Researcher Ryan Corey recently spoke with a friend who teaches at a school where some of the students have hearing. A friend wanted to know if she had any ideas to help her communicate with these students while wearing a mask to slow the spread of COVID-19. Corey, who also has hearing loss, did not know what to tell her. So he headed to the Illinois Extended Listening Laboratory to look for solutions.
Corey, a postdoctoral researcher in electrical engineering and computer engineering with Professor Andrew Singer of the University of Illinois Urbana-Champaign, leads a team studying audio signal processing, especially for hearing aids such as hearing aids. The results of a new study by a team evaluating the acoustic effects of face masks on speech have been published in Journal of the American Acoustic Society.
“Previous research on this topic has focused on medical masks worn in healthcare facilities,” Corey said. “But no one has looked at the acoustic effects caused by different types of fabric masks, so I focused our study on that.”
The team tested medical masks, disposable surgical masks, masks with transparent plastic windows around the mouth and homemade fabric masks bought in stores from different types of fabrics and the number of layers.
The researchers used a special speaker, made to measure by Uriah Jones, a graduate engineer in industrial and business engineering, and shaped like a human head so that the sound radiates as if it were coming from a human mouth.
“We put different masks on the head-shaped speaker and played the same sound for each test,” Corey said. “We also placed a speaker on the turntable to add a directional component to our data.”
The team also collected data from a human speaker wearing a mask.
“Using a real person makes the sounds less repetitive because we can’t say the same thing every time. However, it allows us to explain the actual shape of the head and the actual movements of the lips,” Corey said. “Although these two sets of data are slightly different, both show which sound frequencies are most affected by wearing masks and which masks have the strongest effects.”
Their data showed that all masks muffle the quiet high-frequency sound that occurs when a person utters consonants. “These sounds are already a challenge for those with hearing loss, with or without a mask, and they even become a challenge for those without hearing loss when you mix in a mask.”
Masks also block visual signs such as facial expressions and lip movements, so reading speech is no longer an option when wearing most masks. Almost everyone uses speech reading to some degree, with or without hearing loss, Corey said.
“That’s why we tested open-window masks that have become very popular,” he said. “Unfortunately, the trade-off is that you can see them, but they block out the most sound of all the masks we’ve tested.”
The study found that disposable surgical masks offer the best sound performance among all tested, Corey said. Loosely woven 100% cotton masks also perform well, but, as shown in a study by other Illinois researchers, they may not be as effective as surgical masks in blocking breathing drops. That study showed that tightly woven cotton and blended fabrics can block more droplets, but Corey’s team found that they also block more sound. Based on the droplet study, Corey suggested that multilayer masks made of loosely woven cotton could offer a reasonable compromise between droplet blocking efficiency and sound performance.
The good news is that most masks do not block the sound completely, but simply repel it from the mouth. This detail means that simple amplifiers can make masked speech more accessible to everyone. Face masks have a particularly mild effect on lapel microphones, which are already used in many classrooms and lecture halls. Many hearing aids support a remote microphone accessory that is also worn near the lapel.
“Most people don’t walk around with lapels and microphones while wearing a mask, but it can help in environments where it makes sense, like classrooms and meetings,” Corey said.
University of Illinois at Urbana-Champaign