Sound pressure level is how loud a sound is, expressed as a ratio rather than an absolute amount. It compares the pressure a sound wave creates against a fixed reference pressure of 20 micropascals, and states the result in decibels. A reading of 60 dB SPL means the sound pressure is 1,000 times the reference.
Every number you see on a sound level meter, a phone app or a noise chart is an SPL. The formula behind it:
SPL (dB) = 20 × log10(p / p0), where p is the measured sound pressure and p0 is 20 micropascals (µPa).
Why sound is measured on a logarithmic scale #
Human hearing covers an enormous range of pressures. The quietest sound a healthy young ear can detect is around 20 µPa; a jet engine at close range is roughly a million times that pressure. Writing those numbers out would be unwieldy, so the decibel compresses them into a scale from 0 to about 140.
The compression has consequences that trip people up:
- +3 dB is double the sound energy. It sounds only slightly louder.
- +10 dB is ten times the energy, and is usually described as sounding about twice as loud.
- Decibels don’t add like ordinary numbers. Two machines at 80 dB each produce 83 dB together, not 160. See how decibels add up.
That’s also why hearing-damage rules count time. Under the NIOSH recommended exposure limit, 85 dBA is safe for 8 hours, and every 3 dB above that halves the allowed time, because 3 dB doubles the energy reaching your ear.
What does 0 dB SPL mean? #
Zero on the SPL scale is not silence. It’s the reference point, 20 µPa, chosen to sit near the quietest sound an average healthy young person can hear at 1 kHz. Sounds quieter than that reference produce negative dB values, which is why measurements inside anechoic chambers are sometimes reported below zero.
In practice you’ll never see a negative number on a phone. Microphones have their own electrical self-noise, and on a phone that noise floor typically sits well up into the 20s or 30s of dB, so very quiet rooms read higher than they really are. Readings near the extremes of the scale deserve the most doubt.
dB SPL vs dBA vs dBC: which one is on your meter? #
Raw SPL treats a 30 Hz rumble and a 3 kHz whistle as equally significant. Your ear doesn’t. Frequency weighting corrects for that.
| Notation | What it means | Where it’s used |
|---|---|---|
| dB SPL (or dBZ) | Unweighted, flat response | Acoustic engineering, machine diagnostics |
| dBA | A-weighted, approximating the ear’s sensitivity at moderate levels | Hearing rules, noise ordinances, workplace limits |
| dBC | C-weighted, nearly flat but rolled off at the extremes | Peak levels, concerts, low-frequency complaints |
Nearly every published noise limit is in dBA. OSHA’s permissible exposure limit and NIOSH’s recommendation are both A-weighted, and so is the WHO’s safe-listening guidance. If a number is quoted without a letter, it’s usually dBA. dB vs dBA explains the difference in more detail.
SPL vs sound power vs loudness #
Three related terms that get mixed up:
- Sound pressure level is what a microphone at a particular spot measures. It depends on distance, the room and where you stand. This is the number on a meter.
- Sound power level is how much acoustic energy a source emits in total, independent of where you listen. Appliance labels quote it in dB(A) as a power level, which is why a dishwasher rated at 44 dB doesn’t read 44 on your phone in the kitchen.
- Loudness is the subjective perception, measured in units such as sones and phons. Two sounds at the same SPL can be perceived as very different in loudness depending on frequency content and duration.
Because SPL depends on distance, any measurement is meaningless without a position. “Chainsaw: 110 dB” implicitly means at the operator’s ear. The same saw might read 80 dB across the yard.
What SPL values look like in real life #
| Sound | Typical level |
|---|---|
| Quiet library | around 40 dBA |
| Normal conversation | 60 to 70 dBA |
| Busy restaurant | around 70 to 80 dBA |
| Heavy traffic from the curb | around 85 dBA |
| Motorcycle, lawn mower | around 90 dBA |
| Rock concert, chainsaw | 100 to 110 dBA |
| Fireworks at close range | 140 to 160 dBA |
The last two rows come from the National Institute on Deafness and Other Communication Disorders, which puts headphones at maximum volume and concerts at 94 to 110 dBA and fireworks at 140 to 160 dBA. NIDCD also states that sounds at or below 70 dBA are unlikely to cause hearing loss even after long exposure, while long or repeated exposure at or above 85 dBA can. A fuller list is in our decibel chart of common sounds.
How a phone turns microphone data into dB SPL #
A phone doesn’t measure pressure. It produces digital samples, and the level of those samples is expressed in dBFS, decibels relative to full scale, which tops out at 0 and runs negative. A meter app applies frequency weighting and time weighting to that signal, then adds an offset to move the dBFS number onto the dB SPL scale.
That offset is the weak link. It depends on the sensitivity of that particular microphone in that particular phone, and manufacturers don’t publish it. An app ships with a reasonable default for typical hardware, which is why two phones side by side can differ by several decibels.
Sound Meter dB does the standard meter math on the raw signal: an A-weighting filter built from the IEC 61672 curve, Fast (125 ms) or Slow (1 s) time weighting, and a true energy-average Leq rather than an arithmetic mean of the displayed numbers. It then maps the result onto an approximate dB SPL scale, and says so plainly in the app. If you have access to a calibrated meter, Settings has a calibration offset of up to ±30 dB so your phone can be lined up with it, and the offset used is stored with each saved session. Our guide to calibrating a sound meter app walks through the process.
Frequently asked questions #
Is dB the same as dB SPL? #
In everyday noise talk, yes. A decibel on its own is just a ratio and can describe voltage, power or gain. When someone says a room is “60 dB,” they mean 60 dB SPL, almost always A-weighted.
What is the loudest sound pressure level possible in air? #
An undistorted sound wave in normal atmospheric conditions can’t exceed roughly 194 dB SPL, because at that point the wave’s rarefaction reaches a vacuum. Louder events, like blasts, exist as shock waves and are described with peak pressure rather than ordinary SPL.
Why do noise limits use dBA instead of dB SPL? #
Because A-weighting tracks the ear’s sensitivity reasonably well at moderate levels, so it correlates better with annoyance and hearing risk than a flat measurement. Low-frequency rumble that would inflate a flat reading is discounted, which is closer to how the sound actually affects you.
What SPL is dangerous? #
Risk depends on level and time together. NIOSH recommends no more than 8 hours a day at 85 dBA, halving the time for every 3 dB more. Very high peaks, such as fireworks or gunfire, can injure hearing almost instantly. If you have ringing, muffled hearing or pain after noise exposure, see a doctor or an audiologist.
What’s the difference between SPL and volume? #
Volume is a control on a device; SPL is a measurement at a location. Two phones at the same volume setting produce different SPLs, and the same setting produces a different SPL at your ear depending on the headphones and how they seal.