Sound Waves

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DECEMBER 1, 2000 ISSUE

Soundwave Basics

BY KEN LANYON

Most people who take up recording music don't often take time to think about the basic properties of sound itself. They are content to dive right into the gear and get to work recording that fantastic new song they have been working on, and there really isn't anything wrong with doing this. I myself am guilty of fumbling around a 4-track without a care in the world as to what is happening with these soundwaves. I just wanted to get my music to tape and make it sound ok. But to master good recording techniques, one must first understand the basic properties and principles of soundwaves.

First of all, sound is created and it begins to travel away from the source. Think of these sounds as disturbing the air pressure through which it moves. These disturbances travel in waves which consist of a compression (higher than normal air pressure) and a rarefaction (lower than normal air pressure). Following that, the compression is the high part of the wave, and the rarefaction the low part. Each compression and rarefaction make up one cycle, the length of which is called the wavelength. Our eardrums register these waves by being pushed in on a compression wave, and out on a rarefaction.

Soundwaves have two specific properties that we can relate to in everyday recording: frequency and amplitude. The frequency of a soundwave can be defined as the number of these cycles passing a specific point in a period of time. The faster the wave is moving (vibrating), the more cycles pass by and this results in a higher frequency. These have more energy since they are moving faster, as opposed to lower frequencies which have longer wavelengths, and hence less energy. Frequency is denoted by the units Hz, or Hertz. The human hearing range is between frequencies of 20Hz and 20kHz (20,000Hz). However, our ears are most sensitive to frequencies between 2kHz and 4kHz.

Amplitude also represents the energy level of a wave. Amplitude is how loud a soundwave is, characterized by the height of the compressions and rarefactions. The higher the waveforms, the more energy they have. Amplitude (volume) is measured in dB, or decibels, and the human hearing range in decibels is -1/4dB at the quietest to 140dB, where the sound actually becomes physically painful to listen to.

People often have a hard time understanding decibels at first because decibels aren't true numbers but really a ratio based on a logarithmic scale. It's not as if you can relate decibels to each other as you can with frequencies, which are arranged on a linear scale (for example from 20Hz to 21Hz to 22Hz), having equal distances between each other. The energy between each decibel increases or decreases logarithmically, and to be quite honest, I don't want to get into explaining that kind of math here because I may have confused you enough already. Just remember that decibels are ratios!

. This leads us into another term that you will see when looking at microphone specifications. This would be SPL, or Sound Pressure Level, and it is also measured in decibels. This is a measurement of the intensity of a sound. A microphone's diaphragm is described as being able to handle a certain amount of sound pressure energy (SPLs) before it distorts. The higher the SPL rating, the more sound pressure it can handle.

Phase is another topic involving soundwaves that I want to cover. Imagine that you have two separate soundwaves of equal frequencies. Each one is vibrating independently of the other. Phase, then, is the difference in the timing between these two waves...meaning where the waveforms are rising and falling when compared to each other. One may be on its highest point and the other at its lowest. With this example, these two waves would then be considered "out of phase," specifically 180 degrees out of phase. They would be "in phase" if they were both at the same point at the same time. We can also say that two waves are 90 degrees out of phase with each other, so that one is at its highest (or lowest) point when the other is halfway between. However, 90 and 180 are the only two degrees that are ever really used.

This finally brings us to nodes and antinodes. These involve a mix of the original soundwave, and the original's reflected soundwave. When the two waves mix and are 180 degrees out of phase, they cancel each other out and create a pocket of silence called a node. An antinode is where the original and reflected wave are in phase with each other. The two waves serve to reinforce each other and as a result, the volume is increased.

I am sure that we have all walked into a room where some loud music was playing and noticed as you walked around that there were places in the room that the music seemed louder and softer. Those are nodes and antinodes. If you have not noticed, give it a try. You might be surprised!

By the way, I forgot to mention that when two waves meet in the same space, this is called interference. Nodes and antinodes are types of interference. A standing wave is a type of antinode which keeps getting reinforced due to a rooms shape and it's ability to reflect soundwaves. This knowledge comes in handy when you are trying to mic up a bass cabinet and you are getting strong volume levels from certain notes. You could try changing the position of the mic and amp to reduce reflections and eliminate the problem.

So these are the basic principles regarding soundwaves. Hopefully I was able to make these points clear to you, and that you now better understand the dynamic relationships these properties have with each other. Knowing and applying the basic principles of soundwaves will make you a better recordist, and I encourage you to take the time to build upon the foundational principles I've shared here.

Published in Recording Engineer's Quarterly and Alexander magazines with permission

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