Piezoelectric ceramic directional speaker
Audio directional speakers in the industry, also known as beam speaker (Audio beam loudspeaaker), Audio Spotlight (Audio spotligth), ultrasonic frequency sound system (Hypersonic sound system), directional acoustic system (directional acoustics systerm), parametric speaker (parametric loudspeaker ), parametric acoustic array (parametric acoustics array), etc., as a new speaker is currently no unified name.
Due to its use of ultrasonic nonlinear interactions in the generation of airborne self-demodulation high directivity audible sound, can achieve directional propagation of audio, so audio directional loudspeaker personal call is more appropriate. Sign on the basic principles of the domestic industry for such speakers, development history, current research, technology and application of hot prospects for a more comprehensive introduction to the acoustic power industry reference.
A basic principle and system features
Research on audio directional speakers can be traced back in the mid-18th century psychoacoustics found Tartini tones that sound when two tones together if it is stable, while the third issue is the sound audible. Tartini tone is a subjective tone, even the two frequencies close to the ultrasonic signal also allows people to hear the voice of a linear superposition of change.
Later, Helmholtz confirmed the two tones can spread in the air generated by the nonlinear interaction of their difference frequency, and frequency of the signal, its theories and formulas to predict consistent with the measured data to support this conclusion is correct. According to Helmholtz's theory, the two plane-wave propagation in an inhomogeneous medium, second-order nonlinear field relationship can be described as the relationship shown in Figure 1. When the input to the ultrasonic transducer into electrical signals of two frequencies f1, f2, the ultrasonic transducer transmitting frequency of the two columns by mechanical vibrations in the air are f1, f2 ultrasound. These columns ultrasonic nonlinear interaction in the air, thereby generating a final signal including the original ultrasound f1, f2 and f1 + f2, and the frequency difference between the signals of complex acoustic signals of f1-f2.
Since the sound absorption coefficient is proportional to a square of the frequency, the higher the frequency of ultrasonic signals f1, f2, f1 + f2 of air will be absorbed very quickly, leaving a difference signal is within the audible frequency range f1-f2 in the high directivity in the air sexual transmission. Mechanism of new frequency components of sound waves produced by Bell Labs for this acoustic in the air "deformed communication" made a reasonable interpretation of the nonlinear interaction.
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Whether the acoustic directivity is closely related to the size of the acoustic wavelength and the ratio of the sound source. When the acoustic wavelength is much larger than the size of the sound source, there is no directional sound waves; close until the time when the acoustic wavelength is much smaller than the scale of the sound source, sound waves will gradually showing more and more strong directivity. Acoustic wave length is 17mm-17m between, most are much larger than the wave length of audio sound source size, so the audio range is generally no directivity. And Figure 1, the difference signal is within the audible frequency range f1-f2 why it has a strong directivity?
December 1962, professor of physics at Brown University's Peter Westervelt in his paper first proposed parametric array (parametric array) concept, to make a reasonable explanation of this phenomenon, the basic principle is shown in Figure 2, transducer to send a strong modulation of ultrasound into the air medium, ultrasonic propagation along its axis by advancing the process of continuous acoustic signal is demodulated nonlinear effect, these ever-demodulated audio accumulated together, and thus, an end-fire virtual sound source array (end-fire virtual array) achieved in this way.
The virtual sound source array called parametric array. Parametric array so that the energy in the acoustic wave frequency of the sound wave advancing direction constantly enhanced, and since the ultrasound has a strong directivity, the propagation axis in a direction other than the strengthening effect of this superposition is very weak, which ultimately leads to acoustic waves having a propagation axis direction in the main a high directivity. Parametric array is proposed to generate high-frequency sound waves directivity provides a theoretical basis.
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1965, on the basis of Westervelt parametric array studies, HOBerktay further raised questions about parametric array of accurate, more complete theoretical explanation, more importantly, he deduced from the demodulated sound parametric array of frequency wave is proportional to the original input the second time derivative of the square of the signal envelope, which provides a theoretical basis for the future orientation of the speaker audio signal processing. Audio directional speaker is the use of parametric array principle to generate high directivity audible sound.
Directional speakers from the audio works can be seen. It has the following system features:
1) can transmit high directivity acoustic wave, which is not possible with conventional speakers;
2) with the traditional speaker incentives air through mechanical vibrations audible sound emitted directly compared, it is the first issue ultrasound, and then by the ultrasonic action by nonlinear self-demodulation audible sound, namely indirect audible frequency wave in the air;
3) the frequency of the sound wave energy is concentrated in the communication process most of the energy is located near the propagation axis, as compared with the non-directional sound source to all aspects of the radiation energy, which decay much more slowly, which makes it sound frequency propagation farther;
4) in the conventional speaker port loudest speaker, and the sound of the audio speaker is oriented along the propagation axis formed gradually demodulated and continuously superimposed, so when it reaches a maximum close to the energy parameters of the effective length of the array.
5) emitted by the transducer ultrasound and ultrasound due to nonlinear effects produced by rapidly attenuated in the air, leaving only the high point of the audio wave propagation in the air.
6) through multiple transducer array group (in fact, currently the vast majority of audio directional loudspeaker is studied transducer arraying the way), the beam can be achieved through the principle of phased array transducer plane before half directional propagation space at any angle, this feature can swing out of control mechanical characteristics, purely to circuit design and signal processing rapidly changing acoustic beam propagation angle.
Second, audio directional loudspeaker hotspot focused on the following aspects:
1) signal processing and algorithms: current signal processing system is mainly realized by analog circuits, FPGA, DSP, DSP + FPGA hardware, such as several ways, by comparison DSP processing has high flexibility and low cost; signal processing is mainly square root, double integral prescribing, DSB modulation, truncated DSB algorithms, its fundamental purpose is to achieve directional acoustic transducer high electro-acoustic efficiency and low distortion, and reasonable bandwidth, the performance of any two of these three are a good solution, but the cost is to maximize third party.
2) Transducer Design: ready bimorph transducer array elements is often a mismatch between the sideband amplitude disorders with subharmonic falls outside the audible range, the array is expensive, and high distortion, ultrasonic bandwidth insufficient, the output power is small, and many other problems, so the design of new transducers audio directional loudspeaker research became a major key.
3) ultrasonic linear power amplifier section of key technologies: requires a good amplification efficiency at low crest factor, active power into the load impedance, the amplifier has a good linear amplification capabilities within the effective frequency band so as not to generate a waveform distortion, with ultrasonic effective reactive load.
4) Ultrasonic harm to human body, system-level integration and matching electronic beam deflection method and implementation methods, such as sound field calculation and simulation research has attracted widespread attention.
Three. Audio directional loudspeaker application prospects
Audio directional speaker with a very wide range of prospects, such as can be used on cars, boats, planes, trains and other modes of transport, so that these vehicles speaker / siren only spread along its direction of travel, which can fundamentally weaken or eliminate noise pollution due to transport to the environment.
It can also be used to require the local acoustic wave propagation and inter-regional mutual interference does not produce sound occasions cars, planes, trains, restaurants, games room, internet cafe, multi-national meetings, for example, it can seat between different modes of transport on offer different backgrounds voice services without interference.
In addition, audio directional technology can also be used in car phone secrecy connected, bank machines voice prompts guide the large office buildings lead the way, guiding the blind, intelligent building security, listening to TV sound directivity and many other occasions, its broad prospects given its strong vitality.
Audio is a first directional loudspeaker can achieve acoustic wave source distribution control and position control (through the interface reflection, the formation of virtual sound sources) in the space of a new generation of airborne speakers. Currently, only the United States ATC company has a series of commercial products, but still in the early stages of the international and domestic market is still basically in undeveloped state.
