CSL Postdoc researches speech acoustics, face masks

8/21/2020 Lizzie Roehrs, CSL

Written by Lizzie Roehrs, CSL

All over the world, people are taking precautions to protect themselves and the people around them from COVID 19. The CDC has said time and again that the most effective precautions one can take if they plan on being around others are to wash their hands and wear a mask. To most Americans, these precautions are common sense and almost entirely non-intrusive; however, one demographic may be suffering by the use of masks: people with hearing loss.
CSL Postdoc Ryan Corey
CSL Postdoc Ryan Corey

According to the Hearing Loss Association of America, approximately 48 million Americans have some degree of hearing loss. CSL Postdoc Ryan Corey, one member of this community, is researching different types of masks to determine the impact different materials have on verbal communication.

“Face masks muffle high-frequency speech sounds and block visual cues,” says Corey, an alum of Illinois’ Dept. of Electrical and Computer Engineering (PhD ’19). “Our research team measured several face masks in the Illinois Augmented Listening Laboratory to find out which are the best for sound transmission, and to see whether amplification technology can help.”

Corey’s group measured face masks in two ways. One method used a loudspeaker covered by different masks to measure sound transmission from six feet away. The second method recorded a human speaking while wearing each mask. The person was fitted with lapel, cheek, and forehead microphones as well as one placed directly in front of their mouth to test the effects of masks on recording devices. All masks tested muffled high-frequency sounds above 1kHz.

“These high-frequency signals are important for hearing sounds like “s,” “f,” and “th,” which are usually the most challenging sounds for people with hearing loss,” says Corey. “The performance of the masks appears to depend mostly on material and weave, rather than thickness.”

The best masks overall for verbal speech were the surgical mask and the KN95 respirator, though light cotton masks followed closely behind. The group also included masks with clear windows which have been suggested by hearing loss advocates as they allow for lipreading and better facial cues. Unfortunately, the tradeoff for these masks is that they strongly degrade high-frequency sounds which means people with hearing loss must make a choice between better sound or visual cues.

Graphic showing different kinds of masks and ranks them by how speech friendly they are
Graphic showing different kinds of masks and ranks them by how speech friendly they are

Corey’s group does offer an overall solution. Verbal speech is muffled directly in front of the speaker but doesn’t not seem to be affected in other directions.

“A talker with a clear mask could wear a microphone to help amplify high-frequency sounds,” says Corey. “Lapel microphones are already widely used for amplification systems in classrooms and auditoriums, and some hearing aid users have remote microphones that clip to the chest. These systems should work well with face masks, including masks with clear windows, with no special modifications required.”

In all, Corey’s research found that more breathable masks such as cotton and surgical masks allow for the best transmission of sound. However, safety should always be top priority when choosing one’s mask.

“It is important to note that we did not evaluate how effective the masks are at blocking droplet transmission; it is possible that the most effective masks against the virus are the least effective for sound, and vice versa,” says Corey. “Fortunately, all masks seem to work well with lapel microphones, which could benefit teachers and others who will need to be heard while wearing a face mask.”

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This story was published August 21, 2020.