Abstract:
A hearing protector assembly has a muff and an ear-plug for hearing protection while allowing reception of audio communication. The muff has a receiver for audio signals, a modulation circuit, and a set of LEDs for transmitting the audio signal as a light signal. The ear-plug has a photovoltaic cell for receiving the light signals, a demodulator circuit, and a transducer for converting the received signal into an acoustic signal.

Description:
STATEMENT OF GOVERNMENT RIGHTS  
       [0001] The United States Government has rights in this invention as provided for by the terms of Grant Number R44 OH04132 awarded by the National Institute for Occupational Safety and Health. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention pertains to hearing protectors.  
           [0003]    Hearing protectors are designed to prevent external sound from reaching the ears. They are used in a variety of situations, and are used, for example, by people who are exposed to high levels of noise for extended periods of time. They can also be used during hearing testing, in which case they are used to reduce the interfering effects of background noise in a test environment.  
           [0004]    The two most common forms of hearing protectors are muffs and earplugs, which can be used separately or together for maximal sound attenuation. In many situations, it is desirable for users of hearing protectors (such as helicopter pilots) to continue to receive audio communications while blocking harmful sounds. Some types of muff-type protectors are therefore equipped with earphones to form a communication headset. If both ear-plugs and muffs are needed but the user must also receive audio signals, a communication headset does not serve the purpose because acoustic signals are attenuated by the plugs before they reach the user&#39;s ears.  
         SUMMARY OF INVENTION  
         [0005]    The invention relates to hearing protectors and to methods for providing acoustic signals to users wearing hearing protectors. Described embodiments include an apparatus and method that use optical signal transmission between a signal transmitter in a hearing protecting muff and a receiver connected to an ear-plug and inserted into the ear canal.  
           [0006]    The embodiments of the present invention can include at least some of the following benefits: the circuitry in the ear-plug requires no battery and can operate at low power levels; optical transmissions are easily contained within the muff and therefore do not pose a risk of interfering with other systems; and an optical receiver can be made immune to interference from external electromagnetic fields. The method of transmission used in the present invention is simple and efficient. The light can be transmitted with light emitting diodes (LEDs) powered by a simple battery and therefore provide many hours of use. Other features and advantages will become apparent from the following detailed description, drawings, and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a pictorial representation of an embodiment of the assembly of the invention.  
         [0008]    [0008]FIG. 2 is a block diagram according to an embodiment of the invention.  
         [0009]    [0009]FIG. 3 is a schematic of a circuit according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 1 shows a pictorial view of a hearing protection system  100  which includes a muff assembly  102  and ear-plug assembly  104 . Muff assembly  102  includes a sound-isolating muff  106  and LEDs  108 . Muff  106  has a simple shell and may have a custom fit enclosure or active noise cancellation circuitry (not shown). Muff  106  is worn over a user&#39;s ear. Ear-plug assembly  104  includes plug  110 , a mono-crystalline silicon photovoltaic (PV) cell  112 , and circuitry. Plug  110  can be made of any suitable compliant material in a generic shape or can be a custom-cast earmold for improved fit and sealing ability. Plug  110  is inserted into the user&#39;s ear canal, while PV cell  112  is outside the ear canal.  
         [0011]    Referring also to FIG. 2, an external audio input is provided to modulator  114  for modulating the signal, and then to a buffer  116 . The buffered signal drives a light source, such as LEDs  108 , to emit light signals representative of the audio input. Plug assembly  104  has a PV cell  112  that is positioned to receive the light signals from LEDs  108 . PV cell  112  receives the light signals from LEDs  108  and passes them to a demodulator  118  and a miniature loudspeaker transducer  120  to provide to the user an audio output that is representative of the audio input. Demodulator  118  and transducer  120  are thus part of plug assembly  104 .  
         [0012]    [0012]FIG. 3 shows an example of a schematic of circuits  130  and  128  that correspond to circuits shown in block form in FIG. 2. An audio input signal is provided to a modulator  114 , such as a pulse width modulator (PWM). The PWM includes an input circuit consisting of a high pass filter and load, formed by capacitor  148  and resistor  150 , a comparator  134 , and an oscillator  132 . The filtered input is provided to the inverting input of comparator  134 . The non-inverting input to comparator  134  is a 30 kHz triangle wave carrier signal generated by oscillator  132 .  
         [0013]    Muff assembly  102  may physically include modulator  114  and buffer  116 , or may be physically remote from them. Unless otherwise indicated, these circuit components are considered part of the muff assembly even if physically remote.  
         [0014]    The modulated audio signal is provided to buffer  116 , which is formed in this example by a transistor  136  and a low-output resistor. Transistor  116  has one side coupled to ground, the other side coupled to one side of LEDs  108 , and a control lead coupled to the output of comparator  134 . A power source  138 , such as a 9-volt battery, is coupled to the other side of LEDs  108 . As transistor  136  turns on and off in response to the comparator, light output of LEDs  108  follows.  
         [0015]    As shown in FIG. 3, ear-plug circuit  128  includes a parallel connection of PV cell  112 , a load resistor  140 , and capacitor  142 . Load resistor  140  and capacitor  142  form a low pass filter that serves as demodulator  118  of FIG. 2, and are in parallel with a series coupling capacitor  144  and a low impedance hearing aid transducer  146 .  
         [0016]    With low impedance, transducer  146  effectively utilizes the power received from PV cell  112 . The impedance is preferably on the order of 50 ohms rather than a more common 500 ohm. The demodulation/filtering that occurs in circuit  128  is determined partly by the discrete capacitor and resistor components and partly by the intrinsic characteristics of the PV cell (internal resistance, capacitance) and acoustic transducer (resistance, inductance, acoustical parameters). Using the component values specified to drive an acoustic circuit replicating a human ear canal, one can achieve 90 dB SPL and distortion levels under 1%.  
         [0017]    Circuitry  128  of PV cell  112  is compact enough to be enclosed in the space between muff  106  and the wearer&#39;s ear without adversely affecting the fit or effectiveness of muff  106 . The ear plus circuitry can be provided, for example, on the back of the PV cell using surface mount technology, or combined with the PV cell in a three-dimensional circuit board.  
         [0018]    Due to propagation characteristics inherent in optical transmission, the present invention is immune to interference from adjacent transmitters and does not cause interference for other apparatus. This feature is useful because it allows for binaural presentation (i.e. two independent channels, one for each ear), and also eliminates interaction between any one system and neighboring systems.  
         [0019]    For binaural presentation over separate channels, each muff would have its own modulator and buffer, although it would be preferable to share the oscillator and power supply. A monaural system provided to both ears would share one modulator and buffer, but have separate LEDs and ear-plug assemblies.  
         [0020]    Other embodiments of the present invention are within the claims. For example, the light used in transmission can be visible or infrared, with red being particularly useful for demonstration purposes. While LEDs have been used as an exemplary light source, other sources could be used, preferably a source that is capable of being modulated at an adequately high frequency to accommodate the modulation scheme. While one photocell per ear-plug assembly is preferred, a stack of photocells can be used. The cells can be rectangular in shape or shaped to conform to the ear. Other forms of modulation, such as pulse density and frequency modulation, can be used as an alternative to pulse width modulation. The frequency of the carrier waveform used for modulation can be different from the 30 kHz shown, but should be above the Nyquist frequency of 15 kHz and not be too high to create loss from capacitive effects in the PV cell.