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An Introduction To Resonance in Audio

When Galileo Galilei was experimenting with a pendulum back in the 17th century, he discovered that if he purposely pushed the weight at the end of a string right at the right time, it would continue to sway back and forth for as long as he gave it a tiny push at the right time.

It was he who discovered that you can create potential energy and kinetic energy by simply exerting a small amount of effort. It has been known as resonance since this phenomenon occurred. Resonance doesn’t just affect pendulums though – it affects everything. In fact, everything resonates frequencies. As a musician or producer, you’ll hear resonances as a ring, or a beautiful harmony, and resonance is the sole reason your instruments have tone, character, and depth.

Resonance can be a good and a bad thing… 

For instance, your room may be filled with resonances in the wrong places, causing audio recordings to sound bad if the sound isn’t dampened using acoustic foam or bass traps.

However, resonance in recordings or instruments can be useful for tone or character, and removing all resonance in the mixing stage can be a bad thing if done incorrectly. EQ’ing resonance with sidechain or dynamic EQ is the best way to do this.

In this article, we’ll talk about the practical side of resonance, which is do to with room treatment. In this case, it’s a good thing. In part two, we’ll talk about EQ’ing resonance, and how to remove it properly without destroying your audio’s character and tone.

A History of Resonance

nikola tesla resonance

Tesla was an archetype of a mad scientist who invented alternating current, radio broadcasting, as well as the Tesla coil, which is the subject of many conspiracy theories.

This resonance principle was discovered several centuries later by another guy named Nikola Tesla, who dedicated his life to discovering its application. 

Tesla discovered that through the use of a pocket-sized mechanical oscillator, you could cause mass destruction to buildings, bridges, and other huge structures, just by resonating them.

This helped him to discover that everything in existence resonates.

The power of resonance lies in its ability to multiply force continuously, and a little bit of input energy can result in a great deal of output energy if resonated through a building or structure. For instance, all that is needed is a tiny piston-driven oscillator that propels tuned vibrations into say… steel foundations.

Tesla, despite not being the first to discover resonance, became obsessed with it and it’s functions for human development. He developed some of the most amazing demonstrations of it. 

Studying both mechanical and electrical versions of resonance, he managed to re-create earthquakes using resonance, artificial lightning storms, destroy an entire power plant in Colorado and almost entirely collapse a skyscraper, by resonating the steel frame.

The Importance of Resonance In Audio

There is resonance in everything. Resonance is why we hear rattles and vibrations when the subwoofer hits a certain note, and why the right audio frequency and loudspeaker can shatter a wine glass.

This works because you can send a sound wave through a glass, which resonates at the right frequency to vibrate the glass. 

Doing this at the correct resonance and interval, causes the sound waves to amplify each other, pushing and vibrating the glass more and more until it can no longer take the pressure and smashes.

This is the same for a ceramic bowl placed in the microwave, the microwaves can cause cracks and even smash the entire bowl due to the vibrations created by those waves.

So why does this affect audio?

Well… take a square room with walls, for example – sound waves propagate through the air molecules and are reflected off them.

These sound waves can amplify each other through each bounce of the wall because they are merging with one another. In the same way, the waves in the glass amplify each other, the sound waves do the same. This can create amplification at certain frequencies and cause horrible ringing noises.

Additionally, the waves can phase-cancel each other (a common problem in recording and mixing).

Take a snare, for instance, if it sounds weak, it’s a potential recording issue in which you would have to flip the phase because the two frequencies are clashing and taking away the power from each other.

Take seeing for an example too, we can detect electromagnetic waves through our eyes, which certain frequencies can pass through to detect vibrations that occur and come from space.

When we are on earth, we can detect these vibrations through the passage of our bodies, where electromagnetic waves radiate outward and travel infinitely.

We are actually tuned to the atmosphere because the visible spectrum of light corresponds to a spectrum of frequencies that pass through the filter of the Earth’s atmosphere. (Interestingly, our eyes are actually tuned to the atmosphere). 

All of our senses can be thought of as vibration sensors that are tuned to different frequency bands.

This is the same for hearing. Our ears vibrate from sound waves being sent to the ear drums. 

These vibrations are processed by a bone that moves around the inner ear to wiggle fluid, which stimulates small hairs and then transmits signals to the brain, which transfers that information into what is known as sound.

This then begs the question… does sound exist without ears to transfer that information? 

We’ll leave that philosophical question for you to decide, but without anything to receive and convert the sound, surely you could say it doesn’t exist.

There are an infinite number of resonances happening in every single system in the universe. One can think of it like this: your body is composed of cells, and all of those cells have their own resonance frequencies, which naturally vibrate at certain frequencies.

In the same way that organ pipes, flutes, or guitar bodies all have a collective resonance frequency, so does your body. Each different system will resonate at a certain frequency or vibrate at a certain frequency. 

This is why there is the interesting phenomenon of the “brown frequency”, which is a frequency that can be used to make someone defecate themselves.

However, this was Myth Busted in 2009 and the following was reported during a test: 

“The test subjects all reported some physical anxiety and shortness of breath, even a small amount of nausea, but this was dismissed by the hosts, noting that sound at that frequency and intensity moves air rapidly in and out of one’s lungs. The show declared the brown note myth busted”

A series of progressively quieter overtone frequencies within the system are also vibrating in resonance, but at lower amplitudes, which are referred to as harmonics.

This applies to acoustics and sound design because when one or more frequencies are added together, they create an entirely new waveform with overtones. 

In synthesis, this is how new waveforms are created. You can use multiple sine waves at different frequencies to re-create a saw, square, triangle, and many other waveforms found in modern synthesizers.

How To Calculate An Object’s Resonance

When you put the object next to a speaker and a microphone attached to an oscilloscope next to the object, you are going to be able to identify the resonant frequencies most easily. 

The speaker should play a tone at a certain volume and then gradually change the pitch (or frequency) without increasing the volume.

You will notice that when you are watching the oscilloscope, you will notice that the amplitude of the wave at certain frequencies, which is inversely proportional to the volume of sound being picked up by the microphone, is significantly greater than it is at surrounding frequencies. 

This is when the sound energy absorbed by an object is re-emitted more efficiently at these pitches, and these are the resonant frequencies.

If you prefer, you can perform the same procedure in a low-tech way, although it may not be as precise. 

Try holding a large bowl or a coffee can, or any other object with a resonance that you have been hoping to create, in front of your face. Slowly sing a tone with an increasing pitch while holding the object in front of your face.

You should be able to hear the tone that is emitted back to your ears at a particular pitch if there is a resonant frequency in the audible range. 

The strings of a piano will vibrate when you sing their resonant frequencies into them. If you happen to have one available to you, try singing into it and you will see the strings vibrate.

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