---
title: "Bell horn"
description: "A bell horn is the conical or exponential flare bolted to the driver of an industrial sonic horn. Its job is to transform the high-impedance, small-area pressure pulse from the diaphragm or piston-whistle into a lower-impedance, larger-area sound wave that couples efficiently into the gas inside the vessel."
canonical_url: "https://sylio.co/glossary/bell-horn"
last_updated: "2026-06-28T02:29:29.025Z"
---

A **bell horn** is the conical or exponential flare bolted to the driver of an industrial [sonic horn](/glossary/sonic-horn). Its job is to transform the high-impedance, small-area pressure pulse from the [diaphragm](/glossary/diaphragm-horn) or [piston-whistle](/glossary/piston-whistle-horn) into a lower-impedance, larger-area sound wave that couples efficiently into the gas inside the vessel.

## Why the geometry matters

The bell is not decorative. Its flare profile — usually exponential, sometimes catenoidal or tractrix — sets the horn's cut-off frequency: below the cut-off, the bell stops behaving as a horn and the radiated sound power collapses. A 60 Hz [low-frequency acoustic cleaner](/glossary/low-frequency-acoustic-cleaner) therefore needs a physically larger bell than a 230 Hz unit, which is why low-frequency horns are noticeably bulkier and heavier. Mounting orientation, flange standard ([DN](/glossary/flange-standards-dn-ansi) or [ANSI 150](/glossary/flange-standards-dn-ansi)) and the bell's projection distance into the vessel are all selected to match the cleaning target geometry.

## Materials

- **Carbon steel** for ambient-temperature mounting on cool-side ducts, [silos](/glossary/silo) and [hoppers](/glossary/hopper)
- **316 stainless steel** for corrosive or food-grade environments
- **Inconel 625 or 718** for hot-side service above 350 °C, including [SCR reactors](/glossary/selective-catalytic-reduction), [air heater](/glossary/air-heater) penthouses and [recovery-boiler](/glossary/recovery-boiler) flue paths

## Related terms

- [Sonic horn](/glossary/sonic-horn)
- [Diaphragm horn](/glossary/diaphragm-horn)
- [Piston-whistle horn](/glossary/piston-whistle-horn)
- [Acoustic horn](/glossary/acoustic-horn)