An Introduction to the Properties of Sound for Recording Musicians

An Introduction to the Properties of Sound for Recording Musicians

by Clay Butler

Obviously, if you're reading this article, you have an interest in recording.   I'm sure you also have some notion as to what sound is.  Sound is what we hear, right?  Well, yes.  But there’s a lot more to sound.  Knowing what sound is and how sound works is the key to getting a quality recording.  Let's get our feet wet in sound waves (pun intended).

WHAT IS SOUND?

Simply put, sound is Acoustical Energy, or vibration.  More specifically, sound is vibration propagated through a medium, which is then received by our ears and interpreted by our brain as sound.  The reason I say the vibrations are interpreted by our brain as sound is because acoustical energy still exists that we cannot perceive as sound (more on this later).  For example, while a dog might go nuts from hearing a dog whistle, we as humans cannot interpret those vibrations as sound.  Radio, cell phones, Wi-Fi, microwave ovens, and RADAR all use sound waves that we as humans just can’t hear.  So, in essence, when we refer to sound, we really refer to our perception of vibrations.

Sound vibrations that are propagated through the air (or any other medium for that matter) are in the form of transverse waves.  Thus, you could also say that sound is really rapid fluctuations in air pressure.  The vibrations from a vibrating body, such as a guitar string, push and pull on the surrounding air, creating positive and negative pressures.  These waves occur as the molecules of air are slammed closely together as they are pushed by the vibrating body.  The part of the wave consisting of positive pressure, where the air molecules are slammed together, is called a compression.  Negatively pressured parts of the wave, where the air molecules are pulled apart from each other, are called rarefactions.

THE PROPERTIES OF SOUND

How loudly a sound is perceived is determined by how hard the air gets pushed by the vibrating body.  The harder the air gets pushed, the louder the sound.  Although this is typically referred to as volume, in reference to acoustical energy, it is called Sound Pressure Level (SPL).  The scale used to measure Sound Pressure Level is the Decibel scale, or dB SPL (more on the other Decibel scales in a later article).

The pitch of a sound is a function of frequency.   How frequently a vibrating body pushes the air determines how high or low the tone of the sound is perceived.  The more frequently the air gets pushed, the higher the tone of the sound.  As the air gets pushed less frequently, the tone sounds lower.  Therefore frequency is expressed as the number of sound waves occurring over time.  The scale used is known as Hertz (Hz), which signifies the number sound waves per second.  For example, the note “A” below “Middle C” on a piano is 440Hz.

The frequency spectrum is broken up into three parts.  The Audible Range for humans is roughly 20Hz to 20,000Hz (or 20 Kilohertz, abbreviated as KHz).  The frequencies below 20Hz are categorized as Infrasonic.  All frequencies above 20KHz are referred to as Ultrasonic.

ACOUSTICS

Let’s look again at sound being all about our perception.  Generally, we don’t hear the sound emanating directly from its source.  The majority of the time, we hear sound after it bounces off the surrounding walls, along with any other nearby surfaces, and interacts with the room.  We call this acoustics.  Understanding how acoustics influence sound, especially those sounds you’re trying to record, better enables you to get the sound you want without any surprises.  Each time a sound wave is reflected back into an acoustic space, our perception of that wave changes, especially when you hear sound coming directly from a sound source in addition the reflected waves.  This is why it’s not uncommon for a pro recording studio to spend hundreds of thousands of dollars on architectural design and acoustic treatment, so that those extra reflections and wave interference are eliminated.

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