What is Sound?

Sound is vibration transmitted through a medium. When we think and talk about sound, though, we mean those vibrations in air molecules that can be picked up by human ears.

Sound as a Wave

Sound energy moves the molecules within the substance through which it travels, just like ripples in a pond. Sound waves spread in a circle from the source, becoming less intense with distance.

Sound is like water. It doesn't have a shape or form, but molds itself to its surroundings. And like water, sound can be absorbed by some materials and contained by others.

Sound waves need a medium through which to travel. There's no sound in space, and the most common sound conductor on earth is air. But sound waves can also pass through solid materials. Which is why Soundproof Cow is in business!

How is Sound Measured?

You can look at water and see that either the surface is smooth, or it has a lot of big waves. You can even pour it into a container and measure it. Since sound doesn't look like anything, how can you tell how "big" or "loud" it really is?

Sound is measured in two fundamental ways: frequency and amplitude, or as they are more commonly known, pitch and volume.

Frequency

Frequency, also known as pitch, is the space between waves of sound, or how many times a sound wave oscillates (moves up and down) in one second. The frequency of a sound is like the current in a body of water. The current tells you how fast the water is moving from one point to another.

Frequency is expressed in Hertz (Hz). Hertz is a measure of frequency per unit of time. For example 1 Hertz = 1 sound oscillation per second. The average person can hear sounds with frequencies between 60Hz and 23000Hz. The average cow can hear sounds with frequencies between 20Hz and 35000Hz.

A high-pitched sound is like a ripple in a pond: it's small, has low power, moves quickly and can easily be controlled and redirected. A low-pitched sound is more like a tsunami or tidal wave: large, slow moving, strong and difficult to control or redirect.

We perceive frequency as pitch: how "high" or "low" a sound is. A tea kettle emits a high-frequency whine; a high school kid's "bumping" car stereo emits a low-frequency thump.

The human ear can only hear a specific range of frequencies. Think of a tea kettle: when the water boils, the sound gets higher and higher until you can't hear it anymore. The kettle is still emitting sound, but once it reaches a certain point, the frequency is so high that your ear cannot register it.

High frequency sounds can be easily blocked. But very low frequency sounds (like the music from a teenager's car) are more difficult. That's why you can hear a loud car on the street from inside a basement, but can't hear the people talking upstairs.

Amplitude

Amplitude, also known as volume, is like depth of water. The deeper that you dive, the greater the pressure the water exerts on your body. In sound, the greater the number of decibels produced, the greater the pressure exerted on your ear drums, and the louder the sound.

Frequency and volume, much like depth and current, are equally important in measuring sound, and are entirely independent of one another. A sound with a high Hz rating is just as high-pitched at a low volume as it is at a high volume. For example, the current in a stream is just as fast at 2 feet of depth as it is at 4 feet. Altering one measurement does not impact the other.

Because frequency measures the characteristics of a particular sound wave, it's an unchanging number. But amplitude depends on the environment where the sound is located and is trickier to measure.

You hear sound by registering changes in the pressure of the air within your ear. The louder the sound, the greater the pressure change.

This change in pressure is measured in pascals (Pa). The pascal officially represents "the deviation from local ambient pressure at the time of hearing."

The human ear can respond to a wide range of pressure changes: from 2×10⁻⁵ Pa to over 1 trillion Pa to be exact. Numbers in such a huge range are difficult to use.

To make measuring sound easier, scientists have converted these huge numbers into more manageable ones, expressed in decibels (dB).

You can't perceive anything lower than 0dB, and anything over 130dB is past the human threshold of pain. The decibel scale is a logarithmic scale, meaning the sound pressure level doubles with every 3dB increase.

So a change of only 35dB represents a massive difference. In fact, 35dB is the difference between a TV set and a 4-lane highway from 30 feet away.

Here are the decibel levels of some sounds:

Quiet Library

30 db

Phone Dial Tone

80 dB

Train Whistle at 500 ft.

90 dB

Jet Engine at 100 ft.

140 dB

Krakatoa Explosionat 100 mi

180 dB

Soundproofing materials must control sound pressure level (volume) at many different frequencies.