Photo by George Heinrich
Orchestra Hall
Orchestra nerds know: even the finest brass musicians use a water key (affectionately known as a spit valve) to drain moisture from their instruments. In the same way that breathing on a chilly window causes the glass to fog up, a musician’s exhalation travels through the instrument, releasing aerosols from the end and condensing water inside.
Though this may sound like a germy process, a new study by University of Minnesota College of Science and Engineering researchers has found that aerosols expelled by wind instruments usually don’t spread farther than one foot. These findings are of critical importance to the Minnesota Orchestra, which is innovating new ways to ensure the safety of its musicians during rehearsals and performances.
In October, the CDC updated its guidelines on how COVID-19 spreads: they say the virus’s principal mode of infection is through exposure to respiratory droplets produced by coughing, sneezing, singing, talking, or breathing. People can also catch COVID through airborne transmission of smaller particles that linger in the air, though this is less common. These determinations present a unique challenge to orchestras, which are in the business of exhalation: at press time, the Minnesota Orchestra is only giving performances for radio, television, and livestream, without in-person audiences and limited to 25 musicians tops. Pre-COVID, their usual concerts featured 90 people on stage.
But the findings from the U of M study open new doors for the orchestra: they suggest that strategies such as social distancing, using portable air filters, and putting masks over instruments (you read that right—go buy a cute one for your French horn) can help to reduce the risk of spreading COVID on stage. The U of M research team is working hand-in-hand with the Minnesota Orchestra to refine its safety measures, which could set precedents for orchestras and musical ensembles worldwide.
The study compared the number of aerosols emitted by a range of instruments, and considered the effects of factors like articulation and slurring patterns, play intensity, and special musical techniques on aerosol concentration.
In the first research phase, the U of M team measured the aerosol concentration emitted by 10 different brass and woodwind instruments, which were then categorized as low, intermediate, or high risk. The tuba, despite its large bell circumference, ranked as low risk: it actually produced fewer aerosols than an average person’s exhalation. The bassoon, piccolo, flute, bass clarinet, and French horn all released aerosol concentrations comparable to normal breathing, the clarinet to speaking, and the oboe and bass trombone at levels slightly above the range of speaking. To the surprise of no one in the brass section, trumpets were singled out as the highest-risk instrument: they expelled significantly more aerosols than a person speaking.
“All of this information I think is very useful for planning,” Department of Mechanical Engineering Associate Professor Jiarong Hong, who led the study, said in a press release. “Once we understand the risk level of different instruments, we can actually target the higher risk instruments. You certainly don’t want to have a group of trumpet players playing in a confined room because that will be a very high risk activity.”
The second phase of the study considered how the aerosols travel: Hong’s team used probes to track the aerosols’ journey through the instruments and into Orchestra Hall. They found that the flow was limited, and the aerosols dispersed very quickly: no instruments showed a significant influence of flow past 30 centimeters. This is due in part to the human thermal plume effect, in which human body warmth creates an upward air flow.
Knowing that most of the aerosols are traveling vertically, said Hong in a press release, will help the team to strategically place air filters above the musicians. This, they say, would yield a 95 percent particle extraction rate. Another tactic is to bring down the temps in Orchestra Hall, which would make the plume effect stronger, and the air filters more effective.
The research team also explored direct mitigation options like portable filters and “bell barriers”—essentially masks that cover the instruments’ bells and block the passage of aerosol particles. A single-layer blocked 60 percent of particles without much change to instruments’ sound, but a 92 percent reduction through three layers meant a significant sacrifice in sound quality.
Based on the results of this study, the Minnesota Orchestra is creating a plan to gradually bring more musicians back on stage. They plan to extend their stage and seat players six to nine feet apart, and may implement bell barrier masks and air filters. All musicians who don’t need their mouths to play—percussion and strings, that is—will wear masks onstage. They’ll also work with U of M Department of Mechanical Engineering Assistant Professor Suo Yang to model patterns of airflow in Orchestra Hall. This study marks the first time the Minnesota Orchestra has partnered with the U of M for a comprehensive scientific study.
