Myth busting the Myth Busters
Lately we have seen a number of videos trying to demonstrate that wind and brass instruments cannot possibly be implicated in spreading Covid-19. Typically, they use a candle flame, sheet of paper or a plastic ball, which is displaced by someone speaking but is undisturbed by an instrument, even when played up close. No air comes out, only sound. Soundwaves do not transmit virus. Compelling evidence, even conclusive. So we should be able to get back to playing these instruments now.
Why all the fuss then? Because these videos ignore two small things.
Physics
We’ll concentrate on brass instruments, since many of these videos do. Brass instruments are basically nicely wrapped pipes. The player blows air into one end. The air cannot escape the tube and there is nothing in the instrument to contain air at pressure, so it must come out the other end.
The reason a trumpet doesn’t blow out a candle is the Venturi effect. The player blows air under pressure into the leadpipe, which has a very small diameter. The airstream slows as the tube widens towards the bell. We can estimate the factor by which the airstream slows as it reaches the end of its travel through the instrument. The dimensions given here are diameters, not bores, and the figures ignore any friction losses but the principle is demonstrated.
| Instrument | Leadpipe diameter (mm) | Bell diameter (mm) | Area of leadpipe (sq cm) | Area of bell (sq cm) | Factor by which bell is larger | % of initial airspeed at the bell |
| Trumpet | 11.65 | 123 | 1.07 | 118.82 | 111 | 0.90% |
| Trombone | 12.7 | 204.4 | 1.27 | 328.13 | 259 | 0.39% |
| Tuba | 17.5 | 386 | 2.47 | 1,170 | 287 | 0.21% |
This is oversimplifying but air entering the leadpipe at 4m/s[1] will leave a trumpet at about 3.6cm/s, a trombone at about 1.6cm/s and a tuba at slightly over 8mm/s.
Woodwind instruments –including recorders, fifes, tin whistle and ocarinas – are similar, except that air also escapes through the holes along the length of the instrument, reducing the air pressure and therefore the velocity of the airstream at the bell. Flautists and fife players blow air directly forwards as well as into the instrument, so air is expelled in two directions.
Biology
The other inconvenient truth that the would-be myth busters do not acknowledge is that exhaled breath carries water droplets which are known to contain and transmit Covid-19. The smallest (those below 5μm) are light enough to be picked up by air currents in the room and widely dispersed. These are known as bioaerosols. Vibrations in the instrument break up water droplets, so there will be a larger number of these aerosols than are exhaled by breathing. It is believed that these bioaerosols may remain airborne for extended periods of time and when inhaled by an uninfected person, they are an efficient way of transmitting the disease.
Larger water droplets fall quickly under gravity but still carry and can transmit Covid-19, usually through surface contact. This is known as fomite transmission, the fomite being the object on which the water droplets land.
Conclusion
However much we wish it otherwise, playing woodwind and brass instruments has not been shown to be safe and may only be carried out under limited circumstances. The same applies to singing, which also creates an aerosol-laden air stream. Studies are underway in America, Germany, Japan, Australia and the UK to understand it better but for now, caution must be exercised. For now, the guidance to amateurs remains that they should not meet to play woodwind or brass instruments or to sing in groups although this does not preclude making music at home.
That does not mean that these instruments cannot be played at all. The government has issued guidance for professional musicians, for out-of-school settings and for the full opening of schools in September 2020. We eagerly await music-specific guidance from the Department for Education.
Schools and similar settings can do music using all instruments and singing, although additional measures will be required to control the risks of woodwind, brass and singing. The Music Unlocked guidance on the Music Mark website is being maintained to reflect up-to-date understanding.
[1] Derived from airflow entering a trumpet of 200-600ml/s as measured by a 2010 study at McGill University.