Musical instruments don’t spread aerosols as far as you might think

MINNEAPOLIS/ST. PAUL (10/19/2020) – A new study by University of Minnesota College of Science and Engineering researchers has found that wind instruments typically do not spread aerosols farther than one foot. The researchers suggest that mitigation strategies including social distancing, putting masks over instruments, and using portable filters can help reduce the risk of spreading COVID-19 on musical stages.

The U of M team is working closely with the Minnesota Orchestra. These findings are helping the orchestra establish and refine measures for maximizing the safety of its musicians during rehearsals and performances and could provide new insights for orchestras and bands worldwide.

Led by Department of Mechanical Engineering Associate Professor Jiarong Hong, the research team also studied the number of aerosols emitted by different instruments, as well as how aerosol concentration varies depending on articulation and slurring patterns, intensity of play, and use of special techniques. For example, trumpet was the highest risk instrument, producing significantly more aerosols than a person would by breathing or speaking.

“All of this information I think is very useful for planning,” Hong said. “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 researchers tested various strategies to mitigate the spread of the particles, including using portable filters and instrument masks called bell barriers. They found that a single-layer mask blocks 60 percent of the particles without reducing much sound quality. Two layers block 75 percent with a slight drop in sound quality, and three layers block 92 percent but causes a substantial loss of sound quality.

In the first phase of the study, published in the Journal of Aerosol Science, Hong’s team tested 15 Minnesota Orchestra musicians playing 10 different woodwind and brass instruments. After measuring the aerosol concentration released by each instrument, the researchers categorized them into three risk categories: low, intermediate, and high. The tuba was categorized as low risk because it produced fewer aerosols than the amount people release while exhaling. The bassoon, piccolo, flute, bass clarinet, and French horn produced aerosol concentrations in the range of normal breathing. The clarinet produced concentrations in the range of speaking, the bass trombone and oboe produced concentrations just above the range of speaking, and the trumpet produced a significantly higher number of aerosols than breathing or speaking.

In the second phase of the study, Hong’s team used probes to measure how far the aerosols travel from each instrument inside Orchestra Hall. They found that the flow was very confined, and the aerosols dispersed quickly. At only 10 centimeters (about 4 inches) away from the instrument outlet, the aerosol concentration was less than 10 percent of what it was at the source, and no instruments showed an appreciable influence of flow beyond 30 centimeters (about 1 foot).

Part of this, the researchers said, is due to the human thermal plume effect, which refers to the upward air flow created by a person’s temperature being higher than the air around them. The majority of the aerosols are carried upward by this draft.

“The second part of the study is to help understand where the aerosols go,” Hong explained. “They’re not necessarily spreading horizontally—they are rising vertically. So, this will help us to optimize the placement of filters and the social distancing between individuals.”

Because of the thermal plume effect, they found the most efficient placement of filters would likely be above the musicians—resulting in a 95 percent particle extraction rate. Another strategy could be to reduce the temperature inside Orchestra Hall, which would increase the temperature difference between the people and the environment, ultimately making the plume stronger and the filters more effective.

Right now, the Minnesota Orchestra is performing concerts without in-person audiences—for television, radio, and livestreams—with small groups of up to 25 musicians. In a pre-pandemic world, the orchestra would typically feature about 90 musicians on stage.

Using information from this research, the orchestra has identified a plan to gradually increase the number of musicians onstage, although not to pre-pandemic levels. Musicians will be spaced out at distances of at least 6 to 9 feet onstage, with a large extension being added to the lip of the stage to increase its size. Bell barrier masks and air filters will potentially be utilized as well, and all string and percussion players onstage will wear masks when they perform.

This is the first time the orchestra has ever worked with University of Minnesota researchers on a comprehensive scientific study. Hong and his team connected with the orchestra through U of M Associate Professor Jon Hallberg, who is the Medical School’s Director of the Center for Arts in Medicine and recently served as the orchestra’s tour physician.

“We are grateful to have had the opportunity to work with these accomplished University researchers to better understand how the orchestra can responsibly bring musicians together onstage,” said Minnesota Orchestra President and CEO Michelle Miller Burns. “We are encouraged by the findings of the research. Using this information and other public health advice, our strategy is to put in play a multi-layered approach to safety onstage and backstage that involves COVID testing, light quarantining, wearing masks, maintaining distance between musicians, and investigating bell barriers and air purifiers— all in the interest of mitigating as many risks as possible. This important research will benefit organizations beyond ours, and we are pleased that the University’s findings can now be shared with school groups and other ensembles to help inform and guide their decisions and safety strategies.”

In the coming weeks, the orchestra will also be working with Department of Mechanical Engineering Assistant Professor Suo Yang, who will be modeling patterns of airflow and exchange in Orchestra Hall. Yang will use the aerosol output measurements from Hong’s research to show how aerosols travel throughout the auditorium and onstage when the musicians rehearse and perform.

In addition to Hong, members of the team include mechanical engineering researchers Ruichen He (Ph.D. student), Aliza Abraham (Ph.D. student), Siyao Shao (Ph.D. student), Santosh Kumar (Ph.D. student), Rafael Grazzini Placucci (master’s student), Linyue Gao (postdoctoral associate), Changchang Wang (visiting scholar), Buyu Guo (visiting scholar), and Maximilian Trifonov (visiting scholar). This research was funded by the University of Minnesota Medical School.

Read the phase one research study, entitled “Aerosol generation from different wind instruments,” in the Journal of Aerosol Science.