The technical definition of sound is a mechanical disturbance or a vibration that is travelling through an elastic medium. The most common of elastic mediums is of course air, but other elastic mediums could be wood, metal, glass or water. Audio on the other hand refers to the recording of a sound using a piece of electronic equipment. Audio can be manipulated, the analogue or digital information that is stored on a medium such as a CD or tape is considered audio.
Firstly, when sound is produced either from a speaker, or even your mouth, a sound wave is created. For this discussion we will assume the sound wave is travelling through the air. As this wave passes through the air the particles in the air vibrate. This vibration is what determines the frequency of the sound wave.
But how is frequency measured? Well, if we could look at how many times the particle vibrated back and forth in a given amount of time you would have the frequency. The term used to measure these back and forth vibrations is Hertz (Hz), and one single vibration per second is equal to 1 Hz . As humans we have the ability to hear sounds in a broad range of frequencies. The human ear can hear sounds anywhere from 20 Hz all the way up to 20,000 Hz.
Let's take a closer look at a sound wave. To understand wavelength we need to first understand a few terms. First off there are regions in the air where the air particles are compressed together. These regions are referred to as compressions. There are also regions where the air particles are spread apart. These regions are known as rarefactions. The degree of change in pressure from these compressions and rarefactions is amplitude. Therefore a sound wave with a greater amplitude will produce a greater change in high to low pressure. Amplitude is commonly known as volume. The greater the amplitude the higher the volume.
The wavelength is the length of one complete wave. The top of a wave is known as the peak or Crest; and the bottom of the wave is referred to as the trough. The wavelength can be measured either from peak to peak or trough to trough, it doesn't matter as long as it's one complete wave.
Acoustics is a term that is used to describe the science as it relates to the behaviour of sound, how it's created, how it's received and how it travels through the air. Also different types of materials react differently to various types of sounds. For example hard surfaces, such as concrete and brick will reflect sound. Whereas sound absorption panels, made of medium density fibre board and fibre wool insulation will absorb sound. When designing a room, special attention should be given to the materials used, to give the most optimal acoustic environment.
Ambient noise refers to any unwanted sound. This could be as minor as the sound that is heard from the air rushing through air vents or even as loud as car horns honking out in the street. With good acoustic treatments, ambient noise could be reduced significantly. However to eliminate ambient noise completely is quite difficult.
Now let's talk about audio. Let's say an audio file is of a popular song. At some point the singer recorded this song in a studio, all of the vocals and an instrument. Sounds were captured by microphones and the sounds were converted to electrical signals. This electrical information can now be manipulated however the recording artist wishes. It can be edited, delays or echoes could be added, or tracks could be mixed together, the possibilities are endless. Once the audio file is complete it can be stored on some medium such as CD.