Uniform attenuation (also called flat attenuation) is a technical term used to describe devices that are capable of reducing (attenuating) a sound, light, or other waveforms across all of its frequencies.
The term is usually used to refer to sound attenuation specifically. Ear plugs (and other noise protection devices) that provide uniform attenuation are able to absorb all sound frequencies (within a given range) equally. Conversely, ear plugs that do not have uniform attenuation will absorb frequencies at different rates, which means that they will protect against some types of noise less effectively than others.
Hearing protection is an important aspect of occupational health and safety. Hearing loss due to excessive occupational noise exposure (noise-induced hearing loss) is one of the most common occupational injuries.
Occupational health and safety agencies in most advanced jurisdictions require that workers be provided with personal protective equipment (PPE) against any hazard that they are exposed to insufficient enough quantities that it poses a health or injury risk. Noise hazards are no exception; workers in excessively noisy environments must be provided with adequate hearing protection.
However, not all forms of hearing protection are equal. Many types of hearing protector, including both ear plugs and over-the-ear noise mufflers, do not block all frequencies uniformly. When an individual uses these non-uniform hearing protectors, they may perceive themselves to be exposed to a lower-volume of noise; however, certain frequencies (typically low frequencies) may still be penetrating the ear plugs at unsafe volumes. This excess noise is the result of what's called “frequency leakage."
Hearing protection that provides uniform attenuation is defined by its lack of frequency leakage. Because of this, a given piece of uniform hearing protection may provide the same amount of protection as a given piece of non-uniform hearing protection even if the uniform protection has a lower noise reduction rating (NRR) than the non-uniform protection does.
When determining whether a given piece of equipment provides adequate protection, it is important to take this factor into account when calculating the actual level of protection that is provided. ANSI-standard S12.68 describes the calculations necessary to determine whether a given piece of hearing equipment provides sufficient protection. The difference between a uniform and non-uniform attenuating piece of protection that have the same noise reduction rating (e.g., an NRR of 35 decibels/dB) is often between 10-15 dB.