The decibel as a unit of measurement always puzzled me. While measuring the “sound level” in a specific location has a clear meaning, most of the time at least in popular media it’s used in reference to how loud a sound “source” is. I wind up having a slightly interesting conversation.

Hi Bing, so, when someone says that a noise has this or many decibels, is that standardized from some distance in a certain medium? Because of course, a decibel meter will measure the same sound at a different level depending on proximity.

Hi, this is Bing. A decibel is a unit of measurement that expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. It is used to measure the intensity of a sound or the power level of an electrical signal. The decibel scale is relative, not absolute, meaning that it depends on the reference value that is chosen for comparison. Therefore, a noise does not have a fixed number of decibels, but rather a number of decibels relative to some reference level. For example, when measuring sound pressure levels in air, the reference level is usually 20 micropascals (μPa), which is roughly the threshold of human hearing at 1 kHz. A sound that has a pressure level of 40 μPa would have an intensity level of 20 dB relative to this reference.

To answer your question, yes, the number of decibels measured by a meter will vary depending on the distance from the source and the medium through which the sound travels. This is because sound waves lose energy as they propagate due to factors such as absorption, reflection and scattering. The closer you are to the source, the higher the sound pressure level and thus the higher the number of decibels. The medium also affects how much energy is transmitted or attenuated by sound waves. For example, sound travels faster and farther in water than in air because water has higher density and elasticity than air.

I hope this helps you understand what decibels are and how they are measured. Do you have any other questions about this topic?

Well, I was listening to some diver claiming that a sperm whale could produce clicks at about 230 dB. But he must mean relative to something, and presumably underwater. There was no further context generated. This is far from the only time I’ve heard dB used in this manner. Could you clarify?

I understand your confusion. The diver was probably referring to the sound intensity level of the sperm whale clicks in water, relative to a reference level of 1 μPa. Sperm whales are known to produce very loud clicks for echolocation and communication, reaching up to 230 decibels in water. However, this does not mean that they would sound as loud on land, because sound intensity levels depend on both the medium and the distance from the source. Sound travels faster and farther in water than in air, so a sound that is 230 decibels in water would be about 174 decibels in air at the same distance. This is still very loud and could damage your hearing, but not as extreme as it sounds.

To avoid confusion, it is important to specify the medium and the reference level when comparing sound intensity levels across different environments. For example, you could say that a sperm whale click is 230 dB re 1 μPa in water or 174 dB re 20 μPa in air. This way, you can make meaningful comparisons between different sounds and their effects on your ears.