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EP-4735804-A1 - NOISE ATTENUATOR

EP4735804A1EP 4735804 A1EP4735804 A1EP 4735804A1EP-4735804-A1

Abstract

The invention provides an attenuator for ducted air flow systems such as heating, ventilation and air conditioning systems arranged to direct the airflow into the attenuator by reducing the airflow path geometry into a central channel surrounded by absorbing material to damp resonant modes to reduce noise more uniformly across the desired frequency spectrum than resistive attenuators.

Inventors

  • COLAM, STUART

Assignees

  • FISH INNOVATION LTD

Dates

Publication Date
20260506
Application Date
20240628

Claims (20)

  1. CLAIMS 1. An attenuator comprising: an outer skin; an inlet for receiving an air supply; an outlet; a channel passing through the attenuator connecting the inlet and outlet for the passage of air therethrough; and a porous filler arranged around the channel, wherein the thickness of the filler at different points around the channel differ from each other.
  2. 2. An attenuator according to claim 1 wherein the centre of the channel is offset in a first direction corresponding to a first transverse axis, relative to the centre of a transverse cross section through the attenuator, such that the respective thicknesses of the filler along said first transverse axis of the attenuator, are different on each side of the channel.
  3. 3. An attenuator according to claim 2 wherein the attenuator has a primary suppression frequency of sound to be attenuated and the combined thickness of the filler at each side of the channel and the width of said channel, along said first transverse axis of the attenuator, correspond to half of the compound wavelength of said primary suppression frequency.
  4. 4. An attenuator according to according to claim 3, wherein the thickness of the filler along said first transverse axis of the attenuator, on at least one side of the channel is greater than a quarter of the wavelength of the primary suppression frequency in the filler.
  5. 5. An attenuator according to any preceding claim, wherein the centre of the channel is offset in a second direction corresponding to a second transverse axis, relative to the centre of a transverse cross section through the attenuator, such that the respective thicknesses of the filler material between the channel and the outer skin, along said second transverse axis of the attenuator, are different on each side of the channel.
  6. 6. An attenuator according to claim 5 wherein the attenuator has a second suppression frequency of sound to be attenuated and the thickness of the filler material and the width of said channel combined, along said second transverse axis of the attenuator, corresponds to half of the compound wavelength of said second suppression frequency.
  7. 7. An attenuator according to according to claim 6, wherein the thickness of the filler along said second transverse axis of the attenuator, on at least one side of the channel is greater than a quarter of the wavelength of the second suppression frequency in the filler.
  8. 8. An attenuator according to according to any one of the preceding claims wherein the minimum cross-sectional area of the channel is at least 40% of the cross- sectional area of the inlet.
  9. 9. An attenuator according to according to any one of the preceding claims wherein the filler is a melamine foam.
  10. 10. An attenuator according to according to any one of the preceding claims wherein the filler defines the walls of the channel.
  11. 11. An attenuator according to any one of the preceding claims wherein the channel has a rectangular cross-section, and said filler is provided between each of the four sides of the channel and the outer skin of the attenuator.
  12. 12. An attenuator according to any one of the preceding claims wherein the channel has a central portion and at least one transitional portion between the central portion and one of the inlet and outlet where the dimensions of the transitional portion vary between the dimensions of the central portion and the dimensions of the respective inlet or outlet.
  13. 13. An attenuator according to claim 12 wherein the transitional portion is formed of one of more sheet sections forming the sides of the transitional portion.
  14. 14. An attenuator according to claim 13 wherein the sheets are flat.
  15. 15. An attenuator according to claim 13 wherein the sheets are curved.
  16. 16. An attenuator according to claim 13 wherein the sheets are polygonal having a plurality of flat sections.
  17. 17. An attenuator according to according to any one of the preceding claims where a longitudinal axis of the channel extends through the attenuator in a direction substantially perpendicular to a face of the attenuator in which the inlet is provided.
  18. 18. An attenuator according to according to any one of the preceding claims where a longitudinal axis of the channel extends through the attenuator at an angle offset from the perpendicular to a face of the attenuator in which the inlet is provided.
  19. 19. An attenuator according to claim 15 wherein the channel traverses the width of the attenuator along the length of the channel.
  20. 20. A method of constructing an attenuator comprising providing: an outer skin; an inlet for receiving an air supply; an outlet; and a central channel passing through the attenuator connecting the inlet and outlet for the passage of air therethrough, the method further comprising configuring the attenuator for preferentially damping a predetermined primary frequency by: forming the outer skin with a first dimension between the outer edges of the filler along a first transverse axis; providing filler around the central channel, wherein the thickness of the filler is selected so that the compound wavelength of a signal having the primary frequency along said transverse axis corresponds to twice the first dimension.

Description

NOISE ATTENUATOR FIELD OF THE INVENTION [0001] This invention relates to attenuators, in particular attenuators used to manage noise in air flow systems such as heating, ventilation and air conditioning (HVAC) systems. BACKGROUND [0002] The management and mitigation of noise generated in ventilation ductwork presents a fundamental challenge to HVAC systems. It is undesirable for HVAC systems to introduce noise into the user environment. The use of fans for driving airflow through the system can lead to significant noise generation which is carried through the ductwork. In order to prevent this noise from passing into the user environment, noise mitigation is necessary. [0003] Conventional methods for managing noise have not significantly changed for many decades. The main element used for reducing noise propagation is the so-called ‘splitter attenuator’. This kind of attenuator is typically provided as an element in the flow path with ducting feeding air into and out of it. They tend to be structures with a generally cuboid shape with several thin slabs of porous material interposed in the airpath, such that the free open area is reduced compared to the size of the airpath in the feed ducting. Figure 1 shows a typical example. [0004] This kind of attenuator is inserted in the airflow path and fed from one duct supply. The air flow path is narrowed so that air flows through narrowed channels surrounded by porous material acting to absorb elements of the sound passing down the duct work. [0005] The thin slabs of material (or splitters) are typically formed from a fibrous material such as mineral wool or fibreglass which is encapsulated by perforated metal sheets with an interposed glass tissue to limit fibre migration into the airpath. The resultant open area of the air flow path is often in the range 30-50%, as compared with the connecting ductwork. [0006] The noise generated in such systems and the reduction thereof tends to focus on the typical range of audible frequencies which are encountered in buildings, typically in the range from around 63Hz up to around 8kHz. [0007] A problem with splitter attenuators is that they exhibit an acoustic performance or (dynamic) “insertion loss” (IL) that is generally poorly suited to the environment which it is serving. There is typically pronounced insertion loss at the mid-frequencies (e.g. around 1kHz), but much less attenuation at lower and higher frequencies. [0008] Figure 2 gives an example of the sound level in a room for a conventional splitter attenuator (1600mm splitter attenuator). The sound level across the range of typical frequencies is shown at 110 for a typical 1600mm long splitter attenuator such as that shown in Figure 1. It can be seen that the sound level at 1kHz is much lower than at higher and lower frequencies representing much higher insertion loss. In comparison, the insertion loss at 4kHz and 125 Hz is much less. In the UK, it is common for the target noise criterion to be defined using an internationally recognised set of spectra (so-called noise rating curve, or NR), shown in Figure 2 at 112. [0009] Acousticians typically find themselves specifying an attenuator according to its low frequency insertion loss. This inevitably leads to a noise spectrum in the ventilated room that is spectrally unbalanced. The excess mid-frequency insertion loss results in much lower noise levels in that frequency range compared to that at the lower and higher ends of the spectrum. This may not appear to be problematic as long as the noise is reduced sufficiently overall. However, reducing the mid-frequency noise levels more effectively has the subjective effect of emphasizing the lower frequencies. This can lead to occupants of the ventilated room feeling that the lower frequency rumble is worse than it actually is, even though it meets the target noise levels. [0010] A typical 1600mm long splitter attenuator such as that shown in Figure 1, meets the specification, by being lower than the NR35 curve 112. As seen in Figure 2, the curves 110 and 112 are roughly coincident at the 125Hz level so that the performance target is just met for that frequency. However, the resultant spectrum is significantly divergent by virtue of the excessive insertion loss in the mid- and upper-frequency bands. [0011] Conventional splitter attenuators work on a purely resistive basis which results in poor low frequency performance. This is expected given that the porous absorption used typically increases with frequency with worst performance at lower frequencies. [0012] Another problem with splitter attenuators is that they introduce significant resistance to the airflow. This is compounded when the attenuator length is specified to satisfy the lower frequency criteria (thus overcompensating at the mid frequencies), as this results in the attenuator being more intrusive on the airflow than it would otherwise be. This causes greater losses in the airflow path. For example, the 1600m