Schlieren’s techniques show patterns of the propagation of exhaled air by wind instruments and singers

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The aerial spread of pathogens took on great importance in the public eye after the onset of the 2019 coronavirus disease (COVID-19 pandemic). In an interesting new research paper published recently on the bioRxiv * preprint server, scientists describe the dispersal of exhaled, potentially infected air from singers and woodwind players, using Schlieren techniques, a visual process that is used to photograph the flow of density fluids variable. This could help assess metrics to assess the actual spread of infectious droplets or aerosols in such situations.

It is now known that droplets and aerosols,> 5 µm and

Previously, several studies concluded that the propagation of such particles is almost zero at 0.5 m from the mouth of a professional singer, as indicated by the presence of only minute disturbances observed at the level of a candle flame placed at this distance from the source of exhaled air. . Later, it was observed that the exhalation of air is much more forced during professional singing than during speaking or breathing.

With wind instruments, the air escape pattern is similar to that of vocals, the propagation distance being determined by the speed at which the air escapes from the mouth or instrument and the diameter Release.

The present study applies flow visualization and anemometry techniques to study the dispersion of exhaled air in terms of the pattern of propagation and rate at which the air escapes. Scientists used two methods to observe the flux, namely schlieren imaging using a schlieren mirror and a background oriented schlieren (BOS).

Schlieren refers to a method of photography applied to the visualization of flux of varying density by exploiting the curvature or refraction of light rays as they pass through an interface separating two substances of different densities.

The advantages of these techniques are the possibility of observing density gradients in transparent media, due to variations in temperature or pressure, without distorting the flow field. The measuring field of the schlieren imagery is limited by the size of the mirror, i.e. 100 cm. To properly visualize the spread of exhaled air beyond these limits, the BOS was used.

Breathing air is warmer and more humid than ambient air, resulting in gradients that can be captured by these techniques. Researchers looked at wind instruments, which release air in an initial laminar pattern followed by turbulence, and eventually mix with ambient air. With singers, air spreads most at the start of sound production and is highest when singing consonants or when precise articulation is required.

Researchers observe that the distance at which exhaled air travels and the angle at which the air escapes are both different with the instrument and the player, or singer.

Installation of the single mirror schlieren system (left) and the BOS system (right) at the Department of Building Physics of the Bauhaus University in Weimar

Installation of the single mirror schlieren system (left) and the BOS system (right) at the Department of Building Physics of the Bauhaus University in Weimar

Wind instruments

With wind instruments, air escapes from the bell, from the tone holes, and is blown (flutes) or leaks near the mouthpiece (with the oboe or bassoon). Playing the oboe or bassoon also requires intermittent exhalation through the mouth and nose, as not all air can escape from the tone holes. Air moves fastest when treble is used, but also when exhaling intermittently. With the latter, the speed decreases steadily thereafter.

Convective flow can also occur, representing air movements of about 0.02 m / s at 85 cm from the bell. It is the most distant sensor. With the most proximal sensor, the highest velocity is observed at 45 seconds, corresponding to very transient jets produced by large emissions of breathing air.

Air escapes from the bell over much shorter distances than air escapes from the instrument at various points, or during intermittent blowing, and other sound production practices. Air leaks can travel approximately two feet into the room due to the intermittent exhalation of air through the mouth and nose between two sentences. However, it moves within 30cm when reading various notes. At highs, air barely escapes from the bassoon bell, while the greatest airflow velocity of the bell is in the lower notes. Since most of the key holes are discovered in high notes, these produce maximum airflow from these holes.

The air escaping from the bell travels different distances depending on the width of the bore and the breathing pressure when playing.

Brass

With brass instruments, schlieren imagery shows that with most of these instruments the air escaping from the bell is very turbulent due to the larger diameter of the bell. The air blown into the mouthpiece blows into the bell.

Breathing air rises either by natural convection or mixes with the ambient air. Factors that determine the shape and distance of the air escaping from the bell include the musician’s physique and blowing technique, as well as the angle of the instrument to the mouth. Distances were measured from the bell, mouth, or mouthpiece. Breathing air comes out of the bell about 25 cm at low sounds and a little more at highs. Air can escape from the mouthpiece when the musician’s lips are tired, when playing choppy, or when musicians are untrained or older.

With the use of shock absorbers, air leakage is greatly reduced, except with the F tuba and French horn when a stop mute is used.

The anemometric results confirmed the results of the Schlieren visualizations, showing that the flow values ​​are always above about 0.02 m / s. The reasons may include movements of the fingers or hands during play, air leakage from the key holes, breathing between musical phrases, or other convective airflows in the same room. With some instruments, the measured speed first decreases as the distance from the instrument increases, and then begins to increase. This effect may be due to turbulent flow, producing small vortices which cause a variation in speed.

To reduce such flow of all kinds of brass, the researchers said that a simple filter could be used, made of cellulose and glued to the bell of the instrument. This will work because the air that is breathed through such instruments escapes entirely through the bell.

With wind instruments, air escapes from tone holes and even leaks from the mouthpiece, in addition to the bell. A filter will therefore not interfere with the diffusion of air.

What are the implications?

These data could help uncover the range over which exhaled air, potentially containing infectious particles, could spread during outbreaks of airborne infectious diseases. However, studies only show the range of propagation of the largest droplets, as small droplets or aerosols are not visualized by Schlieren’s methods. These results show that the air flow does not travel more than 1.2 m in the room.

Second, these models relate to the air blown by professional singers and musicians. Hobbyists and learners can produce very different exhalation and leak patterns, which can cause more air to diffuse into the room.

The movement of the player can also change the speed of the breathing air, which also varies with the diameter of the bell and the breathing pressure. Air escaping from the mouth or leaking at the mouthpiece shows a higher velocity of up to 0.15 m / s.

Using this data, the range of propagation, the dimensions of the escaping air and the speed at which it escapes and propagates can be estimated for woodwinds, brass instruments and professional singers. . This would help quantify the risk of viral transmission during such performance in order to develop the best safety precautions for such situations.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.


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