Abstract:
An acoustic signal level limiter provides a telephone handset/headset user protection against loud audible signals generated within a communications system. The acoustic signal limiter comprises an acoustic signal level attenuation circuit and at least one acoustic signal level relay circuit. Once activated, the acoustic signal level attenuation circuit creates an attenuation network that attenuates an electrical acoustic signal transmitted through the communications system. Acoustic signal level relay circuits are activated to further attenuate the electrical acoustical signal to prevent the acoustic signal level attenuation circuit from operating in a deep saturation mode and provide further hearing safety for the telephone headset/handset user. A fuse in series and “Zener Zap” shunting transistor diodes provide assured higher signal level protection.

Description:
BACKGROUND 
   A. Technical Field 
   The present invention relates generally to acoustic signal level detection and control, and more particularly, to hearing safety protection in communication devices by limiting the level of an acoustic signal that will be transmitted to a listener through a communication device. 
   B. Background of the Invention 
   Proper control of electrical acoustic signal levels within a communication system is desirable in order to ensure high quality communication and hearing safety to individuals using the system. Generally, a telephone handset or headset includes a microphone and a speaker. For example, a telephone headset provides a microphone on an arm that is positioned proximately to a user&#39;s mouth and a speaker within an earpiece that is positioned over the user&#39;s ear. In order to provide proper communication, the sound level of the audible signal emitted from the speaker should fall within a range of sound intensity. If the sound level is below this intensity range, then a user may not be able to hear or understand what is being said by the remote talker. Comparatively, if the sound level is above this intensity range, then a user may be subjected to increasingly uncomfortable sound levels finally reaching the point of injury. Loud audible signals are of concern in communication devices, such as telephone handsets and headsets, that position a speaker near a user&#39;s ear. 
   These loud audible signals may be caused by a variety of different events. For example, accidental disturbances within a communication connection may cause an electrical signal level to increase dramatically (e.g., a malfunctioning amplifier, intense feedback, incorrect signal source, or phone line shorted to power line). Oftentimes, the transient time of a signal to reach a high level may be very short and a listener may have little or no time to remove an earpiece or other listening device away from her ear before she is exposed to that high level. In the case of a telephone headset user, the listener may have to bring her hand to her ear, which may require a second or two before the earpiece is removed. Also, many headset users spend a large amount of time on the phone. For example, headset users such as telemarketers, receptionists, and operators often spend eight hours a day answering telephone calls and speaking with people over a telephone. Due to this length of time, these individuals are at a higher risk of sound exposure caused by a sudden or constant loud audible signal. Thus, due to the extended time headset users are on telephone calls and the extra time required to remove the headset, headset users are particularly vulnerable to exposure to loud audible signals generated from the headset speaker. 
     FIG. 1  depicts a varistor  100  that has been used to reduce the maximum level to the speaker and hence the sound exposure. Basically, a varistor  100  is a surge protector that senses peaks within a signal and, in response, creates a shunt path, using diodes, that attenuates these peaks. As shown in  FIG. 1 , a voltage source  105 , with corresponding source impedance  110 , provides an electrical signal to an attached load  125  via the varistor  100 . Examples of this voltage source  105  include a telephone or telephone adapter and examples of the load include a telephone handset or headset speaker. The varistor  100  has a first diode  115  connected, in parallel, to a second diode  120 , each having a corresponding turn-on voltage. This turn-on voltage activates the diode and, typically, is between 0.5–0.7 volts. These diodes  115 ,  120  are used to shunt the positive and negative halves of an electrical signal through their corresponding paths, after the diode turn-on voltages are reached by the electrical signal received from the voltage source  105 . 
   The first diode  115  is used to shunt the negative half of the electrical signal current and is activated by a magnitude of the negative half exceeding the first diode&#39;s turn-on voltage. If this magnitude is below the turn-on voltage, then the first diode  115  remains cut off. However, if the magnitude is above the turn-on voltage, then the first diode  115  is on and conducts current from the voltage source  105  creating a shunt path. The amount of current actually conducted through the shunt path depends on the impedance of the load  125 . In any event, the level of the negative half of the electrical signal current received by the load  125  is reduced because of this activated negative shunt path. 
   The second diode  120  operates in a similar manner as the first diode  115  except that it creates a shunt path for the positive half of the electrical signal received from the voltage source  105 . As a result, the magnitude of each half of the electrical signal is attenuated after diodes  115 ,  120  are turned on. However, as is apparent, the voltage across the load  125  is not totally clamped if the electrical signal power increases. Rather, voltage across the load  125  may continue to increase, although at a much lesser rate due to the clamping effects of the positive and negative shunt paths. This voltage increase across the load  125  is caused by the fact that the resistance levels of the diodes  115 ,  120  are finally fixed. The level of shunting provided by the diodes  115 ,  120  is dependent on the ratio of the diode resistance and the load resistance, both of which are finally fixed, resulting in the diodes being unable to adjust the level of shunting provided by these diode paths. Thus, the voltage across the load  125  may continue to rise and produce a very loud audible signal if the electrical signal level is sufficiently high. 
   The above-discussed problems are caused primarily by the non-linear effects of the two diodes  115 ,  120 . Specifically, the relationship between the current through a diode and the voltage across the diode is nonlinear. As the voltage source reaches the turn on voltage of the two diodes  115 ,  120 , the diodes  115 ,  120  are activated and shunt current from the load  125 . Additionally, the nonlinear characteristics of the activated diodes  115 ,  120  cause higher distortion across the load  125 . Thus, the quality of any audible signal generated by the load  125  is compromised as turn on voltage is approached. 
     FIG. 2  depicts another circuit that has been used to reduce the electrical signal peaks. As shown in  FIG. 2 , a discrete transistor circuit  200  is placed in parallel to the first and second diodes  115 ,  120 . The discrete transistor circuit  200  comprises a first transistor  220 , a second transistor  225 , a first resistor  215  and a second resistor  230 . The first transistor  220  base and emitter are connected across voltage source  105 . The second transistor  225  is coupled, in parallel, to the voltage source  105  in a similar manner. This discrete transistor circuit  200  is placed in front of the two diodes  115 ,  120  and is activated by an electrical signal voltage level from the voltage source  105  being above the turn-on voltage of the first and second transistors  220 ,  225 . 
   The first transistor  220  attenuates the positive half of an electrical signal after the voltage level between the first transistor  220  emitter and base exceeds the first transistor&#39;s turn-on voltage. Typically, this transistor turn-on voltage is between 0.5–0.7 volts. Once this first transistor  220  is turned on, an attenuation network is created comprising the first resistor  220  and the resistance (Rce 1 ) between the emitter and the collector of the first transistor  220 . This attenuation network functions to decrease the positive voltage at the load  125  by allowing current to flow through the first transistor  220 . As the voltage level of the electrical signal increases, the transistor  220  goes into saturation mode and the resistance (Rce 1 ) between the emitter and collector of the first transistor  220  decreases. As a result, because this resistance (Rce 1 ) forms a divider network with the first resistor  215 , the current flowing through the first transistor  220  increases; thereby, limiting the relative voltage across the load  125 . As the voltage source increases, the saturation level within the first transistor  215  deepens. Hence, the voltage across the load  125  decreases. 
   The second transistor  225  operates in a similar manner as the first transistor  220  except that it operates on the negative half of the electrical signal from the voltage source  105 . Specifically, after the second transistor  225  is turned on, an attenuation network is created comprising the second resistor  230  and the resistance (Rce 2 ) between the collector and emitter of the second transistor  225 . Thus, the negative half of the electrical signal is attenuated, resulting in limiting the relative voltage across the load  125  as the voltage level on the electrical signal from the voltage source  105  increases. 
   Although the use of the two transistors  220 ,  225  provides better attenuation characteristics than the varistor  100  shown in  FIG. 1 , the two transistors  220 ,  225  are unable to sufficiently attenuate a very large signal. This failure results from the saturation characteristics of the two transistors  220 ,  225 . Specifically, if the voltage on the electrical signal increases to a sufficiently high level to drive the transistors  220 ,  225  into a deep saturation mode, the resistance (Rce 2 ) between the emitter and collector of the transistors  220 ,  225  becomes fixed. As a result, the two transistors  220 ,  225  function similar to diodes that operate within the varistor  100  and a resistor network is created. Thus, as the voltage level on an electrical signal increases driving the transistors  220 ,  225  into deep saturation, the voltage across the load  125  increases. 
   The first diode  115  and the second diode  120  are still required to provide the Zener short circuit to prevent the output from increasing. In order to compensate for the above-described problem, a varistor  100  may be placed behind the discrete transistor circuit  200 . However, the above-described problems regarding varistors are reintroduced. The resistor network resulting from the diodes  115 ,  120  is unable to clamp the voltage across the load  125 , rather only decrease the slope of any voltage increase of the load  125 . 
     FIG. 3  graphically depicts the inability of the above-described prior devices to effectively create a maximum voltage level across the load  125 . As shown in  FIG. 3 , an electrical output voltage signal  305  operating below a turn-on voltage  320  is relatively unaffected by the discrete transistor circuit  200 . However, after the turn-on voltage is reached, the rate at which the electrical output voltage signal  310  increases is reduced. If only the varistor  100  is used, then the voltage across the load  125  will continue to rise  325  as the voltage source  105  increases. If the discrete transistor circuit  200  and the varistor  100  are used, then the voltage across the load  125  will initially decline  330 . However, once the voltage source  105  is sufficiently high to drive the transistors  220 ,  225  into deep saturation, then the voltage across the load  125  will start to rise  335 . Due to the fixed resistance of the deep saturation characteristics of the transistors  220 ,  225 , the voltage across the load  125  is not absolutely clamped but slowly increases in relation to an increase in the source voltage  105 . It is important to note that the output voltage  310  may still continue above a desired clamping level  315 . 
   Accordingly, there is a need for an apparatus that limits an output voltage as the voltage source increases. Additionally, because this apparatus will likely operate in small devices with a limited power, there is an additional need that the apparatus minimize cost, power consumption, and distortion. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the deficiencies and limitations of the prior art by providing a hearing safety system and method that limits an acoustic signal level to a predetermined threshold. In one embodiment, an acoustic signal level limiter circuit is coupled between a voltage source and a load. Within the field of telecommunications, the voltage source may be a telephone or telephone adapter, and the load may be a telephone handset or headset. The acoustic signal level limiter circuit limits electrical signal levels from the voltage source by implementing multiple attenuation networks that are activated as the electrical signal crosses voltage levels. If the electrical signal level is sufficiently high to activate at least one of these attenuation networks, then the electrical signal is attenuated accordingly. As a result of this signal attenuation, the acoustic signal level limiter circuit is able to maintain a maximum signal level threshold through which the electrical signal is prevented from crossing. Ultimately, when the signal level is high enough to exhaust the attenuation potential of the limiter, a combination series fuse and/or shunt short disconnects all output to the load. This maximum signal level threshold protects a user of a communication device (e.g., a telephone headset) from signals that could produce undesirable audio levels. 
   The acoustic signal level limiter comprises an acoustic signal level attenuation circuit and at least one acoustic signal level relay circuit. The acoustic signal level limiter may be designed to handle various levels of signal overload by adjusting the number of acoustic signal level relay circuits. As the number of acoustic signal level relay circuits integrated within the acoustic signal level limiter increases, the ability of the acoustic signal level limiter is enhanced to protect against extremely high electrical signal levels within the communication system. Thus, the acoustic signal level limiter can be flexibly designed by increasing or decreasing the number of relays to handle different levels of signal overload. Finally, the series fuse and/or shunt short mechanism will disconnect output signals to the speaker load. 
   The acoustic signal level attenuation circuit is a discrete transistor circuit that operates to detect high electrical signal levels, and in response to a detected high level, attenuate the signal level to a proper level. However, the transistors within the acoustic signal level attenuation circuit will operate in a deep saturation mode if the electrical signal level is too high. Deeply saturated transistors are unable to effectively attenuate the electrical signal resulting in the attenuation network, in which the transistors operate, to function similar to a resistor network. Specifically, the effective resistance of transistors in deep saturation is constant resulting in a fixed resistor network that cannot adjust the amount of attenuation on the electrical signal. As a result, the voltage across the load increase, although at a lesser rate, as the electrical signal level from the voltage source increases. 
   An acoustic signal level relay circuit is integrated within the acoustic signal level limiter to enhance the operation of the acoustic signal level attenuation circuit. The acoustic signal level relay circuit inhibits the acoustic signal level attenuation circuit from entering a deep saturation mode. Specifically, once activated, the acoustic signal level relay circuit further attenuates the electrical signal from the voltage source in order to prevent the electrical signal from deeply saturating the transistors within acoustic signal level attenuation circuit and further reduce the voltage level across the load. However, like the transistors in the acoustic signal level attenuation circuit, the transistors within the acoustic signal level relay circuit may be deeply saturated if the electrical signal level is sufficiently high. The integration of the acoustic signal level relay circuit increases the required level at the voltage source to saturate the acoustic signal level attenuation circuit due to the fact that it is further attenuating the electrical signal from the voltage source. 
   Additional acoustic signal level relay circuits may be integrated within the acoustic signal level limiter in order to further increase the required level at the voltage source to saturate the acoustic signal level attenuation circuit. As a result, a manufacturer may design the acoustic signal level limiter according to the environment in which it operates and the particular needs of the user. For example, a telephone communications headset may require only a certain number of acoustic signal level relay circuits in order to protect against large signals propagating through a telephone line. At signal levels well beyond what is to be expected in an application, the series fuse/shunt short mechanism of the invention provides the ultimate fail safe, disabling the headset speaker drive. In addition to establishing a maximum electrical signal level threshold, the acoustic signal level limiter offers other performance advantages over current devices that are being implemented. 
   The acoustic signal level limiter takes advantage of specific characteristics of transistors within the acoustic signal level attenuation circuit and the acoustic signal level relay circuit(s) to provide enhanced signal level clamping effects. For example, the transistor network introduces less distortion than the varistor on the electrical signal as the electrical signal voltage level approaches the turn-on voltage. Also, the transistor circuits, rather than providing a simple flat topped clipping of the diodes, generate a concave topped waveform containing less energy. 
   The characteristics of the transistors may be modified by using various types of transistors (e.g. bipolar junction transistors (BJTs) or field effect transistors (FETs)). For instance, junction field effect transistors (JFETs) may be implemented to designate different pinch-off voltages, and correspondingly, manipulate the maximum signal level threshold allowed across the load. Additionally, the acoustic signal level limiter may be fabricated to provide a Zener diode effect on specific transistors. This Zener diode effect or “Zener Zap” is an integrated bipolar technology effect that provides a fuse that will trigger if a particular current level is reached within the circuit. Thus, a “Zener Zap” or other fuse device may be provided to isolate the load from the voltage source in the event the electrical signal level is too high for the acoustic signal level limiter to effectively attenuate the electrical signal. 
   The features and advantages described in this summary and the following detailed description are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a varistor operable within various communication devices. 
       FIG. 2  is a circuit diagram of a discrete transistor/varistor circuit operable within various communication devices. 
       FIG. 3  is a graphical diagram of the output voltage response of the varistor, and discrete transistor/varistor circuit. 
       FIG. 4  is a general block diagram of an acoustic signal level limiter apparatus. 
       FIG. 5  is a graphical diagram of the output voltage response of the acoustic signal level limiter apparatus. 
       FIG. 6  is a circuit diagram of a first embodiment of the acoustic signal level limiter apparatus. 
       FIG. 7  is a circuit diagram of a second embodiment of the acoustic signal level limiter apparatus. 
       FIG. 8  is a circuit diagram of a third embodiment of the acoustic signal level limiter apparatus. 
   

   The figures depict a preferred embodiment of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structure and devices are shown in block diagram form in order to avoid obscuring the invention. Moreover, it should be noted that the language used in this disclosure has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. 
   A. System Overview 
   The present invention limits the level of an incoming electrical acoustic signal to a particular maximum threshold.  FIG. 4  is a general block diagram showing an embodiment of the present invention. As shown, this embodiment operates between a voltage source  105  and a load  125 . Although the present invention may operate in a number of different environments, the following description will focus on telephone communication applications. However, the present invention should not be limited to the environment, rather, it may be applied in any environment wherein control of electrical signals is necessary. Additionally, the present invention will be described in terms of hardware components (e.g., transistors, diodes and resistors), but should not be limited to this description. Specifically, the present invention may be implemented using a number of different types of technology including hardware, software and firmware (e.g., digital signal processing implementations). 
   The voltage source  105  transmits an electrical signal that is eventually delivered to an electrical load  125 . Examples of this voltage source  105  include telephones and telephone adapters. Examples of the electrical load  125  include telephone headset and handset speakers. The voltage source  105  is coupled to an acoustic signal level attenuation circuit  410 . This acoustic signal level attenuation circuit  410  is coupled to one or more acoustic signal level relay circuits. For example, the acoustic signal level attenuation circuit  410  may be coupled, in parallel, to a first acoustic signal relay circuit  415 . This first acoustic signal relay circuit  415  may be coupled, in parallel, to a second acoustic signal level relay circuit  420 . These parallel connections of acoustic signal level relay circuits may continue depending on the requirements of the voltage source  105  and the load  125 . Finally, the load  125  is coupled, in parallel, to the Nth acoustic signal level relay circuit  430 . It is important to note that there may be only the first acoustic signal level attenuation circuits  415  placed behind the acoustic signal level attenuation circuit  410 . 
   The acoustic signal level attenuation circuit  410  initially detects peaks within an electrical signal from the voltage source  105 . In response to a peak being above a first threshold, the acoustic signal level attenuation circuit  410  reduces the level of the electrical signal. This signal reduction reduces the rate at which the signal level increases at the load  125  in relation to a signal level at the voltage source  105 . Thus, although the acoustic signal level attenuation circuit  410  may reduce the intensity of the acoustic signal from the load  125 , this acoustic signal intensity may still cross an undesirable threshold if the voltage source  105  transmits a sufficiently strong electrical signal. Accordingly, one or more acoustic signal level relay circuits are used to further attenuate the electrical signal transmitted from the voltage source  105  in order to further reduce the rate at which the voltage across the load  125  increases relative to the voltage source  105 . 
   The first acoustic signal level relay circuit  415  is activated in response to peaks within the electrical signal crossing a second threshold. Upon activation, the first acoustic signal level relay circuit  415  attenuates the electrical signal; thereby, further reducing the rate at which voltage across the load  125  increases relative to the voltage source  105 . Upon activation, the first acoustic signal level relay circuit  415  opens an attenuation network that adjusts the level of attenuation dependent on the level on the electrical signal. (In contrast, a varistor acts as a resistor network that cannot adjust attenuation levels because the resistance levels of the diodes within the resistor network are fixed.) However, if the electrical signal level becomes too high, the attenuation network in the first acoustic signal level relay circuit  415  will deeply saturate. As a result of this deep saturation, the attenuation network operates as a resistor network; thereby, allowing the voltage across the load  125  to increase (although at a relatively lesser rate). 
   In the illustrated embodiment, a second acoustic signal level relay circuit  420  is coupled, in parallel, to the acoustic signal level attenuation circuit  410  and the first acoustic signal level relay circuit  415  in order to compensate for electrical signals driving the attenuation network in the first acoustic signal level relay  415  into deep saturation. The second acoustic signal level relay circuit  420  provides a second attenuation network that is activated when the electrical signal crosses a third threshold. This third threshold is below the level required to drive the first attenuation network into deep saturation. Once activated, this second attenuation network further attenuates the electrical signal. As a result, the activation of the second acoustic signal level relay circuit  420  further increases the rate at which the electrical signal is attenuated. Thus, the voltage across the load  125  is further reduced in relation to the voltage source  105 . 
   The voltage across the load  125  may be further suppressed by coupling more acoustic signal level relay circuits. A maximum electrical signal level threshold may be created by the addition of these acoustic signal level relay circuits. Thus, as shown in  FIG. 4 , these acoustic signal level relay circuits may be coupled, in parallel, to each other up to the Nth acoustic signal level relay circuit  430 . Finally, the load  125  is coupled to the array of acoustic signal level relay circuits (if multiple attenuation circuits are used). The acoustic signal level relay circuits may be activated sequentially or activated virtually instantaneously. 
     FIG. 5  shows this maximum electrical signal level at the load  125  by plotting the output voltage (voltage across the load  125 ) versus the input voltage (voltage source  105 ). Prior to activating the acoustic signal level attenuation circuit  410 , the output voltage  505  increases approximately at the same rate the input voltage increases. In response to the electrical signal reaching the activation level  520  of the acoustic signal level attenuation circuit  410 , the attenuation network within acoustic signal level attenuation circuit  410  is activated and the electrical signal is attenuated accordingly. This effect is shown on  FIG. 5  by the drastic reduction in the slope of the curve at the activation level  520 . As the input voltage increases beyond the activation level  520 , other acoustic signal level relay circuits are activated. Thus, as the number of activated acoustic signal level relay circuits increases, the relative attenuation on the electrical signal will increase and cause the load voltage to continually decrease  530 . 
     FIG. 5  shows that this maximum electrical signal level at the load  125  occurs as a result of these activated acoustic signal level relay circuits. In fact, as the number of activated attenuation circuits increases, it was shown that the voltage level across the load  125  decreased due to the total effects of the attenuation circuits and that a higher voltage source  105  was required to deeply saturate the transistors causing the load voltage to rise  525 . In order to prevent deeply saturating the transistors and the resulting increase in load voltage at this high voltage source  105  level, additional acoustic signal level relay circuits may be added to ensure the load voltage does not increase at this high voltage source  105  level. As a result, the emitted audible signal from the load  125  never becomes loud enough to exceed the desired limits. The following sections explain each of the above-described circuits in more detail. 
   B. Acoustic Signal Level Attenuation Circuit 
     FIG. 6  is a circuit diagram of an embodiment of the present invention. An acoustic signal level limiter circuit  600  is coupled between a load  125  and a voltage source  125 . As was the case above, this embodiment will be described as operating in a telephone communication environment. However, the present invention should not be limited to this environment, but rather, it may function in numerous situations where control of electrical signals is necessary. Thus, within this environment, the load  125  may be a telephone headset or handset. The voltage source  105 , with a corresponding source impedance  110 , may be a telephone or telephone adapter. 
   The voltage source  105  is coupled, in parallel, to an acoustic signal level attenuation circuit  680  and a first acoustic signal level relay circuit  670 . The acoustic signal level attenuation circuit  680  is activated by an electrical signal peak from the voltage source  105  crossing a first threshold. The first acoustic signal level relay circuit  670  is activated by an electrical signal peak from the voltage source  105  crossing a second threshold that is higher than the first threshold. 
   The acoustic signal level attenuation circuit  680  comprises a first transistor  630 , a second transistor  635 , a first resistor  625  and a second resistor  650 . The base of the first transistor  630  is coupled to the positive side of voltage source  105 , and its emitter is coupled to the negative side of the voltage source  105 . The second transistor  635  is coupled in parallel to the voltage source  635 , with its base coupled to the negative side of the voltage source  105  and its emitter coupled to the positive side of the voltage source  105 . The acoustic signal level attenuation circuit  680  is activated by the electrical signal from the voltage source  105  being sufficiently high to turn on the first transistor  630  and the second transistor  635 . Typically, this level is between 0.5–0.7 volts. Once activated, the acoustic signal level attenuation circuit  680  attenuates the electrical signal in the following manner. 
   The first transistor  630  attenuates the positive half of an electrical signal from the voltage source  105  after the voltage level between the first transistor  630  emitter and base exceeds a transistor turn-on voltage. Once this first transistor  630  is turned on, a first attenuation network is created comprising the first resistor  625  and the resistance (Rce 1 ) between the emitter and the collector of the first transistor  630 . This first attenuation network functions to decrease the positive voltage at the load  125  by allowing current to flow through the first transistor  630 . As the voltage level on the analog signal increase, the resistance (Rce 1 ) between the emitter and collector of the first transistor decreases. As a result, because this resistance (Rce 1 ) forms a divider network with the first resistor  625 , the current flowing through the first transistor increases; thereby, limiting the relative voltage across the load  125 . Specifically, as the voltage level of the electrical signal increases, the resistance (Rce 1 ) decreases. Because the first resistor  625  is fixed, a larger percentage of the electrical signal flows through the first transistor  630  and a lesser percentage of the electrical signal flows through the load  125 . 
   The second transistor  635  operates in a similar manner as the first transistor  630  except that it operates on the negative half of the electrical signal from the voltage source  105 . Specifically, after the second transistor  635  is turned on, in a similar manner described above, a second attenuation network is created comprising the second resistor  650  and the resistance (Rce 2 ) between the collector and emitter of the second transistor  635 . Like the first attenuation network, the second attenuation network adjusts the percentage of the electrical signal flowing through the second transistor  635  depending on the strength of the electrical signal. Thus, the negative half of the analog signal is attenuated, resulting in limiting the relative voltage across the load  125  as the voltage level on the analog signal from the voltage source  105  increases. 
   The acoustic signal level attenuation circuit  680  effectively attenuates the electrical signal as long as the electrical signal does not put the first transistor  630  and the second transistor  635  into deep saturation modes. Deep saturation occurs when the electrical signal voltage level is sufficiently high to drive a collector-emitter resistances (Rce) to their minimum resistance levels. Thus, at deep saturation, the collector-emitter resistances (Rce) of the first transistor  630  and the second transistor  635  remain constant as the electrical signal voltage level increases. The attenuation network operates as a resistor network causing the voltage across the load  125  to increase. In order to attempt to prevent the acoustical signal level clamping circuit  680  from going into deep saturation, an acoustic signal level relay circuit  670  is coupled to the acoustic signal level attenuation circuit  680  and the voltage source  105 . 
   C. Acoustic Signal Level Relay Circuit 
   The acoustic signal level relay circuit  670  comprises a third transistor  640  and a fourth transistor  645 . The third transistor  640  emitter is coupled to the positive side of the voltage source  105 , the collector of the fourth transistor  645 , the base of the first transistor  630  and the first resistor  625 ; the base is coupled to the first resistor  625  and the collector of the first transistor  630 ; and the collector is coupled to the negative side of the voltage source  105 , the emitter of the fourth transistor  645 , the emitter of the first transistor  630 , and the second resistor  650 . The fourth transistor  645  emitter is coupled to the negative side of the voltage source  105 , the collector of the third transistor  640 , the emitter of the first transistor  630 , and the second resistor  650 ; the base is coupled to the second resistor  650  and the collector on the second transistor  635 ; and the collector is coupled to the positive side of voltage source  105 , the emitter of the third transistor  640 , the base of the first transistor  630 , and the first resistor  625 . The acoustic signal level relay circuit  670  is activated when the voltage drops across the first resistor  625  and the second resistor  650  are sufficient to turn on the third transistor  640  and the fourth transistor  645 . As described above, this voltage drop required to turn on the transistors is generally between 0.5–0.7 volts. Once activated, the acoustic signal level relay circuit  670  applies further attenuation to an electrical signal from the voltage source  105 . 
   The third transistor  640  further attenuates the positive half of the electrical signal from the voltage source  105  after a voltage drop across the first resistor  625  exceeds the transistor turn-on voltage. Once this third transistor  640  is turned on, a third attenuation network is created comprising the second resistor  625  and the resistance (Rce 3 ) between the emitter and the collector of the third transistor  640 . This third attenuation network functions to decrease the magnitude of the positive voltage at the load  125  by allowing current to flow through the third transistor  630 . As the voltage level of the electrical source  105  increases, the resistance (Rce 3 ) between the emitter and collector of the third transistor  640  decreases. As a result, the current flowing through the third transistor  640  increases; thereby, limiting the relative voltage across the load  125 . Specifically, as the voltage level of the electrical signal increases, the resistance (Rce 3 ) decreases. Then, a larger percentage of the electrical signal flows through the third transistor  640  and a lesser percentage of the electrical signal flows through the load  125 . 
   The fourth transistor  645  operates in a similar manner as the third transistor  640  except that it operates on the negative half of the electrical signal from the voltage source  105 . Specifically, after the fourth transistor  645  is turned on by a sufficient voltage drop across the second resistor  650 , a fourth attenuation network is created comprising the first resistor  625  and the resistance (Rce 4 ) between the collector and emitter of the fourth transistor  645 . Like the third attenuation network, the fourth attenuation network adjusts the attenuation level on the electrical signal, but instead, operates on the negative half of the electrical signal. Thus, the negative half of the electrical signal is attenuated, resulting in limiting the relative voltage across the load  125  as the voltage level on the electrical signal from the voltage source  105  increases. 
   This first acoustic signal level relay circuit  670  provides an extra layer of protection and raises the voltage level from the voltage source  105  to put the acoustic signal level limiter  600  into a deep saturation mode. In order to protect a user from electrical signals above this level, a fuse  615  is coupled to the voltage source. The fuse  615  is triggered when the voltage level on an electrical signal crosses a particularly large level that may still injure a user in spite of the acoustic signal level limiter  600 . However, the protection offered by the acoustic signal level limiter  600  may be heightened by providing multiple layers of acoustic signal level relay circuits. 
   The first acoustic signal level relay circuit  670  offers a number of advantages over currently used varistors. First, as already described above, the transistors  640 ,  645  within first acoustic signal level relay circuit  670  provide a variable resistance that adjusts as the level on the electrical signal fluctuates. Thus, as the electrical signal becomes stronger, a higher percentage of the electrical signal is attenuated. Comparatively, the diodes  115 ,  120  within varistor  100  have a fixed resistance and the attenuation within the corresponding resistor networks is fixed. 
   Second, the transistors  640 ,  645  within the first acoustic signal level relay circuit  670  operate as an automatic attenuation control. The higher the voltage source  105  rises the higher the resulting attenuation on the electrical signal. As a result, the voltage across the load  125 , and subsequent audible signal, is better controlled and users will not be exposed to very large electrical signal levels transmitted by the voltage source  105 . 
   D. Multiple Integrated Acoustic Signal Level Relay Circuits 
     FIG. 7  shows a circuit diagram of an acoustic signal level limiter  700  comprising an acoustic signal attenuation circuit  680 , a first acoustic signal relay circuit  670 , and a second acoustic signal relay circuit  790 . The second acoustic signal level relay circuit comprises a fifth transistor  770 , a sixth transistor  735 , a third resistor  765 , and a fourth resistor  745 . The fifth transistor  770  emitter is coupled between the second resistor  750  and the fourth resistor  745 ; the base is coupled to the collector of the second transistor  635 , the collector of the sixth transistor  735  and the fourth resistor  745 ; and, the collector is coupled to the base of the sixth transistor  735 , the collector of the first transistor  630 , and the third resistor  765 . The sixth transistor  735  emitter is coupled between the first resistor  720  and the third resistor  765 ; the base is coupled to the collector of the fifth transistor  770 , the collector of the first transistor  630  and the third resistor  765 ; and, the collector is coupled to the base of the fifth transistor  770 , the collector on the second transistor  635  and the fourth resistor  745 . The second acoustic signal level relay circuit  790  is activated when the voltage drops across the third resistor  765  and the fourth resistor  745  are sufficient to turn on the fifth transistor  770  and the sixth transistor  735 . As described above, this voltage drop required to turn on the transistors  735 ,  770  is generally between 0.5–0.7 volts. Once activated, the second acoustic signal level relay circuit  790  applies even further attenuation to an electrical signal from the voltage source  105 . 
   The second acoustic signal level relay circuit  790  operates similarly to the first acoustic signal level relay circuit  670  to provide further attenuation on the electrical signal from the voltage source  105 . Specifically, the fifth transistor  770  further attenuates the negative half of the electrical signal from the voltage source  105  after a voltage drop across the fourth resistor  745  exceeds the transistor turn-on voltage. Once this fifth transistor  770  is turned on, a fifth attenuation network is created comprising the fourth resistor  745  and the resistance (Rce 5 ) between the emitter and the collector of the fifth transistor  770 . This fifth attenuation network functions to even further decrease the magnitude of the negative voltage at the load  125  by allowing current to flow through the fifth transistor  770 . As the voltage level on the electrical source  105  increases, the resistance (Rce 5 ) between the emitter and collector of the fifth transistor  770  decreases. As a result, because this resistance (Rce 5 ) forms a divider network with the fourth resistor  745 , the current flowing through the fifth transistor  770  increases; thereby, limiting the relative voltage across the load  125 . Specifically, as the voltage level of the electrical signal increases, the resistance (Rce 3 ) decreases, and because the fourth resistor  745  is fixed, a larger percentage of the electrical signal flows through the fifth transistor  770  and a lesser percentage of the electrical signal flows through the load  125 . 
   The sixth transistor  735  operates in a similar manner as the fifth transistor  770  except that it operates on the positive half of the electrical signal from the voltage source  105 . Specifically, after the sixth transistor  735  is turned on by a particular voltage drop across the third resistor  765 , a sixth attenuation network is created comprising the third resistor  765  and the resistance (Rce 6 ) between the collector and emitter of the sixth transistor  735 . Like the fifth attenuation network, the sixth attenuation network adjusts the attenuation level on the electrical signal, but instead, operates on positive half of the electrical signal. Thus, positive half of the electrical signal is attenuated, resulting in limiting the relative voltage across the load  125  as the voltage level on the electrical signal from the voltage source  105  increases. 
   The second acoustic signal relay circuit  790  further increases the required voltage level on the voltage source  105  to drive the acoustic signal level attenuation circuit  680  into a deep saturation mode. This increase is due to the further attenuation provided by the second acoustic signal relay circuit  790 . Thus, the trigger level on the fuse  615  may be increased because of the heightened protection provided by the acoustic signal level limiter  700  having a second acoustic signal level relay circuit  790 . 
   The acoustic signal level limiter  800  may be further enhanced by adding even more layers of acoustic signal level relay circuits.  FIG. 8  shows an example of an acoustic signal level limiter apparatus with N number of acoustic signal level relay circuits. As shown, the acoustic signal level limiter may include a large number of acoustic signal level relay circuits to increase the level of attenuation on the electrical signal from voltage source  105 . The acoustic signal level relay circuits are coupled within the acoustic signal level limiter  800  in a similar manner as described above. For example, third and fourth acoustic signal relay circuits  830  are coupled within the acoustic signal relay circuits in the same manner as the first acoustic signal relay circuit  670  and the second acoustic relay circuit  790 . Accordingly, N and N-1 acoustic signal level relay circuits  840  follow the same coupling procedure within the acoustic signal level limiter  800 . 
   The characteristics of these acoustic signal relay circuits may be modified by adjusting the resistance levels of the resistors operating within the attenuation networks. Also, different types of transistors (e.g., bipolar junction transistors (BJTs) and field-effect transistors (FETs)) may be implemented within the design to adjust turn-on levels and pinch-off voltages. For example, junction field-effect transistors (JFETs) may be used to adjust the level of attenuation and turn-on voltages provided by transistors within an attenuation network. Specifically, the pinch-off voltages on JFETs may be adjusted to change the maximum voltage level threshold across the load  125   
   A finite number of rely circuits in a given application means that a current level can be reached that will result in limiter failure modes. It is necessary to insure that any failure mode is prevented from delivering unsafe voltage output to the load. It is essential that signals are not passed to the load if the limiter function is not operational. The invention provides two methods to achieve ultimate failsafe operation. These are series fusing and shunt shorting of the signal with relation to the load. 
   Firstly, proper series fusing will insure that the load is disconnected before the limiter circuit establishes a failure mode. When the limiter circuit conducts large current over a short amount of the time, the lowest thermal mass item in the circuit experiences the most energy. For example, in a typical IC package, 1 mil diameter gold bond wires fuse open or melt. These wires connect the package pins to the ASIC die bonding pads so when these input wires fuse, the headset earphone signal path is opened. 
   Secondly, a process called a “Zener Zap” can be incorporated in a manner that provides short circuiting of the output at a desired high energy level. The “Zener Zap” occurs when sufficiently high power is produced across semiconductor diode junctions in either a forward or reverse direction. The semiconductor diode junction will breakdown if this power level is sustained for a sufficient period of time and the junction will be finally breached. Specifically, the conducting semiconductor may be heated by this power level in excess of 200 degrees Celsius. The on-chip interconnect metal which contacts these junctions is also heated until the metal liquefies and the metal atoms are drawn through the semiconductor junction. This process may occur very quickly because the metal atoms create a metal bridge or short at the junction. 
   This resistance of this short depends on various factors including the amount of metal atoms at the semiconductor junction and the amount of time the energy remains above the thermal melting point in the junction prior to breech. The short is permanent and its resistance level may vary from a couple ohms up to approximately 25 ohms, much less than the impedance of the acoustic signal level limiter and load. In one design of the acoustic signal level limiter, the corners of the transistor layout base and emitter contacts have been optimized for both current crowding in order to force localized contact heating and to provide a large source of contact metal to precipitate a low ohmic short. 
   As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, the particular division of functionality between the various modules, circuits, or components may differ from that described herein, given the variety of software hardware platforms that may be used to practice the invention. For example, the hardware implementations include custom ASICs, discrete logic, FPGAs, PLAs, or DSP with appropriate software programming. Finally the particular naming of the circuit elements is not mandatory or significant, and the mechanisms that implement the invention or its features may have different names or formats. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.