“Absorbs Up to 82% of Sound” “LL35” — How to Read the Numbers in Soundproof Mat Ads

As you scroll through social media, advertisements for soundproof products like “soundproof carpets” often pop up. Stylish room photos, stories from parents struggling with their children’s footsteps, and impressive figures. As someone who makes a living in soundproofing, I can’t help but stop and look.

These claims are not necessarily false. Many of them are likely based on test report data. However, there is often a significant gap between what the written numbers mean and what buyers expect.

In this article, I will decode the figures seen in advertisements for floor soundproofing products from the perspective of an acoustic engineer.

Part 1: LL and LH Refer to Completely Different Types of Sound

The L-value, which represents floor sound insulation performance, is divided into two categories: LL and LH.

An important point to understand is that cushioned floor finishing materials like carpets or mats primarily work for lightweight floor impact sounds (LL). Heavyweight floor impact sounds are dominated by the building structure—such as the thickness and rigidity of the floor slab—and laying something on the surface will not lead to significant improvement.

This is where the discrepancy arises. The most common consultation regarding noise trouble in multi-family housing is “the sound of children running upstairs,” which is a typical heavyweight floor impact sound. However, it is quite common for advertisements to feature only lightweight-side figures like “LL35” or “ΔLL-◯.”

The correct way to interpret products without LH figures listed is that they do not claim LH performance.

Part 2: A Flooring Rating Is Not “Your Room’s Rating”

Another common point of confusion is the difference between **”the performance of the floor material alone” and “the performance of the building as a whole.”**

In the past, the performance of floor materials was also displayed using estimated L-ratings such as “LL-45” and “LL-35.” However, the actual floor impact sound insulation performance of a building varies depending on factors such as floor slab thickness, room size, shape, and ceiling specifications. As a result, discrepancies occurred between the floor material rating and actual spatial performance. Furthermore, differing test conditions among manufacturers made product comparison difficult.

Therefore, following the 2007 JIS revision, the Japan Building Research Institute established a committee at the request of three flooring industry associations, leading to the enactment of the “Guidelines for Rating Floor Impact Sound Reduction Performance of Flooring Materials” in April 2008. Consequently, performance ratings for flooring materials shifted from estimated L-ratings to “ΔL-ratings (floor impact sound level reduction amount).”

In short, a floor material labeled “LL35” does not mean that laying it will make your room LL-35. It is a reference value measured on a standard laboratory floor (150 mm thick concrete slab); if laid in a wooden apartment, the result will be completely different. This is precisely why the industry moved away from this misleading display to ΔL-ratings.

This discrepancy is not limited to flooring materials; similar gaps occur with wall materials as well.

Note that the ΔL-ratings defined in the guidelines consist of 5 levels for lightweight floor impact sound (ΔLL-1 to 5) and 4 levels for heavyweight floor impact sound (ΔLH-1 to 4). Larger numbers indicate higher performance, and ratings above these are not defined. If you see a rating, check whether it falls within this range and whether it refers to LL or LH.

Part 3: What Is the Percentage in “Absorbs ◯% of Sound” Relative To?

Personally, this “%” expression is what concerns me the most.

Sound volume is expressed in decibels (dB), which is a logarithmic unit. In a logarithmic world, standard arithmetic rules do not apply. Placing two 50 dB machines side by side does not equal 100 dB; it only results in about 53 dB. This is due to the definition of decibels itself.

This means that dividing dB values directly to express a reduction as “◯%” holds little physical meaning. If sound drops from 60 dB to 50 dB, saying “50÷60 means about a 17% reduction” is incorrect. Because dB is a relative logarithmic value based on a reference point, ratios do not correspond directly to sound energy ratio, sound pressure ratio, or perceived loudness ratio.

Incidentally, in terms of sound energy, a 10 dB reduction is actually a 90% decrease, which perceptually feels like “about half the volume.” If expressing in %, this would be the proper context. In any case, percentage figures without clear specifications of “at which frequency” and “what is used as the denominator” cannot be taken at face value.

Small annotations in ads such as “*Measured at specific frequency bands” or “*Division result of dB values” reveal the true nature of these figures. Reading the footnotes is far more useful than looking at the numbers themselves.

(Decibel calculations are also explained in our article [Adding Decibels]).

Therefore, professional soundproofing companies do not use percentages.

Part 4: Who Decides What Is “World-Class”?

Finally, a quick word on advertising representation rules.

Under the Act against Unjustifiable Premiums and Misleading Representations, representations that mislead general consumers into believing a product is significantly better than it actually is (misleading representations of quality) are prohibited. When using expressions like “world-class” or “No. 1,” businesses must be able to demonstrate objective and reasonable evidence backing their claims, including the scope of comparison and survey methods.

When you see “world-class,” look for what it was compared against, under what conditions, and who measured it. If evidence is clearly provided, it can be trusted; if not found after searching, it is wise to reconsider how much weight to give those numbers.

Understand the True Nature of Sound Before Laying a Mat

Just to be clear, I am not implying that soundproof mats or carpets are useless. They certainly have an effect on lightweight floor impact sound, and offer value in terms of convenience and cushioning.

The key question is whether the type of noise troubling you matches the type of sound the product is designed to address. If someone troubled by heavy thuds buys a product looking at figures meant for lightweight sounds, they will end up with “no noticeable change despite laying it down.” This is a matter of product selection rather than product quality.

Our company specializes in professional anechoic chambers, soundproof rooms, and noise control solutions, and we do not handle residential soundproof mats. Nevertheless, the importance of measuring and distinguishing how sound travels remains the same whether for factories or apartment buildings. Verifying what conditions produced those numbers is, in our view, the best shortcut to avoid wasting money on soundproofing.

References

• General Incorporated Foundation Japan Building Research Center (GBRC) “About Floor Impact Sound Level Reduction Rating Method for Flooring Materials (ΔL Grade)” https://www.gbrc.or.jp/test_research/acoustic/sound02/
• “Guidelines for Rating Floor Impact Sound Reduction Performance of Flooring Materials” (Enacted April 2008)
• JIS A 1440 “Measurement of floor impact sound level reduction of floor coverings in laboratory”
• JIS A 1418-1 / 1418-2 “Acoustics — Measurement of floor impact sound insulation of buildings”
• Consumer Affairs Agency “Act against Unjustifiable Premiums and Misleading Representations” Article 5 (Prohibition of Misleading Representations)