The concept of decibels, especially negative values, often leads to confusion. Many wonder if a negative decibel reading indicates a louder or quieter sound. This article clarifies the decibel scale, explains the meaning of zero decibels, and demystifies what negative decibel values represent in sound.
Understanding the Decibel Scale
The decibel (dB) measures sound intensity or power. It is a logarithmic scale, expressing a ratio between two values, not an absolute measurement. This allows it to compress a vast range of sound intensities into a manageable numerical range. For instance, the quietest sound a human can hear and a sound loud enough to cause pain can differ by a factor of a trillion in intensity.
The logarithmic scale reflects how human hearing perceives sound. The human ear responds to sound intensity non-linearly; a large change in physical intensity is perceived as a smaller, proportional change in loudness. On this scale, an increase of 10 dB typically corresponds to a sound being perceived as roughly twice as loud.
The Significance of Zero Decibels
Zero decibels (0 dB) does not signify the absence of sound. Instead, 0 dB on the sound pressure level (SPL) scale represents a specific reference point: the approximate threshold of human hearing. This reference is defined as a sound pressure of 20 micropascals (µPa), the faintest sound a young, healthy ear can detect, often at 1000 Hz.
Because the decibel scale is relative, 0 dB acts as a baseline against which other sound levels are compared. It is possible for sounds to exist that are even fainter than this established threshold, leading to negative decibel values.
Deciphering Negative Decibels
Negative decibel values indicate sounds quieter than the 0 dB reference point. A sound at -10 dB, for example, is softer than the threshold of human hearing and likely inaudible. These values are a direct consequence of the decibel scale’s logarithmic and relative nature. Just as temperatures can fall below freezing, sound levels can fall below the established 0 dB threshold.
Negative decibels are used in various contexts, such as describing very faint sounds that exist but are below human perception, like the inherent noise floor of highly sensitive audio equipment in a quiet recording studio. In audio engineering, negative decibels also denote attenuation, where a signal is reduced in strength; for instance, a mixer fader might show negative dB values when lowering the volume from a reference level. A sound at -10 dB is louder than a sound at -20 dB, as -10 dB is closer to the 0 dB reference and represents greater sound intensity.
Relating Decibels to Everyday Sounds
The decibel scale provides a practical way to understand the intensity of sounds encountered daily. Sounds below the 0 dB threshold, such as those at -10 dB or -20 dB, are generally inaudible to the human ear, existing in the realm of extreme quiet or electronic noise floors. As sound levels increase, they become progressively louder and more perceptible. A quiet room might register around 40 dB, while a normal conversation typically falls within the 60-70 dB range.
Louder environments include busy traffic at about 70 dB or a vacuum cleaner at 60-85 dB. Sounds at or above 85 dB, such as heavy city traffic or a noisy restaurant, can potentially cause hearing damage with prolonged exposure. Very loud sounds like a rock concert (120 dB) or a jet engine (120 dB) are at or near the threshold of pain and can cause immediate hearing damage. Higher positive decibel values indicate increasingly intense and potentially harmful sounds.