Abstract:
An acoustic detector that includes active noise cancellation is presented. An acoustic detector converts sound waves, which include sound waves of interest as well as noise, to an electronic signal. An operator listens for a characteristic sound, for example the sound characteristic of fluid leaking from a pipe, as she varies the position of the transducer. At least one other acoustic detector is positioned in order to monitor noise. The electronic signal from the noise monitoring acoustic detector and the electronic signal from the acoustic detector are combined in a processor in order to cancel the noise. In some embodiments, the operator may adjust a cancellation band so that some frequencies are not cancelled. For example, the operator may adjust the cancellation band so that a co-worker&#39;s voice may be heard.

Description:
BACKGROUND 
   1. Field of the Invention 
   The invention relates to the field of acoustic detectors, and in particular, to leak detection equipment with active noise cancellation. 
   2. Related Art 
   When water or other fluids leak from underground pipes, quick and accurate determination of the site of the leak is necessary to reduce the amount of damage caused by leaking fluid. Acoustic sensing methods are used to locate leaks in underground pipes by detecting the vibrations caused by leaking fluids. Fluids leaking from underground pipes under pressure typically produce acoustic vibrations with a frequency in the range of about 40 Hz to about 4000 Hz. 
   In order to detect the acoustic vibrations, a transducer placed in contact with the ground converts the mechanical vibration into an electrical signal. The electrical signal is filtered to block most noise at frequencies below about 400 Hz and above about 2000 Hz. The signal can also be amplified before and/or after filtering. In some detectors, individual band-gap filters can be selected in order that ranges of frequencies can be monitored. The range of frequencies being monitored depends on the nature of the pipe, the material leaking from the pipe, the size of the leak, and the characteristics of the earth in which the pipe is buried. 
   The processed electrical signal can then be input to one or more speakers in a set of headphones, where it is converted back into acoustic vibrations. An operator wearing the headphones listens for the characteristic tone of the leak. The position of the transducer on the ground is varied in order to find the source of the leak. The operator must be able to accurately determine the spot at which the characteristic tone has a maximum volume in order that an accurate location of the leak is determined. 
   During leak detection, the sound reaching the operator&#39;s ear is primarily a combination of the sound attributable to the leak, noise picked up by the transducer and passed through the electronics to the headphones, and ambient noise transmitted through the air and through the headphone structure. An operator&#39;s ability to locate sound precisely depends in part on how well the sound of the leak can be distinguished over other sounds. 
   Ambient noise protection headphones, which are commonly utilized to reduce ambient background noise, have several disadvantages. First, the attenuation efficiency is limited by the quality of the seal to the operator&#39;s ears and by the characteristics of the foam cushions. Therefore, ambient noise protection headphones may not attenuate noise sufficiently to allow an operator to accurately determine the source of the leak. Second, the attenuation is indiscriminate. Sounds that the operator may need to hear, such as the sound of a co-worker&#39;s yelling a warning, are attenuated along with the unwanted background noise. Therefore, ambient noise protection headphones may pose a safety risk to the operator. 
   Active Noise Cancellation (ANC) headphones, which cancel unwanted noise instead of merely attenuating it, provide a better solution. ANC headphones contain microphones that convert environmental noise to an electrical signal that can then be utilized to produce sound of equal amplitude but opposite phase of the ambient noise. The signal from the leak detector, as with a CD or DVD player, can be input and the operator can monitor the sound produced by the leak detector while canceling ambient noise at the headphones. 
   ANC headphones are marketed by, for example, Bose Corporation of Framingham, MA, Sony Corporation of Tokyo, Japan, and Sennheiser Electronic Corporation of Old Lyme, CT. However, existing ANC headphones are not designed to operate effectively in the frequency spectrum of interest for detecting leaks, e.g. about 40 Hz to about 4000 Hz. For example, the Sony MDR-NC5 has active noise cancellation that operates to a maximum frequency of 1500 Hz, but has a 15 dB noise cancellation only at frequencies less than 300 Hz. The Sennheiser HDC451-1 has 10 dB noise reduction between 400 Hz, and has maximum operating frequency of 1000 Hz. Additionally, these noise cancellation headphones do not cancel noise detected by the acoustic detector of the leak detector. Also, providing noise cancellation for all noise in the spectrum may present a safety hazard to the operator, who then can not hear warning shouts or traffic noise. 
   Therefore, in order to increase operator safety and accuracy, an acoustic leak detection system with active noise cancellation is desired. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an acoustic leak detector with active noise cancellation is presented. An acoustic leak detector includes an acoustic sensor to convert acoustic waves to electronic signals. The acoustic sensor is then utilized to detect leak noises from an underground pipe. In some embodiments, a second acoustic sensor is provided to monitor background noise and provide an electronic signal that cancels the noise from the electronic signal provided by the acoustic sensor. In some embodiments, the second acoustic sensor can be placed in contact with the earth away from the pipe in order to cancel noise that is transmitted from surrounding sources through the earth. In some embodiments, the second acoustic sensor monitor can be placed such as to detect noise in air in the vicinity of the acoustic leak detector. 
   In some embodiments, the acoustic leak detector can further include active noise cancellation at the headset. The active noise cancellation at the headset can be frequency dependent so that certain sounds, for example traffic noise, can be monitored by the operator. 
   In some embodiments, an acoustic leak detector according to the present invention can include several noise cancellation systems. For example, the acoustic leak detector can include a noise cancellation system with a first acoustic detector positioned on the earth away from the pipe in order to cancel noise that is transmitted through the earth, a noise cancellation system with an acoustic detector positioned close to the acoustic detector in order to cancel ambient background noise at the acoustic detector; and an acoustic detector located in the headphones to selectively cancel ambient noise in the headphones. 
   The acoustic sensors may be of any device for detecting sound waves, such as piezoelectric transducers, microphones, or other acoustic sensors. The electronic signals output by the acoustic sensors are input to an electronic processing module. The electronic processing module amplifies and filters the signals detected from the acoustic sensors and combines the signals from the acoustic detectors to reduce the acoustic noise heard by the operator, making the sound produced by the leak more easily discernable. 
   In some embodiments, the headphones have two insulated shells which are held snugly to an operator&#39;s head by a headband. Each shell includes a microphone, which detects acoustic waves and converts them to an electronic signal. This electronic signal, along with the modified electronic signal from the electronic processing module that corresponds to the acoustic waves of interest, are input to the processor. The processor compares the electronic signals from the microphones to the modified electronic signal from the processing module and produces a cancellation signal. The cancellation signal is opposite in phase and of the same magnitude as the portion of the electronic signal from the speaker of the ANC headphones attributable to noise rather than the signal of interest. In some embodiments, the operator can mute the signal of interest and enhance the signal from the microphones in order to better hear the ambient noise signal which would have been cancelled. 
   In some embodiments, the electronic signal from the microphone in the headphones can be processed through a filter. Some of the noise measured by the microphone in the headphones, then, is not canceled. For example, the noise cancellation system may include an adjustable filter, letting the operator choose a frequency band that will not be cancelled by the processor. Alternately, the operator may choose to cancel only a specific frequency band. For example, to increase operator safety during acoustic detection, the operator may choose to cancel only those frequencies close to the frequency band of interest, so that noise outside this band is not cancelled but merely attenuated by the insulation of the headphone shells. The operator may also choose to allow sounds within selected bands of frequencies to not be cancelled. Alternately, the cancellation band may be set automatically or by a person other than the operator; for example, the acoustic detection system may be calibrated prior to use. 
   According to some embodiments of the invention, the acoustic sensors can be transducers such as piezoelectric transducers. The transducer utilized to measure the leak noise can be mounted on a support, which holds the transducer so that it is acoustically coupled to the surface of the ground. The electronic processing module may be mounted to the support, or may be carried by the operator; for example, it may be carried from a shoulder handle. The operator wears the headphones, which are connected to the electronic processing module. In some embodiments, the headphones can also provide some active noise cancellation. 
   According to some embodiments of the invention, an acoustic detector includes an acoustic sensor such as a transducer, and an acoustic barrier to shield the transducer from ambient noise transmitted through air. The acoustic detector may also include active noise cancellation circuitry to cancel noise inside the acoustic barrier. 
   According to some embodiments of the invention, an operator may use the embodiments of acoustic detection systems described above to find the position of a water leak in an underground pipe. The operator places the transducer so that it is acoustically coupled to the surface and listens for leak sounds. The operator may adjust the operating parameters of one or more noise cancellation systems. For example, the cancellation frequency band may be adjusted to optimize the operator&#39;s ability to detect leak sounds while maintaining a level of safety for the operator. 
   A more complete understanding of embodiments of the present invention will be appreciated by those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended drawing that will first be described briefly. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows operation of an acoustic leak detector according to the present invention in the presence of noise producing equipment. 
       FIG. 2  shows a block diagram of an embodiment of a noise cancellation circuit according to the present invention. 
       FIG. 3A  shows a diagram for active noise cancellation headphones of an embodiment of an acoustic leak detector according to the present invention. 
       FIGS. 3B and 3C  show block diagrams of active noise cancellation circuits for canceling noise at the headphones shown in  FIG. 3A . 
       FIG. 4  shows an embodiment of an acoustic leak detector according to the present invention with multiple active noise cancellation systems. 
   

   Use of the same or similar reference numbers in different figures indicates the same or like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a schematic diagram of an acoustic detection system, according to an embodiment of the invention. An acoustic detection system  160  of  FIG. 1  shows a system for detecting the position of a leak  100  in an underground pipe  101 . in some embodiments, acoustic detection systems according to the invention may be used for other purposes, such as, for example, to detect the position of a buried pipe where mechanical vibrations applied to an exposed portion of pipe may be detected above an underground portion. 
   A leak  100  in a pipe  101  emits leak sounds  102 . The frequency of leak sounds  102  generally lies in a frequency range between υ min  of approximately 40 Hz and υ max  of approximately 4000 Hz, although specific leaks in specific pipes (e.g., water leaks in water lines) may emit leak sounds in much more narrow frequency ranges. The frequency range of acoustic waves produced by other systems may lie in a different frequency range with a different υ min  and υ max . 
   An acoustic detector  110  converts leak sounds  102 , as well as noise picked up by acoustic detector  110 , into an electronic signal, which is coupled into processing unit  120 . Acoustic detector  110  may, for example, be a piezoelectric transducer, a microphone, or other acoustic detector capable of converting acoustic waves to electronic signals. A second acoustic detector  111  can be placed away from leak  100  to monitor background noise. In some embodiments, as shown in  FIG. 1 , acoustic detector  111  can be placed in contact with ground  103 , as is acoustic detector  110 , in order to detect noise transmitted through ground  103 . In some embodiments, acoustic detector  111  can be placed on or around processor  120  in order to detect noise. Acoustic detector  111 , as is acoustic detector  110 , can be any device which converts acoustic waves to electrical signals, for example piezoelectric transducers, microphones, or any other device capable of converting acoustic waves to electronic signals. In some embodiments, multiple acoustic detectors may be placed around acoustic detector  110  to provide electrical signals for noise cancellation in processor  120 . 
   Noise generator  150  shown in  FIG. 1  depicts any noise producing device. For example, generator  150  can be heavy equipment (e.g., backhoes, bulldozers, trucks, etc.), can be permanently installed units such as pumps or air conditioners, or can be such devices as jackhammers. Noise produced by generator  150  can be coupled into earth  103  or transmitted through air and degrades the ability of operator  104  to detect leak noise  102 . 
   Processing unit  120  processes the electrical signals from acoustic detectors  110  and  111  to produce a signal which can be reconverted into an acoustic signal at earphones  121 . Operator  104  monitors the acoustic signal at earphones  121  in order to detect leak sounds  102  from pipe  101 . 
     FIG. 2  shows an example block diagram of a signal processing circuit of processor  120 . The electrical signal from acoustic detector  110  is received in an amplifier  201 . In some embodiments, the gain of amplifier  201  can be controlled by operator  104 . In some embodiments, the gain of amplifier  201  can be preset. The output signal from amplifier  201  can be input to a filter  203 . Filter  203  can be a band-gap filter set to one of a set of preselected bands, which can be operator selected or may be pre-determined. In some embodiments, filter  203  can be set to pass only signals in a narrow band corresponding to leak sounds of a particular type of leak. 
   The electrical signal from acoustic detector  111  is input to amplifier  202 . The gain of amplifier  202 , in some embodiments, can be operator selected. In some embodiments, the gain of amplifier  202  can be preselected. In some embodiments, the gain of amplifier  202  can be selected to be the gain of amplifier  201  plus a user-selected gain. The output signal from amplifier  202  is input to filter  204 . Filter  204  can be set to pass signals within one of a preselected set of bands or may be fixed. In some embodiments, external noise within a certain band can be passed so that the operator can monitor certain background noises, for example, surrounding traffic. 
   The output signal from filter  204  is subtracted from the output signal from filter  203  in summer  205 . In some embodiments, the output signal from summer  205  is input to amplifier  206 . In some embodiments of the invention, filtering may occur after summer  205 . In other words, filter  203  may be positioned after summer  205 . In some embodiments of the invention, certain bands of frequencies in the signal received from acoustic detector  111  are not cancelled. 
   Amplifier  206  can have a user-controlled gain, which is utilized to select the volume of sound produced by headphones  121 . In some embodiments, headphones  121  can be a standard set of headphones or earphones. In some embodiments of the invention, processor  120  can include a microprocessor and processing of signals (including filtering and noise cancellation) can be accomplished digitally. 
   According to some embodiments of the invention, headphones  121  can include active noise cancellation to further improve the measurement process. As shown in  FIG. 3A , headphones  121  can include two insulated shells  320 L and  320 R, which may be semi-hemispherical. Insulation  334 L and  334 R attenuates environmental noise, although some noise passes through the insulation. In embodiments with active noise cancellation, microphone  332 L and  332 R convert ambient sound waves at shells  320 L and  320 R, respectively, to electronic signals. The electrical signals produced by microphones  332 L and  332 R include both ambient environmental noise and attenuated leak sounds provided to the interior of shells  320 L and  320 R, respectively, through speakers  333 L and  333 R, respectively. 
   As shown in  FIG. 3B , a portion of the electrical signals from microphones  332 R and  332 L can be subtracted from the signal provided to speakers  332 R and  332 L, respectively, to cancel the ambient noise. In the block diagram shown in  FIG. 3B , the signal from amplifier  206  (see  FIG. 2 ) is split into left (L) and right (R) channels. Each channel differentiates between the portion of the signal from speakers  332 R and  332 L that is due to ambient noise and which is leak noise (i.e., the output signal from amplifier  206 ) and produces an electrical signal which includes the leak noise and which will cancel the ambient noise. Ambient noise is cancelled by producing sound at speakers  333 L and  333 R that has the same amplitude and opposite phase from the ambient noise. 
   The output signal from amplifier  206 , as shown in  FIG. 3B , is summed in summer  338 R with the inverse of the signal from microphone  332 R. Amplifier  336 R amplifies the output signal from amplifier  206  by substantially two (2). Amplifier  340 R amplifies the output signal from microphone  332 R such that the leak signal has an intensity approximately that of the output signal from amplifier  206 . In some embodiments, the gains of amplifiers  336 R and  340 R can be user adjusted to maximize performance. In some embodiments, the gains may be fixed. The output signal from summer  338 R, then, is the leak signal output from amplifier plus a signal corresponding to the inverse of the noise signal. The inverse noise signal, then, will cancel the noise in shell  320 R. The left channel, which includes amplifiers  336 L and  340 L as well as summer  338 L, operates identically as described above with regard to the right channel. 
   By using active noise cancellation at headphones  121  instead of increasing the noise attenuation (by thickening insulation  134  or increasing the force with which headphones  130  are held to the operators head), unwanted noise is eliminated more efficiently. Furthermore, the noise cancellation characteristics of acoustic detection system  160  may be varied to prevent noise cancellation of sounds that the operator may need to hear. 
     FIG. 3C  shows an embodiment of an active noise cancellation circuit which can selectively cancel noise in headphones  121 . Instead of, in effect, subtracting the signal output by microphone  332 R from a multiple of the signal output from amplifier  206  to form a signal with the output from amplifier  206  plus an inverse of signal due to noise, the noise signal is isolated in summer  342 R and filtered in filter  344 R before being subtracted from the output signal from amplifier  206  in summer  338 R. Filter  344 R can be a stop band filter which passes all frequencies except those in the band that operator  104  needs to hear, for example shouts from colleagues or traffic noise. In some embodiments, operator  104  can select the characteristics of filter  344 R to optimize the ability to safely detect leaks. Additional amplifiers can be provided to adjust the amount of cancellation at summers  338 R and  338 L. In  FIG. 3C , amplifiers  345 R and  345 L are shown, but one skilled in the art will recognize that other parts of the circuit shown in  FIG. 3C  may also include amplification. 
   In some embodiments, the operator may adjust the magnitude of cancellation using a noise cancellation magnitude control, thereby decreasing but not eliminating residual environmental noise. The noise cancellation magnitude control can adjust the gain of amplifiers  340 R,  340 L,  345 R,  345 L and any other amplifiers as well as the proportion of the output signal from amplifier  206  that is subtracted in summer  342 R and  342 L. Additionally, in some embodiments, the operator may adjust the cancellation band by controlling the characteristics of filter  344 R and  344 L to allow important background noise to be heard through headphones  130 . For example, the operator may adjust the cancellation band to cancel signals in the frequency range of interest, while background noise at other frequencies is merely attenuated by insulation  134 . In such a case, a sound such as a warning shout of a co-worker would not be cancelled by the noise cancellation circuitry but merely be attenuated by insulation  134 . In some embodiments, the operator may actually enhance the ambient signal in the frequency range of interest in order to hear some background noise better. 
     FIG. 4  shows an embodiment of leak detector  160  with several active noise cancellation components. As discussed above, headphones  121  may be active noise cancellation headphones. Further, acoustic detector  111  can provide a noise signal to cancel noise from the signal of acoustic detector  110  that is transmitted through ground  103 . The embodiment of acoustic detector  110  shown in  FIG. 4  can be housed in a sound insulating dome  401  that can be mounted on a carrying handle  402 . Sound insulating acoustic barrier  401 , when placed flat on earth  103 , contacting acoustic detector  110  with the surface of earth  103 , can attenuate some external ambient noise transmitted through air. 
   Acoustic barrier  401  is positioned to attenuate noise but to not attenuate the acoustic waves of interest. Acoustic barrier  401  may be semi-hemispherical, so that acoustic detector  110  is placed in acoustic contact with a surface  103 , the acoustic barrier blocks noise from above surface  103 . At least a portion of acoustic barrier  401  may be flexible rather than rigid, so that upon pressure, the lower surface of acoustic  401  barrier conforms to the contours of surface  103  for more effective noise attenuation. 
   In some embodiments, an external acoustic detector  403 , provided in the vicinity of or on dome  401 , can provide a signal for canceling ambient noise. In some embodiments, signals related to ambient noise as well as signals from acoustic waves travelling through earth  103 , are provided by acoustic detector  111 . In some embodiments, acoustic detector  111  can be mounted in a second domed acoustic barrier  406 , which is similar to dome  401 . Although acoustic barrier  401  attenuates ambient noise, it does not remove it completely. In the region near surface  103 , the ambient noise may include an appreciable component in or near the frequency range of interest which will not be filtered out as the signal from the detector passes through the amplification and filter stage. 
   Separate operator controls for controlling parameters of the noise cancellation, for example filter characteristics or amplifier gains, can be located anywhere on leak detector  160 , including on the earphones or on processing unit  120 . As shown in  FIG. 4 , controls  404  are mounted on processor  120 . Controls  404  provide individual controls to individual noise cancellation circuits and to provide input controls for those circuits. In some embodiments, an external interface  405  on processing unit  120  can include a communication line. A signal from the communication line at interface  405  can be mixed into the signal acoustic detectors  110  so that operator  104  can communicate with co-workers or receive emergency warnings or instructions without removing or switching off noise cancellation. 
   In some embodiments, microphone  111  can be a contact microphone. A contact microphone typically includes a piezo-electric modulator mounted on a metal rod. The tip of the metal rod can be brought in contact with, for example, a hydrant or other structure to monitor ambient noise. 
   The embodiments described above are exemplary only and are not intended to be limiting. One skilled in the art may recognize various possible modifications that are intended to be within the spirit and scope of this disclosure. As such, the invention is limited only by the following claims.