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Sound Engineering Basics – The Physics of Sound!

You are aspiring to be the next sought-after, well-known, highly acclaimed, ass-kicking (okay, maybe that’s too much) sound engineer right? But how much do you actually know about the most important thing that you will be working with, which is “Sound”? In today’s article, we will talk more about sound engineering basics, more specifically, the physics of sound!

I intend this post to be the start of a series of articles containing valuable knowledge and information, relevant to the world of sound. I hope this series will come across as an easy-to-understand guide to understanding the basics of sound and that people who are relatively new to this will have a solid grasp of the fundamentals. So, let’s not waste any more time, and start!

Sound – What is it exactly?Waves

Sound is such an important aspect of everybody’s lives and this is especially so, for people who work with sound professionally. Whether you are an engineer, working to produce the latest headphones, or a technician, working to maintain sound systems, you need to know the understanding of sound in physics.

To put it simply, sound is a vibration. It is described as a mechanical wave of pressure and displacement, that propagates through mediums such as air and water.

For our folks studying physiology and psychology, sound is described as the reception of such waves and their perception by the human brain.

A more accurate description of sound by the American National Standards Institute would be, “(a) Oscillation in Sound Wavespressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a).”

Having a headache yet? Okay maybe we should just stick with the former definition yeah?

How does it “travel” to our ears?

Human Ear

Photo by Chittka L, Brockmann / CC BY 2.5

When we talk about sound “reaching” our ears, technically we are talking about “propagation”. Sound propagates through compressible mediums such as air, water and solids as “longitudinal” waves and also as “transverse” waves (in solids). Essentially, sound waves are generated by a sound source (such as the vibrating diaphragm of a stereo speaker).

The sound source then causes vibrations to happen in the surrounding medium (in this case, air), and as the source continues vibrating, the vibrations will continue propagating away from the source at the speed of sound, thus forming the sound wave.

Sound Travel

Photo by Nzachariah3 / CC BY 3.0

In case you do not already know, different mediums (air, water, solid) are made up of particles, and that it is important to note that the particles of these mediums do not travel with the sound wave. Intuitively, this is an obvious fact for a solid medium, and the same theory applies for liquids and gases (that is, the average position of the particles over time does not change and they only serve as a medium of transportation for the vibrations).

When sound waves propagate, they can be reflected, refracted, or attenuated by the medium.  Also, the mechanical vibrations (also known as sound) are able to travel through all forms of matter (gases, liquids, solids, and plasmas). The matter in which the sound waves are propagating in, is called the medium. Also take note that sound cannot travel through a vacuum.

Types of Waves

As mentioned above, we have learned that sound can travel as two forms of waves:

  • Longitudinal Wave
  • Transverse Wave

Let’s discuss more about what these two waves are all about.

Longitudinal Wave 

Also known as “l waves”, the defining characteristic of this wave is that the displacement of the medium, is in the same direction as, or the opposite direction to, the direction of travel of the sound wave.

Longitudinal Wave

Photo by sciiat.wikispaces.com / CC BY-SA 3.0

Mechanical longitudinal waves produce compression and rarefaction when travelling through a medium. Due to this reason, it is also often identified as “compressional waves” or “compression waves”.

Longitudinal waves do not just include sound waves, but also seismic P-waves (created by earthquakes and explosions). The most important aspect of  longitudinal waves that you need to understand is that the displacement of the medium is parallel to the propagation of the wave.

To have a good understanding, try to imagine a wave along the length of a stretched Slinky toy, where the distance between coils increases and decreases. Also, take note that sound waves in air are longitudinal, pressure waves.

Transverse Wave

The main characteristic of this wave is that it consists of oscillations occurring in perpendicular (or right angled) to the direction of energy transfer (direction of the moving sound wave). Other than sound, light is also an example of a transverse wave.

The properties of transverse waves remains the same in matter (in which the displacement of the medium is perpendicular to the direction of propagation of the wave). A good visualization would be to imagine a ripple in a pond or a wave created on a string.

Transverse waves

Photo by cnx.org / CC BY 4.0

You can try experimenting on your own by anchoring one end of a ribbon or string and hold the other end in your hand, you can create transverse waves by moving your hand up and down. You will come to realize that you can also create the same waves by moving your hand side-to-side. These are the two independent directions in which this type of wave motions can occur.

Here’s a great video illustrating how sound waves travel!

How fast can sound travel?

A lot of times, people seem to think that their ears can detect sound almost immediately and don’t realize that it actually takes time for sound waves to reach your ear drums.

The speed of sound does not always stay the same and this largely depends on the medium that the waves passes through. The physical properties and the speed of sound is also affected by ambient conditions. For example, temperature plays a part in determining the speed of sound in gases. To further illustrate; in 20 °C (68 °F) air at sea level, the speed of sound is approximately 343 m/s (1,230 km/h or 767 mph), and this is calculated by using the formula “v = (331 + 0.6 T) m/s”.

Speed Of Sound

Using the same formula, you can also find out the speed of sound in fresh water; at 20 °C, the speed of sound is approximately 1,482 m/s (5,335 km/h; 3,315 mph). Similarly in solid matter such as steel, the speed of sound is about 5,960 m/s (21,460 km/h; 13,330 mph).

Okay, I’ll stop here for now (I know you guys probably regretted reading this right?). I hope this article have been very informative (especially to people who are new to this subject) and I will elaborate more on different aspects of sound in follow-up articles in this series! So stay tuned!

Thanks for reading and do leave a comment below!

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Farhan

When I'm not rocking out to great music, I'd prefer to be sleeping on a field on a windy day =)

4 Comments

  1. Hi,

    I am a home based musician. Mainly electronic music such Deep House, Tech House, Minimal and Techno.

    Although I gained a strong knowledge of everything related to music production I can’t call myself a music engineer (even thought I would like to).
    I learned everything by practising myself and it wasn’t easy I have to say. I am still learning every day but some thing are beyond my knowledge .

    The basics for a music producer is to know what is sound and you describe it in a way that’s easy to understand so thank you for that 🙂

    I am happy to have discovered your website because I see a lot of useful information that I will be able to use in my productions.

    I’ll be back for sure!

    Keep up the good work,
    Angélique

    • Hey Angelique, thanks for your comments.

      I too, for the most part, am a self-thought musician. So I definitely can relate to you on how hard it is to learn something through trial and error.

      However, it is great to know that you did not give up your passion and continue to produce music!

      I’m glad you got something out of this article, and be sure to come back any time!

      Cheers =)

  2. Greetings. What do you mean refracted or attenuated by the medium? I get the picture on sound waves being reflected but not the other two.

    I don’t take psychology but I am just wondering, what so important for psychologist to study such area?

    Talk about speed, the fastest between the three medium you illustrated is the one travel through solid matter?

    • Hello Tar, thanks for dropping by =)

      “Refraction” basically means that a wave (in this case, sound waves) changes its direction of propagation, due to the properties of the medium that it is travelling in.

      “Attenuation” basically refers to any loss in intensity of any physical property that is travelling through a medium (In this case, it is sound waves travelling through air).

      Yes, generally, sound waves travel faster in solids. This is due to the particles in solids being positioned very close to one another, as compared to air and liquid.

      The branch of science that is studying the perception of “sound” is called “Psychoacoustics”. This is applied to various branches of music psychology, where scientists basically research on how different kinds of music affects the human brain, and therefore could use this information for health treatment etc.

      I try my best to answer your questions to the best of my ability and knowledge. I do not claim to be an expert in this field in anyway whatsoever.

      Hope that helps =)

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