Ear Defenders

Abstract

An ear defender in which acoustic elements are added which in conjunction with the shunt capacitance provided by the volume enclosed by the shell thereof and the wearer's head constitute a low-pass or band-pass filter.

Description

This invention relates to ear defenders.

Ear defender assemblies are frequently an integral part of other head-worn equipment such as protective helmets, and in such cases it is difficult to remove the headgear for the occasions on which the wearer wishes to hear normal airborne sounds. This difficulty has been reduced by arranging a port or ports suitably disposed in the material of the ear defender shell so arranged that the port or ports can be opened or closed at will. When the ports are closed the ear defender assembly provides maximum attenuation to airborne sounds and noises, but when the ports are opened airborne sounds are permitted to reach the wearer's ears in a fairly normal manner.

Such arrangements are liable to suffer from the disadvantages that spurious noises can be generated by the movement of air over the sound inlet arrangements, as for instance occurs in windy conditions, and furthermore frequency distortion is liable to be introduced into the original airborne sound. It is accordingly an object of the present invention to provide an improved ear defender in which one or more of the aforementioned disadvantages are reduced or overcome.

According to the invention there is provided an ear defender including a shell of rigid material for enclosing a volume of air around a wearer's ear, a passage in said shell for sound waves to said volume of air, at least one element providing acoustic impedance between the outside of said ear defender and said passage, said impedance being selected in conjunction with the acoustic impedance of said volume of air so as to constitute therewith an acoustic filter which attenuates at least higher frequency components of sound passing to said wearer's ear.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 shows a cross-sectional elevation of an embodiment of an ear defender according to the invention;

FIG. 2 is an end view of said ear defender;

FIG. 3 is the equivalent electrical circuit to the acoustic circuit of said ear defender;

FIG. 4 is a frequency response curve for said ear defender;

FIG. 5 is a cross-sectional elevation of an ear defender in accordance with another embodiment of the invention;

FIGS. 6 to 10 are electrical equivalent circuits illustrative of the operation of the invention; and

FIGS. 11 to 15 are graphs representing the characteristics of the respective electrical equivalent circuits of FIGS. 6 to 10.

Referring to FIG. 1, the member 1 is a shell of rigid and dense material which can enclose a volume of air around the wearer's ear (not shown). Member 2 is a sealing annulus whose function is the sealing of the forward annular face of the shell to the side of the wearer's head and the elimination of acoustic leaks between the internal volume of the shell and the outside air. Said member 2 consists preferably of a liquid-filled tube, but alternatively it may be filled with some resilient material such as plastic foam. In conjunction with member 1 the shell 2 provides a significant degree of attenuation of sound in accordance with known principles.

In the rear face of member 1 ports 3 are provided, and bearing against the outside face of the rear of the shell 1 is a disc member 4 which possesses ports 5 disposed so as to align with the ports 3 when said disc 4 has a suitably relative disposition with the rear face of the shell 1. Said disc 4 is arranged to be rotatable by means of the axial mounting 6 and thus if the disc 4 is suitably rotated the holes 3 and 5 are no longer in alignment. The disc 4 is covered on its outer face by a disc or pad 7 of suitably porous material such as felt or preferably foamed plastic.

In operation when the member 4 is rotated so as to be suitably positioned relative to the rear face of member 1 then the ports 3 and 5 are not in alignment, and so there is no airway from the outside air into the internal volume enclosed by member 1. Under these conditions the full noise and sound attenuation of the ear defender assembly is achieved.

When the member 4 is rotated so as to bring ports 3 and 5 into alignment then an airway exists from the outside air into the volume enclosed by member 1 through the disc 7 and ports 3 and 5. Disc 7 which is of suitably chosen material and dimensions constitutes an acoustic resistance. Its purpose is to absorb the acoustic energy in any turbulence which is created when the assembly is present in the path of moving air, for example in windy conditions, and reduce the acoustic noise which would otherwise be produced by such turbulence and which would reach the wearer's ear. Furthermore the acoustic resistance, which is directly in the path of sound waves passing from the outer air into the volume, 20, enclosed by member 1 and the ear of the wearer constitutes one element of a low-pass filter of the resistance-capacitance type. The capacitive element of said low-pass filter is formed by said air volume 20. Said filter is illustrated in FIG. 3 wherein P.sub.in represents the input pressure of the system and P.sub.out represents the output pressure. The frequency response of this filter is illustrated in FIG. 4. The precise value of the effective acoustic resistance in the path of incident sound waves is defined not only by a careful choice of material and dimensions of the disc 7 but also by the size and number of the ports 3 and 5. According to the well known considerations which apply to the performance of resistance-capacitance type low-pass filters it is possible to define the effective acoustic resistance of disc 7 to incident sound waves such that transmission of such sounds to the wearer's ear is substantially unattenuated up to a chosen frequency f.sub.1. Above this frequency the transmission will fall eventually at the rate of 6 db. per octave. If the frequency f.sub.1 is suitably chosen to equal the maximum frequency at which speech sound must be passed substantially unattenuated, for example 3,000 Hz., the arrangement brings about a great improvement in the intelligibility of speech reception under noisy conditions by substantially eliminating all sounds and noises which are not within the frequency range which is necessary for the transmission of intelligible speech.

The disc 7, because of its nature, may require protection by a protective member 8 which consists of a fine mesh grill. If it is necessary to exclude moisture such as may be encountered in conditions of rain then member 8 may consist of a thin disc of suitably flexible moisture-repellent material such as polyethylene. The acoustic impedance of such a plastic disc can be made suitably small compared with the other acoustic impedances in the system. An outer housing 9 serves the functions of maintaining in close proximity the relevant elements, and of providing a suitable grip for the required rotation of the acoustic valve assembly. A spring member 10 is provided and is arranged to ensure that when the acoustic valve is in the closed position member 4 and the rear face of member 1 are maintained in intimate contact so as to avoid the inlet of sound via any unintentional gap between said member 4 and the rear face of said member 1.

A telephone receiver 11 is incorporated in the assembly for the reception of speech or other signals via an electrical communications system. The telephone receiver is so mounted that it does not obstruct the sound path from the outside air to the wearer's ear when the acoustic valve arrangement is in the open position.

A suitable quantity of acoustic absorbent material 12 is incorporated into the volume enclosed by member 1 so as to reduce or effectively eliminate any unwanted acoustic resonances within the volume.

Referring to FIG. 5, parts similar to those shown in FIG. 1 have the same reference numeral. In this embodiment of the invention the construction is similar to that described above with regard to the shell 1, sealing annulus 2, ports 3, disc member 4 and ports 5.

However, it differs from that described above in that there is added an element of volume 13 and a member 14 which is a thin resilient diaphragm having the mechanical properties of stiffness and mass. Member 15 is an operating means for rotating disc 4, but other means such as, for example, an axially mounted knob, may also be used.

The manner of operation of this embodiment will be described by considering the effects of the individual acoustic elements as they are incorporated into the system in turn.

Referring to FIG. 6, the enclosed volume 20 shown in FIG. 1 is represented by a shunt capacitance C.sub.1 by classical analogy. As before, P.sub.in represents the input pressure of the system and P.sub.out represents the output pressure. It can be shown that such a system has zero attenuation provided that the source impedance associated with the energy input is small compared with the impedance of the element C.sub.1, and this is the case when we are concerned with the propagation of sound in free air. The response of such a system is shown in FIG. 11.

Referring to FIG. 7 if we add an impedance C.sub.2 then this system will provide an attenuation independent of frequency and of degree dependent upon the relative impedances of C.sub.1 and C.sub.2 according to known principles. The response of such a system is shown in FIG. 12, and in this embodiment the impedance C.sub.2 is realized by the stiffness of the diaphragm 14 shown in FIG. 5.

Referring now to FIG. 8, if the element of mass M.sub.1 of the diaphragm 14 is added, then the response of the system will be of the form shown in FIG. 13 in which the peak of response occurs at a frequency determined by the impedance of M.sub.1 in conjunction with the combined impedances of C.sub.1 and C.sub.2. It will be appreciated that over a small band of frequencies the attenuation is now zero or may be even less than zero.

Referring now to FIG. 9, an element R.sub.1 associated with diaphragm 14, but not necessarily so, has been added in order to eliminate the undesirable enhancement of the response of the system shown in FIG. 8. The response of the system shown in FIG. 9 is as shown in FIG. 14. It can be seen that we now have an approach to a band-pass filter. It will normally be the case that when the resilience of diaphragm 14 of FIG. 5 has been suitably chose so as to provide the required degree of low-frequency attenuation, and when the mass M.sub.1 of diaphragm 14 shown in FIG. 5 has been chosen to provide reduction of this attenuation at the chosen frequency, then the higher frequency cutoff of the system will be undesirably low.

Referring to FIG. 10, three additional elements namely C.sub.3 representing volume 13 in FIG. 5, M.sub.2 representing the acoustic inertance of the ports 3 and 5 in FIG. 5, and R.sub.2 representing the acoustic resistance of member 12 of FIG. 5 have been added. Member 12 also damps resonances in the cavity 20 of FIG. 5. By suitable proportioning of all of the elements now included in the system according to this invention, the higher frequency resistance of the system may be somewhat extended to provide an overall response as shown by curve A in FIG. 15. It will be seen that over a certain band of frequencies the ratio P.sub.out /P.sub.in is unity or very nearly so and thus there is substantially no attenuation over this range of frequencies. At lower frequencies, however, a significant attenuation is introduced and at higher frequencies a significant attenuation is also introduced.

The frequency range over which there is substantially no attenuation can be suitably chosen to include the range of frequencies which are most important with respect to the intelligible reception of speech, for instance from 1,000 Hz.- 3,000 Hz. The fact that at both lower and higher frequencies there is significant attenuation results in a desirable improvement in signal/noise ratio and intelligibility when the wearer of the ear defender assembly is wishing to listen to speech whilst in a noisy situation.

The curve B in FIG. 15 shows the response of the system when the impedance of element C.sub.2 has been increased due to increased air pressures in the environment surrounding the ear defender assembly. It can be seen from a comparison of curves A and B that significant additional attenuation is introduced over a useful frequency range with consequent improved protection of the wearer's ears against the explosive loud noises which produce such an increased air pressure.

Member 14 is preferably but not necessarily made of a suitable plastics material. It also serves the useful purpose of sealing the acoustic valve assembly against the ingress of moisture and of preventing discomfort to the wearer when the ear defender assembly is used in windy situations.

If the impedance of C.sub.2 is reduced, by making the diaphragm 14 of flexible material such as sheet polyethylene which is mounted in a nontaut manner, so that its impedance is negligibly small with respect to the impedance of the other acoustic elements in the system, the system reverts to a low pass filter with characteristics similar to those of the embodiment described with respect to FIG. 1. However when the diaphragm is forced into a taut condition by increased air pressure due to explosive noises the attenuation is even greater because there was little initial attenuation of the low frequencies. The same effect can be achieved in a modification of the embodiment shown in FIG. 1 in which the member 8 is a sheet of polyethylene mounted in a nontaut manner. This has small impedance compared with the other element in the system and therefore effects substantially no attenuation. However in the presence of increased air pressure due to explosive loud noises the sheet is driven taut and then acts as a substantial attenuating impedance. In both cases when the member is in its taut condition the attenuation in the ear defender is similar to that when the acoustic valve is closed.

The complete ear defender assembly may be simply supported on the head by means of a headband or it may be incorporated into a protective helmet such as worn by aircrews and the crews of fighting vehicles.

Claims

What is claimed is:

1. Ear defender apparatus for protecting the ear of a user against undesirable sound, comprising

a shell (1) adapted to enclose a first volume of air (20) about the user's ear, said shell containing passage means (3) for conducting sound waves to said volume of air;

a layer of acoustic absorbent material (12) lining the interior surface of said shell to reduce unwarranted resonance in said volume of air;

an acoustic impedance element (7, 14) connected with the external surface of said shell across the orifice of said passage means for controlling the transmission of sound to said volume of air through said passage means, said impedance element cooperating with said first volume of air to define an acoustic filter that attenuates at least the higher frequency components of sound that would otherwise pass to the user's ear; and

acoustic valve means (4) arranged between said shell and said impedance element for operation between open and closed positions relative to said passage means, said acoustic valve means including a disc (4) connected for coplanar movement relative to said shell, said disc containing at least one aperture (5) arranged for selective alignment with the orifice of said passage means.

2. Apparatus as defined in claim 1, and further including spring means (10) biasing said disc in the direction of sealed engagement with said shell.

3. Apparatus as defined in claim 1, wherein said acoustic impedance element comprises a diaphragm (14) connected in spaced relation with said shell to define a second volume of air (13) on the opposite side of said passage means from said first volume of air.

4. Apparatus as defined in claim 3, and further including valve-operating means connected at one end with said disc, said valve-operating means (15) extending through said second volume of air and through an opening contained in said diaphragm.

5. Apparatus as defined in claim 1, wherein said acoustic impedance element comprises a porous pad (7).

6. Ear defender means for protecting the ear of a user against undesirable sound, comprising

a. a shell (1) adapted to enclose a first volume of air (20) about the user's ear, said shell containing passage means (3) for conducting sound waves to said volume of air; and

b. an acoustic impedance element connected with the external surface of said shell across the orifice of said passage means for controlling the transmission of sound to said volume of air through said passage means, said impedance element cooperating with said first volume of air to define an acoustic filter that attenuates at least the higher frequency components of sound that would otherwise pass to the user's ear, said acoustic impedance element comprising

1. a resilient diaphragm (14) having properties of stiffness and mass, and

2. means mounting said diaphragm in an initial nontaut condition to enclose a second volume of air (13) in communication with the end of said passage means remote from said first volume of air, said diaphragm being operable to a taut condition by the increased air pressure produced by explosive noises, thereby increasing the attenuation afforded by said acoustic impedance element.

7. Apparatus as defined in claim 6, and further including a sealing annulus of flexible material connected with the peripheral edge portion of said shell for sealing said shell to the wearer's head.

8. Apparatus as defined in claim 6, and further including a layer of acoustic absorbent material lining the interior surface of said shell to reduce unwanted resonance in said first volume of air.

9. Apparatus as defined in claim 6, and further including acoustic valve means (4) arranged between said shell and said impedance element for operation between open and closed positions relative to said passage means, said acoustic valve means including a disc (4) connected for coplanar movement relative to said shell, said disc containing at least one aperture (5) arranged for selective alignment with the orifice of said passage means.

10. Ear defender means for protecting the ear of a user against undesirable sound, comprising

a. a shell (1) adapted to enclose a first volume of air (20) about the user's ear, said shell containing passage means (3) for conducting sound waves to said volume of air;

b. an acoustic impedance element connected with the external surface of said shell across the orifice of said passage means for controlling the transmission of sound to said volume of air through said passage means, said acoustic impedance element including

1. a resilient diaphragm (14) having properties of stiffness and mass, and

2. means mounting said diaphragm in an initial nontaut condition to enclose a second volume of air (13) in communication with the end of said passage means remote from said first volume of air, said diaphragm being operable to a taut condition by the increased air pressure produced by explosive noises, thereby increasing the attenuation afforded by said acoustic impedance element; and

c. acoustic valve means (4) arranged between said shell and said impedance element for operation between open and closed positions relative to said passage means, said acoustic valve means including a disc (4) connected for coplanar movement relative to said shell, said disc containing at least one aperture (5) arranged for selective alignment with the orifice of said passage means.