Sound reaches our ears as vibrations traveling through the air. The perception of how loud or soft these sounds are is known as loudness. To measure this, the decibel (dB) unit is used. This unit provides a standardized way to quantify sound levels.
The Decibel Explained
The decibel is a relative unit that expresses the ratio of two values on a logarithmic scale. This logarithmic nature is useful because the human ear perceives a vast range of sound intensities. A logarithmic scale compresses this enormous range into manageable numbers. For instance, a 10-fold increase in sound intensity corresponds to a 10 dB increase.
The decibel unit is one-tenth of a Bel, a larger unit named after Alexander Graham Bell. The Bel was initially conceived to quantify signal loss over telegraph wires but was too large for practical sound measurements. This led to the adoption of the decibel. A one-decibel difference is considered the smallest change in loudness detectable by the human ear.
The decibel scale is relative, comparing a measured sound intensity to a reference point. For sound pressure level, the reference point (0 dB) is set at 20 micropascals (µPa), which is the quietest sound an average human ear can hear. This reference serves as the threshold of hearing.
Measuring Sound
Loudness, expressed in decibels, is measured using a sound level meter. These handheld instruments quantify sound pressure levels in a given environment. The core of a sound level meter is its microphone, which detects minute changes in air pressure caused by sound waves.
When sound waves hit the microphone’s diaphragm, these pressure fluctuations are converted into electrical signals. The meter amplifies these electrical signals and converts them into digital data. This digital data is displayed as a decibel reading. Sound level meters are commonly used for monitoring noise in workplaces, assessing environmental noise, and analyzing building acoustics.
Common Sound Levels
Common sound levels in decibels include:
The softest sound an average person can hear, the threshold of hearing, is 0 dB.
Normal breathing is around 10 dB, while a soft whisper is about 30 dB.
A normal conversation is 60-65 dB.
Everyday appliances like a vacuum cleaner are around 70 dB.
Busy city traffic is 70 dB.
Sounds above 85 dB can harm human hearing over prolonged exposure. Power tools are about 90 dB, and a rock concert is 110-120 dB. A jet engine taking off nearby is 120-150 dB. Each increase of 10 dB represents a sound that is perceived as roughly twice as loud due to the logarithmic nature of the scale.
Weighted Decibels
Various “weightings” are applied to account for how the human ear perceives sound differently across frequencies. The most common is A-weighting, dBA or dB(A). A-weighting adjusts measured sound levels to better reflect the human ear’s sensitivity, which is less sensitive to extreme frequencies and most sensitive to mid-range frequencies. This weighting is widely used for environmental and occupational noise assessments, as it correlates well with the risk of hearing damage.
Another common weighting is C-weighting, dBC or dB(C). C-weighting provides a flatter frequency response compared to A-weighting, including more low-frequency sound. While A-weighting is used for measuring sounds at lower to moderate levels, C-weighting is used for very loud sounds, typically above 100 dB, or for peak sound measurements like impulsive noise. Also used in entertainment noise measurement where bass transmission is a concern, measuring with both A and C weightings provides a more comprehensive understanding of noise exposure.