When considering design choices for places meant for social gathering, such as restaurants and reception halls, as opposed to places meant for performance or spoken word, there’s an important distinction to be aware of. Clearly, in social gathering spaces, people want to interact to each other. When speech contributes to background noise, several competing factors are activated and compile in causing the reduction of the quality of verbal communication.
There is an inverse relationship between background noise level (ambient) and speech intelligibility. The verbal communication quality degrades increasingly as the ambient level increases, particularly for people with reduced hearing capacity. Research has confirmed that normal-hearing people will have an involuntary tendency to increase their vocal effort when speaking in loud noise to enhance the audibility of their voice. This involuntary reflex includes not only loudness but also other voice features such as pitch, timber, and the duration of syllables. This effect is called the Lombard effect or Lombard reflex, named after the French Otolaryngologist Etienne Lombard (1869-1920) who was the first to observe and report the phenomena. One needs to take the Lombard effect into account when designing for social spaces.
Lombard quantified that there is a half decibel increase in vocal output for every one decibel of increase in ambient noise. The International Standards Organization (ISO) characterizes vocal effort in 6-step decibel levels, starting at “relaxed” when ambient level is 54dBA and ending at “very loud” when ambient level is 78 dBA. The Lombard phenomenon takes effect at an ambient noise level of 45 dBA and speech level of 55 dBA. Doubling the amount of people present increases the ambient noise by 6 dBA. The quality of verbal communication is also standardized by ISO and it’s characterized with the signal (speech level) to noise (ambient) ratio. (See chart below.)
Since the ambient level of a space depends on its volume and its reverberation time in addition to the number of people and the type of gathering (as well as other factors like alcohol consumption and background music), the acoustic capacity of a facility is introduced and defined as the maximum number of people to avoid having the quality of verbal communication at or lower than “insufficient”.
It would be my advice to restaurant designers and other architects of social spaces to hold acoustic capacity as a priority consideration and check their design, in terms of space volume and total absorption amount, against the values that would be required to achieve the acoustic capacity of the room.
| can imagine a world where restaurant owners make deliberate decisions about acoustic capacity, and even post this number--similar to posting fire code maximums--so customers make informed decisions about where they want to spend their time, knowing that they'll be able to hear and be heard. In fact the latter part of this vision is already happening since so many venues are occupied past the acoustic capacity, yielding a negative effect on the enjoyment of the space and thereby negatively impacting the business. By being informed and making your clients informed, you move toward better outcomes for all stakeholders.
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