The post explains the construction of a ultrasonic directive speaker system also called parametric speaker which may be used to transmit an audio frequency over a targeted spot or zone such that the person situated exactly at that spot is able to hear the sound while the person next to him or just outside the zone stays completely untouched and unaware of the proceedings.
Invented and built By Kazunori Miura (Japan)
The outstanding results obtained from the testing of Long Range Acoustic Device (LRAD) inspired the American Technology Corporation to adopt a new name for it and was changed to LRAD corporation on March 25th 2010. Also called the Audio Spotlight, it is a product of Holosonic Research Labs, Inc and is used for non-military applications.
The device is designed to generate intensely focused sound beams over a targeted area only. The unit may be well suited in places such as museums, libraries, exhibition galleries where its sound beam may be used for sending a warning message or instruct a particular mishaving person, while others around are allowed to carry on in perfect silence.
The focused sound effects from such a parametric speaker system is so accurate that anybody who is targeted with it becomes hugely surprised to experience the focused sound content which is heard only by him while the guy just beside him stays completely unaware of it.
Working Principle of a Parametric Speaker
Parametric speaker technology employs sound waves in the supersonic range which have the characteristic of travelling through almost the line-of-sight.
However one may wonder that since supersonic range may well be beyond the 20kHz mark (40kHz to be precise), could be absolutely inaudible to human ears, so how does the system is able to make the waves audible in the focused zone?
One method of implementing this is to use a two 40kHz beams with one having an audio frequency of 1kHz superimposed and angled to meet at the directed point where the two 40kHz content cancel each other leaving the 1kHz frequency audible at that particular spot.
The idea may look simple but the result could be too inefficient due to the low volume sound at the directed spot, not good enough to stun or incapacitate the targeted people, quite contrary to the LRAD.
Other modern methods of producing audible directive sound using supersonic waves are through amplitude modulation (AM), double sideband modulation (DSB), single sideband modulation (SSB), frequency modulation (FM), all concepts depend on the recently researched parametric speaker system technology.
Needless to say, a 110 dB+ supersonic wave could be nonuniform with its sound force distribution while it's in the course of propagation across a long air mass "tube".
Due to the non uniformity of the sound pressure an immense amount of distortion could be experienced which could be highly undesirable for applications in peaceful places such as in museums, galleries, etc.
The above non-linear response is produced due to the fact that air molecules take relatively more time to arrange themselves to their previous original density compared to the time taken for compressing the molecules. Sound created with higher pressures also results in higher frequencies which tend to generate shock waves while the molecules collide with the ones being compressed.
To be precise since the audible content is constituted by the vibrating air molecules that are rather not entirely "returning", therefore when the frequency of the sound increases, the non uniformity forces the distortion to become much audible due to the effect which could be best defined as "air viscosity".
Therefor manufacturer resort to the DSP directive speaker concept which involves much improved sound reproduction with minimum distortion.
The above is complemented with the inclusion of highly advanced parametric transducer speaker arrangement for getting an unidirectional and clear sound spots.
The high directivity created by these parametric speakers is also due to their small bandwidth characteristics which could be enlarged as per the required specification by simply adding many number of these transducers through a matrix arrangement.
Understanding Parametric 2-Channel Speaker Modulator Concept
DSB could be easily executed using analogue switching circuits. The inventor initially tried this, and though could achieve a loud sound, it accompanied with a heck lot of distortion.
Next, a PWM circuit was tried, which employed the concept akin to FM technology, although the resultant sound output was much distinct and free from distortion, the intensity was found to be a lot weaker compared to DSB.
The drawback was ultimately solved by arranging a double channel array of transducers, each array including as many as 50 numbers of 40kHz transducers connected in parallel.
Understanding the Audio Spotlight Circuit
Referring to the parametric speaker or ultrasonic directive speaker circuit shown below we see a standard PWM circuit configured around the PWM generator IC TL494.
The output from this PWM stage is fed to a half bridge mosfet driver stage using the specialized IR2111 IC.
The IC TL494 has a built-in oscillator whose frequency could be set through an external R/C network, here it's represented through the preset R2 and C1. The fundamental oscillating frequency is adjusted and set by R1, while the optimal range is determined by appropriately setting up R1 and R2 by the user.
The audio input which needs to be directed and superimposed on the above set PWM frequency is applied to K2. Note that the audio input must be sufficiently amplified by using a small amplifier such as LM386 and must not be sourced via headphone socket of an audio device.
Since the output from the PWM stage is fed across a twin half bridge IC set up, the final amplified supersonic parametric outputs could be achieved via two outputs across the shown 4 fets.
The amplified outputs are fed to an array of highly specialized 40 kHz piezo transducers via a optimizing inductor. Each of the transducer array may consist of a total of 200 transducers arranged through a parallel connection.
The mosfets are normally fed with a 24V DC supply for driving the piezos which may be derived from a separate 24V DC source.
There could be a host of such transducers available in the market, so the option is not limited to any specific type or rating. The author preferred 16mm diameter piezos assigned with 40kHz frequency spec typically.
Each channel must include at least 100 of these in order to generate a reasonable response when it's being used outdoors amidst high level of commotion.
Transducer Spacing is Crucial
The spacing between the transducers is crucial so that the phase created by each of them is not disturbed or cancelled by the adjacent units. Since the wavelength is mere 8mm, positioning error of even 1mm could result in a significantly lower intensity due to phase error and loss of SPL.
Technically, an ultrasonic transducer imitates the behavior of a capacitor and thus it could be forced to resonate by including an inductor in series.
We have therefore included an inductor in series just to achieve this feature for optimizing the transducers to their peak performance limits.
Calculating Resonant Frequency
The resonant frequency of the transducer may be calculated by using the following formula:
fr = 1/(2pi x LC)
The capacitance of the transducers could be assumed to be in between 2 to 3nF, thus 50 of them in parallel would result a net capacitance of about 0.1uF to 0.15uF.
Using this figure in the above formula we get the inductor value to be in between 60 and 160 uH which must be included in series with the mosfets driver outputs at A and B.
The inductor uses a ferrite rod as may be witnessed in the figure at the bottom, the author could peak up the resonant response by adjusting the rod by sliding it within the coil until the optimal point could be struck.
Circuit idea courtesy: Elektor electronics.
Transducer wiring details can be seen in the below given figure, you will need two of these set-ups to be connected with points A and B of the circuit:
Caution — Health Hazard. Appropriate measures must be taken to prevent long term exposure to high ultrasonic sound levels.