# Shielding attenuation

> Decibels of field reduction provided by a material. Every 10 dB is a factor of 10 in power density: 30 dB removes 99.9%, 10 dB removes 90%.

- Canonical URL: https://emfbase.com/quantities/shielding-attenuation
- Collection: Quantities & units
- Record id: `quantities:attenuation`
- Status: verified
- Last verified: 2026-08-12
- JSON: https://emfbase.com/api/quantities/shielding-attenuation
- Also known as: dB attenuation, shielding effectiveness

## Key values

- **Symbol:** A
- **SI unit:** dB
- **Units in use:** dB

## What it measures

Ratio of incident to transmitted field, band-dependent and geometry-dependent. Manufacturer dB figures are measured on flat samples in a test fixture; installed performance is usually lower because of seams, edges and reflections.

## Units

Multiplier to the SI unit (dB).

- **dB (decibel)** — 1 dB = 1 dB

## Relationships to other quantities

- **S_after = S_before / 10^(dB/10)** — For field strength in V/m the divisor is 10^(dB/20).

## Sources

- `nist-si` National Institute of Standards and Technology. *The International System of Units (SI), NIST Special Publication 330*. https://www.nist.gov/pml/special-publication-330 — government, link checked 2026-08-12

Data licence: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Dataset version 2026.08.1. Retrieved from EMFBase, https://emfbase.com.

This record is reference data, not medical or legal advice. Regulatory limits, scientific guidelines and precautionary values are labelled separately and answer different questions.