Sonar ping

The classic submarine “ping”: a metallic ringing tone that decays over a couple of seconds.

patch("sonar_ping", "one_shot",
    mix([
        noise("white", 3.0).bandpass(1000.0, 200.0).gain(60),
        noise("white", 3.0).bandpass(2050.0, 200.0).gain(20),
    ])
    .lowpass(6000.0, 0.4)
    .tremolo(4.0, 0.2)
    .env(0.008, 1.0)
    .fade_out(0.6)
    .gain(0.25));

The core trick: high-Q bandpass on white noise

The whole sound is built around one realisation: a very high-Q bandpass on white noise reads as a tone, not a band of hiss. White noise has flat spectral content; pushing it through a bandpass(f, Q) with Q ≈ 200 extracts a sliver of energy around f — bandwidth roughly f / Q, so ~5 Hz wide at 1 kHz — which the ear hears as a pure pitch with a tiny amount of chaotic micro-modulation. That last detail is the point: a real sonar transducer rings with mechanical imperfections; a pure sine sounds synthetic, a filtered-noise tone sounds struck.

Two of these ringing tones at 1000 Hz and 2050 Hz stack into a roughly-harmonic interval (an octave plus a quartertone), which gives the ping a chord-like body rather than a single-tone whistle. The lower band gets more energy (gain(60) vs gain(20)) so it sits forward in the mix; the upper band adds shimmer.

Why these numbers

bandpass(1000.0, 200.0).gain(60). The huge gain compensates for the resonant filter’s narrow passband — most of the white noise is discarded by the filter, so we crank the survivor back up to a usable level. The Q=200 is what gives the tonal character; lower it to 20 and you hear noisy hiss instead of pitch.

Second band at 2050 Hz. Slightly offset from a true octave to avoid mechanical sameness with the lower band. Gain dropped to 20 so the upper band sits underneath the lower one as harmonic colour rather than competing with it.

.lowpass(6000.0, 0.4). Cleans any residual high-frequency hash the bandpass filters let through. The low Q (0.4) is intentional — it’s a gentle slope, not a resonant cut.

.tremolo(4.0, 0.2). A 4 Hz amplitude wobble at 20% depth adds a slight pulse to the tail that reads as “this tone is alive, not a sample loop” — sonar listeners hear similar modulation from the transducer’s mechanical hum.

.env(0.008, 1.0). 8 ms attack avoids a hard onset click. Decay constant 1.0 gives a 1/e time of ~1 s — the tone rings out audibly over the following 2-3 seconds.

.fade_out(0.6). Smooths the very end of the buffer so it doesn’t cut abruptly into silence. The env decay is asymptotic; the fade_out brings it cleanly to zero over the last 600 ms.

.gain(0.25). The summed bandpass-noise gains are large numbers (60 + 20 = 80 before normalisation). The final gain dials the whole thing back into the safe -1.0..1.0 range with headroom for the reverb taps you might add later.

Why not just a chirp?

A linear-FM chirp is the textbook explanation of how real LFM sonar pulses work, but it sounds artificial in a game. You can hear the sweep — the pitch slides over the duration, and the ear immediately labels it as “synthesised effect” rather than “metallic transducer ringing in seawater.” Bandpassed noise has the opposite quality: the pitch is stable, but the micro-structure of the source is chaotic, so it reads as a physical object that’s been struck.

If you do want the swept character — for a wider, more “active” sonar — chirp(280.0, 400.0, 1.0) is still available; see the chirp DSL chapter. The two designs cover different moods: filtered-noise for the iconic submarine ping; chirp for a modern active-sonar pulse.

Variants

Brighter / more urgent. Move both bandpass centres up: bandpass(1400, 200) + bandpass(2800, 200). Reads as a smaller boat, lighter weapon.
Heavier / older sub. Drop both centres: bandpass(600, 200) + bandpass(1250, 200). Sounds like a WWII-era ASDIC set.
Pure single tone. Drop the upper band entirely. The ping sounds more like a research pinger or a beacon.
Tighter interval (octave). Set the upper band to exactly twice the lower (1000 → 2000). The harmonic alignment makes the two bands fuse into one tone with extra brightness rather than reading as a chord.
Longer ring. Halve the env decay constant: .env(0.008, 0.5) gives a ~2 s 1/e time. Pair with a longer fade_out(1.2).

Test in sndlab

Drop the patch into the editor pane, press F5. The scope’s upper pane should show a slowly-decaying ringing waveform; the lower spectrum pane should show two narrow peaks near 1 kHz and 2 kHz — the two bandpass centres — sitting above a low noise floor. If you see broadband hiss instead of distinct peaks, the bandpass Q is too low.