This invention relates generally to noise-canceling microphones and related devices. More particularly, this invention relates to a bi-directional noise control device for use in environments that have random ambient noise.
Microphone units typically operate in environments where unwanted noise is present. For example, a person listening to someone talking on the telephone may be distracted from the speaker's voice because of background noise emanating from machinery, traffic, appliances, or other ambient sounds. Background noises may be reduced for the listener if the person talking into the telephone is using a noise-canceling type microphone.
Many noise-canceling microphone element designs employ front and rear sound ports which allow sound to enter both sound ports and impinge upon the diaphragm simultaneously in opposite directions resulting in little or no signal being generated by the microphone. This technique is applied in a wide variety of cardioid microphones as well as telephone handset transmitters and headsets. Some of these microphones employ acoustic tuning to the rear port to make the microphone more frequency-responsive.
Noise-canceling microphones depend upon two factors for their operation. The first factor is the polar pattern of the microphone (usually bi-directional) and the assumption that the noise to be reduced is not on the maximum sensitivity axis of the microphone. The second factor is the different responses of the bi-directional microphone for a sound source close to the microphone, such as sound entering the front sound port, and a sound source at a distance to the microphone, such as sound entering the front and rear sound ports.
When the sound source is close to the front sound port of the microphone, the sound pressure will be several times greater at the front sound port than at the rear sound port. Since the microphone responds to the difference of sound pressure at the two entries, someone talking close to the microphone will provide a substantially higher signal strength than a remote sound, where the sound pressure is equal in magnitude at the two entry ports
Because of construction restraints inherent in front and rear sound port microphone designs, one port of the microphone is always more sensitive than the other. This results from the need to provide a supporting structure for the diaphragm and the resulting impedance that the structure presents to sound entering the rear sound port microphone element. It is common practice for the more sensitive port to be faced forward to capture the desired sound while the less sensitive port is utilized for capturing and reducing or nullifying the undesired background noises.
If the front and back sensitivities of the microphone element were equal, then theoretically 100% noise rejection would be possible whenever noise of equal pressure were subjected to both entrances to the microphone. In practice, however, only 10-20 dB noise reduction is possible using the currently available microphone elements for frequencies below approximately three KHz.
Frequency response is another factor that differentiates noise-canceling microphones. Frequency response is essentially flat in the near field (a sound source close to the front sound port) over the audio band. In the far field (a remote sound source), the frequency response increases in frequency until the pressures at the front and rear sound ports of the unit are 180 degrees out of phase, at which point resonance occurs. At some frequency, the microphone becomes more sensitive to axial far-field sounds than axial near-field sounds. This crossover frequency will occur at a higher frequency for a microphone with a shorter port separation than a microphone with a longer port separation.
Several devices, both electrical and mechanical, used for noise-cancellation purposes exist but have potential drawbacks such as the need for preprocessing. The negative effects of reflections, calibration difficulties, high costs, and operating environments also pose problems. For example, in environments in which human speech is the ambient noise, signal-processing techniques such as filtering cannot effectively be used because the ambient human speech is at the same frequency as the desired speaker's voice and because the ambient noise is random, non-constant or non-periodic.