What is sound, and how does it travel?
Acoustics is the study of sound. But what exactly is sound? When you tap on a table or shout “Hello,” sound waves are created. Sound is a vibration that travels as a wave through a medium, usually air. For example, when a speaker plays music or you clap your hands, the surrounding air is set in motion. These sound waves spread out in all directions from their source—similar to the circular ripples on the surface of water when you throw a stone into it. The ear picks up the waves, and the brain processes them into the sound we hear.
Important: Sound needs a medium such as air, water, or a solid material. Sound cannot travel in a vacuum.
What affects sound?
Sound does not always travel in the same way. Several factors influence its propagation:
Frequency (pitch):
High-pitched sounds behave differently from low-pitched sounds. High frequencies (e.g., the chirping of a cricket) are usually attenuated more quickly in air than low frequencies (e.g., distant rumbling thunder). That is why bass sounds can often still be heard from a greater distance. Walls also allow low bass frequencies to pass through more easily than high-pitched sounds.
Medium and Temperature:
The speed of sound depends on the medium. In air, it is about 343 m/s (at around 20 °C). In water, sound travels much faster, at about 1,500 m/s. This is why whales and dolphins can communicate over long distances. Sound also travels quickly through metals. In addition, warm air usually carries sound farther than cold air, since particles move faster in warm air—so temperature also influences sound propagation.
Did you know?
Blue whales produce extremely loud and low-frequency sounds—some of which fall below the human hearing threshold. With these infrasound calls, they can communicate over distances of 1000 miles. For comparison: That is the straight-line distance from New York City to Miami.
Obstacles and Reflection:
When sound waves hit an obstacle, they are partially reflected or absorbed. Hard, smooth surfaces such as walls or rocks reflect sound, creating an echo or reverberation. Soft, porous materials (e.g., curtains, carpets, or upholstered furniture) dampen sound. That is why sound reverberates more strongly in empty rooms than in furnished ones. Forests and tree canopies also often act as natural sound absorbers.
In summary:
Sound propagates as a wave in all directions, requires a medium, and is altered by environmental factors. This explains why a walk in the woods often feels quieter than a stroll through the city.
Spectrogram – What does a spectrogram show, and how does it work?
What does a sound look like?
A spectrogram provides the answer. It is a visual representation of a sound signal and shows which frequencies occur at which times and how strong they are. The horizontal axis usually represents time, while the vertical axis represents frequency (pitch). Intensity is represented by brightness or color. This makes invisible sound visible as an image.
An example:
If a bird utters several calls, multiple parallel lines or bands appear in the spectrogram. They show which pitches are present in the song. A short, loud whistle appears as a distinct line at a specific time and frequency. In ornithology, spectrograms are frequently used to analyze bird songs. Many species have a typical pattern, an acoustic fingerprint. Experts and trained AI models can use this to distinguish between species.
What is this useful for?
Spectrograms are used in many fields: in music and speech research, in animal research (e.g., whale songs and bird calls), and in technical analysis. In bioacoustics, they are particularly important for comparing animal sounds and discovering new sound patterns. Researchers compare unknown recordings with known patterns. AI systems for bird song recognition also use spectrograms to identify relevant features. Simply put: this turns “invisible” sound into a clear visual representation.
A spectrogram thus answers the question: Who is singing what and when?
It shows which frequencies occur at what times and how strong they are. This makes it possible to determine whether low or high tones dominate, whether a sound consists of individual clicks or continuous noise, and whether multiple sounds occur simultaneously. In the spectrogram of a forest recording, for example, you would see if a low-pitched frog was croaking and a cricket was chirping at the same time—two different frequency bands occurring simultaneously. You can hear this, of course, but the image makes it much clearer.
In summary: A spectrogram is a time-frequency map for sound. It helps to visualize, analyze, and compare sounds. Whether in biology, music, or engineering: Many fields gain insights from it that are difficult to detect with the ear alone.