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Where Is One Supposed to Measure?

An expert to the court was required to perform some noise level measurements in a bedroom of a multidwelling building where the tenant complained about various noises coming from other parts of the building. When he came to perform the measurements he was presented with an empty room where he performed the usual sound level measurements at a height of

1.5 m from the floor. No significant noise event could be identified.

Then it suddenly hit him: If this was supposed to be a bedroom, where was the bed? The tenant looked blankly at him and opened a small cupboard from where he picked up his braid (well yes, he was of Asiatic descent)! When measuring close to the ground, a number of events were at once identified.

Lesson Learned: When somebody complains, it is interesting to perform a measurement at their usual location and not limit oneself to the typical locations indicated in the standard.

Is There an Insulation Problem?

On performing the sound insulation measurements between two cinema projection theatres, the acoustician felt quite confident as he had already designed and built similar facilities along the same design lines. In this particular case, the separating wall between those halls was made of a double concrete wall with an expansion joint in between that should ensure the required performance. So sure was he that he sent a young engineer to perform the measurement while he talked with the end user. To complicate matters, the brand new amplifiers were smelling a bit too hot for comfort, and it was decided that one should not push the volume too far for the sake of safety of the sound equipment (well, it wouldn’t have done to burn the amplifiers before the official inauguration by the authorities, would it?). While this was supposed to be a bit close to the limit in the high-frequency range, it was supposed to be usable. The young engineer came back and stated that he felt the measurements were OK.

To everybody’s surprise the sound insulation result turned out to be rather bad, so bad that spectators complained soon after the opening of the facility. On coming back, the acoustician had the sound system turned on in earnest. While a low-frequency rumble could be heard on the upper part of the wall, a rather high-pitched noise could be heard close to the screen.

When the acoustician pointed out that such a noise could only come from a hole in the walls, the architect was adamant that it was not possible, as nobody in his right mind would have drilled a hole through two concrete walls, and it would be luck that they would match each other. But the acoustician held his ground: This did sound like a hole. Well, exploring the wall with his hand, the end user suddenly found a very soft spot and the noise was suddenly considerably reduced. Such a double hole had really been drilled (for which purpose nobody knew!). While he was wondering why his young colleague had not identified the problem, the end user came up with an explanation that was duly tested and proved conclusive: With a lower sound level on the emission side, it was really much harder to distinguish the noise radiated by the hole.

That did solve the high-frequency part of the problem (and a good thing that was, as the speech signal in a projection theatre could then be heard in the other theatre because of it). Regarding the low-frequency rumble, it eventually turned out that the contractor had left in place the material used to cast the double concrete wall, and this created a surface coupling that significantly decreased the low-frequency sound insulation performance.

Lessons Learned: Use a high sound level pink noise on the emission side to be able to spot weak spots. Do not rely on a young unsupervised collaborator to make the assessment, and do not take for granted that something will not be done because it is not logical (it might not be at the time of analysis, but it briefly made sense to the contractor for reasons of his own).

Listening to Conversations through a Floor

A large rehabilitation project involved the demolition of a few concrete structural elements. In order to ascertain whether some parts of the building could still be kept in operation during the deconstruction phase, it was decided to perform a diagnosis on one unoccupied floor. It involved banging on the floor and walls of a room at one end of the building and checking how much sound energy was lost at each expansion joint using vibration measurements.

When the acoustician analyzed the tapes, he found that after the second expansion joint, the banging was no longer much discernible. On the other hand, the conversations held in the room underneath were totally understandable!

Lesson Learned: A properly executed expansion joint is an efficient attenuator; also, eavesdropping using accelerometer measurements is possible.

 
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