Understanding Decibels: Why They Are Measured as a Relative Unit

Decibels (dB) are widely used to measure the intensity of sound. Unlike other measurements which are absolute, decibels are relative units. This article delves into the reasons behind using decibels, the logarithmic scale, and the perceived relevance of such a measurement system.

Logarithmic Scale and Decibels

Decibels are a relative unit designed to express the ratio of one value to another, typically in the context of sound intensity or power. One of the key reasons for measuring sound in decibels is the use of a logarithmic scale. This logarithmic scale compresses a wide range of sound intensities into a more manageable format, making it easier to compare and understand these levels. This is particularly important given the vast range of sound pressures that humans can perceive, from the faintest whisper to the roar of a jet engine.

Relative Measurement

The decibel scale often compares a sound level to a reference level. For instance, the reference point 0 dB is typically defined as the threshold of hearing, which is 20 μPa (microPascals) in air. Other sound levels are then measured relative to this baseline. This relative measurement method highlights how much louder or softer a sound is compared to a standard reference. This method of measurement is more practical and easier to understand for people who are not acoustics specialists.

Perceptual Relevance

Human perception of sound is not linear. We perceive changes in sound intensity logarithmically. This means that a sound perceived as twice as loud is not actually twice the intensity in physical terms. The decibel scale aligns with this perceptual experience, making it a more intuitive way to express sound levels. This is why decibels are a preferred unit in various fields such as audio engineering, telecommunications, and environmental noise measurement.

Application Across Contexts

The relative nature of the decibel scale allows it to be applied in various contexts without needing to redefine a fixed unit for each scenario. This flexibility is a significant advantage, as it ensures consistency across different applications. For example, in environmental noise measurement, a decibel reading can provide a clear indication of how noisy a particular environment is relative to established noise levels, making it easier to implement effective noise control measures.

Decibel Scale and Dimensional Analysis

The choice to use a relative unit like decibels also addresses a fundamental issue in physics. The logarithm of an absolute value is mathematically possible, but it lacks physical dimension. To overcome this, the original value is divided by a reference value, and the logarithm is taken from this ratio. This ratio is then multiplied by 10 to form decibels. The reference values have been conventionally chosen to match critical sound parameters such as:

The n-sound pressure (threshold of hearing at 1000 Hz) at 20 microPascals. The n-sound intensity, which corresponds to the reference sound pressure of 1 pW/m2. The n-sound power, which corresponds to the sound intensity through one m2 of 1 pW.

For sound insulation, which is a ratio itself, the process is simpler. The logarithm of the attenuation factor between 0 and 1 is taken and then multiplied by -10 to get a positive number. This step ensures that higher decibel readings correspond to better insulation, as the insulation factor decreases as decibels increase.

In summary, measuring sound in decibels as a relative unit allows for a more practical, perceptually relevant, and dimensionally consistent way to express sound intensity. This measure accommodates the vast range of human hearing and facilitates easy comparison between different sound levels across various applications.