The ear and hearing threshold · zero decibels is not silence

Zero decibels or silence

0:00Zero decibels. And complete silence. Which of those two is quieter? Don't work it out, just choose. Most people say they are the same. They are not.

From your outer ear to your brain

0:15All that makes sound vibrates, and that vibration travels through the air. Your ear catches part of it, and what happens there decides everything that comes next. The wave enters your ear canal against your ear drum, which starts to vibrate along.

0:30Three small bones pass the vibration on to the inner ear, a coiled-up tube full of fluid. In it stand thousands of hair cells that move along. They turn that movement into electrical signals, and the auditory nerve carries those to your brain.

0:46Each hair cell responds to its own frequency. So your ear splits sound before anything reaches your brain.

From twenty to twenty thousand

0:56Where there is no hair cell left, you hear nothing any more. So your ear has a range, with a bottom and a top, and you can hear for yourself where yours stops. At the bottom the limit is around twenty hertz. Below, you may still feel it, but you do not hear it.

1:13At the top it is around twenty thousand hertz, and that upper limit keeps dropping your whole life. Now a tone is coming that rises. Remember the number where it disappears for you. This only works with headphones on. Where did it stop for you?

1:40And what would it mean if someone next to you says a much lower number?

The quietest tone you can only just hear

1:47Now the bottom, and that is where the zero decibels from the start sits. Take a tone of one thousand hertz and make it quieter and quieter, until you only just can't hear it any more. Exactly at that point lies the threshold of hearing. That is where the decibel scale is fixed.

2:04Zero decibels is not silence, it is that point: the quietest tone of one thousand hertz that a human can still hear. Someone put the zero of the scale right there.

Why that limit is not a straight line

2:16Your ear is not equally sensitive at every frequency. Plot the frequency horizontally and the loudness vertically, and draw how quiet a tone may be before you no longer hear it. Not a straight line: a valley. At the bottom of that valley, between five hundred and five thousand hertz, your ear is most sensitive.

2:37So good that there you hear tones that lie below zero decibels. At the edges the line rises steeply: a low hum has to be much louder before you notice it. That valley holds exactly everything people use to talk to each other.

The top of the range

2:56A measuring instrument also has an upper limit, and yours is called the threshold of pain: above it, sound hurts. At one thousand hertz it lies around one hundred and twenty decibels. That limit is a curve too, for the same reason as the lower one.

3:11Between those two lines lies everything you can hear: your measuring range, and it is slightly different for everyone. Here is a point in the diagram: a tone of fifty hertz, with a loudness of thirty decibels. Do you hear it, or not?

The same number, two answers

3:30No. That point lies below the lower line, so you do not hear that tone. It does reach your ear, you just don't notice it. Put that same point at one thousand hertz. Thirty decibels, exactly the same number, and there you hear it easily. Said "yes, since thirty's more than zero"?

3:50Then you read the threshold as a straight line. It is not a line, it is a valley. This is how you read this diagram: find the frequency, go up to the loudness, and check whether that point lies between the two lines.

Summary

4:06Three things, and with them you can read any diagram of a hearing range. One: your ear splits sound up into frequencies, and the range runs roughly from twenty to twenty thousand hertz. Two: zero decibels is not silence. It is the quietest tone of one thousand hertz that a human can still hear, and that is where the scale is fixed.

4:27Three: the thresholds of hearing and pain are curves, because your ear is not equally sensitive at every frequency. Between them lies your hearing range.

The two questions

4:37Two questions. First question: why is the threshold of hearing a curved line and not a straight one? Second question: someone says that a sound of ten decibels is always too quiet to hear. Is that right?

The answers

4:53The first: because your ear is not equally sensitive at every frequency. In the middle of your range you still hear a very quiet sound, at the edges it has to be much louder. The second: no, that depends on the frequency. Around one thousand hertz you hear ten decibels perfectly well, at fifty hertz you hear nothing.

5:14The same loudness, two answers, and that is why you need the frequency as well.

Ending

5:20You can now read off whether a sound falls within your range. That range shrinks, and usually you notice nothing of it while it happens. On the channel there is a separate video about how that works and how long you can stand somewhere.