Hearing Sounds

Detectors of sound include the ears of humans and animals, as well as microphones. In previous lessons, you learned about Wave Motion and Sound Waves. Later, you will learn about Reproducing Sounds. But right now, we will look at hearing those sounds.

Animals and humans hear sounds with their ears. These sound detectors have a number of interesting characteristics, such as:

• Range of frequencies
• Range of loudness or amplitude
• Direction detection
• Distance detection

This lesson will explain the characteristics for these interesting and unique detectors of sound. There is a mini-quiz at the end of the lesson.

Frequencies

The frequency of a sound is also known as its pitch. The higher the frequency, the higher the pitch and the shorter its wavelength. There is a range of frequencies that humans and animals can hear or detect. This is also true for any detectors of sound, such as microphones.

Frequency is measured in Hertz—which means cycles per second. The abbreviation for Hertz is Hz.

Humans have limitations

A normal human can hear between 20 Hz and 20,000 Hz. That is quite a range. 20 Hz is a very low pitch sound or very deep tones. 20,000 Hz is a high pitch sound. Besides hearing low frequencies, you can often feel them. An example of that is when you can feel the bass from a loudspeaker.

When humans get older, they have trouble hearing the high pitched sounds and their detection range diminishes.

Dogs can hear high notes

Dogs and some other animals can hear sounds at higher frequencies than 20,000 Hz. That is why people can’t hear a special dog whistle, while the dog will react to it. Being able to hear the higher frequencies may be useful in hunting for the animal. Cats probably can hear higher frequencies also, but most are too independent to respond to such a whistle.

Elephants can hear low notes

Elephants can hear frequencies much lower than the range of humans. Some researchers who were studying elephants in Africa discovered this. The story goes that a herd of elephants they were observing suddenly perked up their ear and ran off in a direction, as if they heard someone calling them. The researchers said they didn’t hear anything, but some said they felt something strange in their bodies, just before the elephants ran off.

After studying this situation, they found that elephants could make a very low frequency sound that humans can’t hear. Elephants are able to hear this sound coming from up to 5 miles away. It is amazing the distances from while they can communicate with each other.

Loudness

Your ears can detect the loudness or amplitude of sounds within a certain range. Loudness is measured in decibels, which really measures their energy. Each unit has 10 times the energy of the smaller unit (deci = 10).

A human can hear from very quiet sounds at 0.1 decibels to sounds at 120 decibels, which actually can cause pain. At the high end of the scale, the energy can cause damage to the sensitive membranes in the ear.

Danger of loud sound

Listening to very loud music can start to permanently damage hearing. This is a problem with many rock musicians, as they start to go deaf from their own music—thus forcing them out of their business.

Another potential hazard is in using earphones. Although you may not have the volume up too high, inexpensive earphones do not filter out spikes of sound that can damage your hearing. These spikes are of a very high volume, but they happen so quickly that you don’t even notice them—except for the possible damage done.

Some can hear low volumes

Younger people can usually have better hearing at the lower volumes than older people do. For example, when I was 15 years old, I noticed that our blinking Christmas lights made a slight buzzing sound before they blinked. This is because such lights usually have a tiny micro-switch, which turns the electricity on an off. It would buzz turning the switching process.

I could hear with the lights switched on or off across the room, with my back facing the lights. Meanwhile, adults would have trouble hearing the sound with their ear up close to the lights. This is not to mean that my hearing was so fantastic. Rather it is to show how age diminishes the ability to hear low volume sounds.

Animals hear low volumes

Animals can hear lower volumes and are more sensitive to louder noises than humans. One reason is because of the way their ears are shaped. Cats are an example of an animal that has ears that gather in sound and focus it into the ear channel. They use the ability to hear quiet sounds to catch mice and such.

You can increase your ability to ear low volumes by cupping your hands by your ears. Your hands are reflecting the sound into your ear channel just like the cat’s ear reflects sound for them.

Direction detection

Because you have two ears—one on each side of your head—you are able to distinguish the direction of the sounds, whether they come from the left or the right. This is because the volume is slightly higher on the side closer to the object. Another factor may be that the ear furthest from the sound gets some sound reflected off objects. The brain processes this information to let you know from what direction the sound is coming.

Many people can tell—with their eyes closed—if the sound is in front or behind or above them. How they do that is a puzzle to me.

Indications are that it is more difficult to tell direction with low frequencies. This is probably due to the fact that the ear is less sensitive to volume variations with low frequencies and the fact that at low frequencies the wavelength is greater than the distance between the ears. This could also muddle the sound heard.

Distance detection

Some animals such as bats and dolphins use sound to measure distance, as well as direction. They create very short chirps and are able to estimate the distance of an object, as well as to get other information about the object from the reflected sound they hear. This is very similar to sonar which fishermen use to find fish or that is used by submarines.

The bat can tell the direction of the object by comparing the sound detected from its two ears. It can tell the distance from the time it takes for the reflected sound to be heard, after being emitted. The bat can also tell if the object is hard or soft—like a tasty moth—by the frequencies that are received, as compared to what was emitted.

Different objects reflect sound frequencies selectively, just like different objects reflect different colors or frequencies of light.

Dolphins and porpoises use sonar the same way as bats—except they use it underwater. It is a shame that humans don’t have this capability, because it might be real handy to have.

In conclusion

Animals and humans can hear sounds in limited range of frequencies. Some animals can hear sounds that humans can’t. Sounds can be heard from very low volumes to so loud that they cause pain and damage. Volume is measured in decibels.

We compare sound with our two ears to determine direction. A few animals can also determine distance and characteristics of object with sound.

Use your knowledge of sound to do some good in this world.

Mini-quiz to check your understanding

1. What sounds can’t humans hear?

2. What helps cats to hear low volume sounds?

3. How do bats detect objects to eat?

If you got all three correct, you are on your way to becoming a champion in science. If you had problems, you had better look over the material again.

What do you think?

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