Three Things You Need to Know When Conducting On-Site Heavy-Weight Impact Testing of Surfaces

AUTHOR:
Mike Raley
Ecore’s INCE Board Certified Acoustic Engineer & Chairman of ASTM WK57850 – Field Measurement of the Reduction of Impact Sound from Heavy Impact Sources When Using Floor Coverings

A twenty-five-pound dumbbell dropped on the floor of a fitness center can make an awful lot of noise. If there are hotel rooms or apartments near the fitness center, the transmitted impact noise can lead to complaints and even tenants moving out of the building. It takes a lot of deflection, or “squish,” to manage the significant energy of these falling weights. Common surfaces like LVT and thin rubber floors are not up to the task, when high levels of impact noise reduction are required.


Often, the best way to select suitable floor surfaces is to conduct on-site heavy-weight impact testing. That testing can be as simple as dropping a weight on various flooring options to see how loud the impacts are in the adjacent spaces of concern. However, while on-site testing can be a simple and effective way to evaluate flooring performance, there are some important details to keep in mind to ensure the test yields truly useful results.

Conduct Test with a Hard, Spherical Object
Commonly, people conduct on-site testing using a dumbbell. This seems logical because free-weights are often the main source of complaints, but there are important issues with the use of dumbbells for comparative testing.

Dumbbell Shape
To get a good comparison, one needs to control the variables in the test, so the only thing that changes is the flooring surface being impacted. One of the most important things to control is how the weight contacts the floor, and the odd shape of a dumbbell makes this difficult.



In a 2015 InterNoise paper (John LoVerde, 2015), John LoVerde and his co-workers at Veneklasen showed that dropping a typical hexagonal dumbbell results in poor test repeatability. This occurs because the dumbbell, when dropped from a horizontal orientation, can rock on multiple axes, causing it to hit the floor in different orientations from one drop to the next.

Duration of Impact
The different orientations change the surface area impacting the floor, but they also change the duration of the impact. If the dumbbell hits as shown in Figure 1, the impact surface area is at its maximum and the impact duration is minimized because both sides of the dumbbell hit at the same time. However, when the dumbbell hits as shown in Figure 2A, the initial impact surface area is minimized, and the impact duration is increased because the upper half of the dumbbell hits slightly after the lower half (Figure 2A to 2B).

Modified Kettle Bells
To eliminate this variability, many acoustical consultants use modified kettlebells. Consultants modify the kettlebells by removing the handle and grinding that area so there is a smooth spherical surface. When used for testing, the consultants drop the kettlebell so that the spherical surface is what hits the floor. Using a spherical impact surface ensures that even if the kettlebell changes position during its fall, the impact surface area, shape, and duration will not change.

Of course, not everyone is going to go to the effort of modifying a kettlebell for this kind of testing. In a paper I wrote for the Acoustical Society of America (Raley, 2019), I showed that good test repeatability can be obtained by dropping a typical dumbbell with the handle oriented vertically instead of horizontally. In this orientation, there is far less chance for the dumbbell to change position as it drops, and the impact area and duration are, therefore, kept much more consistent.

Ensure Impact Height is Consistent
Another factor affecting the validity of heavy-weight impact noise testing is the drop height, which should be as consistent as possible to ensure a valid comparison. The higher you drop the weight, the harder it hits the floor. When we do laboratory testing, we often use a laser level to set our drop height, and we lower the weight just until it breaks the laser line to get the same drop height every time. When testing in the field for simple demonstration purposes, it is not necessary to use a laser level. You can simply stand with your back straight and knees and elbows locked to achieve a fairly consistent drop height.

An important thing to consider when measuring drop heights is the thickness of the products being tested. For instance, a test could use a baseline of the existing floor, but another product could raise the floor height 2.5 inches. To ensure a consistent drop height, measure it from the top surface of each flooring material.

Conduct Weight Drops at Same Location
The last factor to consider is impact location. It is important to keep the actual impact location within the room as consistent as possible. It might seem logical to line up all the flooring samples to be tested, and to just go down the line dropping the weight on each one, but this can lead to inaccurate results and, possibly, incorrect conclusions about which product performs best.

Impact Sound: A Plate Illustration
Most often, fitness centers have a concrete slab floor structure that is the same composition in all directions, unlike the wood joist floors typical of single-family homes. For simplicity’s sake, we can think of the concrete slab as a simple plate. In the late 18th century, a German physicist and musician Ernst Chladni invented an ingenious method for studying the vibration of simple plates, which this video demonstrates quite well.

When a plate vibrates, some points on the plate move up and down vigorously, while other points do not move at all. In Chladni’s research, he poured sand onto a metal plate to show these areas. When a violin bow is used to make the plate vibrate, the sand moves away from the points where there is significant movement and it all gathers at the points where there is little or no movement, creating beautiful patterns. When we bow the plate at different locations, the pattern changes.

Impacting the concrete slab at different locations is akin to bowing the plate at different locations, as it causes the concrete slab to vibrate in different ways. The different vibration patterns can sound different, and some can be louder than others. Therefore, changing the impact location introduces another variable that could potentially lead to incorrect conclusions about which product provides the best noise reduction for a space.

In conclusion, here are three things to remember when conducting on-site heavy-weight impact testing.
1.    Use a hard, spherical object or drop a dumbbell with the handle held vertically for a consistent impact area and duration.
2.    Make sure the impact height is consistent, as measured to the top of each flooring surface.
3.    Do all drops at the same location in the room. Switch out flooring samples, so you are always dropping in the same spot.

For more information on acoustics and Ecore’s heavy-weight impact sound test data, visit Ecore's Acoustic page.


References
John LoVerde, W. D. (2015). Field acoustical measurements of heavy weight impacts associated with weight drops in fitness centers. InterNoise. San Francisco, CA.
Raley, M. (2019). Heavy-weight impact testing: Test repeatability for a modified kettlebell and comparison of impact source weights and drop heights. The Journal of the Acoustical Society of America, 1711.