Drilling in concrete, without a sound?

Anybody who has been in a building where construction work is going on knows how the sound of hammer drills and diamond saws can get on one's nerves. Even if you are some distance from the work site, the sound and vibrations are propagated in such a way that it feels as if they are going on right above -- and sometimes even inside -- your head. Nordland Hospital in Bodø is about to begin a major expansion similar to that recently completed at St. Olav's Hospital in Trondheim. The work will include the renovation of a ten- story concrete tower block. The plan is to carry out the work while the patients are in their rooms -- by renovating half the block at a time, and by moving the patients from one half of the building to the other.

What noise levels are acceptable?

Terje A. Olsen, Nordland Hospital's Development Manager, realised that the renovation work might present a problem and that he had to take the bull by the horns. So last autumn, before inviting tenders, he contacted acoustics researchers at SINTEF. He wanted to investigate what noise levels were acceptable, both to the patients and not least to hospital employees who will have to spend three years in the middle of the project. The study was also designed to investigate what methods could be recommended to satisfy the noise level requirements.

Dramatic noise reduction

Truls Gjestland and Anders Homb at SINTEF know most of what there is to know about sound and the way it propagates in buildings. They distinguish between what they call "air sound," which is what we experience when we're listening to somebody talking or moving about in another room, and "structural sound," which is propagated in the form of vibrations through floors and walls. Such vibrations can cause sound to "radiate" from completely different places than where they arise.

Measurements were carried out at different locations in the building in relation to where work involving concrete was taking place, and the researchers are currently completing their study before sending it to the hospital. The conclusion will be that the use of diamond-tipped drilling tools can significantly reduce noise problems.

"A lot of holes have be drilled during renovation. The use of hammer drills was shown to generate noise in the next room at a level of 85 decibels, which is almost enough to cause hearing impairment," says noise researcher Truls Gjestland, "In contrast, a diamond-tipped drill resulted in an intensity of only about 60 dB."

According to researchers, reducing the sound level by ten decibels will approximately halve the perceived intensity. A reduction of 20-25 dB will therefore be experienced as a dramatic improvement, and 60 dB is about the same intensity as hearing a conversation between two people.

The difference in noise level results from the way a diamond drill grinds its way into a concrete wall, as compared with the impacting action of a hammer drill.

What should the requirements be?

"When sound spreads outwards, it normally becomes weaker with increasing distance. In an office building, for example, you will only faintly hear people walking in the corridor or fetching a drink from the coffee machine. The noise level is perhaps around 50 dB," says Anders Homb. "The sound of a diamond drill will in other words be so faint that it will be drowned out by other sounds."

The two acousticians who went round measuring sound levels in the different rooms of the hospital have discussed what is reasonable when it comes to noise requirements in this sort of workplace. How much can patients, doctors and nurses tolerate? They point out that international studies, for example, have demonstrated that patient recovery calls for peace and quiet.

"We have little or no information about noise propagation and noise limits when it comes to people indoors, close to where construction work is taking place. All previous noise research has focused on those using the equipment, and their working environment. If you are using a hammer drill, for example, the rule is that you must use ear protectors," says Gjestland. "Otherwise, recommendations exist only for noise outside the building -- as might be experienced by neighbours, for instance."

Not the last word

For this reason, Gjestland and Homb had to do some pioneering work and obtain information from various sources. They don't believe that this will be the last word on noise during renovation projects, or on the requirements that should be placed on it. Both schools and offices are in need of rehabilitation, and it isn't always possible to vacate buildings while work is in progress.

The research report concludes that while work with diamond-tipped drills can be both more expensive and slower than with standard tools, it is recommended because it has so many health and environmental advantages. The researchers also recommend studying the possibility of alternating between different types of equipment and perhaps restricting noisy activities to certain times of the day. It is also emphasised that it has been shown that frequent information and advice helps people tolerate noise.

"Starting work is contingent on this knowledge," says Terje A. Olsen. "We are now working on preparing tender invitations, and it is important that the contractors can show that they can satisfy the requirement."