Ultrasonic microphone for operating with an accumulated layer of mud over its vibratile surface without change in sensitivity within its prescribed frequency range

A microphone is described for operating in dirty environments where a layer of dirt or mud may accumulate over the vibratile surface of microphone. The microphone is designed with its vibratile diaphragm having a mass greater than 0.4 gm/sq cm of diaphragm area so that an added layer of dirt over its surface will not cause any significant change in the sensitivity of the microphone over its prescribed frequency range of operation.

This invention is concerned with the design of a microphone whose 
sensitivity remains unchanged over its operating frequency band when the 
microphone is used in an environment which permits its sound sensitive 
surface to become covered with an accumulation of dirt or mud such as will 
occur when the microphone is mounted on the underneath side of an 
automobile and the automobile is driven through mud puddles or slush. 
An example of an application for this inventive microphone is in its use 
for monitoring the air pressure in a tire while the tire is mounted on a 
vehicle and the vehicle is operating under its normal usual conditions. 
For one such application, an ultrasonic whistle is attached to the rim of 
the wheel so that the whistle communicates with the air volume contained 
inside the tire. A valve arrangement associated with the whistle prevents 
the flow of air through the whistle when the tire pressure is normal. When 
the tire pressure falls to a preset value, the valve opens and the air 
inside the tire activates the whistle so that a sound signal is generated. 
The signal will continue until the tire pressure falls by a preset small 
amount, at which time the valve automatically shuts off the air supply to 
the whistle to prevent the escape of all the air in the tire. 
While the whistle is operating, a microphone mounted in the vicinity of the 
wheel will pick up the sound signal from the whistle and convert it to an 
electrical signal. After suitable amplification, the electrical signal 
will activate a dashboard indicator to show that the air pressure in the 
tire has reached a minimum safe level, and thus alerts the driver that the 
tire must be serviced before he can continue to drive at full speed. The 
microphone used in such an application will be exposed to all types of 
road conditions where dirt, mud or slush can splash over the microphone 
surface. If this happens to a conventional microphone constructed with a 
conventional lightweight diaphragm, such as is generally employed in 
microphone structures designed for use in detecting sound signals in air, 
the accumulation of a layer of dirt over the diaphragm surface will 
materially change the effective mass of the vibrating diaphragm, and thus 
significantly modify the sensitivity and response characteristic of the 
microphone. Under such conditions, the tire pressure sensing system will 
become inaccurate or inoperative. The present invention overcomes these 
objections. 
The primary object of this invention is to design a microphone which is 
capable of operating in a dirty environment such as when it is mounted on 
the underside of an automobile and the automobile is driven through mud 
puddles or slush, and the microphone remains unchanged in its sensitivity 
to sound signals generated within a prescribed frequency band. 
A further object of this invention is to design a microphone such that the 
addition of a layer of dirt or mud over its sound sensitive surface will 
not appreciably change the sensitivity of the microphone to sounds 
generated in the prescribed frequency band of operation. 
A still further object of this invention is to design a microphone for 
operating with a layer of dirt covering its sound sensitive surface and 
insure that the mass of the dirt layer does not significantly change the 
total mass of the vibratile diaphragm portion of the microphone, thereby 
preventing significant changes in microphone sensitivity when the 
microphone is used in its intended application.

Referring to FIGS. 1 and 2, which schematically illustrates an embodiment 
of my invention in which a preferred structural configuration of the 
microphone is shown, the reference character 1 illustrates the vibratile 
diaphragm portion of the microphone, which is shown as a circular piston 
which may be metal, such as aluminum. One electrode surface of the 
polarized ceramic element 2 is attached with conducting epoxy cement 3 to 
the surface of the aluminum diaphragm 1. The other electrode surface of 
the ceramic is similarly attached with conducting epoxy cement 4 to the 
steel inertial mass member 5. A machine screw 6 is employed, as 
illustrated, to provide a mechanical clamp for the assembled elements. A 
spring washer 7 is preferably placed under the head of the screw to 
control the compressive stress on the elements and to isolate the screw 
from the vibrations of the inertial mass member 5. An insulating plastic 
bushing 8 serves the dual purpose of aligning parts 2 and 5 and providing 
electrical insulation between the screw 6 and the steel member 5. 
Electrical conductors 9 and 10 soldered respectively to the screw 6 and to 
the steel member 5 provide electrical connection through the conducting 
epoxy to the ceramic electrodes. The transducer assembly thus far 
described is similar to the construction shown in greater detail in U.S. 
Pat. No. 3,739,327. 
The microphone element assembly is preferably bonded to the inside surface 
of a rubber boot 11, as illustrated in FIG. 2. The face of the vibratile 
diaphragm 1 is preferably vulcanized directly to the inner surface of the 
rubber boot by using part 1 as an insert when molding the boot 11. 
The insulated conductors 9 and 10 are connected to the input terminals of 
the electronic circuit 12. The output connections from the electronic 
circuit 12 are connected to the terminals 13, 14 and 15, which are in turn 
connected to the multiconductor cable 16, as indicated. The output 
terminals are held by an insulating disc member 17, which is located 
within the opening of the boot, as shown. Potting compound 18 is 
preferably used to achieve a waterproof seal for the cable entrance to the 
boot assembly, as shown. A rigid thin-walled tubular member 19 is 
preferably inserted as a liner to the rubber boot 11 before completing the 
assembly, so that the completed structure will have a rigid body. 
The receiving response-frequency characteristic of the transducer assembly 
illustrated in FIG. 2 is shown by the solid line in FIG. 3. The desired 
frequency band of operation of the microphone assembly is indicated by the 
frequency region between f.sub.1 and f.sub.2. In accordance with the 
teachings of this invention, it is desirable to set the resonant frequency 
of the transducer assembly above the frequency f.sub.2, as illustrated by 
the peak resonant response of the solid curve. Also, in accordance with 
the further teachings of this invention, the reduction in resonant 
frequency caused by the accumulation of a layer of dirt or mud on the 
surface of the microphone must not be of such great magnitude that the 
sensitivity of the microphone over its prescribed operating frequency 
range f.sub.1 to f.sub.2 is significantly changed. In other words, the 
addition of the mud layer must not drop the resonance much below the value 
shown by the dotted line response curve in FIG. 3. A specific example will 
be given to more fully describe the teachings of my invention in achieving 
the desired objectives. 
Under typical driving conditions through mud, it was experimentally found 
that a layer of mud about 1/16 inch thick could accumulate over the 
surface of the microphone, and that the added mass contributed by the mud 
layer was approximately 0.2 gms per sq. cm. of diaphragm area. This amount 
of added mass to the surface of a typical light weight diaphragm as 
employed in a conventional microphone would cause a considerable change in 
microphone sensitivity. Therefore, in order to prevent large changes in 
sensitivity in the inventive microphone, it is necessary that the mass of 
the diaphragm portion 1 be made significantly greater than the accumulated 
mass of the mud layer. I have found that satisfactory results can be 
achieved if the mass of the diaphragm 1 is made greater than twice the 
mass of the accumulated mud layer. Therefore the minimum mass of the 
diaphragm 1 that was found necessary to maintain the desired uniform 
sensitivity over the prescribed operating frequency range under all 
driving conditions through mud and slush, and for varying accumulations of 
mud or snow up to about 1/16" thick, was approximately 0.4 grams per sq. 
cm. of diaphragm area. Thus the thickness of the diaphragm 1 in FIG. 2 
should be chosen such that the mass of the diaphragm is greater than 
approximately 0.4 gms/sq. cm. of diaphragm area. 
Applicant has chosen a well-known basic type of ceramic transducer 
construction to illustrate the teachings of his invention, and Applicant 
makes no claim to the basic transducer vibrating system. The invention 
resides only in the novel design combinations that have been described to 
achieve the solution to the problem of maintaining uniform sensitivity for 
a microphone that is required to operate under severe conditions of 
accumulated dirt and achieve the objects of the invention as set forth in 
the specification. 
It will be obvious to one skilled in the art that additional modifications 
may be made to the specific embodiments which have been used for 
illustrating the invention; therefore, the intended claims are intended to 
cover all equivalents that will fall within the true spirit and scope of 
this invention.