Back in 2011, Elektor presented a DIY 50-m range directive sound beam using an array of piezo ultrasonic transducers (up to 200) driven with a PWM signal. While the ultrasonic directive speaker project was strictly experimental, it turned out to inspire thousands of designers to start projects of their own. As you will see, the designer used 50+ piezo transducers to generate an audible sound beam. Let’s take a look.
 
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The DIY Ultrasonic Directive Speaker projecct.

The Circuitry

Refer to the schematic of the two-channel PWM modulator. As you can see, there aren't any special components. The TL494 PWM control circuit and the IR2111 half bridge MOSFET driver are used in their standard application circuits. The TL494 has an internal oscillator whose frequency is determined by trimpot R2 and capacitor C1. The basic pulse width is adjusted with R1. You need to set up optimum modulation with trimpots R1 and R2.
 
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The PWM power driver for the ultrasonic parametric speaker unit. The audio input signal is connected to
jack socket K2. Channel B is optional.
"The audio input signal is connected to K2 (loudspeaker level required, not microphone or line). The board has two outputs, A and B, each driving an array of piezo transducers, optionally through an inductor," the designer explained. "Each channel is suitable for up to 200 transducers. The normal supply voltage is 20-24 DVC to K1. The FET stages may be powered by an external supply via the EXT terminal after removing wire link J1. Heatsinks may be required on the IRF540 FETs depending on the supply voltage and the transducers’ ratings (up to 60 VDC may be possible). The U/S speaker schematic is large but unsurprising. It represents one channel and a ‘mini’ version with just 50 transducers."
 
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A mini version with just 50 transducers.
The “mini” version of the ultrasonic directive speaker consists of 50 ultrasonic piezo transducers connected in parallel and in phase. The step-up transformer and associated resonance caps are optional. The associated PWM driver in its basic form is suitable for up to 200 transducers.

The Original Ultrasonic Directive Speaker Project

The article, “Ultrasonic Directive Speaker,” appeared in Elektor March 2011. You can download the article for free during the two weeks following the publication of this news item. Enjoy!
Editor's Note: This article was first published in a 2011 edition of ElektorMag. Some of the components, products, and links may no longer be available. Nonetheless, the educational content remains valuable, and we think it will inspire you to start a new project.
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