Paul Absolon of CMS Danskin Acoustics looks at solutions to reverberation problems in noisy cafes, which are increasingly prevalent in the UK
The proliferation of coffee shops over the last few decades can’t have escaped anyone’s attention. Even before the arrival of Starbucks in 1998, the growth of the nation’s cafe culture was in full swing. Between 1993 and 1997, the number of coffee outlets in the UK increased by an incredible 847 per cent. As a nation, we consume a startling 95 million cups of coffee every day, and the coffee industry contributes over £17bn to our economy, creating hundreds of thousands of jobs.
Given the ubiquity of cafes – not just on the high street but in workplaces, universities, retail spaces, public buildings – why do so many of them test people’s patience with dreadful acoustics?
There are two contributing factors. The first is reverberation. The second is the Lombard Effect.
Reverberation
When a sound wave strikes a hard surface, it is reflected. This is known as reverberation. The length of time it takes the ‘sound pressure’ of this wave to fall by 60 decibels after the original sound has ceased is known as the reverberation time. In some environments, such as auditoria, a certain amount of reverberation is desirable in order to prevent sound ‘drop-off.’ In most environments, however, it’s problematic.
Reverberation is prevalent in environments with lots of hard surfaces. Now, think of your typical cafe environment: all those wipe-clean tables and countertops; those wooden floors; all that steel and chrome coffee-making machinery; all that crockery. Reverberation is guaranteed.
The Lombard Effect
According to the UC Berkeley Phonology Lab Annual Report (2008), “the Lombard Effect is a phenomenon in which speakers alter their vocal production in noisy environments, such as loud parties or restaurants.”
In other words, in an attempt to be heard over gurgling espresso machines, clattering cutlery, clinking crockery, piped-in music and other people talking, we increase the volume and pitch of our voices. This adds to the problem, leading others using the space to raise their voices, which means we have to raise our voices, which leads to an exponential rise in volume. The high levels of reverberation typical to coffee shops massively exacerbate this problem.
Solutions
The reverberation issue is the simplest to resolve; and by solving this problem, you may effectively combat the Lombard Effect by virtue of reducing the overall noise in the establishment.
As reverberation is caused by sounds bouncing off hard surfaces, minimising the number of those hard surfaces will be a solid first step. If this isn’t possible – and, for hygiene reasons, cafes need wipe-clean surfaces – it’s possible to counter the impact of all these reverberant surfaces by adding absorbent materials. The most commonly employed anti-reverberation product is the sound absorption panel. These are fabric covered and are mounted on the walls or suspended from the ceiling. When the sound wave enters the open cell structure or fibrous composition of the panel, it bounces around like a pinball. The friction resulting from each instance of impact is converted into low-level heat, which is absorbed into the material. When the sound wave re-emerges, its energy – and consequently its ‘loudness’ – is significantly diminished.
A useful measure for the effectiveness of any sound absorption solution is BB93: acoustic design of schools – performance standards, which gives the minimum performance standards required for acoustics in school buildings. Although this document only refers to schools, architects often use it as a benchmark when treating reverberation issues generally. BB93 requires that the Reverberation Time in a classroom be as little as 0.4 seconds in some cases. So, the sound pressure of the reflected noise has to decrease by 60 decibels in less than half a second. Although this might not be necessary in a cafe environment, it’s a worthy target.
Challenges
Some environments are more challenging than others. Recently, CMS Danskin Acoustics had to reduce reverberation issues in the cafe at the Royal Observatory in Edinburgh, a building that is well over 100 years old, with all that entails in terms of adapting to modern Building Regulations.
In such cases, design flexibility is key, fabricating to fit the unusual shapes and angles typical to listed buildings, and even creating sound absorption panels that accommodated antique light fittings.
By taking reverberation out of the ‘noise mix’ – visitors to the cafe can speak clearly over the other acoustic intrusions, without having to raise their voices and set in motion the dreaded Lombard Effect.
Paul Absolon is technical director at CMS Danskin Acoustics