Speech Intelligibility and COVID-19

This article explores the relationship between speech intelligibility and virus transmission. 

Why would speech intelligibility matter?


 The scientific community has emphasized how coughing and sneezing yield the transmission of infectious disease. These dramatic expiratory events expel large quantities of droplets from the person, so large that these droplets are often visible.

Concerningly, regular expiratory events such as talking and breathing, also expel particles. These particles may be invisible to the naked eye but still have the potential to carry large quantities of communicable respiratory pathogens. Although such droplets are smaller in size compared to those that occur due to sneezing and coughing, they are still sufficiently large to carry a variety of respiratory diseases including measles, influenza, tuberculosis and Covid-19 (Coronavirus).

Furthermore, research published by Nature.com shows how “the rate of particle emission during normal human speech is positively correlated with the loudness (amplitude) of vocalization, ranging from approximately 1 to 50 particles per second for low to high amplitudes, regardless of the language spoken” (Asadi et al, 2019). That is to say, the probability in the spread of deadly respiratory pathogens via droplets from the nose and mouth is increased with vocal effort.

The hazards of shouting

In recent weeks, Irish news and media outlets have reported how Covid-19 has spread like wildfire in factories such as those where meat is produced. These factories are noisy environments. For workers to hear and understand each other, they need to shout or at least speak with a raised voice. A workplace environment where employees are within proximity for long durations is already unideal. Factor in everyone shouting and the situation becomes a highly contagious and dangerous one. This not only applies to factories, but to any workplace environment where people may need to raise their voice, including offices, gyms, garages, retail outlets etc.


As the lockdown slowly lifts, people must not be returning to a workplace environment that is hazardous. Social distancing has become part of our everyday lives. We need to ensure that as people separate, they do not overcompensate by raising their voice.

Where do acoustics come in?

So, why does all this concern acoustics? – Speech Intelligibility. We achieve safe workplace environments by ensuring adequate speech intelligibly at all times. Where there is sufficient speech intelligibility, there is no need for people to increase vocal efforts above that which is normal, which keeps droplet projection to a minimum.


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What is speech intelligibility?

 Speech intelligibility, more technically referred to as Speech Transmission Index (STI), describes to a way of measuring and predicting how easily speech will be understood in a given situation. With higher levels of speech intelligibility, people are less likely to raise their voice to be heard and understood.


There are two factors at play when determining the intelligibility of speech in a space: 

  1. background noise levels within the space 
  2. reverberation or the reflectiveness of the space

Background Noise Levels:

Noise can arise due to sources inside the building, or from sources outside the building. For example, in modern architecture, noise may radiate from large heating, ventilation and air-conditioning (HVAC) systems suspended from the ceiling. In older blocks where natural ventilation is employed, traffic noise may break in through open and closed windows. Regardless of the noise source, too much background noise will encourage people to raise their voice. Acoustics is a discipline grounded in noise control. Employing an acoustician to ensure adequate control of HVAC noise, external noise intrusion and control of other such noise sources will directly assist in the reduction of droplet projection. 


In open spaces with free-field-like conditions e.g. grassland in the countryside, sound energy escapes into the atmosphere. Such free-field environments have little influence on a sound’s composition. Its timbre goes relatively unchanged. 

In enclosed spaces, sound energy is retained. Sound waves bounce around a room, reflecting off the various boundaries and surfaces within. These reflections or “echoes” prolong a sound’s duration. Consider the sound of a ball bouncing in a huge sports hall. 

It is desirable to control this reflected energy. Too much reverberation will have the effect of reinforcing unwanted noise, such as chatter in an office space or noise from HVAC systems. Furthermore, speech becomes less intelligible in a room that is too reflective. Long subsequent reverberation tails will mask important syllables used for speech perception and cognizance.  

One of the most significant disciplines in acoustics involves controlling reflected energy. Shorter reverberation times yield better speech intelligibility results which will directly assist in the reduction in the spread of disease transmitted via droplets.

To summarise, lower background noise levels and less reverberation means higher speech intelligibility. Higher speech intelligibility means a safer work environment. 



Asadi, S., Wexler, A.S., Cappa, C.D. et al. Aerosol emission and super emission during human speech increase with voice loudness.Sci Rep 9, 2348 (2019). .