Fundamentals Of Sound For Audio Engineers

Understanding how sound travels and how it behaves is critical for audio engineers and sound engineers. If you understand the fundamentals of sound and use them to develop your thinking patterns as a sound or audio engineer, you will always benefit. Your mixes will translate well, and you will always have a solid and logical reason as to what decisions you make during a mixing or tracking session.

In this article, you will learn the fundamentals of sound that are important to understand for sound and audio engineers.

Definition Of Sound

In physics, sound is a vibration that travels through a transmission medium such as a gas, liquid, or solid as an acoustic wave. The sound may be heard as it moves in waves through the air, water, or other objects.

How Sound Propagates

All of us have thrown a pebble in a pond and seen the water move in concentric circular waves. There are waves formed above the water surface as well as below the water surface.

Similarly, sound moves in the air. Sound forms crests(The forward/positive displacement) and trough(the backward/negative displacement) when propagating in air.

For sound to propagate, it requires three basic elements:

1. Source Of Sound:- From where the sound originates. It can be a person speaking or music playing in the speaker.
2. Medium Of Sound:- Through which the sound travels. Air acts as a medium for sound to travel. Sound can also travel through liquids or solids as a medium.
3. Listener:- Someone who is listening to the sound. It can be a person listening to a lecture or music.

Important: As sound cannot propagate without a medium, there is no sound propagation in outer space due to the vacuum.

Characteristics Of Sound Wave

The important characteristics/ properties of sound that are important to understand as an audio engineer or sound engineer are:

1. Frequency: The frequency of sound is the number of repetitions of a sound wave per second. The higher the repetitions, the higher the frequency.
2. Pitch: Pitch is the quality of sound that distinguishes an acute (or strident) note from a grave or flat note. A higher frequency note has a higher pitch than a lower frequency note. 
3. Wavelength: One of the more simple acoustic topics to grasp is wavelength. It's only the length of a wave from one peak to the next.
4. Velocity: The velocity of the wave, sometimes referred to as the speed, is the amount of distance in meters per second that a wave travels in one second. 
5. Amplitude: The amplitude of a sound wave is a measurement of the wave's height. The higher the amplitude, the louder the sound. The loudness of a sound wave can be described as the maximum displacement of vibrating particles of the medium from their mean position at the time the sound is produced. 
6. Reflection Of Sound: Just like light, Sound bounces off when it hits any solid or liquid surface.

Frequency

The first property of sound that we need to understand is the frequency of sound. The frequency of sound is the number of repetitions of a sound wave per second. Frequency is measured in Hertz (Hz). The higher the repetitions, the higher the frequency. Human ears can hear sound waves that have a frequency between 20 HZ and 20 kHz.

Sine waves are pure tones that can give our ears a good idea of how different sound frequencies sound.

A sine wave has different sound characteristics depending on the frequency of the wave. Lower frequencies are more bass-rich. As the frequency increases, the pitch increases, and the sound turns towards the treble.

Below is a sound frequency and characteristics chart for your reference.

Frequency Range	Sound Characteristic
20Hz—40Hz	Sub Bass
40Hz—150Hz	Bass
150Hz - 250Hz	Upper Bass
250 Hz - 2.5 kHz	Mids
2.5 kHz-5 kHz	Upper Mids
5 kHz-20 kHz	Highs/Treble

Different music instruments generate sounds of different frequencies and different timbers. The different construction of instruments and variety of materials used, result in different fundamental frequencies and harmonics. 

The fundamental frequency of a vibrating item is its lowest resonant frequency.

Harmonics are frequencies that are multiples of the fundamental frequency.
The higher harmonics of a fundamental frequency of 50 Hz are 100 Hz, 200 Hz, 300 Hz, 400 Hz, and so on.
The harmonics of a 500 Hz fundamental frequency are 1000 Hz, 2000 Hz, 3000 Hz, and so on.

The different combinations of fundamental frequency and harmonics are one of the reasons why we hear different tones and timbers of different instruments. If the fundamental frequency of an instrument is below 150 Hz, it will have more low-end characteristics. If the fundamental frequency is between 150 Hz - 2.5 kHz it will have a more mid-rich sound. If the fundamental frequency is above 2.5 kHz the main sound character will be treble.

It is important to understand that nearly all instruments will produce frequencies that will be spread across the frequency spectrum. E.g., a Kick Drum will have its fundamental frequencies in the region of 60 Hz - 80 Hz but will also have frequencies present in the 3 kHz region. Similarly, a piano can produce frequencies across the audible frequency range. Different notes produce different frequencies. 

Pitch

Pitch is the quality of sound that distinguishes an acute (or harsh) note from a grave or a flat note. In music, the phrase 'pitch' is frequently used. It is determined by the frequency of the sound wave. A higher frequency note has a higher pitch than a lower frequency note.

People often confuse pitch and frequency. Frequency is a physical quantity that gives the number of vibrations per second, whereas pitch is a perceptual value that is dependent on the listener; in reality, our brains perceive sounds in pitch rather than frequency.

E.g., if someone plays the note middle C on the piano and then plays the note High C, the listener will be able to tell which note is higher in pitch. But if you ask the same person at what frequency the notes are played, the person will be clueless.

Once you understand Pitch, you should also understand Octaves. In music, an octave is an important interval. An octave is twelve semitones apart in music, and it represents a doubling or halving of frequency. In an Octave, the higher-pitched note will have double the frequency of the lower-pitched note and vice versa.

E.g., If you play the middle C on the piano and then play the next C, higher to middle C. The higher C will be exactly double in frequency. Similarly, if you play the middle C and then play the C, lower to middle C, it will be half the frequency of middle C.

Octaves have the same tonality and characteristics, just the pitch changes.

For music producers, understanding music theory and intervals is essential to write and compose more efficiently. As an audio engineer, an understanding of frequencies and octaves will help you make better mixing decisions. You can check out the below-linked articles if you wish to learn more about music theory and rhythm.

Suggested Read: Music Theory Basics For Beginners

Suggested Read: Understanding Rhythm In Music

Wavelength

Wavelength is simply the length of a sound wave in one complete cycle, i.e. from one peak to the next peak in any given cycle. Wavelength is a relatively easier concept to understand as it simply denotes the length of a sound wave.

The wavelength of a 20 Hz sound wave in the air at room temperature is nearly 56 feet, 200Hz is 5.6 Feet, 2000 Hz is 0.56 Feet and 20000 Hz 0.06 Feet.

So as the frequency of sound wave increase, the wavelength of sound waves decreases.

The wavelength of sound is another important concept that is important to understand for audio engineers. It helps you visualise and understand how different frequencies are produced and behave acoustically. Lower frequencies are longer and require more power to generate.

A 20Hz sound wave requires 56 Feet to complete just one cycle. So if you are in a studio, you will require at least 56 feet long room to completely produce 20Hz. Also, due to longer wavelengths and more power needs, low-end frequencies act as a carrier for mid and high frequencies. These are a few significant reasons why properly managing low-end is crucial for great mixes.

Velocity Of Sound

The velocity of the wave sometimes referred to as the speed, is the amount of distance in meters per second that a wave travels in one second. It is also an essential factor on which the behaviour and wavelength of sound depend.

At 20 degrees Celsius (68 degrees Fahrenheit), the speed of sound in air is approximately 343 metres per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 km), or one kilometre in 2.9 seconds or one mile in 4.7 seconds.

Velocity is highly influenced by temperature as well as the medium through which a sound wave travels.

The speed of sound at 0 °C (32 °F) is approximately 331 m/s (1,086 ft/s; 1,192 km/h; 740 mph; 643 km).

The speed of sound varies as the medium changes.

Amplitude and Loudness

The amplitude of a sound wave is the measurement of the wave's height. In sound, amplitude refers to the magnitude of compression and expansion experienced by the medium through which the sound wave travels. The amplitude of a sound wave is also defined as its loudness, or the amount of maximum displacement of the medium's vibrating particles from their mean position when the sound is produced. Our ears perceive this amplitude as loudness. Loud sounds are associated with high amplitude.  The greater the amplitude, the greater the energy.

The first unit that was used to measure the amplitude of sound was TU (Transmission Unit), later renamed Bel. Bell Telephone Laboratory used TU to quantify the reduction of audio level over one mile (1.6 km) length of standard telephone cable.

Today, dB or Decibel is used to measure amplitude or loudness. Decibel is a measure of the ratio between two quantities. It is a logarithmic unit and is used to express relative output sensitivity.

Different dB measurements units are used for different purposes. Acoustics Measurements: dBSPL, Electric Measurement: dBV, dBu, Radio Measurements: dBm, dBW, dBk, Digital measurements : dBFS.

So if you are working in a digital format, i.e. in a DAW you will mostly be concerned with dBFS i.e. Decibel Full Scale.

If you are working with only analogue sound, i.e. using a mixer without any digital conversion, you will be concerned with dBV and dBu. Both represent voltage. 1 dBV reference is Decibel relative to 1 Volt. 1 dBu reference is Decibel relative to 0.775 Volts

We will learn a lot more about dB in a different lesson, as it is of vital importance for sound and audio engineers.

Reflection Of Sound

Sound waves bounce back when they hit a surface. This is referred to as sound reflection. 

Sound waves, like light waves, are subject to the laws of reflection. 

Sound bounces off the medium's surface, which might be solid or liquid. The surface might be large and either rough or polished for sound to be reflected.

Laws of Reflection of Sound

• The angle of reflection is always equal to the angle of incidence.
• The reflected sound, the incident sound, and the normal sound belong in the same plane.

Reflection of sound is one of the primary reasons why sound studios require acoustic treatment. Reflection of sound is also the cause of phenomena like echo, reverb etc.

Now you understand the fundamentals of sounds and their characteristics. If you have any questions or comments please do post them in the comments section.

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