FR2540912A2 - Passive acoustic absorber device also absorbing low frequencies - Google Patents

Abstract

The invention relates to acoustic absorber devices located in any environment as a panel, wall, baffle or central bulb-shaped casing for a premises or for a pipe. The absorber is made up of two plates forming a Helmholtz resonator with perforated wall and continuous structure. The first plate has a variable thickness and is pierced with precalculated holes. The second plate comprises planes which are not parallel to the first plate in order to obtain a variable length of the cavity of the Helmholtz resonator. The device according to the invention is particularly intended for devices for limiting noises which exist or are propogated in any premises, volume, air or fluid passage such as absorbing wall panels, soundproof partitions, noise-reducing walls, machine cowlings, industrial silencers, ventilation ducts, air-conditioning ducts, exhaust pipes of internal combustion engines, reactor nozzles, fluid pipes, apartment fresh air inlets, various ventilation systems.

Description

 Based on main patent N08209711, the present invention relates to passive acoustic absorbers effective from low frequencies and located in any environment as a panel, wall, baffle or central bulb for a room or for a conduit.

 The technical sector of the invention is that of the construction of devices for combating noise existing or propagating in any room, volume, passage of air or any fluid such as absorbent wall panels, sound barrier partitions, anti walls -noise, machine cowling, industrial silencers, ventilation and air conditioning ducts, engine exhaust pipes, reactor nozzles, fluid pipes, apartment fresh air intakes, various vents, etc.

Currently, the most commonly used absorbers are those based on an absorbent material such as mineral wool or those which use the principle of Helmholtz resonators. For the former, there are two main drawbacks
1) their ineffectiveness in absorbing sound waves of low frequencies.

 2) their impossibility of use for excessively high temperatures (heat engines for example).

 The second can be made of any material (suitable for each condition of use) so they are not limited by temperature. A Helmholtz resonator absorbs a very narrow frequency band around a calculated resonance frequency - It is therefore very frequency selective - To overcome this drawback and trying to widen the band of absorbed frequencies, several new designs of resonators have been created - In all cases, they are Helmholtz resonators whose wall in direct contact with noise has a variable impedance, ie a variable resonant frequency.

Various means have been used for this - Some are bulky and difficult to install (see patent 2488710 / 80.17740) - Others, mobile require permanent adjustment (manual or automatic) with all that that entails the risk of wear, breakdown etc ... (see patent 2482663 / 81.08469)
All of them have either a good and regular absorption but over a still too narrow frequency band (see for example patent 2 201 010 / 73.32775), that is to say a correct absorption over a wide frequency band but having significant holes (see for example patent 2 468 964 / 80.22461, or patent 2 434 450 / 79.18929).

The present invention presents an absorber, the wall of which in direct contact with noise has a variable impedance and the resulting absorption of which is regular and located over the entire audible frequency band,
low frequencies included.

The noise absorber consists of wall-mounted Helmholtz resonators
perforated and with continuous structure of variable neck length and length
of variable cavity, this being achieved by a first thickness plate
variable and by a second plate whose planes or curved surfaces are not
not parallel to the first plate. Having the second plate cons
planes or curved surfaces not parallel to the first plate allow
to be able to use several identical rows of holes side by side while
respecting the length of the resonator cavity and the need for an oblique reflection plane. This promotes the particular absorption of a
frequency band.

The absorber will advantageously be covered when the conditions
use allow (nature of the fluid, pressure, temperature ...) of a
porous material.

 In order to increase the insulation in high frequencies, the space between the two plates of the rock wool absorber can be filled.

An embodiment is given by way of nonlimiting example.
referring to the attached drawings
Figure 1 is an overall sectional view of an absorbing device
according to the invention.

Figure 2 is a cross section of the noise absorber in a
new variant example.

Figure 1 shows an overall sectional view of an absorb panel
bant comprising absorbers according to the invention placed repeatedly-
The parts (1) facing the noise source constitute the first plates
absorbers according to the invention - Parts (2) constitute the second
plates - The part (3) represents the possible porous panel covering the absorber
noise butter according to the invention - The part (4) can either be air or a
any material and the peg (5) will be, in the case of a wall panel, a
support plate - In order to stiffen the assembly, beams, of the same thickness
that the panel, can be added in width or in height
of the panel.

Figure 2 is a cross section of the noise absorber in
a new variant.- The part (7) represents the first plate of the absorber
butter if not the invention - Its thickness, the diameter of its holes and the distances
separating them are calculated according to the nature of the fluid in which it propagates
the noise. The part (8) constitutes the second plate of the absorber according to the in
vention.

 The interest of the improvements brought to the main patent lies essentially in the considerable widening of the field of application.

It is no longer just acoustic absorbers for air passages, but acoustic absorbers located in any environment as a panel, wall, baffle or central bulb for a room or for a duct.

Claims (3)

1) Passive acoustic absorber device which absorbs sound waves  of all frequencies, including frequent bass, as sold  cation 1 of main patent NC 82 097 11, characterized in that the  second plate consists of several planes or curved surfaces  not parallel to the first plate. 2) Device according to claim 1, characterized in that the absorb  butter may be covered with porous material. 3) Device according to claim 1 characterized in that the space  located between the two plates of the absorber can be fitted with a  porous material.