referring to fig1 , there is shown a sound - emitting device 10 . the device 10 comprises an oscillator 20 , an amplifier 30 for increasing the signal received from the oscillator 20 , and a transducer 40 for generating a sound wave at a frequency in the in the range of 16 . 5 khz ± 1 khz , in accordance with the signal generated by the oscillator 20 and subsequently amplified by the amplifier 30 . the amplifier may further comprise an automatic gain control 31 , which measures the level of background noise and adjusts the amplitude of the sound wave accordingly , so that the output level is set to approximately 5 - 8 db above the background noise . the device 10 may be mounted to a wall , near places which would be unsuitable for the congregation of young persons , such as outside shops or near street corners , and arranged to emit a sound , for example a “ beep ”, periodically , for example every 60 seconds , at a frequency of approximately 8 khz , to indicate that the device 10 is working properly . the device 10 is found to be particularly effective over a range of 15 m - 20 m with a directional spread of 50 °- 70 °. moreover , the system is also found to have an effect on young persons wearing personal headphones or similar , while listening to music etc . the device 10 shown in fig2 further comprises an additional oscillator 20 and a mixer 50 for combining the signal received from each oscillator 20 . in some embodiments it may be preferred that the signals from the two oscillators 20 generate signals which are of slightly different frequency , but each having a frequency within the range of 16 . 5 khz ± 2 khz , and a low beat frequency component of typically 1 - 10 hz but in some applications , preferably 3 . 8 hz ± 2 hz . the use of a beat frequency is found to be more beneficial than a single monotone ( i . e . single frequency ) since in order for the device to work , the frequency must be ( preferentially ) noticeable to the young persons . however , it is found that unless there is a constant change or refreshing to a person &# 39 ; s sense of hearing , the noise will not stay in a person &# 39 ; s conscience and will therefore not act to repel the person from the location . however , the beat frequency creates a slowly varying whining to the unpleasant high frequency component and therefore ensures that the noise is not biologically filtered from the persons conscience . alternatively , the signals generated using the oscillators 20 of fig2 can be arranged to generate frequencies of for example , 17 . 5 khz and 18 . 5 khz separately . by switching the output between these two sources 20 at a rate of approximately 3 hz , the resultant signal is found create the required unpleasant noise . when large crowds of young persons gather near shops etc , the activation of the device 10 causes a sound wave to be emitted of a particular frequency , which will be preferentially noticeable to younger persons by virtue of their age . furthermore , by emitting the sound wave with a power of approximately 85 db , which is less than the power level which causes permanent ear damage , it is found that the sound wave develops a ringing in the ears of the younger persons , and this development is found to occur regardless of whether they are wearing headphones or using a mobile phone . the ringing is preferentially unpleasant to the younger persons and the slowly varying beat of the sound wave ensures that the noise remains a nuisance . the device 10 can be set to operate for a particular period , for example 20 minutes , once triggering and at the end of the period , the device 10 will reset and can be retriggered . accordingly , the activation of the device 10 results in the younger persons moving away from that location , while older persons remain oblivious to the sound . the device 10 further comprises an attenuation switch 60 that when operated will reduce the output of the transducer 40 so that the device 10 can be used indoors and in confined spaces such as stairwells . in addition , the device can comprise a switch 70 that is associated with a monitor that monitors the level of background noise , and can be set to automatically trigger in the event that the background noise exceeds a certain threshold . although wall mounted devices have been discussed , the device could be hand held and incorporated in a personal alarm . alternatively , the device could be incorporated in a security system , such as an alarm system aimed at deterring intruders thereby providing an audible alarm that all can hear as well as an alarm that repels intruders that are sensitive to the sound of the repeller device . it is envisaged that the sound - emitter may be included in other types of alarm devices other than those installed in public places , for example , the sound - emitter my be included in an alarm clock or even many provided as part of a mobile phone alarm or ring system . although the invention is discussed with reference to individual embodiments , it is intended that the invention covers any combination of the embodiments discussed . further the skilled person will understand that the invention is intended to cover equivalents to those features discussed which are covered by the spirit and scope of the claims .
a sound - emitting device may be mounted to a wall near places which will be unsuitable for the congregation of young person , such as outside shops and on street corners . the device generates sound waves at a frequency that is detectable by one or more subgroups of individuals within a group of individuals .