The most common method to evaluate impact sound insulation in buildings, is the tapping machine. One of the issues with the tapping machine however, is that it does not resemble actual walking sound at all. If you have ever visited a field measurement of impact sound insulation, you will probably recognize the hammering noise that the acoustician creates. Another method that is less common, is the so called impact ball. The impact ball might very well be the most expensive ball in the world, with its four-digit price tag (five if you’re swedish)!
The principle of the impact ball is very simple. Release the ball from a height of 1,0 meters and catch it after the impact, so that you get only one bounce. The sound pressure level from the impact is recorded in the room below, using several fixed microphones positions to get a room average. The ball is dropped on multiple locations on the floor above, becuase you might excite different modal frequencies depending on the location of your drop. We strongly recommend that you use a measurement system of at least two channels, which will halve the required time on site. We usually measure four ball drops, with two fixed microphone positions in the room below. Then we move the microphones to two new fixed positions, and repeat the dropping sequence. This procedure gives us 16 measurements in total of maximum level from each ball drop/microphone combination. Detailed instructions on how to use the ball can be found in ISO 16283-2.
One interesting aspect of the ball, is that it sounds pretty close to ”real” walking sounds, when used in timber constructions. The ball has been developed to replicate the force spectrum from a human footfall as closely as possible, when dropped. It sounds similar to when you stand on your toes and let the weight of your body fall and land on your heels (without shoes). Another interesting aspect is that the ball is quite soft compared to the metal hammers of the tapping machine. In theory, this means that when the hammer hits the floor, the force excitation has an extremely short duration in time, which should mean that we get more high frequency content and less low frequency content in the frequency domain. The ball on the other hand, has a force excitation with a longer duration in the time domain, which in the frequency domain should translate into more low frequency content and less high frequency content.
We have conducted measurements with both the tapping machine and the ball in two studies with about ten different floor structures in each study. One of the studies indicated something interesting: It seems as if the tapping machine can show greater differences between the best and the worst floor structure, whereas the same comparison using the ball indicated a smaller difference. For example, the best/worst difference might be ~10 dB with the tapping machine, but only ~5 dB with the ball. When dealing with wooden constructions and the sound from footfalls, the region 20-100 Hz contains almost all the energy. It is likely that walking noise complaints will relate to this frequency region. In the other project, the ball and tapping machine showed similar relative improvements between the worst and the best floor. This could perhaps mean that the result is sensitive to the construction under study.
Is is possible that the tapping machine might over-estimate the subjective difference between the best/worst floor structure, and the ball gives a more pessimistic result that perhaps is closer to the real perceived annoyance? These results are preliminary at this stage and would be interesting to verify with further measurements. What is your experience from using the impact ball? We would love to get the conversation started.