2341742 Acoustic absorber and emitter The present invention primarily
relates to an acoustic absorber, but also relates to an acoustic emitter. 5 The desire to absorb acoustic energy is encountered in a wide variety of situations. The need for sound absorbers commonly stems from a desire to prevent or reduce transmission of sound to a particular region or to prevent or reduce reflection of sound waves by a surface. For example, uses of sound absorbers include the treatment of room acoustics and the reduction in noise annoyance produced by traffic travelling on urban motorways.
A sound wave incident on a perfect absorber will produce no reflected wave, normally a situation that can only exist if there is no change of impedance. For example, in the case of a plane wave, propagating in one direction only, in free air there is practically no reflection of the sound wave. A similar situation exists with a plane wave in a waveguide of constant cross section and infinite length. However, neither of the two above-described cases of perfect absorption of sound is of any practical use in reducing the intensity of the sound waves reflected by a surface. For example, when attempting to improve the acoustic performance of a room, where the relevant surface might be the internal surface of a wall of the room, to take advantage of the second of the two above-described cases of perfect sound absorption, the wall would have to be completely replaced by a waveguide of a length sufficiently long as to be practically equivalent to a waveguide of infinite length.
Practical methods and apparatus for absorbing sound have, of course, been proposed and used.
An object of the present invention is to provide a sound absorber, which is of practical use and has improved absorption characteristics over sound absorbers of the prior art, yet without unduly increasing manufacturing costs. A further object of the present invention is to provide an improved sound emitting apparatus. A yet further object of the present invention is to provide a simple method of manufacturing either a sound absorber or a sound emitter.
According to the present invention there is provided a sound absorber comprising an array of a multiplicity of tapered horns.
Each of the horns of the array acts as an absorber within certain frequency ranges, determined partly by their dimensions, and thus the array of horns acts as an acoustic absorber.
It should be understood that the term "array" in the present context means that the horns are arranged with their mouths (the mouth of a tapered horn being at its wide end) being in an ordered arrangement adjacent to one another including, but not necessarily limited to, an arrangement where the mouths of the horns are arranged in a grid of rows and columns. For example, in the context of the present invention, an array of horns may comprise a multiplicity of horns arranged with their mouths in a single row. The mouths of the horns do not necessarily have to be the same shape. The cross-sectional area may be constant along a part of the length of the horn.
3 - The sound energy decreases along the length of each horn (in a direction towards its end) owing to energy lost through movement of the air in the tube, since air has a degree of viscosity. Thus, generally, the longer each horn is, the better its sound absorbing characteristics.
Sound waves having a wavelength comparable to or less than three times the smallest dimension of the mouth of each horn are, to some extent, reflected, because the mouth end of the horn acts as a reflective surface, particularly when the direction of the incident wave is normal to an internal surface of the horn in the region of the mouth. The effectiveness of the absorber therefore decreases as the frequency of sound increases. There is therefore a cut-off frequency, above which the effectiveness of the absorber begins to decrease significantly. The cut-off frequency, fc, is to a first approximation inversely proportional to the circumference of the mouth of each horn.
Thus, the frequency response of the sound absorber can be improved, within limits (dictated by, for example, the dimensions and materials of the walls) by decreasing the circumference of each horn. Thus by providing a relatively large number of horns, over the area that the acoustic absorber spans, the absorbing characteristics of the absorber are improved at high frequencies.
Because the volume occupied per unit length of each horn decreases towards its end it is possible for the horn to bend away from the central axis of its mouth, so that the straight line distance between the mouth and the end of the horn is less than the length of the path down the centre of the horn. Advantageously, a portion of a horn of the sound absorber is curved. Thus, it is possible to provide an acoustic absorber that has a thickness less than the path length of each horn. Preferably, at least a portion of a horn of the sound absorber has opposing walls that bend in the same direction. To increase the saving in space, and thus the reduction in thickness of the sound absorber per unit length of each horn, it is necessary to increase the curvature of each horn. However, if the curvature of each horn is too great, there may be some reflection of sound, thus compromising the performance of the absorber. The maximum degree of curvature is to some extent determined by the desired performance of the acoustic absorber over the range of wavelengths for which sound is to be absorbed.
There may be four or more horns in the array. An absorber with as few as four horns is particularly simple to construct and may, in some applications, be a sufficiently efficient sound absorber. Preferably, there are more than 20 horns in the array. More preferably, there are more than 50 horns in the array. There may be more than 100 horns in the array.
Preferably, the sound absorber is so arranged that the mouths of the horns are substantially aligned and define a substantially smooth surface. The surface so defined may be planar or alternatively may be curved. For example, the surface so defined may be a cylinder.
Preferably, there are no gaps between adjacent horns.
otherwise, such gaps might reduce the performance of the absorber.
Advantageously, a wall of one horn serves as one of the walls of an adjacent horn. Preferably there are no gaps between adjacent horns, especially between the mouths of adjacent horns. Otherwise such gaps might reduce the performance of the absorber. For example, if the absorber is mounted against a solid boundary, such gaps could allow sound to leak past the array of mouths to the space beyond and be reflected by the solid boundary, the resulting reflected sound waves being able to pass back through the same gaps.
The walls of the horns should preferably contain the sound pressure with the minimum of vibration or deflection of the walls, within such constraints as cost and weight.
For a given wall type and frequency of sound, the deflection of a wall will be dependent upon the pressure difference across it. If the horns are discrete.. sharing no common surfaces, then because the air to the exterior of the horns is at some constant value, most probably atmospheric pressure, the pressure across the walls of a given horn will correspond to the sound pressure within that horn. If the horns are arranged to share walls and so arranged that the sound waves on either side of a wall are less than 60 degrees out of phase then the pressure difference across a shared wall will be less than the pressure difference had the horns been discrete (i.e. the pressure difference between the external pressure (probably atmospheric pressure) and the pressure on either side). Thus the two walls can be replaced by one wall that may be of lighter construction than either of those which it replaces, especially when the maximum phase difference of waves along adjacent horns is much less than 60 degrees.
The material savings that result from the sharing of the majority of partitions may be of the order of 50%. Preferably, each of the horns has at least one wall that is shared with another adjacent horn. More preferably, the majority of adjacent horns of the sound absorber share a wall. Yet more preferably, each pair of adjacent horns of the sound absorber share a wall.
The horns may advantageously be exponential horns.
An exponential horn can be defined as a waveguide in which the crosssectional area at a given distance along the length of the horn can be approximated as an exponential function of that distance. Such an exponential function can be expressed in the form of A(x)::c: A0e-mx, where A(x) is the cross- sectional area of the horn at a distance x from the mouth, AO is the cross-sectional area of the horn at the mouth and m is the flare constant (m>O), which is characteristic of the rate at which the crosssectional area changes with distance.
The acoustic impedance of a notional constant section waveguide is independent of frequency, so an open end behaves as a perfect absorber at all frequencies. In contrast, an exponential horn has an acoustic impedance, which tends to a constant value above a cut-off frequency, fe, which is proportional to the flare constant, m.
Thus, the frequency response of the sound absorber can be improved, within limits (dictated by, for example, the dimensions and materials of the walls) by decreasing the flare constant.
Each exponential horn, and therefore the acoustic absorber as a whole, has absorption characteristics similar to that of an infinitely long waveguide having a constant cross-sectional area, in respect of sound waves having a 5 frequency between f,, and f c.
Whilst it is preferred that the horns each have an exponential taper, an absorber according to the present invention with tapered horns each having a cross-sectional area that decreases along the length of the horn according to an alternative taper law can still provide many benefits over sound absorbers of the prior art. For example, each of the horns may have a cross-sectional area that decreases linearly along the length of the horn. Alternatively, the taper may be such that the function that describes how the cross- sectional area varies along at least a part of the length of the horn is a hyperbolic function.
The sound absorber advantageously contains sound absorbing material. There will be a certain amount of reflection caused by the rapid change of impedance at the end of each horn, irrespective of whether the end is closed or open. Such reflection is undesirable, since the reflected sound waves might pass back out through the mouth of the horn, thereby reducing the performance of the absorber. The provision of sound absorbing material facilitates the reduction of such reflections caused by the ends, by for example increasing the absorption of sound energy by viscous losses. A finite length tapered horn provided with sound absorbing material behaves more like an infinitely long exponential horn. It is therefore possible for horns of the acoustic absorber to be shorter, without reducing their performance.
Preferably, the sound absorbing material is positioned in a section of the or each horn and the density of the sound absorbing material in a first region within that section is less than the density of the sound absorbing material in a second region that is positioned between the first region and the narrow end of said horn. The density of sound absorbing material within said section preferably generally increases along the length of, in a direction towards the end of, the or each horn. For example, the mass of absorbing material per unit length of the or each horn may be substantially constant within the section. The greater the rate of change of impedance along the length of the horn, the greater the degree of reflection of sound waves: abrupt changes in impedance cause some of the wave to be reflected before the far end of the horn is reached. Thus, by providing a steadily increasing impedance along and up to the closed end of a horn, the percentage by energy of sound waves reflected is reduced. As the horn tapers, the viscous losses caused by the air and horn alone increase to the point where the provision of an absorbing material becomes superfluous; that is to say it is not necessary for the section in which absorbing material is provided to extend to the very end of a horn.
The effectiveness of a horn having a given thickness of absorbing material generally decreases as the wavelength of the incident sound increases above a cut-off wavelength, kc, that is related to the physical structure and dimensions of the absorbing material. For example, in the case of a finite length waveguide having a constant cross-sectional shape, provided with absorbing material extending, by a given length, from the end of the guide, the performance of the absorbing material decreases to an unacceptable degree as the wavelength increases above about four times the thickness of the absorbing material.
Preferably, the sound absorbing material is a fibrous material. For example, the sound absorbing material may be a fibrous tangle of glass fibre or wool. Many further suitable materials are also known per se. The fibrous material may simply be attached to the solid boundary at the end of a horn, which boundary would otherwise be reflective.
It has been observed that a tapered horn provided with a fibrous material that is highly compacted at the narrow end of the horn performs better than a pipe with parallel walls having an identical length and packed with the same mass of fibrous material.
The present invention also provides a sound emitting apparatus comprising a sound wave producing means and a multiplicity of tapered horns arranged in an array, wherein the wall of one horn serves as one of the walls of an adjacent horn. The sound wave producing means may be one or more loudspeaker drive units, a siren or the like.
One particularly advantageous arrangement of the horns of the apparatus is such that the mouths of the horns are substantially aligned and define a substantially cylindrical surface. That arrangement is particularly useful when it is desired to radiate sound in various different directions, for example in the construction of an audible warning apparatus, incorporating, for example, a siren.
Any of the features described above with reference to the arrangement and configuration of the multiplicity of horns of the sound absorber are applicable to this present aspect of the invention concerning a sound emitting apparatus. 5 The present invention also provides a method of manufacturing the above- described sound absorber or sound emitter, the method comprising the steps of providing a plurality of pre-formed sheets and fixing the sheets together in a layered arrangement, thereby forming the multiplicity of tapered horns arranged in an array.
The array of horns may be arranged in rows and columns. A sheet used in the method may then advantageously either form at least one wall of each of a plurality of horns in successive columns or at least one wall of each of a plurality of horns in one column and at least three walls of at least one of those horns. Of course, in the context of the present invention the terms "row" and "column" are interchangeable.
The present invention also provides a pre-formed sheet for use in the method as described above.
The present invention also provides a sound absorber comprising a tapered horn. According to a further aspect of the invention there is provided a method of absorbing sound in a room by providing an architectural sound absorber comprising a tapered horn. The acoustics of the room may thus be improved. It should be understood that an architectural sound absorber is a sound absorber suitable for use in a room in a building. According to that further aspect there is also provided an architectural sound - 11 absorber comprising a tapered horn.
It will of course be appreciated that various features of the aspect of the present invention that require the provision of a multiplicity of horns may, if suitable, be incorporated into the other above-described aspects of the invention. For example the tapered horn may be an exponential horn and/or the absorber may include sound absorbing material.
By way of example embodiments of the invention will now be described with reference to the accompanying drawings, of which:
Figure 1 is a perspective view of a first example of an exponential horn, Figure 2 is a perspective view of a second example of an exponential horn, Figure 3 is a perspective view of a sound absorber according to a first embodiment of the invention, Figure 4 is a perspective view of a sound absorber according to a second embodiment of the invention, Figure 5 is a schematic perspective view of a sound absorber according to a third embodiment of the invention, Figure 6 is a perspective view of a sound absorber according to a fourth embodiment of the invention, Figure 7 is a schematic cross-section of a sound absorber in the region of the mouths of the horns, illustrating the method of the construction, Figure 8 is a schematic cross-section of a further sound absorber in the region of the mouths of the horns, illustrating the method of the construction, Figure 9 is a perspective view of a sound emitter according to a further aspect of the invention, and Figure 10 is a perspective view of an alternative sound emitter with a portion cut away to show the internal structure.
Figures 1 and 2 each show an example of an exponential horn 1. Each horn 1 has a cross-sectional area that tapers, exponentially, from a wide end 4 to a narrow end 3. A mouth 2 is defined by the wide end 4 of the horn 1. The horn of Figure 1 tapers in two dimensions and has a generally square cross-section along its whole length. The horn 1 of Figure 2 tapers in one dimension, the cross- section of the horn having a constant height.
Figure 3 shows a sound absorber 10 according to a first embodiment of the invention. The sound absorber 10 is formed of 36 exponential horns. The mouths of the horns are each square in cross-section and are arranged in 6 rows of 6 horns. The mouth of each horn measures 150mm by 150mm, corresponding to an upper frequency limit of approximately 800 Hz. Each horn is about 1m in length and halves in width every 200mm giving a lower frequency limit of 200 Hz. The mouths of all the horns lie in a plane and define a square.
Each horn tapers in two dimensions in a similar manner to that of the horn illustrated by Fig.l. Glass wool is packed in towards the narrow end of each horn. The mass per unit length of glass wool, in the region where the glass wool is provided, is approximately constant. Thus, the density of the glass wool increases along the length of the horn to a maximum at the narrow end of around 250 Kg /M3 Figure 4 shows a second sound absorber 20 similar to that shown in Figure 3, except that each horn tapers in one dimension only (in a similar manner to that shown in Figure 2). Each pair of adjacent horns in a column share a wall.
Manufacture of the absorber 20 is therefore simpler than manufacture of the horn 10 of Figure 3 and less material is required.
A third sound absorber 30 is shown, schematically in Figure 5. The array of horns may be defined by 7 curved walls and 7 parallel and spaced apart planar walls that intersect the curved walls, each horn having a rectangular cross-section along its length. Figure 5 shows the curved walls which are shown in Figure 5 as being upright and the lower 6 horizontal walls. (Figure 5 does not show the uppermost horizontal wall.) The absorber 30 is made from a flat horizontal top wall (not shown), a flat horizontal bottom wall parallel to and spaced apart from the top wall, 7 curved upright walls 31 extending between the top and bottom walls, and 5 sets of partition walls, each set together defining one of the 5 horizontal walls that are parallel with and lie between the top and bottom walls and that intersect the curved upright walls. The walls are together so arranged to define 36 exponential horns, each having a rectangular cross-section.
All of the walls of the sound absorber are made of a material that is sufficiently stiff and lightweight to keep the resonant modes of the structure above the frequency band in which the absorber is to function. A suitable material is aluminium. Glass wool is provided in the absorber in a similar manner to that as described in Figure 4. Each horn tapers in one dimension only. Each pair of adjacent horns, in a row or in a column, share a wall. Therefore, even less material is required to form the sound absorber 30, in comparison to the sound absorbers illustrated in Figures 3 and 4.
All of the curved walls 31 of the sound absorber 30 curve in the same direction. When the sound absorber 30 is used in outdoor applications, for example, for the purpose of reducing noise pollution near a busy road, the curvature of the walls 31 can be utilised to prevent the ingress of rain water. The sound absorber can be so positioned that, whilst being generally horizontal at its narrow end 32, any water that enters though the mouth of any one of the horns simply flows back out of the mouth of the horn.
Figure 6 shows a fourth sound absorber 40 formed from six preformed corrugated curved sheets 41 held to one another by glue joints. The corrugated sheets 41 are so shaped that together they form an array of exponential horns. The mouths of the horns are arranged in rows and columns, the rows and columns being arranged at 45 degrees to the vertical (as viewed in Figure 6). Each pair of adjacent sheets forms four or five horns, each horn being in different row and column.
Figure 7 shows schematically a partial cross-section of sound absorber, similar to that shown in Figure 6, to show how a plurality of curved corrugated sheets may be arranged to form an array of exponential horns, arranged in rows and columns. An alternative arrangement is shown in Figure 8, wherein one sheet forms three walls of every other horn in a row. Thus when three such sheets are stacked, a single row of exponential horns is formed and with every subsequent sheet added a further row of horns is formed. In each of Figures 7 and 8, one of the sheets is highlighted by being drawn with a thicker width (in practice all the sheets have substantially the same width).
Figure 9 shows a sound emitting apparatus 60 according to a further aspect of the present invention. The apparatus 60 comprises a multiplicity of horns arranged in an array. The emitter is formed from a plurality of curved walls 61 that extend between two flat, generally round, parallel, spaced apart and horizontal walls (not shown) and a plurality of parallel flat partition walls (also not shown) and so arranged to define the multiplicity of exponential horns. The mouths of the horns defined a generally cylindrical surface. The narrow ends of the horns are open and adjacent to an omni-directional siren (not shown). When the apparatus is operated, sound waves are produced by the siren, are guided by each of the horns and then exit mostly in a radial direction (perpendicular to the central axis of the apparatus) from the mouths of the horns.
A further embodiment envisaged, and where the mouths of the horns define a cylindrical surface, is shown in Figure 10. The emitter 70 comprises a stack of identically shaped spaced walls 71 that are funnel-shaped (each rather like a separate horn) all having their central axes aligned along a single central axis. That further embodiment could, but need not necessarily, further comprise vertical walls that extend radially from the central axis extending between each - 16 pair of adjacent funnel-shaped walls.
It will be appreciated that various modifications may be made to any of the above-described embodiments of the present invention without departing from the scope of the present invention. For example, the absorber 30 of Figure 5 could alternatively be manufactured by providing two curved sheets and 7 flat sheets extending therebetween and 30 curved partition walls.
Also, where the absorber includes a set of parallel walls the parallel walls between the outermost parallel walls may be omitted. A sound wave which enters the array of horns at an angle to the direction of the parallel walls will follow a zig-zag path where the total path length is independent of the separation of the walls. Thus the intermediate parallel walls are not acoustically significant and may therefore be omitted entirely. In practice they play an important part in the support of the curved partitions but may be reduced to say thin rods and tubular spacers.
Furthermore, although the horns of all of the abovedescribed embodiments have rectangular cross-sections, other cross-sectional shapes may be suitable, for example, hexagonal. Instead of making the absorber from aluminium, fibre reinforced composite materials could be used. Rather than using glass wool as a sound absorbing material, other fibrous materials or a foamed material could be used. Many other geometrical configurations of exponential horns exist that could be used to provide a sound absorber or emitter. For example, in the case when the mouths of the horns define a cylindrical surface the path followed by each horn can be helical as well as spiral, in which case the pitch of the helix may fall as the distance from the mouth increases.
Either of the geometrical arrangements of the horns of the sound emitters illustrated by Figures 9 and 10 could be incorporated in the design of a sound absorber.
Both the cylindrical arrangements can be turned inside out so that the mouths define an enclosed cylindrical surface (with the horns extending away from the central axis of the cylindrical surface). This might be used as a silencer for an engine exhaust or to line the test chambers for gas turbines.
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