Negative decibels. Can you hear them?

You may have noticed, while fixing the volume on certain speakers or radios, that the display told you it was playing at -10 or so decibels. This might seem a bit strange. Are you able to hear below zero decibels so clearly somehow?

As we’ll briefly discuss in a moment, most audio pieces of equipment do not in fact play at negative decibels, but there is an interesting question here; can sound go below zero decibels? And if so, can people hear at that level at all?

A quick point of clarification

There are technically two different situations in which you can have a negative decibel value.

One way is to have a sound that’s below the relative threshold of zero, which is what the rest of the article will be discussing.
The other is to have a reduction of base volume. This latter situation might be seen on volume sliders for instance, like on a radio or a speaker. What this means is that the equipment is taking the base volume of whatever sound and reducing it by a set amount. So, if a song is 80db and the volume slider says it is at -10db then the sound that is produced by the speaker will be 70db. This can go the other way as well, if the volume slider reads +10db then the speaker would play audio at 90db. Although these readings are more technical than the standard 1-10 scale that some speakers use, it is very helpful for people working with audio, as they can have very reliable control over how loud the sound that is produced is.

When would decibels actually go negative?

So what about that first situation then? What does it mean for a sound to be under the relative threshold of zero? Decibels are what is called a relative scale. Different points of the scale are measured based on how they compare to some neutral value, which is called a “zero”. Another good example of a relative scale is the height above sea level. The sea level acts as the “zero” point, so heights are measured from above that point. In this same way, negative decibels are like heights that are below sea level.

Whereas sea level describes altitude, the decibel scale describes something that is somewhat unexpected; pressure. The decibel scale is based on how much pressure is applied to the ear by a sound; the higher the pressure, the greater the volume heard. What we call 0 decibels is the point where the pressure of a sound is so small that we can’t audibly hear it. There is still pressure though; the atmosphere is constantly applying pressure, and many smaller noises don’t produce enough pressure for our ears to pick it up. These sounds too quiet for our ears to pick up are what would be measured as negative on the decibel scale.

What kind of things would be a negative on the decibel scale?

It’s hard to say what sounds would be a negative on the decibel scale; they’re too quiet to be heard by human ears. It should be noted though that there’s a difference between a sound being inaudible because it is the wrong frequency for human ears, and a sound being inaudible because it is too quiet.

Drumming your fingers on a table softly enough so that it can’t be heard would be an example of the former; if you were to drum your fingers harder, the sound would become audible, so it is a case of negative decibels. A dog whistle, on the other hand, can’t be heard because the frequency is too high for human ears to pick up. You could play a dog whistle at negative decibels hypothetically, but even if you played it as loud as you possibly could it still wouldn’t be audible.

The inaudible point can also vary depending on the frequency. Human ears can detect a specific range of frequencies, but on the ends of this spectrum those sounds become inaudible if they’re played below a certain intensity. So although we can hear 20kHz if it is played at, say, 80 decibels, we won’t be able to hear that same sound if it is played at 20 decibels. The chart below shows the audible intensity of the frequency spectrum that our ears can hear. You might note that the audible intensity dips very low around 1-4 kHz. This is the range at which most human speech lies, hence why we’re especially adept at picking it up. In some cases the sounds that are audible can dip below 0 decibels within this range, although this is highly dependent on how healthy an individual’s ears are.

Is the zero decibels level the same for all animals?

In real life, yes, because it’s us humans who created and defined the decibels scale. But if we look at the logic behind the scale one could argue that each animal has a different zero on the decibel scale.

“Zero” for us is the point where a sound is too quiet for humans to hear, but that level is based on our ears. But what about other animals? Where would their “zero” be?

As with humans, different animals can pick up certain frequencies at different intensities. Many birds, for example, can hear a much wider range of frequencies than people, but often need those sounds to be louder to become audible than a human would. On the other hand, cats, owls, and many other night-time predators can hear at lower intensities than humans can at the same frequencies. These rarely go very far in the negatives though; cats and dogs can both hear below 0 db at certain frequencies, but this is often only about -10 to -15 db.

Below you’ll find a graphical representation of the audiogram for several different animals, which you can compare to the human audiogram shown earlier to see how different animals might hear differently to us.

Note: These audiograms correspond to single individuals, they are not averages. This means that the audiogram for a dog does not represent all dogs, just the one used during the test, and other dogs might have a wider or narrower range of hearing.

Dogs

Cats

Baboon

White-Tailed Deer

Conclusion

Although it might be a little disappointing to realize that hearing the radio when it’s at -20 decibels doesn’t actually mean you have super-hearing, there is something special about the ranges that we do hear. The ranges that we hear the best are also the ranges that we speak in, meaning that humans have evolved to get better and better at hearing each other, even when our voices aren’t that loud.