How SpectroTrace Works

SpectroTrace uses a clever audio technique to hide images inside sound. When you play the generated audio through a spectrogram analyzer, the hidden image is revealed. Here's how it all works.

The Inspiration

This technique has been used by musicians for decades. In the 1990s, electronic artist Aphex Twin famously hid images in tracks like “Windowlicker” — including a haunting face that only appears when you view the audio through a spectrogram analyzer.

I thought this was fascinating, but the leading tool for creating spectrogram art, Photosounder, costs around €87 and offers many professional features that can feel overwhelming for simple experimentation. SpectroTrace was born from a simple idea: make this creative technique accessible to everyone, for free, right in the browser.

What is a Spectrogram?

A spectrogram is a visual way to see sound. It shows three things at once:

• Time — moving from left to right
• Frequency — low sounds at the bottom, high sounds at the top
• Loudness — brighter colors mean louder sounds

Musicians, scientists, and audio engineers use spectrograms to analyze sounds. You might have seen them as colorful waveform displays in music software.

The Conversion Process

SpectroTrace works by reversing the spectrogram process. Instead of turning sound into a picture, it turns a picture into sound:

1. Image Analysis — Your image is converted to grayscale and divided into vertical columns (time slices).
2. Frequency Mapping — Each row of pixels is assigned a musical frequency. The top row becomes high-pitched sounds, the bottom row becomes low-pitched sounds.
3. Brightness to Volume — Bright pixels create loud sounds, dark pixels create quiet sounds (or silence).
4. Additive Synthesis — All these frequencies are combined using sine waves, creating the final audio.

What is Additive Synthesis?

Additive synthesis is a sound creation technique that builds complex sounds by combining simple sine waves. Think of it like mixing colors — by combining basic building blocks, you can create anything.

For each moment in time, SpectroTrace adds together hundreds or thousands of sine waves, each at a different frequency and volume based on the corresponding pixel brightness.

Tips for Best Results

• Use high-contrast images — the clearer the contrast, the clearer the result
• Simple images work better than complex photos
• Text and logos produce excellent results
• Use the crop tool to focus on the most important part of your image
• In Advanced mode, longer durations give more detail but larger files

Privacy and Security

All processing happens directly in your browser using JavaScript and the Web Audio API. Your images are never uploaded to any server. Once you close the page, everything is gone — I never store or see your files.