Everyone knows the old trope where a powerhouse opera singer hits the right note and a crystal glass shatters from the noise, but is it really possible? The situation might seem far-fetched, like something you’d be much more likely to see in movies or cartoons than in real life.
In fact, the phenomenon of resonance means it is technically possible in real life, whether the resonant frequency (the one that matches the natural frequency of the glass) is produced by somebody’s voice or by one or many musical instruments.
Learning more about resonance gives you an understanding of how sound works, the principles underpinning many musical instruments and how to increase or decrease motion in a mechanical system like a swing set or a rope bridge.
Definition of Resonance
The word resonance originally comes from the Latin resonantia, meaning “echo,” and it is closely related to the resound, which means to return an echo or “sound again.” These two definitions already relate to sound waves and give you a basic picture of the meaning of the word in physics too.
However, more specifically, the definition of resonance in physics is when the frequency of an external oscillation or vibration matches an object (or cavity’s) natural frequency, and as a result either causes it to vibrate or increases its amplitude of oscillation.
In mechanical systems, resonance refers to the amplification, reinforcement or prolongation of sound or other vibrations. Just as in the definition above, this requires an external periodic force to be applied at a frequency equal to the natural frequency of motion for the object, which is sometimes called the resonant frequency.
All objects have a natural frequency or resonant frequency, which you can think of as the frequency the object “likes” to vibrate at. For example, if you tap a crystal glass with a fingernail, it will start to vibrate at its resonant frequency and will produce a “ting” with a corresponding pitch. The frequency of vibration depends on the physical properties of the object, and you can predict this pretty well for some things like a taut string.
Examples of Resonance – Sound Resonance
Learning about some examples of resonance will help you understand the various forms of resonance you encounter in your day-to-day life. The most common and simplest example is sound waves, because when you vibrate your vocal cords at the right frequency (for the cavity of your throat and mouth), you can produce speech tones and musical tones that other people can hear.
The vibration of your vocal cords produces the sound waves, which are really pressure waves in the air composed of alternating compressed sections (with a greater than average density) and rarefactions (with a less than average density).
Most musical instruments work in the same way. For example, in a brass instrument, the vibration of the player’s lips against the mouthpiece creates the initial vibration, and when this matches the resonant frequency (or a multiple of it) for the size of the pipe he or she blows into, there is resonance, and the amplitude of oscillation increases notably and produces an audible tone.
In woodwind instruments, there is a “reed” that vibrates as air is passed over it, and again the same process of resonance and amplification turns this small vibration into an audible musical tone. String instruments like a guitar are a little different, but the strings have a resonant frequency of vibration, and the sound waves produced resonate in the cavity (e.g., in the space in the body of an acoustic guitar) to make the noise louder.
A simpler example is when you drop a tool or a plate on the ground. The clang produced is caused by the tool or plate vibrating at its resonant frequency. This simpler way to generate sound is used by carefully designed tuning forks, which are designed so as to produce a specific pitch as their natural frequency, which musicians can then tune their instruments to.
Examples of Resonance – Mechanical Resonance
Although resonance is usually used to refer to sound waves, mechanical resonance is in some ways easier to understand. A simple example is a child learning to pump a swing for the first time. The oscillatory motion of the swing has a natural frequency, and when the child learns to push (i.e., apply a periodic force) at the swing’s natural frequency, their pushing becomes much more effective. As a result of this, the amplitude of oscillation of the swing increases and the person sitting on it goes higher each time.
Hitting the natural frequency of an object isn’t always a good thing, though. For example, soldiers marching across a rope bridge in unison could cause it to vibrate out of control and possibly even collapse if they step at its natural frequency. In cases like this, the general might ask them to “break step” so they aren’t applying a periodic force at the natural frequency of the bridge.
Even more stable bridge designs have resonant frequencies, but this only causes an issue in rare causes (such as with the Broughton Suspension Bridge, a bridge in England that collapsed in 1831, supposedly due to soldiers marching in step across the bridge).
Analog clocks also depend on mechanical resonance and the natural frequency of a component to keep time. For example, pendulum clocks use the natural frequency of the swing of the pendulum to keep time, and a balance wheel works on the same basic principle. Even quartz crystal clocks depend on resonance frequency, but in this case the crystal regulates the oscillation from an electronic oscillator, resulting in huge improvements in accuracy compared to simpler designs.
Other Examples of Resonance
There are many other forms of resonance too, and all of them work on the same basic principle. Two other examples of resonance you’ll be familiar with have to do with electromagnetic oscillations rather than mechanical ones. The first is your microwave.
The waves produced by the microwave produce heat in your food because their frequency matches the resonance frequency of the molecules inside the food (e.g., water and fat molecules), which causes them to wobble and subsequently release energy in the form of heat.
Another example is the antenna for your TV or even a radio antenna. These devices are designed to maximize the absorption of electromagnetic radiation, and when you “tune” the antenna to a specific frequency, you’re adjusting the resonance frequency for the device. When the frequency of the antenna matches the frequency of the incoming signal, it resonates and your TV or radio “picks up” the signal.
So How Does the Crystal Break?
Now that you understand the key points about the definition of resonance and what a resonance frequency is, you can understand the classic example of a singer managing to break a crystal glass by singing at the right pitch. The glass has a resonant frequency, and if the singer produces a sound with a matching frequency, the glass will begin to vibrate. This is called a sympathetic vibration because prior to the singer making a noise, the glass was completely still.
At first, there may be a small vibration in the glass, but actually making it shatter requires a sustained and loud note at the right frequency. If the singer can do this, the amplitude of oscillation of the glass increases and eventually starts to compromise the structural integrity of the glass. It’s only at this point – when the note has been sustained for long enough for the glass’s vibration to reach the maximum amplitude it can support – when the glass will actually break.