# What’s the loudest / softest / highest / lowest sound humans can hear?

Humans can’t hear all sounds. Actually, not even most, in the grand scheme of things. Like how we can only see a narrow band of all wavelengths–hence “visible” light–we can also only hear some of the possible wavelengths. And wave heights. You might remember this from physics, but there are two measurements that are really important on a diagram of a wave: wave length and intensity. Like so:

So you’ve got the wavelength, which is the distance between two peaks or two troughs, and the amplitude, which is the distance between the mid-point of the wave and the tip of a peak. Which is all very well, but it doesn’t tell you much in the grand scheme of things, since most waves aren’t kind enough to present themselves to you as labelled diagrams. You actually have a pretty good intuitive grasp of the wavelength and amplitude of sound waves, though. The first is pitch and the second is what I like to refer to as “loudness”. (Technically, “loudness” is a perceptual measurement, not a… you know, this is starting to be boring.)

So there’s a limit in how loud and how soft a sound can be and a limit of how high and low a sound can  be. I’ll deal with loudness first, because it’s less fun.

Loudness

So we measure loudness using the decibel scale, which is based on human perception. Since 0 decibels is, by definition, the lower perceptual limit of sound for humans, the quietest sound humans can hear is just above that, which is around 20 micro-pascals of pressure. Of course, that’s healthy young humans. The older you get, the more your hearing range decreases, which is why your grandmother asks you to repeat yourself a lot. The loudest is just under 160 decibels, since exposure to a sound at 160 decibels will literally rip your eardrum. That’s things like being right under a cannon when it fires, standing next to a rocket when it launches or standing right next to a jet engine during take off, all of which tend to have other problems associated with them. So… avoid that.

Pitch

Pitch is a bit more interesting. Normal human hearing is generally between 20 hertz and 20 kilohertz–compare that to 15 to 200 kilohertz for dolphins and bats! (Because they both rely on sonar and echo-locution for hunting.) Just like hearing range for loudness, though, this gets narrower as you get older, particularly at the higher end of the range. Here’s a video that runs the gamut of the human hearing range (warning: you might want to turn your speakers down).

If you’re older than 25 (which is when hearing loss usually starts in the upper ranges) you probably couldn’t hear the whole thing. If you did, congratulations! You’ve got the hearing of a normal, young human.

# Seeing noise?

Some of you may be familiar with synesthesia, a neurological condition where you perceive sensory input from one sense as if it were another sense–with synesthesia the color yellow might taste like root beer, or the sound of a bassoon may feel like bread dough. Even without synesthesia, however, linguists (particularly phoneticians and phonologists) see sound all the time. What does it look like? Something like this:

These, boys and girls and others, are what your speech sounds look like. Spectrograms are one of the most useful tools in the speech scientist’s tool shed. Heck, they’re pretty much a Swiss army shovel. You can spend your entire career basically only looking at data in this one form.

Why? Well, there’s a lot of data in a spectrogram. Big things, like whether a sound’s a ‘b’ or a ‘p’ (there’s a big black bar on the bottom if it’s a ‘b’, but not if it’s a ‘p’), but also really small things that we as humans have have a really hard time hearing. Like, remember what I said earlier about your ears lying to you? Turns out it’s a lot easier to sort out the truth if you can see what you’re hearing. Plus, by looking at spectrogram we can quantify things like average vowel frequencies really quickly and easily. (Turns out, by the way, that you can [maybe, kinda, if you squint just right and have just the right voice sample] judge how tall someone is based on their vowel frequencies.)

But spectrograms aren’t just a serious scientific tool; they’re also pretty fun. Aphex Twin, an ambient musician (I mean, he makes music in the ambient genre, not that he provide background music at canape parties. Sheesh.) uses spectrograms as an art form. This song, for example, has a picture of his face encoded in it’s spectrogram. Give it a listen and see if you can find it!

On a more general note, the study of images made with sound is known as cymatics. I’m just going to leave this video here for the more physics-minded among you: