Study On The Directivity Of Difference-frequency Sound Field Generated By A Directional Loudspeaker

Directional loudspeaker, being a new type of loudspeaker, can generate high directional audible sound, which depends on the self-demodulation of non-linear interaction between the ultrasound and air. The directivity of the difference-frequency sound field generated by the non-linear interaction is one of the key features of this directional loudspeaker. Depending on the theoretical analysis and computer simulation, the paper discussed influence to the directivity of difference-frequency sound field, that caused by the parameters of transducer and arrays, which could provide a meaningful theoretical reference to the design of the directional loudspeaker.Firstly, the paper deduced the difference-frequency sound field directivity of a single piston transducer, according the point source field theory and Lighthill equation. And then discussed the influence to the directivity of the difference-frequency sound field, that caused by the transducer area, original wave frequency, difference-frequency. The results show that the directivity generated by the ultrasound difference-frequency is much better than that by the audible sound radiation directly. Larger transducer area, lower original wave frequency and higher frequency of the difference-frequency wave are conducive to optimizing the sound field directivity.Secondly, based on the difference-frequency sound field directivity theory of a single transducer, the directivity function of rectangular arrays comprising circular piston transducers was obtained. According the directivity function, the influence of array element area, inter-element spacing, element-number, array size and the frequency of both original and difference-frequency waves on the directivity of the transducer arrays were discussed. The results showed that the directivity of main-lobe mainly depends on the array size and difference-frequency wave frequency, but on array element area, inter-element spacing, element-number, and the frequency of original waves. Increasing the array size or the frequency of the difference-frequency wave can strongly enhance the main-lobe directivity. Increasing the array size and element area, reducing inter-element spacing and the original wave frequency, increasing the frequency of difference-frequency wave were all conductive to suppress the side-lobe amplitude. But increasing the frequency of difference- frequency wave had a weak influence on the amplitude of side-lobe near to the main-lobe.