Humans are actually somewhat better at telling loud sounds apart than quiet ones- the JND drops from about 1.5 dB at 40 dB to about 0.5 dB at 90 dB. The smallest difference that can be perceived is called a just noticeable difference (JND). For comparison, the “two jet” sound has a sound level that is only 3 dB more than the “one jet” sound. Most people have trouble telling which sound is louder when comparing sounds with levels that differ by less than 1 dB. Humans have trouble distinguishing between sounds with seemingly large differences in intensity- two jet planes don’t sound much louder than one.

If the SPL of a sound is expressed relative to human threshold of hearing, but the SIL is expressed relative to the runway at JFK airport, the SPL and SIL won’t be equal. Negative elevation means you’re not quite at the top yet.Ĭan a sound have a negative sound level and still be heard? When is SIL different than SPL?Ī sound’s SIL and SPL are always equal, unless different reference levels are used. Everest, you might want to choose the top of Everest as the your reference level. But, in some contexts, you could choose to use some other reference. When Wikipedia gives the elevation on the floor of Death Valley as -86 meters, you assume the author means 86 meters below sea level. Elevations are relative to a reference level- almost always sea level. When this is done, it is usually safe to assume the reference is the threshold of hearing.Įlevation in geography works the same way. Decibel values for sounds are often given without a mentioning a reference level (like in the table above). For instance, recording engineers usually use the loudest sound that doesn’t cause distortion as the reference. Sometimes, other references are more convenient. The chart above uses the threshold of hearing as the reference sound. 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/m 2. Every extra 10 dB corresponds to a sound that is ten times more intense than before.ĭoes a 60 dB sound have twice the intensity of a 30 dB sound? Sound levels are relative SoundĪ quick look at the chart reveals that sound level is very a different thing than sound intensity.

(See chart below for sound levels for common sounds).

Sound levelsare expressed in decibels. Sound levels for common sounds cover a range of about 130 dB. Sound pressure is not the same thing as sound pressure level and so on. No matter which term you use, the word “level” is crucial- sound intensity level is not the same thing as sound intensity. Many people use SIL and SPL interchangeably, while others use the phrase sound level instead. There are two common (nearly identical) log scales for expressing sound amplitude: s ound intensity level (SIL)and sound pressure level (SPL). Examples of logs in science include the pH scale (for acids and bases in chemistry), the Richter scale (for earthquakes) and decibel (for sound). Sound levels use the math of logarithms to compress these wide-ranging numbers into something manageable. The numbers are difficult to “grok” and even more inconvenient to work with. Loud sounds can have intensities that trillions of times that of quiet sounds. Sounds humans can tolerate sounds that have pressure amplitudes that are many millions of times larger than the quietest sounds we can hear. Loudness perception 42 Decibels and sound levels Sound Levels

Decibel scale graph