ACOUSTICS
with trucks delivering building materials and concrete, be listed.
Piling
Where piling is to undertaken near to existing buildings – especially where these structures are on a raft-type construction – it is worth checking whether the construction company has carried out the necessary checks to ensure that there will be no detrimental effects on the adjacent buildings due to induced vibration. By this I am referring principally to the limited risk that any activity inducing low frequency sound might cause horizontal motion of floors, walls, and furnishings, due to sympathetic resonance, which can be distressing to patients and staff, especially in a hospital setting.
In the worst cases, the whole building mass can ‘bounce’ on the ground under it, which acts as a spring. Raft-type buildings with foundations on clay soils are particularly susceptible to this induced vibration effect. Buildings with a piled foundation are less susceptible. Figures 1 and 2 show the natural frequency of foundations against soil pressure. If you have a situation that falls between the maximum and minimum lines in either graph, then it is advisable to select a piling frequency that lies outside the maximum and minimum lines to avoid a state of forced vibration (bounce), which can be a very serious situation.
I worked on a project monitoring construction sound levels where the construction company avoided filling in the weekly ‘Acoustic Report Sheets’ that I had issued, despite me having prepared the documents about three weeks in advance.
The point here is that, in some cases, hospital activities being undertaken in adjacent buildings to the construction site can be relocated to avoid sound and vibrations problems impacting on them. Back to my own experience, and on turning up at a hospital on one occasion to check the data on the survey meters, I discovered that something strange had occurred the previous day. On visiting the construction team, I was informed that the hospital site engineer knew all about the incident. In fact, I found out, the building next to where a contractor was pilling had started to ‘bounce’, culminating in the lighting and other equipment in an operating theatre ‘shaking about’.
Key metrics
Readers will have noticed that I have used the unit of measurement, dB(A). Although my intention has been to avoid too much technical jargon, I feel that what is meant by dB(A) warrants some explanation. dB(A) is a sound level criterion method which takes the measured sound and corrects the sound level at each frequency to match the sensitivity of the
40 Health Estate Journal January 2021
normal human ear. While this sensitivity varies from person to person, the ear is less sensitive at lower frequencies, very sensitive in the mid-range frequencies, and a little less sensitive at higher frequencies. After correction has been applied, the final level of sounds are integrated to one single number. This is the most used sound level criterion, not necessarily because it is the best, but because it has been in use for the longest time. Therefore, should a comparison with previous data be required, dB(A) is used. Unfortunately, whenever you integrate something to a single figure, it results in a loss of information.
I hope that by this juncture I have made
it clear that attention to sound and vibration in a hospital environment is an absolute necessity, and, equally, that calling in an acoustic consultant every time there is an issue is not desirable. Instead – I would suggest – a more effective strategy might be to have a
Ken Marriott
Ken Marriott MIOA is a highly experienced mechanical engineer with an HNC in Mechanical Engineering, and a BA in Pure Mathematics from the Open University. A specialist in sound, vibration, and other environmental engineering, he ran an environmental consultancy, ICTC, for many years. He is also a member of the London Mathematical Society. He explains: “Listed below is a selection of the projects I have undertaken which have provided invaluable experience in sound and vibration engineering that has proven useful on many hospital projects.”
Design n Design and development of a range of acoustic attenuators.
n Design and associated work on many generator plants, including generator systems for hospitals, and British Telecom exchanges.
n Work on offshore platforms including Piper Alpha, the Bouri platform, and two 27,000 ton platforms for the Sakahlin Energy project.
n Various drilling and process plant projects, including Shell Corrib and Vankor drilling.
n A primary warning system for gas escapes for a pipeline in China.
n Design work for GCHQ, AWRE, and new parliamentary buildings.
Demolition construction and planning n Harrods of London. n Hospital sites including Guy’s Hospital, Addenbrooke’s Hospital, and The Royal Free.
n Various entertainment venues.
Problem-solving n Low frequency noise problems from power plant installed at Imperial College London, which caused the ‘neighbours’ at the Royal College of Music to complain.
n Vibration problems at Esso Fawley. n Vibration in the control room on the BP ETAP exploration platform.
n Compressor house vibration and shaking at Heathrow Airport.
n Vibration induced in a whole building due to piling carried out adjacent to the building.
Lectures and talks n Lectures on acoustics for MSc university students.
n Presentations for the Institute of Acoustics.
n Papers presented at various international conferences.
couple of facilities engineers attend a short course incorporating, say, a number of presentations on acoustics and vibration, so that there is always an engineer on site with some experience in, and knowledge of, acoustics to call on before summoning an acoustics consultancy.
I hope that this article gives estates and facilities engineers an idea of what to look for when dealing with some of the ‘typical’ sound and vibration issues that can arise on hospital sites.
It is useful to remember, when you do get a problem with noise, that – as in the wider world – there are only three types of problem: n Problems that you have seen before (Good).
n Problems similar, but not identical to, ones that you have seen before. (Not too bad).
n Problems that you have never seen before. (Trouble).
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