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What is ‘A’ weighting?

A-weighted decibels, abbreviated dBA, or dBa, or dB(a), are an expression of the relative loudness of sounds in air as perceived by the human ear. In the A-weighted system, corrections are applied to the decibel values of sounds to simulate the variable response of the human ear to different frequencies. A large negative correction is made to low frequency sounds because the ear is relatively insensitive to them and a slight positive correction is made to frequencies between  1000 and 6000 Hz, which is the approximate frequency region of human speech. A negative correction is made for high frequencies.

Sound pressure measurements should give a reading of the sound pressure in dB SPL, but in the context of human hearing it is more practical to provide also a value which corresponds more closely to the hearing sensation or loudness of the sound. The A, B, and C filters used currently in sound-level meters were aimed at mimicking equal loudness curves over frequency under different conditions of sound intensities, i.e. for sounds of low, medium, and high loudness levels, respectively. The “A” network modifies the frequency response to follow approximately the equal loudness curve of 40 phons, while the “C” network approximately follows the equal loudness curve of 100 phons. A “B” network is also mentioned in some texts but it is no longer used in noise assessments. The popularity of the A network has grown in the course of time. In current practice, the A weighting curve filter is used to weight sound pressure levels as a function of frequency, approximately in accordance with the frequency response characteristics of the human auditory system for pure tones. This means that energy at low and high frequencies is de-emphasised in relation to energy in the mid-frequency range.

The A-filter has been adopted so generally that sound pressure levels frequently quoted in literature simply in dB are in fact A-weighted levels. Many older general purpose sound level meters are restricted solely to A-weighted sound pressure level measurements.