Sound is measured in decibels (dB), a unit that captures how intense a sound wave is compared to the quietest sound a healthy human ear can detect. That baseline, known as the threshold of hearing, sits at 0 dB. From there, the scale climbs logarithmically, meaning each 10 dB increase represents a tenfold jump in sound energy. A quiet natural area with no wind registers around 20 dB, normal conversation falls between 60 and 70 dB, and a jet engine at 100 feet hits roughly 140 dB.
Why the Decibel Scale Is Logarithmic
Your ears don’t respond to sound in a straight line. Doubling the physical energy of a sound doesn’t make it seem twice as loud. Instead, your perception of loudness increases on a curve, and the decibel scale mirrors that curve by using a logarithmic formula: you take the ratio of a sound’s intensity to the threshold of hearing, find the power of ten, and multiply by 10. The “deci” in decibel means one-tenth of a Bel, the original unit named after Alexander Graham Bell. That factor of 10 was added because one decibel is roughly the smallest change in loudness most people can notice.
In practical terms, this means that going from 50 dB to 60 dB isn’t adding a little more noise. It’s multiplying the sound’s energy by 10. Going from 50 to 70 dB multiplies it by 100. Yet to your ears, a 10 dB increase sounds only about twice as loud. This mismatch between physical energy and perceived loudness is exactly why a logarithmic scale makes sense for describing sound.
Frequency: The Other Half of Sound
Decibels measure intensity, but sound also has frequency, measured in hertz (Hz). Frequency determines pitch. A low rumble might be 100 Hz; a high-pitched whistle could be 4,000 Hz. Healthy human ears pick up frequencies from about 20 Hz to 20,000 Hz (20 kHz), though infants hear slightly above that range and adults gradually lose sensitivity at the high end, with the practical ceiling often dropping to 15,000 or 17,000 Hz by adulthood. Sounds below 20 Hz are called infrasound, and those above 20 kHz are ultrasound.
Frequency matters for measurement because your ears aren’t equally sensitive to all pitches. You hear mid-range frequencies (around 1,000 to 4,000 Hz) much more easily than very low or very high ones. A 60 dB bass tone and a 60 dB tone at 1,000 Hz have the same physical intensity, but the 1,000 Hz tone sounds noticeably louder. This quirk of human hearing is the reason engineers developed frequency weighting.
A-Weighting and C-Weighting
When you see a noise measurement written as “dBA,” that “A” means it has been filtered to approximate how human ears respond at moderate volumes. The A-weighting curve reduces the contribution of very low and very high frequencies, giving more weight to the mid-range where hearing is most sensitive. Most noise regulations, workplace limits, and environmental standards use A-weighted measurements because they best reflect what people actually experience.
C-weighting (dBC) applies a flatter filter that keeps more of the low-frequency energy in the measurement. It’s typically used for peak sound levels, like the sudden blast from an explosion or the low-end rumble of heavy machinery. The World Health Organization, for example, recommends workplace noise stay below 85 dBA on average while capping peak levels at 135 dBC.
Phons and Sones: Measuring Perceived Loudness
Because two sounds at the same decibel level can seem like different volumes depending on their frequency, scientists developed additional units. The phon ties loudness to a reference pitch of 1,000 Hz. If a sound at any frequency seems just as loud as a 60 dB tone at 1,000 Hz, it’s rated at 60 phons. This gives a more honest picture of how loud something actually feels.
Phons improve on raw decibels, but they still don’t scale proportionally. A sound rated at 60 phons doesn’t feel twice as loud as one at 30 phons. That’s where the sone comes in. The sone scale is designed so that doubling the number doubles the perceived loudness. Starting from a baseline of 1 sone at 40 phons, 50 phons equals 2 sones, 60 phons equals 4 sones, and so on. For audio engineers and acousticians designing products or spaces, sones provide the most intuitive measure of how people will actually experience sound.
How Sound Drops Off With Distance
Sound from an isolated source in open air follows the inverse square law: every time you double your distance from the source, the intensity drops to one quarter. In decibel terms, that works out to roughly a 6 dB decrease each time you double the distance. So if a lawn mower reads 90 dB at 3 feet, it would measure about 84 dB at 6 feet and around 78 dB at 12 feet.
This only holds cleanly outdoors with no walls, floors, or ceilings bouncing sound back. Indoors, reflections and reverberation keep sound energy from dissipating as quickly, which is why a noisy restaurant can feel overwhelmingly loud even when you’re far from the kitchen.
Instruments Used to Measure Sound
A sound level meter is the most common tool. It uses a microphone to capture sound pressure at a specific moment and location, displaying the result in decibels. Because it only measures one point in time and space, getting an accurate picture of a noisy workplace usually means taking readings at multiple spots throughout the day.
A noise dosimeter works differently. It’s a small device clipped to a worker’s shoulder that records sound levels continuously over an entire shift, then calculates an average exposure for that period. Because it travels with the person, it captures every environment they move through, making it far more accurate for workers who don’t stay in one place. OSHA’s noise monitoring guidelines describe the dosimeter as the preferred approach when employees move between areas or when noise levels fluctuate throughout the day.
Neither instrument handles extreme sounds perfectly. Standard dosimeters and sound level meters tend to max out around 140 to 146 dB. Impulse noises like gunfire or fireworks can hit 170 to 180 dB, and those peaks simply get clipped, meaning the device records a flat ceiling instead of the true spike. Specialized microphones and measurement systems are needed for those environments.
Workplace Noise Limits
OSHA sets the legal permissible exposure limit at 90 dBA averaged over an 8-hour shift. At that level, employers must use engineering or administrative controls to reduce noise. A separate threshold kicks in at 85 dBA: once workers hit that average, employers are required to start a hearing conservation program that includes monitoring, hearing tests, and access to hearing protection.
NIOSH, the research agency behind many workplace safety recommendations, takes a more conservative position. It recommends keeping all worker exposures below 85 dBA for an 8-hour day. The two agencies also disagree on how to account for louder, shorter exposures. OSHA uses a 5 dB exchange rate, meaning that for every 5 dB increase, the allowed exposure time is cut in half. NIOSH uses a 3 dB exchange rate, which is stricter: every 3 dB increase doubles the noise energy and halves the safe exposure time. Under the NIOSH standard, 88 dBA is safe for only 4 hours, and 91 dBA for just 2 hours.
Common Sounds on the Decibel Scale
- 20 dB: A quiet natural area with no wind
- 60 to 70 dB: Normal conversation
- 85 dB: The threshold where prolonged exposure begins to risk hearing damage
- 140 dB: A jet engine at 100 feet, near the pain threshold for most people
- 170 to 180 dB: Gunfire and fireworks at close range
Smartphone apps can give you a rough sense of how loud your environment is, though they lack the calibrated microphone of a professional sound level meter. For casual awareness, checking whether a noisy venue consistently reads above 85 dBA on your phone is a reasonable signal that your hearing is at risk if you stay for an extended period.