FAQ
TL;DR: Expect short links: “not more than 20–30 meter” range, and PWM is the simplest path. “Do you want to go ahead?” [Elektroda, Safwan RAJAB, post #21667843]
Why it matters: This FAQ helps DIYers decide if 25 kHz ultrasonic audio links are workable, how to build a test rig, and what trade‑offs bite.
Quick Facts
- Typical carrier: 25 kHz ultrasonic; users debated AM/FM vs PWM/PPM/PCM for audio transport. [Elektroda, Frank Bushnell, post #21667840]
- Practical line‑of‑sight range: Approx. 20–30 m in flat, unobstructed spaces. [Elektroda, Safwan RAJAB, post #21667843]
- Early FM attempts achieved only a few inches without dishes or modern op‑amps. [Elektroda, Rodney Green, post #21667842]
- Power ideas discussed: audio‑band PA up to ~100 W for the 25 kHz carrier. [Elektroda, Rodney Green, post #21667845]
- Long‑range claims (with dishes and “plenty of power”): up to about 1 km. [Elektroda, Rodney Green, post #21667842]
Can I really send music over a 25 kHz ultrasonic transducer with an electret mic?
Yes, but expect very limited range and heavy directionality. Contributors reported workable links measured in meters, not rooms. PWM is suggested as the simplest modulation for a budget build, while AM was called the worst choice. If you proceed, start small and line‑of‑sight. [Elektroda, Safwan RAJAB, post #21667841]
What range should I plan for indoors?
Plan for roughly 20–30 meters in flat, unobstructed conditions. Walls, soft furnishings, and off‑axis alignment reduce range sharply. “Not more than 20–30 meter in flat area” is a realistic expectation to start with. [Elektroda, Safwan RAJAB, post #21667843]
Which modulation works best at 25 kHz: AM, FM, or PWM?
The consensus: use PWM for simplicity and decent results. PPM or PCM can work but add complexity. AM was described as yielding the worst results for this application at similar cost. Choose PWM to prototype quickly, then iterate. [Elektroda, Safwan RAJAB, post #21667841]
How far did early experiments with FM over ultrasound go?
One builder using narrowband FM in the 1970s reported reception only within a few inches. The setup lacked op‑amps and used LC amplifiers and a ratio detector, highlighting how challenging low‑frequency FM is without modern parts or optics. [Elektroda, Rodney Green, post #21667842]
Could I ever reach 1 km with ultrasound?
A poster believed 1 km might be possible with reflectors at both ends and plenty of transmit power. However, another experienced user countered that 20–30 m is more realistic for practical builds. Expect heavy engineering for any long‑range goal. [Elektroda, Rodney Green, post #21667842]
What’s a simple 3‑step way to try a PWM ultrasonic audio link?
- Build a 25 kHz PWM transmitter and drive a suitable ultrasonic transducer.
- Aim a matching receive transducer at it, line‑of‑sight, a few meters away.
- Demodulate with a low‑noise audio chain and listen; expand distance gradually. [Elektroda, Safwan RAJAB, post #21667841]
What are the main failure modes I should watch for?
Square‑wave carriers from PWM create strong harmonics that complicate transmission and reception. Poor filtering, off‑axis alignment, and room absorption kill SNR quickly. Consider band‑pass filtering and clean drive to limit distortion products. “ON/OFF signalling” increases harmonic trouble. [Elektroda, Rohit Dubla, post #21667848]
Is RF or infrared a better choice for sending audio?
Several contributors advised using proper RF (FM Tx/Rx) for audio links, citing efficiency and easier modulation. IR can work but needs effective amplitude control and careful encoding; otherwise dynamic range is poor. RF remains the practical path for quality audio. [Elektroda, Rohit Dubla, post #21667837]
What front‑end or demod ideas improve an ultrasonic receiver?
One approach mirrors classic radio: a low‑noise preamp into a Gilbert‑cell mixer (e.g., NE/SA602), then audio quadrature phase shifting and AGC. Add a power amp for listening. Parabolic dishes at both ends improve link budget. [Elektroda, Rodney Green, post #21667845]
How much transmitter power should I budget?
A contributor suggested using an off‑the‑shelf audio‑band power amplifier, even up to about 100 W at 25 kHz. Manage thermal limits and transducer ratings carefully to avoid damage. Directional optics (dishes) help more than brute power alone. [Elektroda, Rodney Green, post #21667845]
What is an electret microphone, and can it receive ultrasound?
An electret mic is a pre‑polarized condenser microphone capsule. Many electrets roll off above 20 kHz, so response at 25 kHz can be weak. Use a true ultrasonic receiver transducer for better sensitivity, or verify your mic’s high‑frequency response first. [Elektroda, furious 555, post #21667836]
What does PWM/PPM/PCM mean in this context?
PWM varies pulse width with the audio, PPM shifts pulse positions, and PCM digitizes audio into code words. The thread favored PWM for simplicity and cost, while noting PPM and PCM are feasible but more complex builds. [Elektroda, Safwan RAJAB, post #21667841]
Could AM work if I insist on analog simplicity?
One expert explicitly advised against AM here, calling it the worst choice at roughly the same cost as better options. If you insist, expect lower efficiency and poorer results compared with PWM or well‑designed FM/SSB methods. [Elektroda, Safwan RAJAB, post #21667841]
Any inspiring builds or cautionary tales from low‑frequency projects?
A 22 kHz radio transmitter project struggled with enormous antenna requirements and interference from a distant 1 MW station. The builder had to reduce power to about 100 mW due to component limits. Lesson: physics punishes low carriers. [Elektroda, Rodney Green, post #21667849]
Will high‑power ultrasound bother animals?
One participant joked that kilometer‑range ultrasonic power would drive all local dogs mad. Humor aside, be considerate with ultrasonic SPLs and beam aiming, as animals detect frequencies we do not. Use minimal power needed. [Elektroda, Frank Bushnell, post #21667844]
