Improving microphone technology
Personal microphones used by public safety patrol and response officers are expected to work reliably in extremes of environment: raging forest fires, driving hurricanes and bitter cold. Mics must filter out high-decibel noises ranging from rock concerts to gunfire. Moreover, they are expected to be immune to radio frequency interference (RFI) and electromagnetic interference (EMI) and to be compatible with “incompatible” radios.
The wonder is not that mics don’t always work to satisfaction, but that they work at all.
These devices have worked tolerably well, often under adverse conditions. That is shown by their ubiquity. Public safety officials constantly rely on portable radio systems. This, in turn, has driven manufacturers to work harder to come up new equipment. A number of manufacturers work in this arena, producing good microphones, but each has a different approach. No single “silver bullet” can solve all of the acoustical design problems associated with personal mics.
The way Television Equipment Associates, Brewster, NY, deals with problems such as RFI, EMI and radio incompatibility is to customize each unit.
“Each problem must be dealt with on a case-by-base basis,” said TEA Vice President Steve Tocidlowski.
For instance, dynamic microphones are used for military-type applications and electret mics are used for others. Dynamic mics withstand rugged use and typically have low impedance and a good frequency response. A diaphragm fluctuates an attached coil in and out of a magnetic field, which induces electric currents in the coil. Low-distortion, high-frequency-response electret mics are similar to condenser-type mics, where a diaphram is attached to one plate of a capacitor, except that electrets do not need a bias voltage to create an electric field. Dynamic mics have to be amplified to simulate electret mics. Each mic type is prone to different types of interference.
“The problems are not easily characterized, especially when the microphones are used in different environments,” Tocidlowski said. “And RFI is a very mysterious phenomenon to deal with. You often have to proceed through trial-and-error. It’s not as if you have an empty tire that you simply fill up with air.” Shielding, noise canceling, RFI chokes and capacitors and multi-directional devices are different solutions that may be appropriate in one situation but not in another.
Tocidlowski said that TEA’s mics had to be designed for different radios. “Each department generally tries to standardize its equipment, and as a manufacturer, we work with these interface problems,” he said. “But it’s also very common for public service departments to have more than one brand of radio-along with different models.”
Evanston, IL-based Shure Brothers, on the other hand, has a modular link series to solve radio incompatibility. Applications Engineer Matt Waller said that Shure has a variety of port sets that can hook up to every radio.
“It’s almost like a plug-and-play,” Waller said, “Through the use of the appropriate module you can plug into any radio without the need of hard wiring.” Waller also said that modularity allowed a new cord to be added to a hand mic in seconds, avoiding downtime to send the cord and the mic to the repair shop, as is the case when the cord is hardwired to the mic.
“Cord failure is the single, biggest cause of microphone failure,” Waller said. “This solution allows for the quickest resolution of this problem.”
To meet the demands of ruggedness and reliability, Shure microphones must pass 37 environmental tests, Waller said, including exposure to extreme hot and cold temperatures, ultra-violet light, moisture and salt spray.
Gerald Bloom, president of Earmark, Hamden, CT, said that handling ambient noise was one of the design criteria for his company’s products.
“We’re a very specialized radio company and design only for environments others don’t want to work in,” Bloom said. “A few environments generate more noise that we can handle, but not many.” He said that a front-to-back noise canceler erases, at a minimum, 12dB to 20dB and can be designed to cancel levels as high as 130dB.
The basic technology of enhancing the voice and filtering out unwanted sounds is well-known, Bloom said. The voice goes in one side while the other side cancels out the background noises. Bloom said that the geometry and design of the acoustic cavity, achieved through many years of research, has achieved a high level of efficiency.
Earmark has focused on fire and chemical hazard uses. The company offers a couple of options. One is an external mic that can hook onto the outside of a mask. The patented filtering device responds only to the human voice through special techniques involving nonlinear components used for differentiation. It acts as if the mask were not there. For use with SCBA equipment, like a respirator, which hampers radio communication, Earmark uses the more traditional throat mic. This device picks up the low vibrations from the throat and reshapes them to emphasize high frequencies and cast off low frequencies, so what comes out approximates normal voice.
Karl Redmer, electrical engineering manager, Otto Communications, Carpentersville, IL, said that the growth of portable radio use had not only increased requirements for environmental durability but demands for more features as well, resulting in more buttons and attachments that make sealing the mic case more difficult.
Redmer said that his company’s products go through intensive focus group discussion and beta testing. Two difficult applications being dealt with include sealing the microphones against conditions such as driving rain, or hurricanes, so the water is kept out, but the voice audio emerges as loudly as possible against the background noise. Firefight-ers also work in situations where there is driving cold water from the hoses as well as excessive heat from burning buildings.
These noise-filtering products are also designed for air transportation ground crews, which face both noise and a wide range of weather conditions.
“The three conditions we design our microphones for are environmental extremes, high noise levels and physical durability,” Redmer said. “They have to withstand shock treatment, and the cable has to be very rugged and remain flexible when it’s subjected to abuse, such as being caught in a fence during a chase.”
Redmer added that each microphone requires different sealants. “You can’t simply close all the openings,” he said. Microphones must incorporate subtle differences of design so that they can interface effectively with more than 100 types of portable radios.
Motorola Land Mobile Products Sector, Schaumburg, IL, has also developed a product designed “for the public safety professional who is often in an environment where it is difficult to communicate effectively and privately,” said Charles Backof Jr., vice president. “Sirens, crowd noise and speeding traffic can interfere with hearing or sending messages.”
Backof said that Motorola’s system enhances communications in loud environments while providing a “speak-in-a-whisper” capability. An integrated microphone/receiver system, called CommPort, fastens securely onto the ear. Some “skull mikes,” which pick up bone vibration, often suffer intelligibility because spoken fricatives (such as in words pronounced with an “f” or “s” sound-“fire”or “system”) are not sufficiently sounded. Backof said Motorola’s mic instead picked up sound waves as they crossed the face, enhancing voice quality.
Gail Moody, Motorola marketing manager, said that the company had put a lot of effort into ergonomics. Customer surveys, she said, have demonstrated that “Users want these microphones to be comfortable enough so they can wear them through an entire shift.”