Guo Zhongbao is used to playing for a full house in concert halls. The melodies usually linger on in his head even after the music ends.

But Guo, the principal French horn player of the Shanghai Symphony Orchestra, recently had a special night when he volunteered to work at a local face mask factory in Shanghai.

After he ended his shift on the production line－from 9 pm on March 24 to 3 am on March 25, Guo arrived at home with the sound of the machine still ringing in his head.

"It was the first time that I had worked in a factory, and the machines generated noise, which bounced back and forth in the space for the whole night. The sounds even appeared in my dreams over the next few nights," says Guo, 40, who has been playing with the Shanghai orchestra since he graduated from the Shanghai Conservatory of Music with a master's degree in 2003.

"As a musician, I am very sensitive to sounds. When working on the production line, I tried to follow the beat of the machine, which gave my work a rhythm."

Guo had two main jobs as a volunteer at the mask factory: fixing the ear loops to the masks and trimming the edges of the face covering.