---
title: "Acoustic impedance"
description: "Acoustic impedance is the resistance a medium offers to the flow of acoustic energy. It is the product of medium density and the local speed of sound and is measured in pascal-seconds per metre (Pa·s/m). When sound travels from one medium to another with different impedance, a fraction of the energy is reflected at the interface — the larger the mismatch, the more is reflected."
canonical_url: "https://sylio.co/glossary/acoustic-impedance"
last_updated: "2026-06-28T02:29:22.453Z"
---

**Acoustic impedance** is the resistance a medium offers to the flow of acoustic energy. It is the product of medium density and the local speed of sound and is measured in pascal-seconds per metre (Pa·s/m). When sound travels from one medium to another with different impedance, a fraction of the energy is reflected at the interface — the larger the mismatch, the more is reflected.

## Why bell horns exist

A bare pneumatic [diaphragm](/glossary/diaphragm-horn) is small, stiff and presents a high acoustic impedance. The open volume inside an ESP or boiler is large and low-impedance. A direct coupling would reflect most of the diaphragm's energy back to itself instead of radiating it into the vessel. The [bell horn](/glossary/bell-horn) is an impedance-matching transformer: its exponential flare gradually steps the impedance down from the throat to the mouth, letting acoustic energy escape efficiently into the gas.

## Why air-to-metal interfaces reflect almost everything

Air has an acoustic impedance of roughly 410 Pa·s/m; steel is roughly 47 million Pa·s/m — a five-order-of-magnitude mismatch. Sound waves striking a metal tube reflect with essentially no transmission. Cleaning energy therefore couples to deposits via gas-borne pressure variation, not by transmission into the metal.

## Related terms

- [Bell horn](/glossary/bell-horn)
- [Sound pressure level](/glossary/sound-pressure-level)
- [Attenuation (acoustic)](/glossary/attenuation-acoustic)