In the guest blog, physicist Andrea Navarro-Quezada shows the importance of sound waves for our everyday life and research.

Almost all of us know the concept of a wave. It is a periodic motion or oscillation that can be observed, for example, on the surface of water in a pond (mostly if ducks swin in it), in the sea, or in a glass of water.

In physics, one distinguishes between two types of waves: those that require a medium to propagate (water, air, etc.), and those that can also propagate in vacuum. Light, or electromagnetic waves, belong to the second category. Sound waves to the first.

A wave has an amplitude - or height - and a frequency. The frequency describes a periodic event, such as the shift between day and night due to the earth's rotation around its own axis, or the heartbeat in the body (measured in beats per minute). Normally, the frequency is measured in hertz and this tells how often a process repeats itself in one second.

Sound waves in everyday life

We are surrounded by sound waves every day: music, nature sounds, speech, etc. These sound waves are created by the vibrations that generate changes in air pressure which propagate through air. The speed of a sound wave in air is 343.5 meters per second. However, this value depends on the temperature and the medium in which the sound propagates. For example, the speed of sound in water is 1400 meters per second.

Our vocal cords work analogously to the strings of a guitar: the taut muscles in the larynx cause them to vibrate, producing the sounds that make a person's voice. The higher the tension in these muscles, the higher the frequency and the tone. Actually, the word "tone" comes from ancient Greek and means tension.The human ear is sensitive to frequencies in the range from 20 to 20,000 hertz. Sound waves with a higher frequency are called ultrasound.

Like light waves, sound waves can be absorbed or reflected. Most of us have experienced this unconsciously at some point in our life. For example, when talking in an empty room. The sound waves are reflected off the walls and you hear a reverberation. If there are furniture and plants in the room, the sound waves are reflected and absorbed by the different objects. Thus, the reverberation disappears. This is what we often call the "acoustics" in a room.

Applications of sound waves

The principle of reflection of sound waves has already wide applications. One example is the sonar to locate submarines in the ocean: the sound waves are reflected by the submarine and are measured by a detector. The time it takes for the transmitted sound waves to be reflected back to the detector is used to determine the distance to the submarine. The farther away, the longer it takes for the reflected sound wave to return. The frequency range for sonars lies between 50 and 15,000 hertz.

A second example is sonography, which employs ultrasound waves with a frequency higher than 20 000 hertz for medical imaging. Here, the ultrasound waves, which penetrate centimeters deep in the body, are either absorbed or reflected by the tissue, muscles, organs and bones, depending on their thickness and density. When an ultrasound image is produced, the areas that allow the waves to pass through (for example, fluids) are dark and the areas that reflect them back are bright.

Novel research with ultrasound waves

Although ultrasound waves have been used in medicine for imaging for several years, new applications continue to be explored. As recently reported in The Lancet Oncology, ultrasound waves could also be used to improve the treatment of brain tumors called gliobastomas. To do this, a small device that generates pulses of ultrasound waves is used on the brain. At the same time, small micrometer-sized bubbles are injected into the blood vessels, which are brought into vibration by the ultrasound waves allowing to overcome the blood-brain barrier. Normally, this blood-brain barrier serves as a protective mechanism to prevent any substance from flowing through the brain. However, this also applies for some drugs, hindering in some cases the effect of chemotherapy. Thanks to this novel procedure a more efficient administration of chemotherapy drugs to fight gliobastomas will be available.

This shows that sound waves are very versatile and that despite the already well-established and known applications there is still a lot left unexplored. (Andrea Navarro-Quezada, 22.6.2023)