Sealing against water, dust, air, etc. Sound reflection Vibration damping Tube insulation, parquet underlay, air duct insulation, sealing strips, gymnastic walls, floating floors, sealant backer, expansion joint filler, closure strips... 

Traditional acoustic instrumentation, such as sound level meters, detects the sound pressure using a single microphone that responds to the pressure fluctuations incident upon the microphone. Since pressure is a scalar quantity, there is no simple and accurate way that such instrumentation can determine the amount of sound energy radiated by a large source unless the source is tested in a specially built room, such as an echoic or reverberation room, or in the open air away from sound reflecting surfaces. This imposes severe limitations on the usefiilness of sound pressure levd measurements taken near large equipment that cannot be moved to special acoustic rooms. 

Effects of the environment When the sound power of a noise source is evaluated in the field using sound pressure level techniques, it is necessary to apply a correction to the measured levels to account for the effects of the enviromnent. This environmental correction accounts for the influence of undesired sound reflections from room boundaries and nearby objects. 

Measuring tonal noise sources. Measiuing the sound power of tonal noise sources presents difficulties using traditional techniques (ISO 3740, 1980). Unfortunately, using sound intensity techniques on such sotuces is also fraught with problems. This is because the spatial distribution of the intensity is very sensitive to small alterations in source position and the presence of nearhy sound reflective objects. 

While most of us think of acoustics as the study of sound reflections (such as in a room or an auditorium) it is a very broad field that intersects and affects a number of other disciplines, including audio, architecture, aeronautics, even deep sea diving. 

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