For the study, published in The Journal of the Acoustical Society of America, the team tested medical masks, disposable surgical masks, masks with clear plastic windows around the mouth, and homemade and store-bought cloth masks made of different fabric types and numbers of layers.

 

The researchers used a special loudspeaker shaped like a human head so that sound radiates as it would coming from a human mouth.

 

"We put the different masks onto the head-shaped loudspeaker and played the same sound for every test," said Ryan Corey from University of Illinois at Urbana-Champaign in the US.

 

"We also placed the speaker onto a turntable to add a directional component to our data."

 

The team collected data from a mask-wearing human speaker, as well.

 

"Even though these two data sets are a bit different, they both show which sound frequencies are most affected by mask-wearing and which masks have the strongest effects," Corey said.

 

Their data showed that all masks muffle the quiet, high-frequency sound generated when a person pronounces consonants.

 

"Those sounds are already a challenge for those with hearing loss, with or without masks, and even become a challenge for those without hearing loss when you throw a mask into the mix," Corey said.

 

Masks also block visual cues like facial expression and lip motion, so speech reading is no longer an option when wearing most masks.

 

Almost everyone uses speech reading to some extent, with or without hearing loss, Corey said.

 

The study found that disposable surgical masks offer the best acoustic performance among all tested, Corey said.

 

Loosely woven 100 per cent cotton masks also perform well but, as shown in a study by other Illinois researchers, they may not be as effective as surgical masks at blocking respiratory droplets.

 

That study showed that tightly woven cotton and blended fabrics may block more droplets, but they also block more sound.

 

Based on the droplet study, Corey suggested that multilayer masks made of loosely woven cotton may offer a reasonable compromise between droplet-blocking efficiency and acoustic performance.

 

The good news is that most masks do not completely block sound, they simply deflect it away from the mouth.