Acoustics I - What is Sound and How Does It Behave?
- Matheus Antunes
- Aug 13
- 4 min read
Stepping away from digital audio for a bit, because I feel like talking about acoustics.
In our day-to-day, we talk a lot about digital audio: plugins, DAWs, converters. But to truly master the art of recording and mixing, we need to take a step back and understand the material we work with in its purest form: sound in a physical space. Acoustics isn't a luxury; it's the foundation.
This post kicks off a new series here on the blog, dedicated to unraveling the principles of acoustics. Let's start at the beginning: what sound is, how it travels to our ears, and which physical phenomena dictate how we perceive it in an environment.
Quick Summary:
Sound is a Disturbance: It is a wave of compression and rarefaction of particles (like those in the air) that propagates from a vibrating source.
Energy in Motion: The particles of the medium vibrate, but it is the energy wave that travels, not the particles themselves.
Acoustic Phenomena: When sound encounters an obstacle, it can be reflected, absorbed, refracted, diffracted, or experience interference, drastically altering how we hear it.
The Physics of Sound: From Vibration to Perception
Sound, in its essence, is nothing more than a mechanical disturbance that propagates through a medium, such as air.
Think of a person hitting a table with their hand. Before the impact, the air particles in the environment are relatively stable. At the exact moment of the hit, the table's surface vibrates. This vibration "pushes" the nearest air particles, creating a high-pressure zone, or compression.
Immediately after, the table's surface returns to its original position, creating a low-pressure zone, or rarefaction (also called decompression).
This cycle of compression and rarefaction acts as a pulse that propagates, pushing neighboring particles and creating a wave that travels through space until it reaches our ears, where it is interpreted by the brain as sound.
Visualizing Sound Propagation
In this visual representation, we can see an output transducer (like the driver of a loudspeaker) vibrating. The air particles are represented by the dots.
When the speaker moves backward, it creates a zone of rarefaction, with fewer particles.
When it moves forward, it creates a zone of compression, with more particles.
It's important to note that, as the highlighted red dots in the animation show, the air particles only oscillate back and forth around their resting position. It is the energy wave that travels through the medium, not the particles themselves, and the path it travels varies according to the amplitude of the speaker's movement.
When Sound Meets an Obstacle
When this sound wave encounters an object, like the wall of a room, several physical phenomena occur simultaneously, defining the room's acoustics. The main ones are:
Reflection
Absorption
Refraction
Diffraction
Interference
1. Reflection When the sound wave hits a solid, rigid barrier (like a concrete wall or a pane of glass), a large part of its energy is reflected back into the environment, much like light in a mirror. The angle of incidence tends to equal the angle of reflection. This is the phenomenon responsible for echo and reverberation.
2. Absorption Part of the sound wave's energy is not reflected but is instead absorbed by the surface. This absorbed energy is primarily converted into heat, reducing the intensity of the reflected wave. Porous and soft surfaces, like rock wool acoustic panels, acoustic foams, or even a sofa, are designed to maximize absorption and "dry out" the sound of the room.
3. Refraction (or Transmission) The energy that is neither reflected nor absorbed passes through the obstacle; that is, it is refracted from one medium to another (from the air into the wall, for example). This is why we can hear the sound from an adjacent room. The sound wave makes the wall vibrate, and on the other side, this vibration generates a new sound wave in the air, albeit with much less intensity.
4. Diffraction This is the phenomenon that allows sound to "bend around" obstacles or pass through small openings. If a door is only slightly ajar, you can still hear the sound from the other side because the sound waves bend and spread out as they pass through the gap. Diffraction is more pronounced for low-frequency sounds (bass), which have longer wavelengths.
5. Interference Interference occurs when two or more sound waves meet at the same point in space at the same time. The result of this encounter can be:
Constructive Interference: If the peaks of the waves align, they add up, resulting in a sound of greater amplitude (a louder spot in the room).
Destructive Interference: If the peak of one wave aligns with the trough of another, they cancel each other out, resulting in a sound of lesser amplitude or even complete silence (a cancellation spot in the room).
This last phenomenon is the origin of the infamous standing waves, which cause peaks and nulls at specific frequencies depending on the room's geometry, and is one of the biggest challenges in the acoustic treatment of studios.
Understanding these five phenomena is the first step to stop being a "victim" of your room's acoustics and start being an agent who controls them. Every sound you hear in a room is the complex result of the interaction between the direct sound source and the countless reflections, absorptions, and interferences that occur in milliseconds.
In the next post in this series, I'll talk about wave theory—frequency, wavelength, and amplitude—so we can then move on to practical solutions for acoustic treatment.
Cheers and see you next time!