Welcome back, aspiring cyberwarriors!
Today we are going to show you how to build what is commonly called an audio laser, using inexpensive and widely available components. If you are willing to buy parts from low-cost Chinese platforms and wait for longer shipping times, the total cost of this project can be kept under $20. For demonstration and accessibility purposes, however, we will mostly refer to parts available on Amazon and eBay. You can do your own research and compare prices, as the same components are often significantly cheaper elsewhere.
This project is not about building a perfect or commercial-grade system. The goal is to prove the concept, demonstrate how the technology works, and give you something tangible that you can experiment with and improve. With better components, higher voltages, and a well-designed enclosure, performance can be improved substantially. If you have access to a 3D printer, you can also design and print a proper housing for the device, which opens the door to more advanced and polished builds later on.
The term audio laser is informal but popular. You may also see this technology referred to as an Audio Spotlight or an Ultrasonic Speaker. These names describe the same thing that produces sound that is highly directional, unlike conventional loudspeakers.
Regular Speakers vs. Parametric Speakers
The key difference between a regular speaker and an ultrasonic, or parametric, speaker lies in how sound propagates through space. A traditional speaker radiates sound waves outward in all directions. These waves spread, reflect off walls and surfaces, and quickly fill the entire room. This is useful for music and speech, but it makes precise control over where the sound goes almost impossible.
An ultrasonic or parametric speaker behaves very differently. Instead of directly emitting audible sound, it emits ultrasound, typically around 40 kilohertz, which is far above the range of human hearing. This ultrasonic signal is tightly focused into a narrow beam. When this beam travels through air, nonlinear effects cause it to demodulate, recreating audible sound only along the path of the beam. The result is sound that behaves more like a laser than a floodlight.
This principle should already feel familiar if you followed Parts One and Two of the LRAD series. Directionality, beam shaping, and energy concentration are the same core ideas, just applied at a much smaller and safer scale. It is still important to remind ourselves of these fundamentals, because they explain why small changes in angle, distance, or alignment can dramatically change the result.
A parametric speaker achieves this effect by using ultrasonic transducers and signal-processing techniques. By modulating an ultrasonic carrier with an audio signal, we can focus sound in a way that would be impossible with conventional speakers. The first true audio spotlight was demonstrated in 1998 by Dr. Joseph Pompei at MIT, who showed that intelligible speech could be reproduced through the demodulation of high-frequency ultrasound. At the time, the hardware was expensive and complex. Today, with faster microcontrollers and very cheap ultrasonic transducers, similar results are achievable by hobbyists.
Implementation Options
There are at least two practical approaches you can take when building an audio laser.
The first method uses a single ultrasonic array. In this approach, the audio signal is used to modulate an ultrasonic carrier. When this modulated ultrasound propagates through air, nonlinear effects cause it to demodulate back into the audible frequency range along the beam path. The quality of the resulting sound depends heavily on the modulation method and the linearity of the output stage.
The second method uses two ultrasonic arrays operating at slightly different frequencies. These beams intersect in space, and in the region where they interfere with each other, constructive and destructive interference produces audible sound. While elegant, this method is more complex to implement and align mechanically.
For this project, we will use the first method, as it is simpler, cheaper, and more forgiving. It also maps very cleanly to the block diagram approach we will describe next. If you look up parametric speakers on Wikipedia, you will quickly encounter intimidating equations and nonlinear acoustic theory. While this math is correct and important, it is not required to build a working system. Today, instead of getting lost in equations, we will focus on how the signal flows through the hardware and how each stage contributes to the final result.
Block Diagram
The signal path in this project is straightforward and logical. Audio enters the system through a standard 3.5 mm line-in microphone jack. This signal first passes through a preamplifier circuit, where its amplitude is adjusted to a level suitable for digital sampling. From there, the amplified audio is fed into the analog-to-digital converter (ADC) of the microcontroller.
The microcontroller samples the audio and uses it to modulate a 40 kilohertz carrier signal. This modulation is implemented using pulse-width modulation (PWM). If you have experience with Class D audio amplifiers, this approach should feel familiar. The audio information is encoded in the duty cycle of the PWM signal rather than in its amplitude.
The complementary PWM outputs are then sent to an H-bridge driver, which acts as a power amplifier. This stage drives the ultrasonic transducer array, converting the electrical signal into focused ultrasonic energy.
STM32F103 Microcontroller
The main component responsible for audio sampling and modulation is the STM32F103 microcontroller. This device was chosen because it is inexpensive, widely available, and well supported in the hobbyist community. Running at 72 megahertz, it offers a 12-bit ADC, multiple hardware PWM channels, and programmable complementary outputs, which are ideal for driving an H-bridge.
The STM32 generates a 40 kilohertz PWM signal whose duty cycle is adjusted in software based on the incoming audio. With a 72 megahertz system clock, the duty cycle can be updated at very fine intervals, giving approximately 1,800 possible duty-cycle steps. This corresponds to just under 11 bits of effective resolution. For comparison, CD-quality audio uses 16-bit resolution, so you should not expect high-fidelity sound from this setup. That said, speech reproduction is very achievable. An obvious upgrade path would be a faster processor that allows more frequent duty-cycle updates, but the STM32F103 strikes a good balance between cost and capability for this project.
Parts
Now let’s walk through all the components you will need.
TCT40-16T – 17-20 Pieces
The ultrasonic transducers are the most visible part of the project. The TCT40-16T transducers operate at 40 kilohertz and are designed specifically for transmission. It is important to get the T version, not the R version, which is intended for receiving.
These transducers are inexpensive and often available in bulk for under ten dollars. Using around seventeen to twenty units allows you to form a reasonably effective array, capable of projecting sound up to approximately 18 meters or 60 feet under good conditions. Using more transducers improves directivity and output power. Higher-quality transducers will further improve audio clarity and range.
STM32F103C8T6 – 1 Piece
As discussed earlier, the STM32F103 is the heart of the system. It handles both audio sampling and ultrasonic modulation. Its combination of speed, PWM capability, and low cost makes it well suited for this role.
LM358 – 1 Piece
The audio preamplifier is built using the LM358 operational amplifier. This is a very common and inexpensive op amp. While it is not rail-to-rail and has limited slew rate, it is sufficient for a proof-of-concept design.
If you want to improve performance, you can replace the LM358 with a higher-quality op amp that offers lower noise and better dynamic range. Many suitable alternatives are available at similar prices, especially when sourced in bulk.
TC4427A – 1 Piece
The TC4427A serves as the power driver for the ultrasonic transducer array. It is essentially a MOSFET-based H-bridge in a single package. While not ideal, it is easy to use and readily available.
One important limitation is its maximum operating voltage of around 18 volts. Many ultrasonic transducers, including the TCT40-16T, can tolerate much higher peak-to-peak voltages. If you want greater range and output power, upgrading to a more capable driver stage is recommended.
Misc Passive Components
You will also need a PJ-307 3.5 mm stereo jack for audio input. Only one is required.
An MT33608 step-up module is optional. It can be used if you are not relying on a bench power supply or if you want to experiment with higher drive voltages. Battery operation is also possible with proper design choices.
Ultrasonic Array
The ultrasonic transducer array must be assembled carefully. Alignment matters. A flat, evenly spaced array will produce a tighter beam and better results. This array connects directly to the output stage and is the final element in the signal chain.
Circuit Diagram
Here is the complete circuit diagram. All the functional blocks described earlier are clearly visible. The firmware for the STM32 is available on GitHub and can be modified if you want to experiment with different modulation schemes or carrier frequencies.
Be Creative
Once you understand the fundamentals, this project becomes a platform for experimentation. You can improve the design with better components, more transducers, or a custom enclosure. Some people get a 3D-printed body that allows them to create ergonomic shapes, add switches, and integrate additional features.
With a Bluetooth module, control switches, and a trigger-style button, the final build can resemble a handheld blaster. While this version is wired and requires a constant power source, adding a battery makes it fully portable and much more visually impressive.
Creativity is the real limit here. Better layouts and thoughtful mechanical design can transform this simple prototype into a polished demonstration device. As with all engineering projects, understanding the fundamentals is just the beginning.
Ready-Made Options
If you prefer an off-the-shelf solution instead of building your own system, there are commercial options available online. One example is the Audfly Model B Ultrasonic Speaker, which implements the same focused-audio principles. These units are significantly more expensive than a DIY build. Whether you choose a ready-made device or a home-built system ultimately depends on your budget and how much experimentation you want to do.
Summary
You saw how a highly directional “audio laser” can be built using cheap, easily available components. It explained how ultrasonic speakers focus sound into a narrow beam by modulating ultrasound, applying the same directionality concepts discussed earlier in the LRAD series at a much smaller and safer scale. Rather than aiming for perfect audio quality, we emphasized understanding and experimentation.
With better components, higher power, and batteries, the simple prototype can be expanded into a polished and portable demonstration of focused acoustic technology.
If you enjoy experimenting with frequencies and trying new things, we recommend signing up for our SDR for Hackers training. With Master OTW, you’ll learn how to use your computer and inexpensive SDR hardware to explore and hack a wide range of radio signals.
Source: HackersArise
Source Link: https://hackers-arise.com/long-range-acoustic-device-lrad-part-3-building-your-own-audio-laser/