Acoustic Underlay Performance – You Can’t Design or Buy It Without Knowing These 6 Things

Acoustic Underlay Performance – You Can’t Design or Buy It Without Knowing These 6 Things

So you have looked at the key types of underlay but the variety of impact noise performance data in the product specifications can be perplexing.

Knowing how to read and assess impact noise performance data is the only way to make sure you know if you are specifying or buying the right type of underlay.

This article helps to demystify impact noise performance of acoustic underlayment and the areas that control it so you can make a more educated decision.

1. What is Impact Noise?

Would you ever buy something without knowing what problem it was solving? Hopefully not!

Knowing what impact noise is before specifying or buying Acoustic underlay will also help you to get a better grasp of the other parts of this article.

Impact Noise

When anything (keys, coins, pet claws, toys, furniture, foot steps) falls onto, strikes, or is dragged across a surface it creates a vibration which passes through the floor finish and into the structure below. From here it will then pass through the building structure and come out as noise which you will then hear. This is impact noise in its most basic form.

Acoustic underlay - What is impact noise?
Wooden blocks hitting the floor and creating Impact Noise

If these noises are heard frequently during quiet times of day (or any frankly) and are loud enough, they will become annoying and will disturb and distract from whatever is happening in that space.

Impact noise is most commonly a problem in dwellings (hotels, residential, student accommodations, care homes etc) and this is where acoustic underlay is most commonly installed. It is also a consideration in places like schools, hospitals, offices or anywhere people do not expect to hear the activities of their neighbours because the occupants require quiet conditions.

We can consider impact noise as the amount of noise in our room coming from impacts on the floors in rooms above us.

The louder the noise, the worse the impact noise performance of the structure and vice versa.

What Does Impact Noise Sound Like?

Well let’s listen to some typical sounds!

These items were dropped on a 150mm/6″ concrete slab from a height of 50cm/20″ or roughly knee height.

You will notice that most of the sounds are clicks and clacks which in the constructions industry is described as high frequency sound. In contrast a typical low frequency impact sound might be that of someone walking bare foot and their heel thudding against the floor.

As you might expect dropping items on different floor finishes and structures will result in different noise levels in the rooms below. Drop something on carpet on top of concrete, and more than likely you will not hear the noise below. Drop something on a tiled floor on a concrete structure with no acoustic underlay and it will sound similar to the recordings above.

This leads us onto the next section.

2. Know Your Structural Type -What Is Your Building Made From?

This is crucial as impact noise travels more easily through some floor finishes and structures resulting in different impact noise levels in the space below.

The type of building, or more specifically what your floor structure is made from has a huge impact on how the acoustical underlay will perform and the level and annoyance of impact noise heard below.

Not knowing this could lead to a poor choice of acoustical underlay. You might buy or specify what you consider the best underlay on the market but if used in the wrong structure it may not give the desired result.

Buildings are normally categorized as either lightweight or heavyweight structures, typically wood and concrete respectively, but there are some nuances.

Heavyweight Buildings

Typically this covers any structure made from concrete. Most concrete structures for dwellings will be a minimum of 150mm/6″ thick of either poured or precast concrete but in some areas of the world they can be thinner and then they act/sound more like lightweight structures as they are less stiff and more flexible.

Concrete is heavy, normally 350kg/m2 for an 150mm/6″ slab. This makes it incredibly stiff and as such the vibration from the typical impacts discussed earlier will pass through it very easily when less than 250mm/10″ thick. It is however a simple structure and so dealing with impact sound in concrete structures is relatively straightforward.

As a rule of thumb, assuming a typical concrete slab, the thinner the concrete structure the thicker the underlay that will be required.

If the concrete slab is more than 250mm, it is also possible that no underlay will be required. This is because the lightweight nature of these types of impact noise do not create enough vibration to pass all the way through the structure. But never assume and always check with your acoustic consultant.

Lightweight Buildings

Traditional structures like solid timber joist, open web truss, and I joist have recently been joined by the new kids on the lightweight building block CLT and NLT.

They act and sound completely different to concrete structures because they have much less mass. A typical 150mm/6″ CLT structure will weigh around 75-80 kg/m2 vs 350kg/m3 for the concrete. Therefore sound travels through them far more easily and as such a thicker underlay would need to be considered as well as a resilient ceiling to reduce the transfer impact noise from the ceiling below.

The worst case scenario for lightweight structures would be any type of joisted floor where really the only thing between you and the space below is the floor (maybe plywood or cement board) and the plasterboard ceiling below. This is common in buildings constructed before ~1950s when it was far more common for bricks and wooden joists to be used but it is also common in low rise (below ~8 stories) multi family dwellings in North America.

So it is key to remember that what works to control impact noise in heavyweight buildings will not necessary work in lightweight buildings and that acoustic underlay performance will differ across structures.

3. How Is Acoustic Underlay Performance Determined?

Building vs Lab Acoustic Test 1
Site testing vs lab testing

Knowing where and how acoustic underlay is tested will help you understand how relevant it is to your decision.

Impact Noise Testing in a Laboratory – “Lab Testing”

In this situation a manufacturer will employ a third party independent lab to test their material in controlled conditions.

For impact sound this consists of two similarly sized rooms, one stacked on top of the other with a big hole in the floor (testing aperture). The manufacturer will then decide what flooring system they want to put in the testing aperture with their underlay.

A tapping machine is then placed onto the floor system which is built in the testing aperture.

A tapping machine is a box with a line or 5 hammers which when turned on will drop on the floor one at a time repeatedly. This will create impact noise and thus vibration on the floor in the click clack frequencies discussed earlier. Just imagine someone running very fast on the spot with high heels on and you will get an idea of what it might sound like!

A microphone is then placed in the space below and the tapping machine impact noise is measured. Typically a structure with no acoustical underlay is tested first so when an underlay is introduced we can measure again and compare the two.

The manufacturer may choose to test the same underlay under various floor finishes or various underlays on the same floor finish to understand how the underlay performs.

The laboratories are designed and built so that it is very difficult for the impact noise to go anywhere other than through the testing aperture. Therefore, we can be confident that when the impact noise is measured in the space below, it is only noise coming through the testing aperture and not via indirect or flanking noise.

Impact Noise Testing In A Building – “Site/Field Testing”

As the name suggests this is impact noise testing that is conducted within an almost or completely finished building.

In the majority of cases, this testing is conducted for the design team to confirm that an underlay to be installed meets the acoustic criteria or brand requirements set out for the project. This is sometimes called mock up or pre completion testing.

The fundamentals of the testing are very similar to the lab tests. A tapping machine is placed in one room and the impact noise is then measured in the room directly below.

The testing conditions in this type of testing are not controlled like lab tests, and there will be noise coming via in-direct noise paths.

Should You Request Lab Or Site/Field Impact Noise Test Results?

Site/field testing is very useful for verification purposes but it is 100% unique to that building, its structure and the floor finish. Therefore, using site/field tests for design purposes or for choosing which underlay to buy is a risky business. In fact ASTM  ASTM E1007-14 Section 1.6 stipulates:

“Any single field measurement only represents the performance of the actual assembly tested and cannot be used alone to accurately predict how an identical or similar assembly might perform.” 

Therefore for design and purchase decisions, using 3rd party labs tests is the prudent way to draw real comparisons between products.

For example if comparing two underlay products for use beneath tile flooring both the tests should have been done on the same thickness concrete slab using a similar tiled floor finish with no ceiling below. This would be an apples to apples comparison.

If you are assessing on behalf of your condo board or housing association, using lab test data is the only way of knowing if a suggested underlay will meet the criteria or if a physical mock up test is completed in the condo in question before a full installation is completed.

5. Knowing the Difference Between Material & System Impact Noise Performance Data.

Now we are getting into the nitty gritty!

Manufactures will provide what ever data they see fit. It is then down to the user to know how to dissect it and relate it to project specifications. So let’s dissect it!

IIC and Ln,w
Material vs System Performance


Earlier the process of impact noise lab testing was described. To recap a tapping machine is placed on the floor (with or without underlay/floor finish) and the noise coming through the floor is measured below. Once this data is processed it will give a number which is the performance of the whole floor system (floor finish, adhesive, underlay, screed, ceiling etc).

In territories that use ASTM standards it will represented as an Impact Insulation Class (IIC).

In territories that have standards based on or the ISO then it this is presented as a measured Level (Ln,w).

Be careful! The higher the IIC the better the performance. The lower the Ln,w the better the performance.

These system performances are perfect as for design purposed you can find a test whose constructions is as similar as possible to your use case. This also works for assessing a product submittal. Read in detail the construction that was tested in the lab and compare it against the structure you are working.

Remember always look for apples to apples comparison

Delta IIC/Lw

Unlike system performance (IIC/Ln,w) which assesses all the materials in the structure together, material impact noise performance is literally what effect one material has on the performance of a floor ceiling system.

Earlier we talked about lab testing being done on a slab with no acoustic underlay first then again with an acoustic underlay. This is how we get the material impact noise performance, two tests, removing or adding one material between them.

A delta is simply a difference value.

Performance 1 – Performance 2 = Delta.

To confuse things further for both Delta IIC and Delta Lw the higher the number the better the performance.

6. Knowing The Testing Loop Holes.

We left the best till last.

Loop Hole 1

If it is too good to be true, it probably is!

It is not easy to achieve IIC ratings of 65 (~Ln,w 45 dB) and above with just a standard floor finish and a typical underlay in concrete buildings (let alone lightweight buildings!). So if you see ratings higher than this dig into the details and always request a full test report (should be around 10 pages long). This should illuminate as to how the high impact noise performance was achieved.

Loop Hole 2

Testing with no floor finish.

This again can lead to higher than expected test results especially delta values. Why? Well if we place the tapping machine directly on the underlay, which is normally far softer than a floor finish, it will soak up a lot more impact vibration. This is especially true for low density underlays.

Loop Hole 3

Testing small samples.

Testing a sample which is just larger than the tapping machine as opposed to covering the whole testing aperture can provide increased performance results so check this in the test report.

Loop Hole 4

Quoting a performance number and then not supplying test data.

Never accept a single figure as proof of performance. Always get a test report and make sure it is similar to your use case.

Loop Hole 5

Showing only field/site test data.

Field/Site impact noise data can look very favourable with high FIIC,/L’nTw figures. But as set out in section 3 above field/site test data should not be used on its own to make a judgement so always request further details of exactly what was tested and also some lab tests to back up the performance claims.

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