US 3117575 A
Description (OCR text may contain errors)
Jan. 14, 1964 M. CARRELL ETAL 3,117,575
EAR PROTECTOR Filed Aug. 22, 1961 INVENTORS ROSS M. CARRELL WILLARD E MEEKER BY MARTIN rou R ATTORNEYS United States Patent Ofiice 3,117,575 Patented Jan. 14, 1964 3,117,575 EAR PRDTEQTQR Ross M. Carr-ell, Haddonfield, Willard F. Meeker, Westrn'ont, and Martin L. Touger, Woodhury, N.. assigncrs to the United States of America as represented by the Secretary of the Airlfiorce Filed Aug. 22, 1951, Ser. No. 133,279 Claims. (Cl. 128-152) This invention relates to noise exclusion devices and, more particularly, to an ear cushion or ear protector for protection from intense noise of modern high performance aircraft and all similar situations where the noise level is high.
Other things being equal, the noise exclusion of an ear cushion or protector is proportional to its volume especially at low frequencies. In most situations where ear protectors are used, the size of the ear protector is limited and, therefore, the attenuation of the noise and the noise exclusion obtainable is limited in proportion.
The object of the present invention is the provision of a means whereby the acoustical capacitance of a cavity can be increased without increasing the geometrical volume of the cavity. That is to say, high attenuation of noise and therefore greater protection from the shattering effects of noise is obtained in an ear protector of regular or normal size.
A further object of the invention is the provision of a device for covering the ear wherein absorption of sound waves and the attenuation of noise within the small enclosed area is obtained by means of one or more flexible diaphragm elements forming a Wall of an evacuated cell which is placed in or in proximity to the enclosed area surrounding the ear.
Other objects and advantages will become apparent as the description proceeds.
In the drawing:
FIG. 1 is a transverse sectional view of the device with the inserted cell in perspective and showing its application to a human ear.
FIG. 2 is an enlarged transverse sectional and perspective view of the vacuum cell introduced into the ear protector of FIG. 1.
FIG. 3 is a transverse sectional and perspective view of a modified form of the device.
FiG. 4 is a cross-sectional view of a modified form of cell which can be used with the device of FIG. 1.
Referring more in detail to the drawing, an ear protecting housing It) is provided with a padding element 12 of a configuration adapted to fit the head and envelope the ear. The housing 19 and the pad 12 may be of any standard form. An evacuated cell 14 is secured to the interior wall of the housing by any suitable means. The cell 14 is comprised of a rigid circumferential wall 16 and a pair of flexible end walls 18 which are sufliciently spaced from each other so that they may flex without touching. The flexible end Walls 18 in the embodiment of the invention shown, are provided with concentric corrugations which provide the oil can effect. The cell 4, contains a vacuum. The cell is made of metal, preferably a stable one such as beryllium copper. The end walls are kept from touching under atmospheric pressure by prestressing so that under atmospheric load, the Walls are compressed into a configuration which is compliant. This general property is possessed by oil can surfaces, the negator spring, toggle mechanisms, microswitches and many similar devices. The function of the cell is the negation or absorption of the force of the sound vibrations striking against the outer surface of the housing 1d and entering the cavity 24 surrounding the ear. This is accomplished by the energy required to overcome the stiffness in the flexible walls 18 to a suflicient degree to cause displacement. While retaining a practical and usable size of ear protector, the acoustical capacitance of the protector is thus increased and an effect is obtained comparable to a device much larger in size.
FIG. 3 shows a modification of the device of FIG. 1, incorporating the same principles of attenuation and absorption of noise as described above. A flexible diaphragm capable of receiving sound vibrations and respending to them by displacement is used as one of the walls of a vacuum-containing cell. In this modification, the housing comprising the body of the ear protector is a double walled cell and contains a vacuum, and the interior wall includes a flexible membrane.
An ear protector housing 30 is provided with the usual padding element 32. In this form of the device, however, the housing 30 is a specially constructed ear protector having an outer Wall 34 of plastic, metal, Wood or any other material found suitable, and an inner wall 35. A portion of this inner wall is a flexible diaphragm 36. In the embodiment shown, the diaphragm 36 is provided with concentric corrugations, but flexibility may be obtained in any way found suitable without departing from the scope of the invention.
The padding element 32 and the inner and outer surfaces are constructed in such a fashion that the cavity or area 36 between the walls 39 and 34 will sustain a vacuum.
In the form of the device shown in FIG. 4, the cell 14 as shown in FIGS. 1 and 2 may be replaced by the cell 40, which is of lenticular form and has two outwardly convexed and flexible walls 42. The character and material of these walls are such that the area 44 enclosed between them can sustain a vacuum. Thus, there are two diaphragms contiguous to a vacuum area, which will flex in response to the impact of sound waves.
All of the above described modifications of the device incorporate the same inventive concept, namely, that if the Walls of an evacuated cell are sufficiently flexible, the apparent acoustical capacitance of the cell will be greater than the geomertical volume occupied by the cell. It could also be stated that the cell would be more compressible than a volume of air equal to the geometrical volume of the cell.
For clarification of the involved principle, consider a cylindrical cavity closed at one end with a rigid wall and closed at the other end by a perfectly flexible wall or by a weightless, frictionless, perfectly sealin g piston. The compliance of this piston or perfectly flexible wall is expressed by the equation:
where C, is the acoustical capacitance of the inclosed area or cavity, and A is the piston area. The mechanical compliance of the piston or flexible wall. is also expressed by the equation:
where d is the displacement and f is the applied force.
The acoustical capacitance of the volume of the inclosed area may be expressed by the equation:
where C is the mks. acoustical capacitance, V is the volume, .p the density of the air inclosed and c the velocity of sound. At normal pressure and temperature pc =l.5(10 The mechanical compliance of the piston due to the stidness of the air in the inclosed area then becomes:
f A 1.5(1 )/i We have considered a cylindrical cavity whose volume VzAl. Therefore If a diameter of two inches is ascribed to the cylinder A=3.l4 or 2(1O )m. For a volume of 50 cc., l=(l0* )m. or about one inch C is then 8.35(l0- meters/newton, or l.47(1(lin./lb.
if we consider the cavity or closed area evacuated and this compliance transferred to the piston, it becomes apparent that the geometric volume is no longer important. Since its acoustical capacitance becomes infinite upon evacuation, the diaphragm would experience an atmospheric pressure of (14.7) (314}:461 lbs. The diaphragm would be displaced 0.68".
In the examples given in FIGS. 14, we have considered a diaphragm which is formed in sum a way that the displacement noted would deflect into an essentially flat shape perhaps with corrugations yet which exhibit the same compliance described above. It has become apparent that the geometric volume within the cavity has no acoustical significance after evacuation and it can be made arbitrarily small so long as the back wall, as 3.0 in FIG. 3 of the enclosed area, does not touch the diaphragm or inner wall 36 of diaphragms 18 (FIG. 2). It has become apparent that the acoustical capacitance of this diaphragm is equal to that of a 50' cc. cavityi A cell such as is shown in FIG. 4, where each wall is a diaphragm and the interior of the cell is evacuated, would therefore have acoustical capacitance equal to a 100 cc. air cell. Since the ambient air pressure is not constant but varies over a range of :L one-half p.s.i., the diaphragms, must necessarily flex with this pressure in a a manner which is comparable to the flexing present in an aneroid barometer. For the compliance considered above, a one-half psi. change would move the walls 4.6(l()- inches. This means that the diaphragms must be .042 inch apart if they are not to touch under high atmospheric pressure. This establishes the minimum thickness of the cell. A cavity of equal diameter would be 2 inches long to contain 100 cc. It is thus a gain of 48 in volume. These calculations serve to illustrate that devices of the type shown in FIGS. 14 and described above are within reason so far as the magnitudes of the stiffness desired.
In practice, a cell such as described above and illustrated in FIGS. 14 would effectively double the volume of the protector. Several cells could also be used, each equivalent to 100 cc. Each doubling of the efiective volume would increase the low frequency attenuation by 6 decibels. Large sums of money are being spent in the services and in large electronic companies for the lessening of or protection against noise. Elaborate and expensive electronic means have been contrived to achieve comparable results. The means contrived by the present inventors is simple in construction and operation and can be manufactured at a low percent of such involved costs.
While the invention is shown and described in connection with one form for illustrative rather than restrictive purposes, it is obvious that changes and modifications may be made by those skilled in the art without departing from the scope and spirit of the invention as defined in the accompanying claims.
1. An ear protector comprising a housing element to be placed over the human ear, contact the head and provide a closed space adjacent the ear, said protector comprising spaced inner and outer walls and an evacuated space therebetween, a flexible, circularly corrugated diaphragm incorporated in said inner wall and lying adjacent said evacuated space.
2. An ear covering device for providing protection from sound vibration comprising a cup shaped element for covering the ear, an inner wall and an outer wall on said cup so constructed as to remain out of contact with each other and provide an unobstructed space therebetween under vacuum conditions, said inner wall comprising a flexible diaphragm, circularly corrugated and provided with a calculated mechanical compliance, said wall being capable of absorbing sound vibration by the resilient displacement thereof, thereby increasing the effective acoustical capacitance of said enclosed space by a calculable amount.
3. An ear protector for protection of the ear against sound vibration, said protector comprising a cup shaped element for covering the ear and providing a space between said element and the car, a portion of said space being defined by Walls, said portion of said space being under vacuum conditions, said walls remaining continuously out of contact With each other, at least one of. said walls being flexible and provided with circular corrugations to provide calculable resistance to displacement for increasing the acoustical capacitance of the space occupied by said vacuum conditions.
4. An ear protector as defined in claim 3 wherein the walls defining the space under vacuum comprise a pair of circular disks connected at their circumferences by a rigid cylindrical element to form a cell, and wherein the cell thus defined is attached to the inner surface of said cupshaped element.
5. An ear protector as defined in claim 3 wherein the Walls defining the space under vacuum comprise a cell depending from the inner surface of said cup-shaped element.
References Cited in the file of this patent UNITED STATES PATENTS 2,476,589 Driskill July 19, 1949 2,976,948 Thiessen et a1 Mar. 28, 1961 FOREiGN PATENTS 467,298 Germany Oct. 22, 1928