Most instrumentation for measuring sound has the capability to weight all of the component frequencies of a sound, and sum them into a single dB(A) or dB(C) number. A Bode plot showing the magnitude response of different band-pass filters. We know that human hearing is significantly non-linear, and that our perception of loudness in particular is a complex function of sound pressure level, frequency content, time and other factors. The dB scale is convenient in a context such as this because frequency response plots are intuitive and visually informative when the frequency axis uses a logarithmic scale and the amplitude axis uses a dB scale. Sound levels can also be measured using a frequency-weighted filter which provides a more suitable indication of the low frequency content, for the purpose of evaluating “bass” sound which may travel/penetrate farther than treble sound sounds measured in this way are designated in units of C-weighted decibels dB(C). We have a number of different dB scales for describing levels of phenomena (pressure, voltage, digital amplitude) relative to some reference quantity. For this reason, most guidelines and limits for noise outdoors or indoors, such as ordinances, regulations and By-laws, are specified in terms of a single-number dB(A) level. A dB(A) spectral-sum sound pressure level is a reasonable single-number representation of the perceived overall loudness of a complex sound that contains multiple different frequencies. The change between any one decibel and the next is never the same as the change between another, different, two decibels. The frequency-weighting is referred to as the “A-scale.” Most instrumentation for measuring sound has the capability to weight all of the component frequencies of a sound, and sum them into a single number sounds measured in this way are designated in units of A-weighted decibels dB(A). Even just understanding this much will help you realize how much variation there is within the decibel scale. Therefore, sound levels are often measured using a frequency-weighted filter which emulates the frequency sensitivity of the human ear. Decibels, or dB units, provide a convenient way to express relative levels between powers that differ by many orders of magnitude. The human ear varies in its sensitivity to sounds of different frequency. Decibels are how we measure sound but due to the logarithmic nature of the Decibel scale, it isnt the most intuitive concept to grasp. Most sounds can contain a mixture of many frequencies simultaneously.
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