The differences between dB HL and dB SPL arise from isophonic curves. Pure-tone audiometric thresholds are expressed in dB HL (hearing level) and are referred to hearing thresholds of normal hearing young individuals. Many older general purpose sound level meters are restricted solely to A-weighted sound pressure level measurements.ĭifferent decibel measures are used in audiometry (evaluation of hearing sensitivity) than in sound pressure measurement. It should be noted that the A-filter has been adopted so generally that sound pressure levels frequently quoted in audiology literature simply in dB are in fact A-weighted levels. Since the human ear is much more resistant to noise-induced hearing loss (NIHL) at and by low frequencies A weighting is more in correspondence with NIHL risk. B (or even C) weightings provide a better correspondence between loudness and moderate (or high) acoustic levels, however A weighting differs only from B and C as underweighting frequencies below about 500 Hz. This means that energy at low and high frequencies is de-emphasized in relation to energy in the mid-frequency range.Ĭorrelation between noise effect hearing loss and sound exposure levels measured in A, B, or C weightings would not be very different. 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. The popularity of the A network has grown in the course of time.

A “B” network is also mentioned in some texts but it is no longer used in noise evaluations. 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. for sounds of low, medium, and high loudness levels, respectively (IEC 651, 1979). The A, B, and C filters used currently in sound-level meters were aimed at mimicking isoloudness curves over frequency under different conditions of sound intensities, i.e. While sound pressure measurements should give a reading of the sound pressure in dB SPL, 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 in phones. Normal equal-loudness-level contours for pure tones For frequencies lower than 3-4 kHz and higher sound frequencies, the ear becomes less sensitive. As it could be seen in the figure below, the frequencies 3-4 kHz are the most sensitive within sound frequency range from 20 Hz to 20 kHz that can be heard by human ear. Isophonic curves relate the characteristic of a given tone expressed in dB SPL to its subjective loudness level expressed in phones (see figure 1 below). In order to assess loudness of a sound the isophonic curves are explored. The loudness of a sound is not equal with its sound pressure level and differs for different frequencies. This subjective or perceived magnitude of a sound by an individual is called its loudness. The human ear is not equally sensitive to sounds (tones) of the same sound pressure levels but different frequencies.

Table 1: Typical sound pressure levels for daily life soundsģ.3.3.2. In the logarithmic scale the range of human ear’s audible sounds is from 0 dB SPL (hearing threshold) to 120-140 dB SPL (pain threshold) (see table 1 below). Since using such a large scale is not practical, a logarithmic scale in decibels ( dB) was introduced which is also in agreement with physiological and psychological hearing sensations.ĭB of sound pressure level ( dB SPL) is defined as: 20 log 10 p1/p0 where p1 is actually measured sound pressure level of a given sound, and p0 is a reference value of 20μPa, which corresponds to the lowest hearing threshold of the young, healthy ear. Human ear’ audible sound pressure levels range from 20 μPa ( hearing threshold) till 20 Pa (pain threshold), resulting in the scale 1:10,000,000. One parameter of the acoustic ( sound) wave which is generally used to assess sound exposure to humans is the sound pressure level expressed in μPa or Pa.

Equal loudness spl hl decibel scale