EP1321725A1

EP1321725A1 - Device particularly for reducing the noise emitted by air conditioners

Info

Publication number: EP1321725A1
Application number: EP02027607A
Authority: EP
Inventors: Alberto Doria; Paolo Tarquini; Stefano Strapparava
Original assignee: Liebert Hiross SpA
Current assignee: Vertiv Srl
Priority date: 2001-12-21
Filing date: 2002-12-11
Publication date: 2003-06-25
Also published as: ITPD20010297A1

Abstract

A device (10) particularly for reducing the noise emitted by air conditioners (11), comprising at least one acoustic resonator (18), with noise reduction effectiveness over a low-frequency range, arranged at an air passage cavity located inside the air conditioner (11), in a region where the air flow is substantially uniform.

Description

The present invention relates to a device particularly for reducing the noise emitted by air conditioners.
In air conditioners, the most significant source of noise is generally constituted by the fan of the air conditioner.
Various kinds of fan can be used depending on the application and on the type of conditioner.
The most widely used fans in the air-conditioning field are centrifugal fans, whose impeller can have forward-curved blades or backward-curved blades.
Fans with impellers that have backward-curved blades, for example, are preferable owing to their electromechanical characteristics and their great flexibility.
From the constructive standpoint, fans with backward-curved blades have an impeller that is characterized by a very small number of blades (5-10), and accordingly the emitted noise spectrum has a peak, due to the transit of the blade, in the low-frequency range (below 500 Hz).
For example, a fan with nine blades and a rotation rate of 1500 rpm has a blade frequency of 225 Hz.
Due to the impeller, the system is excited by a tonal excitation in the low-frequency range, and due to the various acoustic modes that are present in the internal cavities of the conditioner the response is high and occurs as peaks of noise in a frequency range where it is difficult to intervene with the reduction methods conventionally used.
Conventional devices suitable to reduce the noise emitted by conditioners generally use insulating or absorbent materials.
With acoustic insulation, one acts on the propagation of sound, which is hindered by means of materials that partly reflect it, allowing the smallest possible amount to pass.
The materials used are high-density materials, which reduce the transmitted component, which varies at the various frequencies and is obviously a function of the material and of its geometric characteristics.
With acoustic absorption, the incident sound wave is converted into thermal energy inside the structure of the sound-absorbing material.
Materials having an open-cell porous structure are used for this purpose.
The thickness between the surface of the material and the rigid support affects the absorption characteristic, which is highest for thicknesses equal to one quarter of the wavelength.
The absorbing materials are normally placed inside ducts arranged inside the delivery and intake circuits of conditioners.
Devices of this type exhibit good performance when the noise to be reduced is characterized by an acoustic spectrum with significant components for frequencies higher than 500 Hz.
The effectiveness of these systems, however, decreases when the noise has components at medium-low frequencies (100-250 Hz), unless one uses very long ducts that are longer than the wavelength of the sound to be reduced.
The aim of the present invention is to provide a device particularly for reducing the noise emitted by conditioners that is particularly effective in the low-frequency range.
Within this aim, an object of the invention is to provide a device that is effective over a wide frequency range.
Another object is to provide a device that has a simple structure.
Another object is to provide a low-cost device that can be manufactured with systems and technologies of a known type.
This aim and these and other objects that will become better apparent hereinafter are achieved by a device particularly for reducing the noise emitted by air conditioners, characterized in that it comprises at least one acoustic resonator, with noise reduction effectiveness over a low-frequency range, arranged at an air passage cavity located inside said air conditioner, in a region where the air flow is substantially uniform.
Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein:
Figure 1 is a perspective view of an air conditioner to which a device for reducing emitted noise according to the invention is applied;
Figure 2 is a perspective view of a component of the device according to the invention;
Figures 3 and 4 are perspective views of two embodiments of the device according to the invention;
Figure 5 is a view of a mechanical model of the device according to the invention;
Figure 6 is a qualitative chart of the effectiveness of the device according to the invention.
With reference to the figures, a device particularly for reducing the noise emitted by conditioners, according to the invention, is generally designated by the reference numeral 10.
Figure 1 is a schematic partially sectional view of a conditioner, generally designated by the reference numeral 11, which conditioner comprises, inside a box-like container 17, a finned battery 12 for cooling air that arrives from outside.
The air is moved by a fan 13, flowing through cavities located inside the box-like container 17 and formed by internal walls 14 and by the walls of the box-like container 17.
Figure 1 further illustrates a compressor 15, which is connected to a corresponding circuit 16, which is per se known, for providing the refrigeration apparatus.
The device 10 comprises a plurality of acoustic resonators 18, which are arranged in mutually opposite arrays on two side walls 14a and 14b, which form a cavity 19 through which the fan 13 aspirates.
Each acoustic resonator 18 is of the Helmholtz type, which comprises a tubular neck 20 that has a circular cross-section and is connected to a box-like body 21 that is shaped substantially like a parallelepiped.
Advantageously, each resonator 18 can be lined internally with sound-absorbing material in order to improve its noise reduction properties.
As shown in Figure 3, the device 10 can comprise a single pair of resonators 18, arranged on two side walls 14a and 14b of the intake cavity 19 of a conditioner 11.
As regards operation, a resonator 18 acts like an equivalent mechanical system constituted by a mass and a spring with a single degree of freedom, as shown in Figure 5.
The fluid contained in the neck 20 behaves like a concentrated mass 23, while the fluid contained in the box-like body 21 behaves like a pneumatic spring 24.
The resonator 18 has a natural frequency that depends on the volume of the box-like body 21 and on the cross-section and length of the tubular neck 20.
It is therefore possible to size the resonator 18 so that the frequency band in which it is effective can be in the low-frequency range.
Sizing is defined by the following function: f = c2π A lV where c is the speed of sound at 20° (c=343 m/s), f[Hz] is the frequency, A [m²] is the area of the cross-section of the neck 20, l [m] is the length of the neck 20, and V [m³] is the volume of the box-like body 21.
As shown qualitatively in Figure 6, the response obtained by a resonator 18 or by an array of resonators is a reduction in noise level in a narrow band that straddles the resonance band.
The resonator 18 is arranged conveniently in a region where the air stream is substantially uniform and is not affected by the presence of the resonator 18, with the opening 22 of the tubular neck 20 arranged parallel to the air stream.
The resonator 18 is sized so that its natural frequency is coupled to the frequency of the main peak of the excitation spectrum, which coincides with the blade frequency.
In order to increase the effectiveness of the device 10, instead of increasing the volumes of a single resonator 18 it is possible, as mentioned, to provide arrays of resonators, as shown also in Figure 4, all tuned to the same frequency, so that the effects of the individual resonators 18 are combined, with the advantage of having a better filling of the spaces available inside the cavity 19.
Advantageously, the cavity in which one or more resonators 18 are arranged have a high frequency response around the blade frequency, i.e., their resonance frequency is close to the blade frequency.
When using arrays of resonators 18 it is possible to differentiate the tuning frequencies of the resonators 18.
In this case, the device 10 has a wider frequency range over which noise reduction occurs.
Advantageously, it is possible to provide devices 10 in which the tuning frequency of the individual resonators 18 and/or of the entire array can be changed as the excitation frequency varies.
For this purpose, it is possible to use variable-geometry resonators 18, for example with replaceable necks 20 that are flanged and fixed reversibly to the box-like body 21, with telescopic necks 20, et cetera.
The resonators 18 can be made of materials of various kinds and thicknesses, but so that the walls that form the internal cavity of the resonator have structural frequencies that are higher than the tuning frequencies, in order to avoid the resonance condition.
Advantageously, it is possible to use plastics or composite materials constituted by walls made of metal plate with interposed sound-absorbing plastic material.
In practice it has been found that the present invention has achieved the intended aim and objects.
A device has in fact been provided which is particularly effective for reducing noise emitted by conditioners.
The device has a simple structure and furthermore has a low cost.
The present invention is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept.
The technical details may be replaced with other technically equivalent elements.
The materials, so long as they are compatible with the contingent use, as well as the dimensions, may be any according to requirements.
The disclosures in Italian Patent Application No. PD2001A000297 from which this application claims priority are incorporated herein by reference.
Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

A device (10) particularly for reducing the noise emitted by air conditioners (11), characterized in that it comprises at least one acoustic resonator (18), with noise reduction effectiveness over a low-frequency range, arranged at an air passage cavity (19) located inside said air conditioner (11), in a region where the air flow is substantially uniform.
The device according to claim 1, characterized in that said resonator (18) is of the Helmholtz type, comprising a tubular neck (20) connected to a box-like body (21).
The device according to claim 2, characterized in that said neck (20) has a circular cross-section.
The device according to claim 2, characterized in that said box-like body (21) is substantially shaped like a parallelepiped.
The device according to one or more of the preceding claims, characterized in that said resonator (18) is tuned to the blade frequency of the fan (13) of said air conditioner (11).
The device according to one or more of the preceding claims, characterized in that the opening (22) of said neck (20) of the resonator is parallel to the air stream.
The device according to one or more of the preceding claims, characterized in that said at least one resonator (18) is arranged in such a position as to not affect the air stream.
The device according to one or more of the preceding claims, characterized in that said resonator (18) is arranged adjacent to the intake of the fan (13) of said air conditioner (11).
The device according to one or more of the preceding claims, characterized in that said resonator (18) is arranged in a cavity that has a high frequency response around the blade frequency or with a resonance frequency that is close to the blade frequency.
The device according to one or more of the preceding claims, characterized in that it comprises a pair of resonators (18) arranged on two mutually opposite walls (14a, 14b) of said cavity (19).
The device according to one or more of the preceding claims, characterized in that it comprises two arrays of resonators (18) tuned to the blade frequency of the fan (13) and arranged on two mutually opposite walls (14a, 14b) of said cavity (19).
The device according to one or more of claims 1 to 10, characterized in that it comprises two arrays of resonators (18) arranged on two mutually opposite walls (14a, 14b) of said cavity (19), said resonators (18) being tuned to different frequencies.
The device according to one or more of the preceding claims, characterized in that it comprises resonators (18) whose tuning is variable according to the blade frequency of the fan (13).
The device according to claim 13, characterized in that said resonators (18) comprise replaceable necks.
The device according to claim 13, characterized in that said resonators (18) comprise telescopic necks.
Use of acoustic resonators (18) particularly for reducing the noise emitted by air conditioners.
The use according to claim 16, characterized in that said resonators (18) are of the Helmholtz type.