Parametric Directional Speaker Array

1. Overview

A parametric directional speaker exploits the air’s nonlinear acoustic properties to carry normal audio on an ultrasonic (40 kHz) beam. By amplitude-modulating the carrier with your audio and blasting it through an array of ultrasonic transducers, the system creates a narrow, “flashlight-like” sound beam. Past a few meters, the audible component self-demodulates in the medium itself, so bystanders off-axis hear almost nothing.

2. Key Components

• Microcontroller (STM32F103 “Blue Pill”)

  • Generates a stable 40 kHz PWM carrier on a GPIO pin

• Audio Input Stage

  • PJ-320 3.5 mm jack

  • Coupling capacitors (0.1 µF) to block DC

  • Unity-gain resistor network (1 kΩ, 47 kΩ) for level setting

  • 10 kΩ potentiometer (VR1) for user volume control

• Amplitude Modulator

  • LM358 dual op-amp

  • One section sums carrier + audio, the other creates an adjustable DC offset

  • 100 µF electrolytic caps smooth supply rails and block low-frequency drift

• Power & Driver Stage

  • L293D dual H-bridge to provide push-pull drive at ±18 V (or your chosen ultrasonic drive voltage)

  • 100 µF decoupling at each supply rail

  • Output resistors (15 Ω–75 Ω) for current limiting and impedance matching

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• Ultrasonic Transducer Array

  • Typically Murata MA40S4S or similar 40 kHz piezo discs

  • Arranged in a line or grid (e.g. 8×2) for beamforming and increased SPL

3. Underlying Science

• Parametric Array Principle

  • In a linear medium, waves pass through unaltered. But air shows nonlinear behavior at high sound pressures.

  • When you broadcast two close ultrasonic frequencies (e.g. 40 000 Hz ± audio), their interaction in the medium produces a difference frequency equal to the audio.

  • Effectively, the air acts as the demodulator, so the listener at distance hears only the audible component.

• High Directionality

  • Wavelength λ = c/f ≈ 343 m/s ÷ 40 kHz ≈ 8.6 mm

  • A loudspeaker array several centimeters across will have a beamwidth on the order of λ/D (D = array aperture), yielding a tight cone (< 10°).

• Self-Demodulation Distance

  • The audible demodulation builds up over a meter or two of path and peaks typically 3–5 m from the source, depending on amplitude and air conditions.

4. Practical Applications

• Museum & Gallery Exhibits

  • Create individual “audio zones” so each exhibit whispers only to the viewer directly in front.

• Retail & Advertising

  • Targeted audio messages over products or display stands without disturbing the whole store.

• Industrial & Medical

  • Non-contact communication in noisy environments, or focused ultrasonic therapy beams that also “speak.”

• Assistive Tech

  • Personal audio channels—e.g. for hearing-assistance booths—without headphones.

• Security & Defense

  • Discreet warning tones or spoken commands delivered as a narrow “sound spotlight.”

By combining standard analog/RF‐style amplitude modulation, a low-cost microcontroller, driver ICs, and off-the-shelf piezo transducers, you get a highly directional speaker system that carries your voice (or music) inside an invisible ultrasonic beam—unlocking novel ways to deliver sound “only where you want it.”