GB2546792A

GB2546792A - A silencer

Info

Publication number: GB2546792A
Application number: GB1601675.0A
Authority: GB
Inventors: Justo Perez Pablos Ignacio; John Lamb William
Original assignee: Dyson Technology Ltd
Current assignee: Dyson Technology Ltd
Priority date: 2016-01-29
Filing date: 2016-01-29
Publication date: 2017-08-02
Also published as: GB201601675D0; WO2017129931A1

Abstract

A silencer comprises a duct 14 having an inner surface 16 defining a bore 18, an outer surface 20 exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the gas flow into thermal energy. Micropassages are defined as having a width w, as measured along the longitudinal axis A of the bore, of less than one millimetre, eg 0.1mm. The micropassages may be microslits 22, or microperforations. The depth d of the micropassages, orthogonal to the axis A, is preferably less than 2mm, eg 1mm. The microslits 22 may extend partially or fully around the longitudinal axis A. If annular microslits 22 are provided, joints are formed between adjacent duct sections. Each micropassage (i) is disposed at one of a plurality of axial locations which are spaced apart along the axis A, and (ii) extends between the inner and outer surfaces 16, 20 of the duct 14. The axial locations register with pressure maxima of a plurality of resonant frequencies of sound waves generated by the source, eg a motor of a fan, fan heater, humidifier, vacuum cleaner or hairdryer.

Description

A SILENCER

FIELD OF THE INVENTION

The present invention relates to a silencer, and to apparatus including a source of acoustic energy and a silencer. In its preferred embodiment, the silencer is in the form of a duct for receiving a gas flow generated by the source of acoustic energy, but the silencer is also usable with sources of acoustic energy which do not create a gas flow. The invention also provides a method of silencing a gas flow generated by a source of acoustic energy.

BACKGROUND OF THE INVENTION

Noise is generated by an acoustic source in the presence of a gas flow through a duct, and chosen frequencies of the generated noise can be targeted by one of a number of known sound attenuation techniques. As one example, the internal surface of the duct may be lined with a porous medium, such as a foam or a felt, to reduce the emission of relatively high frequency sound waves into the external atmosphere. As the gas flow passes through the porous medium, frictional forces between the porous medium and the gas molecules cause acoustic energy within the gas flow to be converted into thermal energy. Depending on the chosen porous medium, there is a risk that the medium may become blocked or “bum out” over a prolonged period of time, thereby reducing its efficiency. The use of porous medium is also associated with a relatively high pressure drop through the medium.

As another example, a number of resonant sound chambers may be provided between the internal and external surfaces of the duct, as described in GB602,859, for attenuating sound waves which enter the resonant chambers. Each chamber comprises a neck which is open to the gas flowing through the duct, and is located at a position which corresponds with the location of a peak of a sound wave within the duct, or a harmonic thereof. The requirement to provide resonant sound chambers around the bore of the duct requires precise tuning of the size and shape of those chambers, and inevitably increases the external diameter of the duct.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a silencer comprising a duct having an inner surface defining a bore, an outer surface exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the bore into thermal energy, wherein each micropassage extends from the inner surface to the outer surface of the duct, and is disposed at one of a plurality of axial locations which are spaced apart along the longitudinal axis of the bore.

As used herein, the term “micropassage” refers to any form of passage formed in the duct, such as a slot, slit or perforation, which has a width, as measured in the direction of the longitudinal axis of the bore, which is less than 1mm. Examples of micropassages include a microperforation, which may have a polygonal or circular cross-section orthogonal to the longitudinal axis of the bore, and a microslit, which preferably extends at least partially about the longitudinal axis of the bore and which has a circumferential length, as measured about the longitudinal axis, which is greater than its width.

Under an acoustic excitation, an alternating pressure difference develops at the side extremities of the micropassage, which causes an oscillating motion of the air particles between the side surfaces of the micropassage. Particles in contact with these side surfaces remain at rest, whereas those at the centre of the micropassage move with a maximum velocity, thereby creating an oscillating laminar flow. Due to air viscosity, a boundary layer is established in a small region close to each side surface. It is in this region, which defines an interface between adjacent air layers moving with different velocities, where frictional forces develop, which convert acoustic energy into thermal energy, and without the relatively high pressure drop associated with porous media. This thermal energy can be dissipated within the duct, conveyed from the duct, for example by a gas flow passing through the duct, or pass through the micropassage and into the atmosphere surrounding the duct.

One or more micropassages may be disposed at each of the axial locations along the bore of the duct. For example, an array of microperforations may be disposed at each of the axial locations. Within the array, the microperforations are angularly spaced, preferably equally angularly spaced, about the longitudinal axis of the bore. Alternatively, one or more microslits may be disposed at each of the axial locations along the bore of the duct. Each microslit is preferably arranged orthogonal to the longitudinal axis of the bore. For example, two, three or four microslits may be disposed in a coplanar arrangement at each of the axial locations, such that the microslits are angularly spaced about the longitudinal axis of the bore. As a further example, a single annular microslit may be disposed at each axial location, with the inner or outer surfaces of the duct on either side of the microslit being connected together by a joint spanning the microslit. As a yet further example, a plurality, or a stack, of closely axially-spaced microslits may be provided at each of the axial locations along the bore of the duct. The stack of microslits may have microslits located to either side of that axial location. The effect of providing a stack of microslits at one axial location is approximately the same as that produced when providing a single microslit of equivalent open cross-section at that location, provided that single microslit has a width which is smaller than the width of the boundary layer (and so less than 1 mm, and typically less than 0.5 mm)

System parameters, including the width of the micropassage and the depth of the micropassage, are selected to facilitate two contrasting processes. One is the diversion of acoustic waves through the micropassage, and the other is the absorption of the acoustic energy of the waves within the micropassage to generate thermal energy. The former process requires the micropassage to be acoustically “permissive” so that sound waves are attracted to it, whereas the latter process requires the micropassage to be sufficiently “restrictive” so that acoustic energy can be converted into thermal energy.

As mentioned above, each micropassage has a width which is less than 1mm. The width of the micropassage controls the acoustic resistance of the micropassage, which in turn is closely related to the acoustic absorption of the micropassage and its ability to convert acoustic energy within the bore into thermal energy. In experiments a width which is less than 0.5mm, and preferably in the range from 0.1 to 0.3mm, has been found to provide optimum silencing.

The depth of the micropassage, as measured orthogonal to the longitudinal axis of the bore, has also been found to contribute to the acoustic resistance of the micropassage. It is preferable though to control the acoustic absorption of the micropassage by means of the selection of its width, and to maintain the depth of the micropassage at a minimum value which takes account of the material from which the duct is formed. For example, the duct may be formed from a metallic material or from a plastics material. The depth of each micropassage is preferably less than 2 mm, and is preferably in the range from 0.5 to 1.5 mm

To facilitate the positioning of the micropassages along the duct, the duct preferably has a substantially constant cross-section. For the same reason, the duct preferably has a constant cross-section, which may be polygonal or circular. The sound waves preferably pass through the duct in the direction of the longitudinal axis of the duct, which direction is preferably parallel to the inlets of the micropassages.

Each of the axial locations at which a micropassage is disposed is preferably selected to register substantially with the location of a maximum of the sound pressure along the duct. The pressure distribution for each resonance frequency of the sound generated by a source of acoustic energy, such as a motor, to which the duct is to be placed in fluid communication, is different. By way of example, for a volume velocity source located at one end of a duct of constant cross-section the fo resonant frequency has a single maximum located at the source position, whereas the fi resonant frequency has an additional maximum disposed between the ends of the duct, and the f2 resonant frequency has two additional maxima disposed between the ends of the duct. The microchannels may be positioned at two or more of the axial locations which each correspond to the position of one of these maximum pressures to maximize the sound reduction achieved through the provision of the microchannels. With this example, for a volume velocity source located at one end of a circular, unflanged duct of constant cross-section, the axial locations of the pressure maxima for a resonant frequency fn are dependent, inter alia, upon the length, L, and the radius, R, of the bore of the duct, and may calculated using the equation:

where x is the position of the maxima from the end of the duct proximate to the source, n is integral number > 0, m is the pressure maxima index, an integral number where 0 < m < n, and the term 0.6R is an end correction of the circular duct, which end correction will vary depending on the shape of the duct. For other source types, such as a pressure source, the location of the pressure maxima may vary, and may be calculated according to the shape of the duct.

The inner surface of the duct may be lined with a porous medium, such as a foam or a felt, for relatively high frequency (greater than 3 x 103 Hz) noise reduction. The provision of such a porous medium has been found to not interfere with the noise reduction achieved through the provision of the microchannels, which tend to provide lower frequency noise reduction. A cover or shield may be provided over the outlets of the micropassages to inhibit the ingress of dust, dirt or other material into the micropassages, provided that the cover does not increase the acoustic resistance of the micropassages.

In a second aspect, the present invention provides apparatus comprising a source of acoustic energy and a duct having an inner surface defining a bore for receiving sound waves generated by the source, an outer surface exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the bore into thermal energy, wherein each micropassage extends from the inner surface to the outer surface of the duct, and is disposed at one of a plurality of axial locations spaced apart along the

longitudinal axis of the bore and which register with pressure maxima of a plurality of resonant frequencies of sound waves generated by the source.

In a third aspect the present invention provides a method of silencing a gas flow generated by a source of acoustic energy, comprising conveying the gas flow through the bore of a duct having an inner surface defining the bore, an outer surface exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the gas flow into thermal energy, and for dissipating the thermal energy into the external atmosphere, wherein each micropassage extends from the inner surface to the outer surface of the duct, and is disposed at one of a plurality of axial locations spaced apart along the longitudinal axis of the bore and which register with pressure maxima of a plurality of resonant frequencies of sound waves generated by the source.

Features described above in connection with the first aspect of the invention are equally applicable to the second to third aspects of the invention, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of part of an apparatus comprising a source of acoustic energy and a silencer;

Figure 2(a) is a schematic illustration of the silencer, and Figure 2(b) is a side sectional view of the silencer; and

Figure 3(a) illustrates an example of the variation in the sound pressure within the duct of the silencer for three different resonant frequencies in the case of a volume velocity source, and Figure 3(b) is a schematic illustration of a duct having micropassages positioned at axial locations which register with pressure maxima of the resonant frequencies.

DF.TATT/FJ) DESCRIPTION OF THE INVENTION

Figure 1 illustrates schematically part of an apparatus which comprises a source 10 of acoustic energy and a silencer 12. The apparatus is preferably an apparatus which generates a fluid flow, and so may be in the form of a fan, a fan heater, a humidifier, a purifier, a vacuum cleaner and a hair dryer. In this example, the source 10 is a motor for generating a gas flow through the apparatus.

The silencer 12 comprises a duct 14 which is in fluid communication with the source 10, and so is able to receive sound waves generated by the source 10. In this example the silencer 12 is located downstream from the source 10, but alternatively the silencer 12 may be located upstream from the source 10. Where the source 10 is in the form of a motor, the duct 14 is preferably arranged to receive a gas flow generated by the source 10.

With reference also to Figure 2, the duct 14 preferably has a constant cross-section along its length L. In this example, the duct 14 has a circular cross-section of internal radius R, but the duct 14 may have a polygonal, regular or irregular, cross-section or a constant non-circular cross-section, such as an elliptical cross-section. The duct 14 has an inner surface 16 which defines the bore 18 of the duct 14, and an outer surface 20 which is exposed to the external atmosphere. The bore 18 of the duct 14 has a longitudinal axis A, the length of the duct 14 being measured in the direction of the longitudinal axis A. In this example, the duct has a length of 200mm and a radius of 16mm.

The duct 14 comprises a plurality of micropassages which extend from the inner surface 16 to the outer surface 20 of the duct 14. In this example, each micropassage is in the form of a microslit 22 which extends about the longitudinal axis A of the bore 18, and is located in a plane which is orthogonal to the longitudinal axis A. The microslits 22 may extend partially about the longitudinal axis A, or fully about the longitudinal axis A, as illustrated. Where such annular microslits are provided, joints are formed between adjacent sections of the duct 14 to hold the duct 14 together.

Each of the microslits 22 has a width w, as measured in the direction of the longitudinal axis A, and a depth d, as measure orthogonal to the longitudinal axis A. The width of the microslits 22 is preferably constant, and is less than 1mm. In this example, each microslit 22 has a width of 0.1mm. The depth of each micropassage is preferably less than 2 mm, and in this example is 1mm.

Each microslit 22 is disposed at one of a plurality of axial locations which are spaced apart along the longitudinal axis A of the bore 18. Each of the axial locations at which a microslit 22 is disposed is selected to register substantially with the location of a maximum of the sound pressure along the duct 14. Depending on the shape of the duct 14, the variation of the sound pressure along the duct 14 can be measured, determined mathematically, and/or can be predicted computationally. With reference to Figure 3(a), the pressure distribution for each resonance frequency of the sound generated by the source 10 is different. As illustrated, for a volume velocity source located at one end of the duct 14, which has a constant cross-section along its length, the f0 resonant frequency has a single maximum located at the source position, whereas the fi resonant frequency has an additional maximum disposed between the ends of the duct 14, and the Ϊ2 resonant frequency has two additional maxima disposed between the ends of the duct 14. With reference to Figure 3(b), the duct 14 has a first microslit 22 positioned at an axial location which is spaced from the source 10 by a distance xi, and which registers with the location of a pressure maximum of the f2 resonant frequency. The duct 14 has a second microslit 22 positioned at an axial location which is spaced from the source 10 by a distance X2, and which registers with the location of a pressure maximum of the fi resonant frequency. One or more additional microslits may be provided at axial locations which register with the second pressure maximum of the f2 resonant frequency, and/or with one of the pressure maxima of any of the other fn resonant frequencies.

Claims

1. A silencer comprising a duct having an inner surface defining a bore, an outer surface exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the bore into thermal energy, wherein each micropassage extends from the inner surface to the outer surface of the duct, and is disposed at one of a plurality of axial locations which are spaced apart along the longitudinal axis of the bore.

2. A silencer according to claim 1, wherein the micropassages comprise at least one of a plurality of microslits and a plurality of microperforations.

3. A silencer according to claim 2, wherein each microslit extends at least partially about the longitudinal axis of the bore.

4. A silencer according to any preceding claim, wherein a plurality of microslits is disposed in a coplanar arrangement at each of the axial locations along the bore.

5. A silencer according to any of claims 1 to 3, wherein a plurality of axially- spaced microslits is disposed at each of the axial locations along the bore.

6. A silencer according to any preceding claim, wherein each micropassage has a width, as measured in the direction of the longitudinal axis of the bore, in the range from 0.05 to 0.5 mm, preferably in the range from 0.1 to 0.3 mm.

7. A silencer according to any preceding claim, wherein each micropassage has a depth, as measured orthogonal to the longitudinal axis of the bore and extending from the inner surface to the outer surface, which is less than 2 mm, preferably in the range from 0.5 to 1.5 mm.

8. A silencer according to any preceding claim, wherein each of the axial locations registers with the location of a pressure maximum of one of a plurality of resonant frequencies of sound waves generated by a source of acoustic energy to which the duct is to be placed in fluid communication.

9. A silencer according to claim 8, wherein one of the axial locations registers with the location of a pressure maximum of the first order resonant frequency of the sound waves generated by the source.

10. A silencer according to claim 8 or claim 9, wherein one of the axial locations registers with the location of a pressure maximum of the second order resonant frequency of the sound waves generated by the source.

11. Apparatus comprising a source of acoustic energy and a duct having an inner surface defining a bore for receiving sound waves generated by the source, an outer surface exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the bore into thermal energy, wherein each micropassage extends from the inner surface to the outer surface of the duct, and is disposed at one of a plurality of axial locations spaced apart along the longitudinal axis of the bore and which register with pressure maxima of a plurality of resonant frequencies of sound waves generated by the source.

12. Apparatus according to claim 11, wherein the source is configured to generate a gas flow through the bore of the duct.

13. Apparatus according to claim 12, wherein the source is a motor for generating a gas flow in the apparatus.

14. A method of silencing a gas flow generated by a source of acoustic energy, comprising conveying the gas flow through the bore of a duct having an inner surface defining the bore, an outer surface exposed to the external atmosphere, and a plurality of micropassages for converting acoustic energy within the gas flow into thermal energy, wherein each micropassage extends from the inner surface to the outer surface of the duct, and is disposed at one of a plurality of axial locations spaced apart along the longitudinal axis of the bore and which register with pressure maxima of a plurality of resonant frequencies of sound waves generated by the source.