When Is a Decibel Not a Decibel?

At NGC Testing Services, our acoustical testing lab is concerned primarily with the measurement of sound power level and sound pressure level.

Sound power and sound pressure are two distinct and commonly confused characteristics of sound. Both share the same unit of measure, the decibel (dB), and the term "sound level" is commonly substituted for each.

A decibel is a unit used to measure the intensity or power of a sound. It's a logarithmic unit, which means that the scale is based on powers of 10. Decibels are often used to quantify sound levels because the human ear perceives sound intensity logarithmically, meaning that a doubling of sound intensity does not result in a doubling of perceived loudness.

The decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal-to-noise ratios are often expressed in decibels.

So, what is the difference between sound power and sound pressure decibels?

Sound Power vs Sound Pressure Decibels

Sound Power Decibel

Sound power level is the absolute measurement of the total acoustic energy or wattage that a sound source is emitting. Since it is the measure of the total acoustic energy being radiated, it’s not affected by the acoustic environment in which it is located. It is an absolute.

Sound Pressure Decibel

On the other hand, decibel sound pressure level is a pressure disturbance in the atmosphere whose intensity is influenced not only by the strength of the source but also by the surroundings and the distance from the source to the receiver.

It is simply the sound pressure level measured of a sound power source at a particular location (orientation and distance) in a particular acoustic environment.

If you know the sound power that is emitted by a sound source, it’s possible to estimate the sound pressure level that would be measured at a certain distance from that source in a certain acoustic environment (room size, dimensions, and absorption).

This is an important tool for the prediction of noise levels when a sound source is introduced to an already noisy environment.

Enhancing Environments Through Acoustical Testing

Understanding sound power and sound pressure through acoustical testing enables engineers and professionals to quantify, analyze, and mitigate the impact of noise in various settings, contributing to improved product quality, environmental sustainability, and human well-being.

If you have a question about our acoustical testing services or would like to get your building product or assembly tested, reach out to our team today. We’ve been in the business for over 55 years, delivering accurate test results — fast.

Editor's Note: This post was originally published in September 6, 2021 by Bob Menchetti and was updated and republished in June 2024.