Airports and railway stations: high spec audio systems5 December 2014
Passenger expectations of public audio are greater than ever before, but architects’ favouring of hard, reflective surfaces often gives rise to intelligibility challenges. David Davies finds out to what extent high-spec audio systems can make all the difference.
Delivering transparent audio in reverberant public spaces such as airports and railway stations is rarely a straightforward business. If you doubt this assertion, just look at the recent controversy surrounding the public address system at Waterloo Station – a London transport hub commonly recorded as one of the 20 busiest passenger terminals in Europe.
In a move that was described as a response to complaints from passengers and locals that the station had become too noisy, South West Trains had proposed a two-week trial in which it turned off the PA system for travel announcements. Not surprisingly, one might think, disability campaign groups argued that the loss of the announcements would raise serious challenges for blind and partially sighted passengers. The scheme was dropped shortly thereafter.
In its coverage announcing the abandonment of the proposal, London’s Evening Standard newspaper reported that South West Trains spent nearly £3 million on a 1,000-speaker PA two years ago, so the fact that a proposal to scrap PA usage for travel information could even receive serious consideration is worthy of note. More broadly, the episode highlights the scale of the challenge that can confront audio manufacturers and integrators in devising systems that are capable of delivering intelligible – but never over-powering – audio in heavy-usage public spaces.
With this in mind, Installation decided to compile a list of critical questions about audio in airports and railway stations – and then go in search of answers from four leading manufacturers.
Are there limits to what an installed system can do in an imperfect acoustic environment?
In general, the answer is ‘yes’, but on the basis that most acoustical issues can be dramatically minimised by a carefully designed and configured system such as that provided by beam-principle loudspeakers.
“It is well-known that very high reverb and background noise tends to impair speech intelligibility from the PA system. However, with accurate directivity control and smart DSP algorithms, correct intelligibility can be achieved even in adverse acoustic environments,” says Xavier Meynial, technical director at Active Audio, whose transport-friendly solutions include the RayOn column speakers.
“There is [a limit] for conventional PA systems as it’s very hard to avoid reverberation, and all too often you are faced with highly reverberant spaces that for one reason or another can’t be acoustically treated to dampen that effect,” says Stuart Archibald, product manager at Tannoy, whose solutions for the transport market include digital beam-steering column array QFlex. “That’s why digital beam-steering products are ideally suited to these environments, allowing you to steer the beam directly to the listening area avoiding the reflective surfaces, which means clarity and intelligibility for the listener.”
Duran Audio is a long-time champion of better audio for transport hubs, and since late 2013 it has been putting forward its case as part of Harman’s JBL. Nick Screen, Harman sales director for Duran Audio Products, also highlights the role of beam products in correcting inherited acoustical problems.
“If you need to achieve speech intelligibility or music clarity in a ‘less than perfect’ acoustic environment, then the ratio of direct sound to reverberant sound becomes critical,” says Screen. “Beam shaping or beam steering” – technologies deployed in JBL Intellivox products – “allow us to aim the sound where it is needed at the listener and reduce reflections/reverberation. Of course there is a limit to what you can do; these types of devices are good tools but they are not magic wands. You may need to look at a combination of acoustic absorption and Intellivox technology to achieve the desired result.”
TOA is another manufacturer with an extensive portfolio of transport-friendly solutions, including the SX-2000 digital voice alarm system – which is suitable for both paging and voice evacuation – and the platform application-oriented Plane Wave Design, whose flat speakers have been developed to deliver clear audio to a limited target area with minimal attenuation.
Like Archibald, TOA technical manager Ian Bridgewater confirms that a careful selection of system can do much to arrest a facility’s acoustical challenges. So for example, “with the use of line array loudspeakers you can put the sound in to the areas you want without lots of unwanted sound causing reverberation issues and resulting in poor quality sound”, he says.
To what extent does ambient noise complicate matters and what can be done to reduce its impact?
There is agreement that ambient noise – which is bound to vary considerably in level as passenger throughput ebbs and flows throughout the day – is a pivotal concern. In many cases, the solution is a design that incorporates a degree of intelligent measurement, such as ambient noise sensing, for adjustment of settings in response to the overall noise level at any given time.
“In the middle of the day the station/airport may be quieter, and the last thing you need is a loud booming public address system; [by contrast] during rush hour the system has to be heard over the increased ambient noise,” says Archibald. “The best way around this is to use automatic noise-sensing microphones placed on or near loudspeakers to measure the ambient noise and automatically increase the level of the system to a preset level above the ambient noise. This is usually controlled by the PA/VA processor used in the system.”
TOA has a similar philosophy. “We always design our systems to be louder than the background noise level by a minimum of 10dB to allow the system to be intelligible,” says Bridgewater. “In any locations with varying background noise levels we use ambient noise sensing. This monitors the background noise levels and automatically adjusts the paging system to the same ratio as the noise. A limit can be set so the system does not become too loud where it could be a nuisance.”
With such responsiveness in mind, Harman/Duran has built an ambient noise-sensing mic into its Intellivox system. Where an installed system combines more specialist and conventional loudspeakers, “similar functionality can be added into the ‘front end’ of the system” – in other words, at the processor level. But Screen notes that caution is always advised “because you have to protect the hearing of workers and passengers within the space. You can’t just keep adding dBs to ensure you have a good direct to reverberant ratio.”