Loudness perception
42 Decibels and sound levels
Sound Levels
Sounds humans can tolerate sounds that have pressure amplitudes that are many millions of times larger than the quietest sounds we can hear. Loud sounds can have intensities that trillions of times that of quiet sounds. The numbers are difficult to “grok” and even more inconvenient to work with. Sound levels use the math of logarithms to compress these wide-ranging numbers into something manageable. Examples of logs in science include the pH scale (for acids and bases in chemistry), the Richter scale (for earthquakes) and decibel (for sound).
There are two common (nearly identical) log scales for expressing sound amplitude: sound intensity level (SIL) and sound pressure level (SPL). SPL and SIL are essentially identical. Many people use SIL and SPL interchangeably, while others use the phrase sound level instead.
No matter which term you use, the word “level” is crucial- sound intensity level is not the same thing as sound intensity. Sound pressure is not the same thing as sound pressure level and so on.
Sound levels are expressed in decibels. Sound levels for common sounds cover a range of about 130 dB. (See chart below for sound levels for common sounds).
| Sound | Sound Level (in dB) | Sound Intensity (in pW/m2) | Pressure Amplitude (in μPa) |
| Hearing threshold | 0 | 1 | 20 |
| Quiet forest | 10 | 10 | 60 |
| Watch ticking | 20 | 100 | 200 |
| Rice crispies | 30 | 1,000 | 600 |
| Library | 40 | 10,000 | 2,000 |
| Floor fan | 50 | 100,000 | 6,000 |
| Conversation | 60 | 1,000,000 | 20,000 |
| Noise inside car | 70 | 10,000,000 | 60,000 |
| Vacuum cleaner | 80 | 100,000,000 | 200,000 |
| Leaf blower | 90 | 1,000,000,000 | 600,000 |
| Chain saw | 100 | 10,000,000,000 | 2,000,000 |
| Machine shop | 110 | 100,000,000,000 | 6,000,000 |
| Loud indoor arena | 120 | 1,000,000,000,000 | 20,000,000 |
| Jet takeoff | 130 | 10,000,000,000,000 | 60,000,000 |
A quick look at the chart reveals that sound level is very a different thing than sound intensity. Every extra 10 dB corresponds to a sound that is ten times more intense than before.
Stop to think 1
Does a 60 dB sound have twice the intensity of a 30 dB sound?
Sound levels are relative
Sound levels show how a sound compares to a reference sound. The most commonly used reference sound is called the threshold of hearing– a barely audible pure tone at 1000 Hz that has a pressure amplitude of 20 μPa and an intensity of 1 pW/m2. The chart above uses the threshold of hearing as the reference sound.
Sometimes, other references are more convenient. For instance, recording engineers usually use the loudest sound that doesn’t cause distortion as the reference. Decibel values for sounds are often given without a mentioning a reference level (like in the table above). When this is done, it is usually safe to assume the reference is the threshold of hearing.
Elevation in geography works the same way. Elevations are relative to a reference level- almost