Acoustic signal level limiter

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 levellimiter 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 limitercircuit 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 leastone 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 electricalsignal 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 levelthreshold 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 byadjusting 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 protectagainst 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, thetransistors 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 theattenuation 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 ofattenuation 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 levelattenuation 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 deeplysaturating 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 signallevel 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 levelattenuation 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 aresult, 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 acousticsignal 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 theultimate 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 toppedclipping 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 beimplemented 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 onspecific 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 beprovided 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 thedrawings, specification, and claims hereof.