Electroacoustical audible noise reducing apparatus

Abstract

 The noise power perceived in an environment of volume larger than and outside a volume including a source (11) of audible noise is appreciably reduced with an active system that includes an acoustical combiner (13) that combines the noise signal to be reduced with an inversely fed back signal provided by a loudspeaker (14) energized by a controlled frequency selective amplifier (16) that amplifies spectral components to be reduced provided by a microphone (15) near the output of the acoustical combiner (13).

Description

[0001] The present invention relates in general to reducing audible noise and more particularly concerns novel apparatus and techniques for using an active electroacoustical system to significantly reduce audible noise with relatively compact and reliable apparatus.

[0002] Exposure to audible noise at significant levels not only causes discomfort and fatigue but may result in temporary and/or-permanent damage to hearing. Accordingly, to meet government-established maximum noise levels, passive techniques, such as noise-reducing enclosures, have been employed to reduce audible noise levels. While reasonably effective at high audio frequencies, attenuating low frequency audible noise passively requires considerable mass to obtain effective attenuation.

[0003] As an example of active methods to reduce unwanted audible noise reference is made to U.S. Patent No. 2,043,416. More recent approaches involve open and closed loop systems. In one open loop system a noise source within a given surface may have a number of microphones just inside the surface and a number of loudspeakers just outside the surface. The microphones pick up the noise at the surface to provide a signal that is amplified and delivered to the loudspeakers with substantially the same intensity but opposite phase as the noise passing across the surface to effectively reduce the noise perceived by an observer remote from the surface.

[0004] A closed loop system may use a microphone near the observer coupled to a nearby loudspeaker through signal processing electronics for receiving the audible noise from a distant source and providing a signal from the loudspeaker of opposite phase, thereby tending to reduce the noise level perceived by the listener. Such a system is described in 28 Journal of the Acoustical Society of America 966-972 (1956) and discloses using a specially designed loudspeaker and microphone separated by a distance of approximately one inch. This approach works only when the listener, loudspeaker, and microphone are separated by a small fraction of a wavelength of the highest frequency audible noise energy to be cancelled as attenuation is provided in only a small volume of space directly around the microphone.

[0005] - According to the invention electroacoustical audible noise reducing apparatus for a source of audible noise having significant audible spectral components in a predetermined frequency range below 500Hz, comprises a first electroacoustical transducing means for transducing an input electrical signal into a corresponding audible signal; an acoustical combining means for combining the audible noise from the source with the audible signal from the first electroacoustical transducing means; a second electroacoustical transducing means responsive to the acoustical signal at the output of acoustical combining means for providing an electrical signal representative thereof; controlled amplifying means responsive to the latter electrical signal for providing an amplified electrical signal to the first electroacoustical transducing means; the acoustical combining means and the first and second electroacoustical transducing means being closely spaced and separated from each other by less than a half wavelength at the highest frequency of the predetermined frequency range, whereby, in the predetermined frequency range, spectral components of noise energy in a volume of the environment, outside, and significantly larger than, the volume occupied by the source of audible noise, the first and second electroacoustical transducing means and the acoustical combining means are significantly less than corresponding components at the output of the source of audible noise.

[0006] The invention thus provides improved apparatus and techniques for reducing audible noise, and over a relatively large region.

[0007] Preferably, the acoustical combining means comprises a duct, the second electroacoustical transducing means is located in the duct and the duct is formed with an opening intermediate its ends coupled to the first electroacoustical transducing means. The apparatus forms a closed loop that tends to minimize the intensity of the acoustical signal output from the duct outlet.

[0008] According to another specific form of the invention, the source of audible noise and first and second electroacoustical transducing means are separated from each other by a distance less than a half wavelength at the frequencies of interest to be attenuated.

[0009] An example of apparatus according to the invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a block diagram illustrating the logical arrangement of the apparatus; and,

Figure 2 is a combined pictorial-schematic-block diagram illustrating the logical arrangement of the apparatus when used as an electroacoustical exhaust silencer system.

[0010] With reference now to the drawings and more particularly Figure 1 thereof, there is shown a block diagram illustrating the logical arrangment of electroacoustical audible noise reducing apparatus according to the invention. Like elements are identified by the same reference symbols throughout the drawings. A source of audible noise 11 is combined, in an acoustical combining means 13, with an audible signal from a source 14 to provide a reduced noise level at output 12. The source 14 typically comprises an acoustical transducing means for transducing an input electrical signal into a corresponding acoustical signal. A second electroacoustical transducing means 15 for transducing an acoustic signal into a corresponding electrical signal, such as a microphone, provides an electrical signal representative of the acoustical signal at output 12. Controlled amplifying means 16 amplifies this signal and energizes the source of audible signal 14 to provide an audible signal. Acoustical combining means 13 combines the noise signal with the audible signal of substantially the same waveform but opposite phase from the audible noise provided by source 11 thereby significantly to reduce the audible noise level on output 12.

[0011] The result is that the total undesired noise power radiated into the environment by the combination of sources 11 and 14 in the frequency range where attenuation occurs is significantly less throughout a region of volume significantly larger than the volume occupied by the source of audible noise 11, the acoustical combining means 13, the source of audible signal 14 and the second electroacoustical transducing means 15. In a specific form of the invention these elements are separated from each other by a distance less than a half wavelength at the highest frequency of interest to be attenuated, typically a frequency that is of the order of 500 Hz or less.

[0012] Referring to Figure 2, there is shown a combined block-schematic-pictorial representation of an electroacoustical exhaust silencer for an engine 11 which comprises the noise source, characterized by appreciable noise reduction with compact structure without loading the engine, thereby allowing the engine to operate more efficiently as compared with an engine exhausting through a conventional passive exhaust silencer system.

[0013] The exhaust pipe 21 may be formed with a T-section 22 having a short stem 23 having an end opening covered by an electroacoustical transducer 14. Microphone 15 between the section 22 and outlet 12 of the exhaust pipe 21 provides an electrical signal to controlled amplifying means 16 representative of the acoustical signal just inside the outlet 12 for amplification by controlled amplifying means 16 to energize loudspeaker 14 with a signal tending to reduce the audible noise from outlet 12 through the use of known negative feedback techniques. Controlled amplifying means 16 preferably includes frequency selective apparatus for transmitting signals within a low audio frequency range, such as 10 to 500 Hertz, thus embracing the spectrum of significant levels of audible noise energy which are to be reduced and which are received at the inlet to the exhaust pipe 21 from the engine, and thus helping to maintain system stability. A portion of the exhaust pipe 21 functions as the acoustical combining means 13. Preferably T-joint 22 is as close to the outlet as practical but sufficiently far from the microphone 15 so that the acoustic wave is essentially plane where the microphone 15 is located. Stem 23 is preferably as short as practical. The distance between transducers 14 and 15 should be less than a half wavelength at the highest , noise frequency to be reduced.

[0014] The maximum span across the duct or pipe is preferably less than a quarter wavelength at the highest noise frequency to be reduced and if circular in section the pipe preferably has a diameter less than a sixth of the wavelength.

[0015] The system exemplified by the electroacoustical muffler of Figure 2 is characterized by significant structural features which allow the distance of the noise source from the transducers to be relatively free from criticality because the noise energy from the source and the transducer providing the noise-bucking field is confined by the structure and released to the outside environment through a limited aperture in this case the inlet of the exhaust pipe. The same system could be used in a ventilating duct. The unconfined system exemplified by a compressor, and including a loudspeaker and microphone preferably has these three elements in close proximity with the microphone preferably midway between the compressor and loudspeaker providing the noise-bucking field to reduce the noise energy radiated by the system.

[0016] The principles of the invention may have many applications, but are especially useful where the audible noise energy sought to be attenuated is in the low frequency range. Thus, it may be used to reduce the low frequency noise present in air ducts or provided by compressors or other motors. For example, audibly perceptible noise reduction of compressors or motors, may be achieved according to the invention with the first and second electroacoustical transducing means located within a half wavelength of the compressor or motor comprising the noise source, a microphone being preferably midway between each noise source and a loudspeaker or other transducer providing the noise-bucking field. The invention may also be used in combination with passive devices capable of significantly reducing audible noise at higher frequencies.

Claims

1. Electroacoustical audible noise reducing apparatus for a source (11) of audible noise having significant audible spectural components in a predetermined frequency range below 500 Hz, the apparatus having a first electroacoustical transducing means (14) for transducing an input electrical signal into a corresponding audible signal; an acoustical combining means (13) for combining the audible noise from the source (11) with the audible signal from the first electroacoustical transducing means (14); a second electroacoustical transducing means (15) responsive to the acoustical signal at the.output (12) of the acoustical combining means (13) for providing an electrical signal representative thereof; controlled amplifying means (16) responsive to the latter electrical signal for providing an amplified electrical signal to the first electroacoustical transducing means (14); the acoustical combining means (13) and the first (14) and second (15) electroacoustical transducing means being closely spaced and separated from each other by less than a half wavelength at the highest frequency of the predetermined frequency range, whereby in the predetermined frequency range, spectral components of noise energy in a volume of the environment outside, and significantly larger than, the volume occupied by the source (11) of audible noise, the first (14) and second (15) electroacoustical transducing means and the acoustical combining means (13) are significantly less than corresponding components at the output of the source of audible noise.

2. Apparatus according to claim 1, wherein the first (14) and second (15) electroacoustical transducing means are a loudspeaker and a microphone, respectively.

3. Apparatus according to claim 1 or claim 2, wherein the acoustical combining means (13) comprises a duct (21), the second electroacoustical transducing means (15) is located in the duct (21), and the duct (21) is formed with an opening intermediate its ends coupled to the first electroacoustical transducing means (14).

4. Apparatus according to claim 3, wherein the opening comprises a T-joint having a hollow stem (23) formed with an opening to which the first electroacoustical transducing means (14) is connected.

5. Apparatus according to claim 3 or claim 4, wherein the duct (21) has a cross-sectional area with a maximum span thereacross that is less than a half wavelength at the highest frequency of the predetermined frequency range, a portion (22) of the duct comprising the acoustical combining means (13), and the second electroacoustical transducing means (15) being spaced from the opening sufficiently far so that the acoustical wave in a cross-sectional plane of the duct (21) passing through the second electroacoustical transducing means (15) is substantially plane.

6. Apparatus according to any of claims 3 to 5, wherein the duct (21) is an exhaust pipe for connection to the exhaust outlet of an enging (11).

7. Apparatus according to any of claims 1 to 6, wherein the controlled amplifying means (16) includes frequency selective apparatus for attenuating signals outside a predetermined low audio frequency range that is within the frequency range of 11 to 500 Hz.

8. Apparatus according to claim 1, wherein the second electroacoustical transducing means (15) is located substantially midway between the source (11) of audible noise and the first electroacoustical transducing means (14).

9. Apparatus according to claim 8, wherein the controlled amplifying means (16) includes frequency selective apparatus for attenating signals outside a predetermined low audio frequency range that is within the frequency.

Drawing