Did you know? Face masks impair speech intelligibility

How do masks affect speech intelligibility and thus our ability to communicate? This was investigated by an interdisciplinary team from the Department of Acoustics. The scientists from the Department of Acoustics have analyzed how speech levels and speech intelligibility change when people wear a fabric mask, disposable mask, FFP2 mask and half mask and have compared these with results obtained without a mask. An “artificial mouth” which emitted a noise with a spectrum and volume similar to those of human speech was used for the tests. A measuring microphone located 1.5 meters away recorded this signal: first without a mask, and then with the respective masks placed in front of the artificial mouth. The team carried out the tests in different rooms at Fraunhofer IBP: in the anechoic chamber with its sound-absorbing walls, in the organ laboratory with acoustics similar to those of a large meeting room, and in the High Performance Indoor Environment (HiPIE) lab, a test environment in which a quiet and a noisy office scenario were simulated acoustically.

Speech intelligibility is markedly impaired

The results show: even with a low background noise of 35 decibels, fabric masks impair speech intelligibility by up to 15 percent, FFP2 masks by 10 percent and disposable masks by 5 percent. Half masks, such as those worn by rescue workers, can impair speech intelligibility by as much as 25 percent. Speech intelligibility is measured on the scale of the Speech Transmission Index STI from 0 (unintelligible) to 1 (perfectly intelligible). Whether “outdoors” or indoors - the value measured with all the masks studied was between STI 0.35 and STI 0.5 and thus significantly impaired.

Acoustically optimized masks

In conversations, the situation is aggravated by the fact that masks cover the mouth and thus render lip-reading impossible. This is particularly difficult for people with poor hearing. The increased distance between the conversational partners due to social distancing further diminishes speech intelligibility. Since people intuitively move closer together when they cannot hear each other very well, impaired speech intelligibility is likely to have a negative impact on compliance with social distancing rules. In everyday working life, poor speech intelligibility can have serious consequences. Regardless of the pandemic, optimizing the acoustic properties of masks is an important matter. This starts with the development of systems and materials. Together with manufacturers, different materials and systems can be acoustically tested and compared with each other at Fraunhofer IBP. It is also important to consider the different ways speech is perceived when people wear a mask. This can be achieved through subject studies. Can people better understand speech if the mask is transparent, for example, so that lip movements and the speaker's facial expressions can be seen? These and other aspects will continue to be addressed by Fraunhofer IBP's research teams together with industrial partners in the future.