Likewise our hearing does not have the same sensitivity at all frequencies. No microphone has the same sensitivity to all frequencies and no speaker reproduces all frequencies equally well, as we will see in Chapter 18 on electronics. The above curves are very much like the frequency response curves of microphones and speakers. Medium loudness doesn't change the perceived pitch very much. Low frequencies are perceived to be slightly lower than expected if they are very loud. High frequencies are perceived to be a slightly higher pitch than normal if they are very loud. It is also the case that intensity has an effect on perceived frequency the same laboratory frequency will appear to be a slightly different frequency if the intensity is different. In the next post, we’ll explore the math behind calculating decibel levels.\( \newcommand\) is due to the tube resonance of the auditory canal (see chapter 12 for tube resonance and chapter 10 for a picture of the auditory canal). So, there you have it: a brief explanation of the nature of the decibel and reference values. All other values are expressed as a ratio to the reference. On a decibel scale, the reference value is set to zero. This makes a good choice for a starting point or reference. For example, the reference sound pressure level of 20 micro-Pascals is the smallest perceptible pressure the human ear can hear. While the reference values may seem arbitrary and oddly specific at first glance, these values have clear origins. Therefore, the benefit of using a level and decibel scale is to make comparing different physical quantities easier. When a power-like physical quantity is presented as a level and decibel scale, this power-like quantity is compared to the reference value. For example, it’s best to write 63 dB as 63 dB re 20 µPa where re means referencing. The definition of levels and the decibel scale requires a reference value. MeasurementĪlthough the reference value is assumed to be known often in speech and even in writing, it is good practice to include the decibel reference value, especially in formal reports and documents. The chart below shows some common reference values for various measurements. Without a reference, a decibel is meaningless.įor sound pressure, the reference value is generally 20 micro-Pascals. Every time you come across the word decibel, you can tack on “with reference to _”. On the decibel scale, a unit of pressure is equal to 10 times the negative value of the logarithm of the ratio of the actual pressure to a reference pressure. Although rarely explicitly mentioned, decibels are always in reference to a chosen value. what?” In other words, twice has no meaning unless you have a reference. However, if you were asked to ring a bell when “twice as much time had passed”, you would be left scratching your head asking, “Twice as much time as. Five seconds at lunchtime is the same as five second before bedtime. Five seconds last five seconds regardless of when you start counting. Absolute units can be counted without a reference. Well, the firstly it’s important to understand that the decibel is not an absolute unit like seconds or meters-it’s a relative unit. How could one unit describe so many different things? Shouldn’t different quantities have different units? While sound intensity (in W/m 2) is the SI unit, the sound intensity level in decibels (dB) is more relevant for how humans perceive sounds. With so many usages, it is no surprise that a cloud of confusion lurks around the term. You may have noticed that when people talk about the loudness of a sound, they describe it in units of decibels rather than watts per meter squared. The word decibel, abbreviated dB, seems to show up everywhere in acoustics: sound pressure levels, sound power levels, intensity levels, and even voltage levels.
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