US2684067A

US2684067A - Soundproof shield

Info

Publication number: US2684067A
Application number: US227136A
Authority: US
Inventors: Pierre A Lienard
Original assignee: Office National dEtudes et de Recherches Aerospatiales ONERA
Current assignee: Office National dEtudes et de Recherches Aerospatiales ONERA
Priority date: 1951-05-19
Filing date: 1951-05-19
Publication date: 1954-07-20
Anticipated expiration: 1971-07-20

Description

July 20, 1954 P. A. MENARD SOUNDPROOF SHIELD Filed May 19, 1951 ntinlllllllllnl il ATIONS IN MICRO z 86 I. 2 3 3 a a MPLITUDE 0F VIBR 5 FIGA lNvNToR Pierre A. Llenard Y ATTORNE Patented July 20, 1954 STATS SOUNDPROOF SHIELD a company of France Application May 19, 1951, Serial No. 227,136

3 Claims. l

This invention relates to soundproof shields or helmets intended to be worn o-ver the ears of a subject as protection against noise and as an aid to telephonie communication, and more particularly to a noise shield constructed of a plurality of lamina-tions of metallic and non-metallic materials having widely different mechanical properties offering a maximum of protection against ambient noise.

Soundprooi shields for the ears are known in 'which cups or shells of soft and flexible materials such as rubber are provided to nt over the ears of the wearer, with or without accommodation for the receivers of a telephone headset. These known devices however provide only an imperfect degree o1" soundprooflng because, being made of materials of low and substantially uniform modulus of elasticity, they transmit sound waves without great attenuation, especially at low frequencies.

According to the present invention instead, there is provided a shield which combines into a unitary structure a plurality of layers of metallic and non-metallic materials, having high coefiicients of internal friction and having widely dissimilar moduli of elasticity. By the selection of materials having these properties and hy their intimate union into a unitary structure, the transmission of sound from the exterior to the interior of the shield is greatly reduced as regards both the propagation of sound waves through the structure of the shield and as regards sympathetic vibrations of the structure or of the various elements thereof as a whole.

according to another feature of the invention the varius layers or laminations of the shield, and in particular the metallic layer or layers thereof, are given a shape which permits the ready conformation of the shield to the head of the wearer or to the heads of successive wearers, so as to effect an improved seal ybetween the shield and the wearers head in spite of the use of metallic layers as elements of the shield.

The invention will now be further described in detail in terms of a preferred embodiment and with reference to the accompanying drawings in which Fig. i is a view partly in section of a soundprooi" shield according to the invention incorporating a telephone receiver;

Fig. 2 is a graph illustrating the sound absorption occurring in the layers of the soimdproof shield of the invention;

Fig. 3 is a sectional View, at a reduced scale, of an insert adapted to replace the telephone re- (Cl. 12S-152) ceiver shown in the shield of Fig. 1 when the shield is to be used otherwise than for communication purposes;

Fig. fl is a view in plan oi a spacer adapted for use with the shield of the invention when employed by a user wearing spectacles, and

Fig. 5 is a sectional View taken on the line 5--5 of Fig. ll.

The shields of the invention are used in pairs, one for each ear of the wearer, and they may be used with or without the receivers of a telephone headset. Fig. 1 shows a sectional View of one such shield in which a telephone receiver has been tted. The shield of Fig. 1 is of a size to surround completely the external ear, preferably without touching it, and is adapted to seal off the auricular canal from exterior space and to protect from acoustic vibrations the portion of the head most sensitive to bone conduction. The two shields are connected together for use by means of a headband, not shown, which may be of any suitable type and to which the shields may be connected by means of forks i and thumb screws 8.

The shield of Fig. 1 comprises a laminated structure of which the major component, in volume, is a lightweight plastic material having a high coeiiicient of internal friction, for eX- ample polyvinyl chloride foam. Polyvinyl chloride is to be preferred to rubber foam which has too low a mechanical hysteresis. A cup or shell I of such plastic material is joined in intimate contact as by gluing with a cup-shaped sheet 3 of lead or of a lead-tin alloy. A second shell or lamination 2 of plastic material similar to the shell i engages the inner side of the metaliic sheet 3 in similar intimate contact. The shell i is encased in a metallic cup 5, made for example of aluminum, which serves both acoustic and mechanical purposes for the shield. The cup 5 provides mechanical protection to the shield as a whole and provides also with the plastic shell I an additional interface between dissimilar materials at which a fraction of the incident sound waves are reflected.

With the four laminations l, 2, 3 and E the sound must over substantially the entire surface of the shield pass through four separate layers of material with three plastic-metal interfaces in order to penetrate into the interior. This produces a great attenuation of incident sounds even as to low frequency components. At each of these interfaces the discontinuity in medium effects reflection of a portion of the incident sound wave amplitude.

The construction of the invention provides a further reduction in acoustic transmission to the interior of the shield by the intimate union of the plastic layers I and 2 with metallic layers 3 and 5. By aixing the plastic layers to the cup and to the lead alloy sheet 3 there is promoted a loss of energy in friction attendant upon the mass vibration of the metallic layers when vibrating as diaphragms. By increasing the damping of these vibrations the points of resonance of the metallic laminations are substantially lowered. According to the invention therefore the damping of vibrations cf the outer casing 5 and of the intermediate metallic sheet 3 is increased by gluing or otherwise fastening thereto in intimate contact the layers of plastic foam material I and 2 which have, for their density, a high internal coefficient of friction or acoustic resistivity.

Attenuation of sound waves passing through the resulting structure is therefore due not only to the alternation of materials having widely different moduli of elasticity but also to dissipation of sonic energy by internal friction in the plastic layers and also to the damping which these plastic layers provide to vibration of the metallic layers 3 and 5.

Apart from the leakage of sound past the edge of the shield where it adjoins the skin of the wearer, which is reduced to a minimum by the shape given to the edge of the shield of the invention, as will be presently described, the acoustic permeability of the shield depends upon three factors, viz. the coefficients of internal friction or the acoustic resistivities of the component layers, the coeicients of transmission of the sound waves across the various interfaces be tween the successive layers of the shield, and the transmission factors representing the flexural vibrations of the laminae as diaphragms.

The coefficient of transmission for sound waves progressively propagated across an interface between two diiferent media is given by 4m (1i-W02 in which is the ratio of the acoustic resistivities of the media, E1 and E2 are their bulk moduli of elasticity and p1 and p2 are their densities. The following table gives values for the coefiicients of transmission through aluminum-rubber foam, rubber foam-lead, and lead-rubber foam interfaces.

The data of the table show that by choice of media on either side of the interfaces having widely dissimilar moduli of elasticity the ratios m of the acoustic resistivities of the two media at the interfaces are made either Very large or very small by comparison with unity so that in either case the resulting coefficient of transmission is very small.

The diaphragm transmission coefficient is without practical importance in the case of the plastic layers because of their large thickness compared to their diameter. In the case of the metallic layers 3 and 5 the effective diaphragm transmission coefficient is greatly reduced by the damping of these layers which are loaded with the plastic layers I and 2. Apart from the loading factor, the diaphragm transmission coefficient decreases with increasing mass of the lamina undergoing flexural vibrations and with its internal coefficient of friction. In order to reduce this transmission coefficient it is therefore advantageous to use for the metallic layers metals having at once a high density and a high coefficient of friction. Lead fulfills both of these requirements and is therefore used in the shield of the invention at the interface between the two plastic layers. The internal coefficient of friction of a material may be measured by the loss of energy per vibration cycle of a `par subjected to transverse vibrations. rlhe curves of Fig. 2 give in logarithmiccoordinates the loss in ergs of energy per cycle and per unit volume of the preferred materials of the shield of the invention, as a function of the amplitude of vibration in microns. The curve 5i applies to lead, curve 52 to polyvinyl chloride foam, curve 53 to foam rubber and curve 54- to aluminum, all for vibrations between and 200 cycles per second. At a vibration amplitude of four microns the losses for lead, polyvinyl chloride foam and rubber foam are respectively 3, 8X10-2 and 1.2 103 ergs. It is apparent from Fig. 2 that lead is highly effective in sound absorption and that polyvinyl chloride foam is much more so than foam rubber.

Referring again to Fig. l, except for the outer protective layer 5 each of the laminations is provided with a broad margin extending laterally in a direction substantially parallel with the surface of the wearers head against which the shield is to be applied. By so shaping the laminations a broad annular area of contact is made available over which an effective seal may be made to exclude the passage of sounds under the edge of the shield. The metallic layer 3 in particular is formed with a flaring border or iiange 4 which because of its malleable nature may be readily con formed by each wearer to the shape of his own head. On either side of the iiange il the plastic layers I and 2 are formed with parallel flanges I3 and 6. With the flange l the flanges i8 and 6 form a pliable border which can be given a permanent set in accordance with the shape of the wearers head, insuring a substantially perfect seal. This annular area of the shield is covered over with a brous material Il which improves the acoustical seal and the acceptability of the shield to the wearer.

Within the cavity formed by the cup-shaped conformation of the laminations, space is provided for the accommodation of a telephone receiver 9. The receiver may be positioned within the cavity in an optimum position with respect to the ear. One preferred form of receiver is provided with a hollow rim Il), of soft rubber circumferentially perforated, which may be filled with cotton or other absorbent material. The cavity is circumferentially lined about the receiver 9 with absorbent material such as cotton which may be retained in position by a sack or pocket l2. The sack may further include a desiccant. The irregularly limited space thus enclosed between the head of the wearer, the external ear, the surface of the absorbing pocket l2 and the reentrant surface of the receiver 9 promotes the absorption of such sounds as may penetrate the shield and retards the appearance or" cavity resonances and reverberations which produce irregularities in the wearers capacity to hear.

if it is unnecessary to provide the wearer with facilities for telephonie communication, the receiver 9 may be replaced with an insert I3 (Fig. 3) having the same exterior shape and preferably manufactured of a plastic foam material similar to that employed for the shield layers I and 2. The insert is hollow and may be nlled with an absorbent material maintained in place by a grating I4.

The invention provides for use with the shield a spacer I5 illustrated in Figs. 4 and 5 which serves to preserve the soundproof qualities of the shield for users who wear glasses. The spacers iii, one for each of the two shields employed, may be made of rubber or of any similar flexible and pliable material and are of generally meniscus section as illustrated in Fig. 5. By virtue of this shape they nt on their concave sides over the malar bones and under the ring of fabric II at the annular edge of the shield. The spacers are provided with groove Il in order to accommodate the limbs of the wearers eyeglasses.

While the drawings and description herein have set forth in detail the construction of a preferred embodiment of my invention, various modications may be made within the scope of the invention, as set forth in the appended claims.

I claim:

l. A noise shield for the human ear comprising an aluminum cup, a nrst cup-shaped member of polyvinyl chloride foam adapted to iit within the aluminum cup and having a :daring brim extending beyond the cup, a continuous thin lead-bearing metallic membrane tting within the rst polyvinyl chloride member and glued thereto, a second cup-shaped member of polyvinyl chloride foam adapted to fit within the membrane and over the outer ear, and an annular sack of sound absorbent material supported .fithin the second polyvinyl chloride member.

2. An ear shield comprising an outer metallic cover, a first flanged cup-shaped member of aerated plastic material ntting exclusively of its ange within said cover, a flanged sheet of metal of high density tting within said first member, a second flanged cup-shaped member of aerated plastic material fitting within said sheet, and means to bond said sheet to each of said members.

3. An ear protector comprising a metallic cover, two hanged cup-shaped members of nonmetallic material of low modulus of elasticity and low density, a hanged metallic cup of high density fitting between said members, and means to join the surfaces of said cup in intimate union to the adjacent surfaces of said members.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,873,864 Ely Aug. 23, 1932 2,037,884 Day Apr. 21, 1936 2,313,379 Wood Jan. 9, 1943 2,478,783 Seher Aug. 9, 1949 2,536,261 Caldwell Jan. 2, 1951 2,566,975 Beranek Sept. 4, 1951 FOREIGN PATENTS Number Country Date 977,197 France Nov. 8, 1950 1,014,280 France May 28, 1952

Claims

1. A NOISE SHIELD FOR THE HUMAN EAR COMPRISING AN ALUMINUM CUP, A FIRST CUP-SHAPED MEMBER OF POLYVINYL CHLORIDE FOAM ADAPTED TO FIT WITHIN THE ALUMINUM CUP AND HAVING A FLARING BRIM EXTENDING BEYOND THE CUP, A CONTINUOUS THIN LEAD-BEARING METALLIC MEMBRANE FITTING WITHIN THE FIRST POLYVINYL CHLORIDE MEMBER AND GLUED THERETO, A SECOND CUP-SHAPED MEMBER OF POLY-