Infrasound, any sound below 20 Hz, sits beneath the normal range of human hearing but can still be perceived as pressure, vibration, or even a faint tone if the level is high enough. Generating it requires moving large volumes of air slowly, which is fundamentally different from producing audible sound. There are several practical ways to do this, ranging from simple DIY builds to specialized equipment.
Why Infrasound Is Hard to Make
A regular loudspeaker cone has to quadruple its back-and-forth travel every time you halve the frequency. Below about 40 Hz, even large subwoofers run out of physical travel and their output drops off sharply. The core problem is an impedance mismatch: air is too thin and light to efficiently receive energy from a vibrating cone at very low frequencies. So making infrasound means finding ways to move much more air than a conventional speaker can.
Subwoofer in a Sealed Room
The simplest approach is a large subwoofer (15 inches or bigger) mounted in a sealed, airtight room. When a speaker is enclosed in a small space, even modest cone movement pressurizes the entire room, effectively turning the walls into part of the system. Feed the speaker a sine wave signal from a tone generator app or function generator set between 1 and 20 Hz, and the room will pressurize and depressurize at that frequency. You won’t “hear” it in the traditional sense, but you’ll feel pressure fluctuations in your ears and chest.
This works because the sealed room eliminates the need for the speaker to project sound outward through open air. The smaller the room relative to the speaker, the stronger the effect. A closet-sized space with good seals around the door can produce noticeable infrasound from a modest subwoofer and a basic amplifier.
Rotary Woofers
The most effective dedicated technology for generating infrasound is the rotary woofer, developed by Bruce Thigpen of Eminent Technology. Instead of a vibrating cone, it uses a set of spinning fan blades whose pitch angle changes in response to an audio signal. A motor spins the blades at a constant speed while a voice coil and magnet assembly (connected to a standard amplifier) tilts the blades back and forth. As the blade angle changes, it pushes more or less air, creating a pressure wave.
The key advantage is that the blade pitch barely needs to increase as frequency drops, unlike a cone speaker that needs exponentially more travel. This makes the rotary woofer efficient all the way down to single-digit Hz frequencies. The unit is typically mounted in a wall or sealed baffle separating two spaces, so one side experiences positive pressure while the other gets negative pressure. These are commercially available, though expensive, and are used in home theaters and research labs that need true sub-20 Hz reproduction.
Pipes and Resonance Chambers
Pipe organs have produced infrasound for centuries. The physics is straightforward: a pipe resonates at a frequency determined by its length. A 32-foot organ pipe produces a fundamental tone around 16 Hz. A 64-foot pipe, the largest ever built, reaches down to about 8 Hz. At that size, you’re talking about a pipe roughly the height of a six-story building.
You can apply the same principle on a smaller scale with PVC pipe. A pipe open at both ends resonates at a frequency where its length equals half the wavelength. For 17 Hz, that’s roughly 10 meters (about 33 feet) of pipe. You’d excite the air column with a speaker or compressed air source at one end. The pipe amplifies the resonant frequency and suppresses others. Coiling the pipe can save space, though tight bends reduce efficiency. Even a straight 4- to 5-meter pipe with a speaker driver at one end can reinforce frequencies in the low 20s, right at the border of infrasound.
Mechanical Oscillators and Fans
Any large surface that oscillates slowly will generate infrasound. A few practical approaches:
- Modified fan: A standard box fan or industrial fan with an uneven number of blades blocked or weighted will produce pressure pulses at its rotation rate. A fan spinning at 900 RPM completes 15 rotations per second, so blocking one blade creates a 15 Hz pulse. Slower fans or motors with eccentric weights can go lower.
- Vibrating panel: A large, flexible sheet of plywood or sheet metal driven by a mechanical shaker or eccentric motor can push enough air to generate infrasound in an enclosed space. The panel acts like an oversized speaker cone.
- Pneumatic pulse: Alternately opening and closing an air valve connected to a compressed air source or shop vacuum at a controlled rate produces discrete pressure pulses. A motorized valve spinning at 10 revolutions per second creates a 10 Hz tone. This is essentially how pipe organs work, scaled down.
Signal Generation
Whatever physical method you use, you need a signal source. A smartphone app or computer running a tone generator can output sine waves at any frequency, but most built-in speakers can’t reproduce anything below about 80 Hz. You’ll need to route the signal through an external amplifier to a capable transducer. Free software like Audacity can generate precise sine waves, square waves, or swept tones in the infrasound range and export them as audio files. Function generators (available for under $30 as USB devices) offer more precise control and can output frequencies down to fractions of a hertz.
Standard audio amplifiers work fine for driving infrasound transducers. The signal is just an electrical waveform like any other audio. The challenge is entirely on the output side: getting a physical device to move enough air at those frequencies.
Measuring What You’ve Made
Standard microphones and sound level meters are not reliable below about 10 Hz. Most consumer equipment rolls off steeply below 20 Hz and won’t register infrasound at all. Specialized measurement microphones designed for low-frequency work can reach into the infrasound range, but they’re expensive lab instruments. Microbarometers, which detect tiny changes in atmospheric pressure, are the gold standard for frequencies below 2 Hz and are used in international nuclear test monitoring networks.
For a practical DIY check, a sensitive barometric pressure sensor (like the BMP280 or BME280 used in Arduino and Raspberry Pi projects) can detect infrasound pressure fluctuations when sampled at a high enough rate. These won’t give you calibrated decibel readings, but they’ll confirm that your setup is producing pressure waves at the target frequency.
Safety Considerations
Infrasound at moderate levels is not inherently dangerous, but high-intensity exposure can cause discomfort, nausea, disorientation, and a sense of pressure in the chest and ears. The ear remains the primary sensing organ for infrasound, and at levels somewhat above the perception threshold, you can feel vibrations throughout the body. Denmark, one of the few countries with specific indoor infrasound limits, caps permissible levels at 85 dB inside residences. For context, that’s a substantial amount of acoustic energy even though you can’t traditionally “hear” it.
Keep exposure times short when experimenting, especially in sealed rooms where pressure can build. If you feel chest pressure, ear discomfort, or nausea, reduce the level or leave the space. Infrasound also travels extremely well through walls and floors, so your neighbors may feel the effects before you realize the sound is escaping your room.