Emergency Siren

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What Is This Sound?

This is a synthesized emergency siren sound created entirely using web audio synthesis — no recordings of real sirens involved. The urgent, sweeping wail you hear is generated in real-time by your browser, replicating the continuous rising and falling frequency pattern used by emergency vehicles and alarm systems worldwide.

Sirens are one of the most psychologically potent sounds in human experience. Designed to be impossible to ignore, the continuous frequency sweep exploits fundamental properties of the human auditory system to command immediate attention. This synthesized version captures those same attention-grabbing properties using pure synthesis.

How Is the Siren Sound Created with Code?

The siren is built from a sine oscillator with its frequency modulated by an LFO (Low Frequency Oscillator). Here is the Tone.js implementation:

Step 1: Create the Sine Oscillator

The siren’s core is a pure sine wave. Unlike complex waveforms, the sine wave produces a clean, piercing tone that carries well over distance — the same reason real sirens use near-sinusoidal tones:

const synth = new Tone.Synth({
  oscillator: { type: "sine" },
  envelope: {
    attack: 0.1,
    decay: 0,
    sustain: 1,
    release: 0.5,
  },
});
synth.toDestination();
synth.triggerAttack("C5");

Step 2: Add the Frequency Sweep LFO

The wailing effect comes from an LFO that continuously sweeps the oscillator frequency between 400 Hz and 1200 Hz at a rate of 2 Hz (two full cycles per second):

const lfo = new Tone.LFO({
  frequency: 2,
  min: 400,
  max: 1200,
  type: "sine",
});
lfo.connect(synth.frequency);
lfo.start();

Step 3: Signal Chain

The signal path is straightforward: Sine Oscillator (C5 base) → LFO Frequency Modulation (2Hz, 400–1200Hz) → Speakers. The sustain value of 1 ensures the tone continues indefinitely until explicitly stopped, mimicking a real siren’s continuous operation.

The Science Behind the Siren Sound

Why Frequency Sweeps Demand Attention

The siren’s effectiveness is rooted in how the human auditory system detects change. Our hearing evolved to prioritize time-varying sounds over steady-state ones — a survival mechanism that helps detect approaching predators, falling objects, and other dynamic threats. A continuous frequency sweep activates this change-detection system relentlessly, making it nearly impossible to habituate to or ignore.

The 2 Hz sweep rate is carefully chosen. Research shows that modulation rates between 1 and 4 Hz produce the strongest sense of urgency. Below 1 Hz, the change is too gradual to feel alarming. Above 4 Hz, the sweep begins to blur into a single complex timbre rather than a perceived motion.

ISO 7731 and Alarm Design Principles

The international standard ISO 7731 provides guidelines for auditory danger signals. Key principles reflected in this siren design include: the use of frequencies between 300 Hz and 3000 Hz (where human hearing is most sensitive), a minimum of 15 dB above ambient noise level, and a sweeping pattern that prevents masking by steady-state environmental sounds. The 400–1200 Hz range of this siren falls squarely within the ISO-recommended band for maximum detectability.

The Doppler Connection

The wailing siren also triggers an instinctive response related to the Doppler effect. When an emergency vehicle approaches, its siren’s pitch rises; as it passes, the pitch falls. Our brains have learned to associate continuous pitch changes with approaching objects, adding a spatial urgency dimension to the alarm that a steady tone cannot achieve.

Frequency Spectrum

Common Uses

• Game Sound Design — Alert and warning systems, police chase sequences, air raid scenarios, and danger zone indicators
• Film & Video Production — Background siren ambiance for urban scenes, action sequences, and tension building
• App Notifications — High-urgency alert sounds for emergency, safety, and monitoring applications
• Theater & Live Events — Dramatic sound effects for stage productions and immersive experiences
• Safety Training — Demonstration audio for emergency response training and fire drill simulations
• Music Production — Atmospheric element in electronic, industrial, and cinematic music compositions

Technical Details

Frequently Asked Questions

Can I use this siren sound in my project?

Yes. The sound is generated by code in your browser. The downloaded WAV file is yours to use freely in any personal or commercial project. Be mindful that playing siren sounds in public spaces may cause confusion or alarm — use responsibly.

How do I change the siren type?

Different siren types use different LFO patterns. For a European-style two-tone siren (“nee-naw”), use a square wave LFO alternating between two fixed frequencies. For a slow air raid siren, reduce the LFO rate to 0.1 Hz and widen the frequency range to 300–1500 Hz.

Can I make it more aggressive?

Switch the oscillator from sine to sawtooth for a harsher, more penetrating tone with added harmonic content. You can also increase the LFO rate to 3–4 Hz for a faster, more frantic sweep that increases perceived urgency.

Why is a sine wave used instead of a more complex waveform?

Real emergency sirens use electromechanical rotors or electronic oscillators that produce near-sinusoidal tones. The sine wave carries the maximum amount of energy at its fundamental frequency with no harmonic spread, making it the most efficient waveform for projecting a clear tone over long distances and through ambient noise.