Friday 22nd Nov, 2019

Cutting out the noise for PA systems

Photo: Shutterstock / tm stagetec

Chatswood railway station in Sydney has implemented a new Digital PA system that promises to cut out the interruptions.

 


Announcements are an important component of train station organisation, but lots can get lost in the noise and bustle of a busy station. PA systems can sometimes overlap due to the sheer frequency of announcements that have to be made at any one time.

Chatswood railway station on Sydney’s North Shore Line was one such station facing the consequences of this. So many trains were passing through the station – which serves roughly 60,000 passengers a day and sees a train pass through every three minutes on average – that it was not a practical solution to have one announcement playing on each platform at a time.

Given the station’s popularity and configuration of four tracks split into two islands, it was making announcements hard to prioritise for Sydney Trains and Sydney Metro, both of which used the station for their North Shore Line and Northwest services respectively.

Since there is only one audio system at the station and both operators needed to play announcements at the same time, Sydney Trains and Sydney Metro worked together with audio specialist supplier tm stagetec systems to find a more practical solution.

Both Sydney Trains and Sydney Metro stated that they wanted their operators to be able to broadcast simultaneous announcements on adjacent platforms so that waiting passengers would be able to hear and understand the announcements on their platform, without being disrupted by similar and simultaneous announcements emanating from the other platform a few metres away.

The project team came up with three proposed solutions in how to deal with the increasingly troublesome occurrence.

The first proposed option was to prioritise the announcements between operators. However, it was decided by the team that due to the frequency of announcements to be played, it was nearly impossible to prioritise the announcements without compromising to other types of announcements such as safety or track works.

The second proposed idea was to install a physical barrier in the middle of the island platform to block the sound spilling over to the other platform. However, since there was a lot of devices already installed along the platforms, and Chatswood is categorised as an Emergency Warning & Intercommunication System (EWIS) station requiring a full fi re assessment before adding new materials or devices to the platforms, the idea was dropped.

The third option, involving sophisticated sound isolating technology, was the one that eventually won out, says Charles Chan, an audio services manager at Sydney Trains who led design and project management on the project alongside colleague Nixon Edward.

“Chatswood Station consists of two island platforms: Platform 1 & 2 and Platform 3 & 4,” Chan says. “Platform 1 and Platform 4 are operated by Sydney Trains where Platform 2 and 3 are operated by Sydney Metro. Both operators are required to play the announcements in accordance to their individual schedule.

“Acoustic modelling and demonstration were conducted with a positive outcome. Therefore, it was decided to proceed with a new Digital Array Speaker solution to meet the business and operational needs for both operators.”

tm stagetec systems settled on Plane Array CS-90 speakers from subsidiary EDC Acoustics, which led distribution and system design integration on the project. EDC had to program the PA systems to a very narrow radiation pattern in the horizontal plane so that they could be steered to the desired target area and maintain that focus over a wide bandwidth. The CS-90 speakers, being tall, narrow column-style speakers, were especially well suited to this task.

The CS-90s could utilise computer-controlled wavefront control in 3D, allowing users to direct and shape wave fronts to meet the needs of the task at hand, in this case to isolate the sound output to a particular island without the risk of overlap from an announcement on a different platform. This not only provided a benefit for the commuters but would help to avoid noise leaking to other areas outside of the station as well.

It was a challenging project for EDC, according to David Connor, the audio and electroacoustic designer charged with leading the acoustic modelling side of the project.

“This was a psychoacoustic challenge as well as a technical challenge because the other platform announcements have meaning and further disrupt the brain when compared to incoherent noise,” he says.

“Delivering this level of isolation in such a short distance is a difficult task that was further compounded by secondary sound arrivals in the form of reflections, echoes and reverberation. In order to achieve the objectives, the loudspeakers had to be focussed very specifically.”

The setup was designed so that the difference in total sound level between the two PA system announcements would be more than 15 decibels (dB) in a range between 250 hertz and 8 kilohertz.

These figures were chosen as they were generally considered to deliver the minimum headroom required to deliver significant intelligibility in the presence of noise, according to Connor.

Acoustic echo cancellation and ambient noise systems were also installed to allow automatic, real-time level adjustment at each individual platform to ensure that passengers could still hear announcements while waiting for either train or metro services.

“The design solution performance was predicted with the goal of more than 15 decibels only being realisable from 630 hertz to 4 kilohertz,” Connor explains. “Less than 10 decibels could be achieved from 400 Hz to 8 kHz.

After much analysis, this was deemed to be the best practical result possible, and the installation proceeded. Since the speaker systems were implemented at Chatswood earlier this year, tm stagetec systems, Sydney Trains, and Sydney Metro have all received significant positive feedback.

“After the system was commissioned the realworld performance was measured by creating a graph comparing the design predictions with the real-world measurements that found they were very similar,” Connor explains. “Hats off to science.”

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