Understanding QCM-D basics through musical analogies
Fredrik Pettersson Jun 17, ’25 > 6 min

Understanding QCM-D basics through musical analogies

New concepts and technical principles are sometimes best explained using analogies that relate to something already familiar. Here, QCM-D technology is explained using a set of different musical instruments.

Key Concepts of QCM-D Technology

QCM-D is an acoustic technology, i.e. based on measurements of sound energy. Therefore, it is intuitive to use musical instrument analogies to explain the QCM-D working principle. To understand the fundamentals of the QCM-D technology, there are a few key concepts that must be understood:

  1. Crystal resonance and mass change: How the crystal resonance relates to the measurement of mass changes
  2. Energy dissipation: How energy dissipation is affected by the addition of soft layers
  3. Material properties characterization: how material properties can be characterized via acoustic analysis

Musical Instrument Analogies

In the webinar Basics of QCM-D, I explain these three concepts using the below instruments

  1. Guitar
  2. Church bell
  3. Electric organ

In brief, picturing these musical instruments, how they work and how they interact with their surroundings, we learn the following:

QCM-D analogy guitar

1) Guitar: a vibrating guitar string helps us understand the sensor resonance and the harmonics

When vibrating, the quartz crystal acts like a guitar string. It would be a short and thick guitar string, only ~0.3 mm long, but it behaves in the same way. To induce a tone, the sensor is stimulated with electricity. Like a guitar string, if the string is made longer, the tone will be lower and if the string is made shorter, the tone will be higher. It is the same for the QCM-sensor, i.e. if the sensor is made thicker, by adding material, the tone will be lower, and if it is made thinner, by removing or desorbing material, the tone will be higher. The QCM sound is in the MHz regime, but it is the same phenomenon as the guitar string.

QCM-D analogy church bell-1

2) Church bell: the analogy of a ringing church bell helps us understand the dissipation and the decay of the crystal oscillation

If you hit a church bell that is mounted in an undampened mounting, it will ring for a very long time. If the ringing motion is dampened, however, for example by some friendly tourists giving the church bell a hug, the sound will decay faster. You can think of the hugging tourists acting like an added soft film which dampens the sound signal. In that sense, the church bell can sense its environment. If it interacts with a rigid film, or no film at all, which would be the case when its is ringing in air, the dissipation will be low, and it will ring for a long time. If it interacts with a soft film, or if a friendly tourist gives it a hug, the energy will dissipate quicker and the ringing will be a short sound.QCM-D analogy electric organ

3) Electric organ: the different tones and sound qualities of an electric organ help us understand different thicknesses and different material properties

Since we are dealing with sound waves and acoustics, we can use physical formulas for how sound waves, i.e. pressure waves, distribute through films with different material properties to establish physical models that will help us understand the relation between the sound and the material properties. For example, the viscoelastic model describes how the pressure wave moves in a layer with certain viscoelastic properties. Measuring the sound, we can then use the model to calculate what the viscoelastic properties of the layer are.

With this model in mind, we can compare QCM-D to an electric organ. The organ can produce different tones of different qualities, and so can QCM-D. We measure the different tones and listening to the different harmonics is like listening to different qualities of the sound. So, the different keys, the different tones, of the electrical organ would correspond to different thicknesses of the film on the sensor, of the mass change. Add to this the possibility of changing the quality of the sound by using the different knob settings. For example, changing from a piano sound to a trumpet or saxophone sound. This would then represent different qualities of the material and how the different harmonics behave. So, in that aspect we can say that QCM-D is like an electric organ.

Learn more about QCM-D

With these analogies in mind, we can better understand the principles of QCM-D technology. To learn more about QCM-D, watch the webinar to delve deeper into the fundamentals of the technology, the theory behind it, and what you can measure and analyze.

Why it is useful to measure multiple overtones
Webinar

Watch the webinar to learn more about the working principles of QCM-D

Webinar

Basics of QCM-D

Watch

Editor’s note: This post was originally published in August 2020 and has been updated.

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