Infrasonic Filter

What Is an Infrasonic Filter

An infrasonic filter is a high-pass filter designed to remove or attenuate frequencies at or below approximately 20 Hz—the lower limit of human hearing. These inaudible ultra-low frequencies consume amplifier headroom and cause excessive cone movement in subwoofer drivers without producing audible output. The filter can be integrated into a subwoofer's internal amplifier, installed inline between a preamp/amplifier and the subwoofer input, or implemented as a standalone external device.

High-Pass Filter Mechanics

A high-pass filter operates by reducing output at and below its cutoff frequency. When a high-pass filter is set to, for example, 25 Hz, it reduces the signal at that frequency by 50% (−3 dB) and attenuates frequencies below that point even more steeply. The steepness of this attenuation is measured in dB per octave—a standard metric for how quickly a filter's response drops off below the cutoff.

Common filter slopes in consumer and professional audio range from gentle (e.g., 18 dB/octave) to steep (48 dB/octave). Steeper slopes provide faster attenuation but introduce phase shift, time-domain ringing, and softer transients; gentler slopes preserve bass clarity but offer less protection against over-excursion. This trade-off—between excursion control and audible bass fidelity—is the core engineering tension in filter design.

Driver Protection in Ported Enclosures

Ported (bass-reflex) subwoofers are particularly vulnerable to over-excursion at frequencies below the port's tuning frequency (F_b). In a ported design, the port loads the driver near F_b, minimizing cone movement. Below that frequency, however, the port's acoustic output falls off and cone excursion rises sharply—without limit. As a general rule, cone excursion roughly quadruples for each octave lower in frequency (in the frequency region well below a driver's resonance) to maintain constant sound pressure level; this is an engineering approximation that holds in the piston-band region but is not a universal law across all driver types and loadings.

At frequencies well below tuning, especially at high amplifier power levels, this uncontrolled excursion can exceed the driver's maximum excursion limit (X_max), risking mechanical damage. An infrasonic filter rolls off response below the tuning frequency to constrain excursion while minimizing impact on audible bass above it. Additionally, ported enclosures can exhibit distortion and driver "pumping" from warped vinyl records or other rumble-frequency energy below tuning; a steep-slope subsonic filter is commonly recommended to prevent this effect for vinyl playback.

Sealed Enclosures and Filter Requirements

Sealed (acoustic suspension) enclosures differ fundamentally in their low-frequency behavior. The air spring inside a sealed box naturally rolls off deep frequencies, providing an acoustic restoring force that limits cone excursion automatically. This natural roll-off significantly reduces the risk of over-excursion at very low frequencies. As a result, sealed subwoofers can safely disable the infrasonic filter or set it to 20 Hz if available. Ported enclosures tuned above 40 Hz, by contrast, typically require an active infrasonic filter to prevent driver damage.

Frequency Selection and Tuning Formulas

Recommended infrasonic filter cutoff frequencies commonly range from approximately 10 Hz to 50 Hz depending on the subwoofer design, enclosure type, and listening environment. A practical starting point for tuning is around 30 Hz, which balances excursion control with minimal audible impact.

For ported enclosures, a common design guideline is to set the filter frequency at approximately half an octave below the port tuning frequency. This can be calculated by multiplying F_b by roughly 0.75. For example, a subwoofer with a port tuned to 35 Hz would use a filter cutoff of approximately 26–28 Hz (35 Hz × 0.75 ≈ 26.25 Hz). This relationship ensures driver protection in the critical region below tuning while preserving the intended bass response at and above the tuning frequency.

Optimal filter settings are system-dependent and may require listening tests to achieve the best balance between low-frequency extension and excursion control. In one modeled subwoofer simulation, a setting of 15–17 Hz achieved this balance for that specific enclosure and driver combination, though such precise thresholds vary considerably with driver compliance, enclosure volume, and damping characteristics.

Product Implementation Examples

Subwoofer manufacturers offer varying filter options. Arendal Sound subwoofers, for example, provide selectable infrasonic filter settings from 12 Hz to 31 Hz, with an internal 10 Hz protection filter active even when filtering is switched off. This dual-layer approach protects the driver against extremely deep rumble while allowing user fine-tuning for room and content preferences.

Inline standalone infrasonic filters are installed between the amplifier or preamp output and the subwoofer input, allowing users to add filtration to subwoofers or systems that lack internal filtering. These devices are typically adjustable across a frequency range and offer selectable slope options to match system requirements.