Abstract:
A photoacoustic detector includes a sensing region for receiving atmospheric samples. One microphone receives acoustic samples from the sensing region. Another microphone receives acoustic samples from a displaced region. Microphone outputs can be subtracted to eliminate common noise and to generate an indicium of gas present in the sensing region.

Description:
FIELD 
     This application pertains to photoacoustic detectors. More particularly, the application pertains to such detectors which include circuitry to remove acoustic noise. 
     BACKGROUND 
     Various types of photoacoustic sensors are known to detect gases. These include, Fritz et al., US Patent Application No. 2009/0320561, published Dec. 31, 2009 and entitled “Photoacoustic Cell”; Fritz et al., US Patent Application No. 2010/0027012, published Feb. 4, 2010 and entitled, “Photoacoustic Spectroscopy System”; and Fritz et al., US Patent Application No. 2010/0045998, published Feb. 25, 2010 and entitled “Photoacoustic Sensor”. The above noted published applications have been assigned to the assignee hereof, and are incorporated herein by reference. 
     Such sensors, while useful, can be affected by acoustic and mechanical vibration noise sources. Such sources can create significant errors when their frequency content contains components at or near the operational frequency of the respective sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a detector in accordance herewith; 
         FIG. 2  is a set of graphs which illustrate operational aspects of the detector of  FIG. 1 ; and 
         FIGS. 3A ,  3 B illustrate exemplary signal processing. 
     
    
    
     DETAILED DESCRIPTION 
     While embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles hereof, as well as the best mode of practicing same. No limitation to the specific embodiment illustrated is intended. 
     In a disclosed embodiment, noise can be eliminated in a photoacoustic detector by using two microphones. A first microphone is connected to a gas cell. This microphone responds to the photoacoustic signal and any acoustic noise signal. The second microphone is not connected to the gas cell but is exposed to the environment. This microphone responds to just the acoustic noise signal. 
     The signals from the two microphones can be combined to remove the common acoustic noise. Preferably, the microphones will be mounted with the same orientation relative to a selected plane so that vibration related noise will act equally on both microphone diaphragms. 
     The cell has an input port which is covered with a diffusion membrane. The second microphone is covered by an acoustic filter which is sealed to the sound port. The acoustic filter is designed to replicate the acoustic behavior of the cell and membrane connected to the first microphone. 
       FIG. 1  illustrates an embodiment  10  of a photoacoustic detector in accordance herewith. Detector  10  can include a housing  12  suitable for portable or fixed use such as by attachment to a wall, ceiling or other mounting structure as desired. Detector  10  can monitor gas concentration in a region R. 
     Detector  10  includes a sensing chamber, or gas cell  20 . The cell  20  can have a variety of shapes as would be understood by those of skill in the art. The shape of the cell  20  is exemplary only. 
     Cell  20  defines an internal region indicated generally  22  with an atmospheric/environmental input port  24   a.  Port  24   a  is covered by a gas permeable membrane  28 . 
     Cell  20  defines a light, or radiant energy input port  24   b  which can receive infra-red radiant energy from a source  30 . Radiant energy from the source  30  can be focused by a reflector  32  and filtered by a filter  34  carried by the cell  20  adjacent to the port  24   b.    
     Cell  20  also defines an acoustic port  24   c  to which is coupled a first microphone  40 . The microphone  40  has an audio input port  40 - 1 . A second, or reference microphone  42  has an audio input port  42 - 1  which is covered by an acoustic filter  28 - 1 . 
     The membrane  28  in combination with the cell  20 , and acoustic filter  28 - 1  have substantially identical acoustic attenuating characteristics relative to respective microphones  40 ,  42 . The microphone  40  responds to audio inputs, including noise, from within the region  22 . The reference microphone  42  is oriented and carried on the housing  12  to respond to audio inputs such as noise from the ambient environment in the vicinity of the detector  10 . Both microphones can have the same orientation relative to a predetermined plane to equalize the effects of vibratory noise. 
     Control circuitry  50 , which could be combinational, or sequential, or both, receives signals, on lines  40 - 2  and  42 - 2 , from both microphones  40 ,  42 . As discussed subsequently, the common mode noise can be eliminated by subtracting the two signals. 
     Control circuitry  50  can be coupled to source  30  so as to modulate same at a selected frequency, as would be understood by those of skill in the art. Also as would be understood by those of skill in the art the control circuitry  50  can include wired or wireless interface circuitry  52  so that the detector  10  can communicate with an associated monitoring system, or diagnostic and test equipment via a wired or wireless medium  54 . 
       FIG. 2  illustrates the cell signal A, labeled, detection channel detected by microphone  40 , reference signal B detected by microphone  42  and the noise free difference signal C. Signal C could be processed to make a gas concentration determination. 
       FIGS. 3A ,  3 B illustrate exemplary processing  100 ,  200  which can be carried out by the detector  10 . Alternately, instead of carrying out the processing locally, the signals from the microphones  40 ,  42  can be transmitted via interface circuits  52  to displaced circuitry for processing. 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope hereof. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.