Ultrasonic Speaker Project: Directional Audio Beam

Table of Contents

• What Is Directional Audio?
• The Science of Ultrasonic Sound Beams
• Components and Transducer Arrays
• Driver Circuit Design
• Applications of Directional Audio
• FAQ
• Conclusion

What Is Directional Audio?

A directional audio speaker (also called a parametric speaker) uses ultrasonic transducers to create a narrow beam of sound that can be aimed at a specific location. Unlike conventional speakers that radiate sound in all directions, a directional speaker delivers audio only to the person standing in the beam — someone a metre to the side hears nothing. This technology has fascinated makers and engineers for decades, and with affordable ultrasonic transducers available in India, you can build one at home.

The principle works by modulating an ultrasonic carrier frequency (40kHz) with the audio signal. The ultrasonic waves, due to their short wavelength, propagate in a tight beam. As they travel through air, nonlinear acoustic effects demodulate the signal, producing audible sound only along the beam path. The result is eerie and impressive — sound that appears to come from nowhere, audible only to the targeted listener.

The Science of Ultrasonic Sound Beams

The physics behind parametric speakers relies on the nonlinear acoustics of air. When two ultrasonic frequencies interact in air, they produce sum and difference frequencies. If one frequency is a 40kHz carrier and the other is 40kHz plus the audio signal (amplitude modulated), the difference frequency is the audio signal itself. Because the ultrasonic waves travel in a tight beam (directivity is proportional to frequency), the demodulated audio also stays within the beam.

The quality of the demodulated audio depends on the number of transducers, the carrier power, and the modulation technique. More transducers create a tighter beam with better low-frequency reproduction. Higher carrier power extends the range but increases distortion. Double-sideband amplitude modulation is the simplest technique; single-sideband modulation with preprocessing produces cleaner audio.

Practical directional speakers use arrays of 40 to 200+ ultrasonic transducers. For a DIY build in India, start with 16 to 36 transducers — this produces a clearly directional beam over 2 to 5 metres. More transducers improve the effect but increase cost and circuit complexity.

Components and Transducer Arrays

Required components:

• 40kHz ultrasonic transmitter transducers — 16 to 36 pieces
• Arduino or signal generator for modulation
• High-voltage amplifier (L298N H-bridge or dedicated ultrasonic driver)
• Audio input circuit (electret microphone or 3.5mm jack)
• Perfboard or custom PCB for the transducer array
• 12V to 24V power supply

Arrange the transducers in a circular or hexagonal array on a perfboard. All transducers are wired in parallel — they receive the same modulated ultrasonic signal simultaneously. The array diameter should be at least 100mm for a noticeable beam effect. Larger arrays (200mm+) produce tighter beams.

Driver Circuit Design

The driver circuit generates a 40kHz carrier, modulates it with the audio input, and amplifies the result to drive the transducer array. A simple approach uses an Arduino to generate the 40kHz PWM carrier and an analogue multiplier circuit (or digital modulation) to apply the audio signal.

For the amplifier stage, an L298N H-bridge module can drive the transducer array at up to 24V. Connect the modulated 40kHz signal to the L298N input, and the amplified output to the transducer array. The H-bridge’s push-pull output doubles the effective voltage swing, increasing acoustic output power.

More advanced builds use a dedicated ultrasonic driver IC or a Class D amplifier configured for 40kHz operation. These provide cleaner driving signals and better efficiency, but the L298N approach works well for initial experiments and demonstrations.

Applications of Directional Audio

Directional audio has practical applications beyond the novelty factor:

• Museums and exhibitions: Deliver audio descriptions to visitors standing in front of a specific exhibit without disturbing others nearby
• Retail stores: Target audio advertisements at customers in specific aisles
• Personal audio zones: Watch TV at night without disturbing a sleeping partner — aim the sound beam at your listening position
• Art installations: Create immersive sound experiences where sound appears to come from unexpected locations
• Accessibility: Deliver audio navigation cues to a specific person in a crowded space

In India, directional speakers have potential in crowded public spaces like railway stations and airports, where conventional PA systems create a cacophony of overlapping announcements. Targeted audio beams could deliver gate-specific announcements only to passengers at that gate.

FAQ

Is ultrasonic sound dangerous?

At the power levels used in DIY builds (under 120 dB SPL ultrasonic), there is no evidence of health risk to humans. However, dogs and cats can hear ultrasonic frequencies and may be disturbed. Do not point the array at pets. Also avoid operating near sensitive equipment like ultrasonic sensors or motion detectors, which may malfunction.

How far does the audio beam reach?

A 36-transducer array produces an audible beam extending 3 to 5 metres. Larger arrays (100+ transducers) with higher-power drivers can reach 10 to 20 metres. The beam width is typically 10 to 20 degrees, narrowing with more transducers.

What is the audio quality like?

The demodulated audio is mid-range focused — bass frequencies below 200Hz are poorly reproduced due to the physics of parametric demodulation. The effect works best with speech and mid-frequency music. Do not expect hi-fi quality; the appeal is the directional effect, not audiophile fidelity.

Conclusion

Building a directional audio speaker is one of the most impressive acoustic projects you can attempt with readily available components. While the audio quality is modest, the spatial effect — sound that only you can hear, coming from thin air — never fails to amaze. For Indian makers with access to ultrasonic transducers from Zbotic.in, this project combines electronics, physics, and creativity in equal measure.