Who’s That Knocking on the Floor?

Who’s That Knocking on the Floor?

by Rob Lilkendey / Published May 2014




Clip clop, clip clop, clip, clop. Screeeech! Thump! Screeech! Sounds terrible, doesn’t it? What could it be? To residents of many multi-family buildings, these are the all too familiar sounds of their upstairs neighbor walking to their dinner table, pulling out a chair, sitting down, and sliding back under the table. A simple act to the person doing it, but often a considerable nuisance to the person living below. And as more and more multi-family residences are sold throughout Florida and buildings begin to reach capacity, the potential for complaints of footfall noise, chair dragging noise, and other impact sounds disturbing residents will grow.

What can be done to bring some relief to residents complaining of excessive exposure to impact sounds from their neighbors with hard surface flooring? As we know, intruding sounds that are reasonable to one person may be intolerable to another, so we cannot rely solely on the opinions of the affected party. Fortunately, there is a standardized test that can be conducted to rate the impact noise reduction performance of a floor/ceiling assembly, often referred to as a “tapping test.” This test yields a single number rating, called an AIIC (Apparent Impact Insulation Class) rating or an NISR (Normalized Impact Sound Rating) of the acoustical performance of the assembly.

For the results of the test to be meaningful and for the parties involved to move toward a resolution, the condominium/homeowners association should have an agreed upon AIIC or NISR criterion already established in the condominium documents to determine whether the existing assembly passes or fails the test. Many condominium documents already include IIC (Impact Insulation Class) rating requirements for hard surface flooring assemblies, but an IIC rating is obtained only in a laboratory, not in the field.

An IIC rating is useful to include as a means to approve or deny proposed, hard surface flooring/resilient underlayment requests by residents who can provide a laboratory test result of the overall assembly demonstrating that the required IIC rating has been achieved. However, it should be accompanied by a field measured criterion such as an AIIC or NISR as well, in the event there are disputes as to whether the correct product was installed or installed properly. Some condominium documents require the use of a specific, hard surface flooring underlayment product believed to achieve the desired acoustical performance. However, the use of that product doesn’t guarantee an acceptable outcome, as installation deficiencies could result in greater transmission of sound than anticipated.

Choosing Acoustical Criteria

So, how does a board of directors arrive at agreed upon acoustical criteria (lab test and field test), or determine if the criteria they already have are appropriate? A combination of research, consulting advice, and mock-up field testing can help a board select criteria that are suited to the cost/grade of residences and expectations of the occupants. Sources such as the Florida Building Code, U.S. Department of Housing and Urban Development, and the guidelines issued by the International Code Council (see sidebar), as well as knowledge of the requirements of other similar buildings in the same geographical area provide a good starting point for understanding the acoustical performance ratings appropriate for different grades of residences.

Field testing—in multiple units in the actual building, not other projects that are claimed to be similar—of floor/ceiling assemblies that use specific products approved by the board that are intended to achieve an agreed upon acoustical criterion will confirm whether the chosen underlayment product or rating is sufficient. It will also help establish a minimum AIIC or NISR requirement for future field tests conducted to resolve disputes or to determine compliance with the condominium document requirements. Hav-ing quantitative test results and an established criterion to which they can be compared, removes the subjectivity from discussions about the severity of the noise impacts.

Testing… 1… 2… 3… Testing

If the tested impact noise reduction performance of a floor/ceiling assembly in question fails to meet the requirement stated in the condominium documents, there are several approaches to consider:

  1. If an underlayment product that is pre-approved in the condominium documents was used but failed the test by a significant margin (more than two or three points), then there is a good possibility that the flooring was not installed in strict accordance with the underlayment manufacturer’s written instructions. There may be hard grout between the tile and the wall, or baseboards resting on top of the flooring and attached to the wall, both of which will create flanking paths for impact sound transfer and lead to reduced acoustical performance. Fixing the deficiencies may solve the problem.
  2. If the installed underlayment was not one that was approved by the condominium documents and fails the test, or a substitute product was approved by the board on the condition that it passes a field test and fails the test, then the underlayment can be replaced with a board-approved product, and retested if necessary.
  3. Thick area rugs and carpet runners can be installed throughout most of the upper residence; however, this is not a permanent solution and does not improve the condition in rooms where rugs are not used, such as kitchens and bathrooms.

If the floor/ceiling assembly passes the test, and the homeowner on the floor below is still seeking relief from the noise, it might be possible to install a special sound-reducing ceiling in the lower residence to further reduce the noise. Due to the costs involved, this should only be done with the advice of a qualified, Acoustical Con-sultant who can explain the limitations of such an approach and develop the details and specifications necessary to maximize the acoustical performance of the ceiling.


Of the many sources of noise in multi-family buildings that have the potential to disturb residents, footfall noise is the one that most often rises to the level of serious complaints and, occasionally, legal action. Having an established set of pass/fail impact-noise criteria for approving and enforcing the use of suitable underlayment products where hard surface flooring is used, tailored to the unique structure and design of the building, and considerate of homeowner expectations, provides a means to avoid potential problems down the road, and to address complaints with unbiased testing when they do occur. These criteria are best derived through consultation with an Acoustical Consultant experienced in these matters and testing of assemblies in the affected building. Now, if only we could do something about the neighbor’s television!