fig1 is a schematic diagram showing a general construction of an example of the public address system according to the present invention . in the example of the invention shown in fig1 a speaker system 1 includes a loudspeaker 2 and also a receiving assembly 12 consisting of two receiving coils 12a and 12b which are located perpendicular to each other . a microphone 4 has a transmitting coil 5 which generates an induction magnetic flux . the microphone 4 and the speaker system 1 are located in a field such as a hall in which a sound is to be broadcast . in such a case , it is assumed that one or a plurality of speaker systems are located in the hall and the microphone 4 can be freely moved by a user in the sound broadcast . as will be described later , the output from the microphone 4 is fed to the speaker 2 through a cable or wireless and through an amplifier . the coils 12a and 12b , which are located perpendicular to each other , are wound on cores 11a and 11b , respectively , as shown in fig2 . an induction magnetic flux h generated by the transmitting coil 5 , in response to an output from an oscillator ( not shown ) in the microphone 4 , is selected so as to be in the plane in which the receiving coils 12a and 12b lie , as shown in fig1 or 2 . for example , if as shown in fig1 the speaker 2 of the speaker system 1 emits a sound in the direction indicated by an arrow 3 , two receiving coils 12a and 12b in the speaker system 1 are located perpendicular with each other in a plane parallel to the plane of the sheet of the drawing and the induction magnetic flux h from the transmitting coil 5 located near the microphone 4 also exists in the plane parallel to the plane of the sheet . according to the invention , further as shown in fig3 a signal e 2 obtained from one of the receiving coils , such as the receiving coil 12b is fed to a phase shifter 13 to be phase - shifted by 90 °, and the phase - shifted signal from the phase shifter 13 and also the signal e 1 from the other receiving coil 12a are applied to an adder 14 to be added to each other . the added signal from the adder 14 is applied through an amplifier 15 to a detector 16 which delivers its detected output to an output terminal 17 . in the above system , if a high frequency current of , for example , 1mh z is supplied to the transmitting coil 5 , the wave length of the electro - magnetic wave from the transmitting coil 5 is 300 m which is much longer than the distance between the transmitter and receiver . when the wave length of the signal from the transmitting coil 5 is sufficiently longer than the distance between the transmitter and receiver , the voltages induced in the receiving coils 12a and 12b are always same or opposite in phase . accordingly , if it is assumed that the distance between the transmitter and receiver is taken as γ , the angle between the magnetic flux h and the receiving coil 12a as θ ( as shown in fig2 and 3 ), and the angular frequency of the magnetic flux h as ω , respectively , voltages e1 and e2 , which are induced in the receiving coils 12a and 12b , are expressed as follows : thus , an output voltage e2 &# 39 ; from the phase shifter 13 is expressed as follows : therefore , an output voltage e from the adder 14 is expressed as follows : ## equ1 ## from the equation ( 4 ) it will be apparent that , according to the present invention , the added voltage e is varied in only phase and is not varied in amplitude or level even if the angle θ between the magnetic flux h and the receiving coil 12a is varied . therefore , a dc voltage , which is not varied in level regardless of the angle θ between the magnetic flux h and the receiving coil 12a but is varied in level in response to the distance γ between the transmitter and receiver , is obtained at the output terminal 17 led out from the detector 16 as a detected output . as shown in fig3 the sound output from the microphone 4 is fed to a pre - amplifier 30 and the output therefrom is fed through a variable attenuator 32 to a power amplifier 34 to which the loudspeaker 2 is connected . the variable attenuator 32 is supplied with the output from the detector 16 through the output terminal 17 for controlling the attenuation factor thereof . as the microphone 4 moves near the speaker system 1 , the magnitude of the output from the detector 16 increases and hence the attenuation factor of the attenuator 32 increases correspondingly . thus , the output level of the attenuator 32 to the power amplifier 34 becomes low . it may be also possible that the output from the microphone 4 is applied to the amplifier 30 by a wireless manner . in such a case , it is necessary that the transmitter is accommodated in the microphone 4 and the receiving circuit , which will convert the signal from the transmitter into a sound signal , is provided at the side of the speaker system 1 . further , in place of the variable attenuator 32 such a technique can be of course used in which the gain of the amplifier 30 or 34 is controlled by the output from the detector 16 . as described above , according to the system of the present invention , the distance between the transmitter and receiver can be positively detected , and when the transmitter and microphone move near the speaker within a predetermined region , the howling can be positively avoided . in applications wherein the signal is of a narrow frequency band , the phase shifter 13 may be formed of , for example , a buffer circuit 21 , inverter 22 , resistor 23 , capacitor 24 and buffer circuit 25 as shown in fig4 . in this case , if it is assumed that the resistance value of the resistor 23 is taken as r and the capacity of the capacitor 24 as c , it is sufficient that ( 1 / rc )= ω is satisfied . as described above , according to the present invention the stable operation is achieved and the howling is positively avoided . further , in the case of a public address system having a plurality of speakers , it is sufficient that the receiver constructed as above is located near each of the speakers . it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the present invention , so that the spirits or scope of the invention should be determined by the appended claims only .