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Public Address System for Heathrow Worldwide Distribution Centre


With a proven record for successfully ‘taming’ acoustically hostile environments, PEL took on Royal Mail’s international mail centre at Langley, a stone's throw from Heathrow. Officially called Heathrow Worldwide Distribution Centre (HWDC) it handles international mail, which is being transferred from existing offices around the UK.


Acoustically Hostile Environment - Requiring a Wide Dynamic PA System

On a site so large and diverse, with segregated areas for Arrivals, Sortation, Despatch/Storage and support services such as Administration, Engineering, Kitchen/Dining and Welfare, locating staff quickly and easily can be difficult. A distributed public address system was seen to be the ideal solution. However, for such a system to be effective, it had to be carefully designed and planned to be intelligible in all areas, ranging from quiet administration office to cavernous constructions housing noisy automated mechanical handling machinery – much of it operating intermittently.

With a proven record for successfully ‘taming’ acoustically hostile environments with public address technology, for organisations such as BBC, Deutsche Bank, Royal Mail, Marks & Spencer, BMA, Hugo Boss,Westfield, HBoS, Debenhams, City Academies and Local Authorities, PEL successfully designed and installed a PA system that fulfills this requirement and meets established performance criteria.


The Client

Royal Mail’s international mail centre at Langley is one of the most technically advanced Offices of Exchange in the world. It handles international mail, which is being transferred from existing offices from around the UK. This was a massive investment of £150 million in a cutting-edge mail centre to transform the way international mail is handled, driving up quality and reducing costs in the process. The site itself is 100,000 square metres or the equivalent of 12 football pitches in everyday terms. Inside there are 2.5km of overhead walkways and 12km of conveyor belts to move more than six million items of international mail each day.

Public Address Loudspeaker Selection

To ensure good performance, PEL specified Constant Directivity Horn loudspeakers, which provide clear sound dispersion uniformly throughout the broadcast areas. The 12cm cone driver is splashproof and housed in a tough ABS enclosure with stainless steel fixing hardware to ensure many years of robust and reliable service. A total of 282 Constant Directivity Horn loudspeakers were installed at high level throughout the complex in strategic positions to give the best possible sound distribution. Approximately 30,000 metres of cable were used to group the loudspeakers in different areas to create a zoned system, reflecting the operating processes.

Other areas, such as Administration and Welfare were equipped with more conventional flush mounted ceiling loudspeakers to suit the quieter environment and suspended ceilings.

Central Equipment Racks

Cables for groups of loudspeakers (zones) feed out from the central equipment racks, which house the power amplification and signal processing and control equipment. Space is incorporated to allow for easy future expansion if required. The amplification is chosen to match the relevant power requirements of individual loudspeaker circuits. When designing systems, PEL typically allow 20% spare capacity for any additional loudspeakers that may be required in the future. Power amplifiers used in the HWDC project include 400W, 240W, 150W, 100W and 50W.

Ambient Noise Sensing


Because ambient noise levels constantly change in the various process areas, a total of 26 Ambient Noise Sensing computers are incorporated into the racks. By constantly monitoring ambient noise in each zone, via a specialised microphone installed therein, the computers automatically adjust the volume output to broadcast at 10dB(A) above the ambient noise level. Announcements are therefore always clearly audible and distinct, but never too loud. Rack 1 also houses PEL’s programmable Audio Processing System (APS) that controls all broadcasts, zone selection and also stores digital messages that may be recalled at any time via digital control consoles.

Audio Processing System (APS)

For system control, PEL used their Audio Processing System (APS) in conjunction with three APS Digital Microphone Consoles. Each console allows for any combination of zone selection for broadcast of live speech and for playback of any four of the digitally stored messages within the APS system. The APS system allows for up to 32 Digital Microphone Consoles to be connected onto each radial microphone circuit and allows for easy future expansion because it is software programmable. Further details about the flexible APS system can be found in the ‘Sound’ section under ‘Public Address & Voice Alarm Systems’.

PA System Project Conclusion

During the installation and commissioning phases, PEL operated as Principal Contractor under CDM regulations, producing and operating to a Construction Phase Health & Safety Plan that was consistent with Royal Mail’s stringent site requirements. Regular project meetings were held on site to ensure the project ran smoothly and ran to schedule.

At final inspection it was agreed that the public address system was a total success, delivering messages that are clearly audible and intelligible to all broadcast areas.


National Design Consultancy, Royal Mail’s Building Services Engineers, reported that: “from start to finish, PEL’s performance was exemplary”.


Download Heathrow Worldwide Distribution Centre Case Study

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