An overview of patents of noise reduction in buildings

Abstract

Noise is an unwanted sound in urban areas. Natural and or recycled acoustic absorbing or insulation materials are invented to reduce air and or impact sounds in buildings. Yet, they are not widely and commercially available. This review presented an overview of relevant studies on natural materials particularly on cotton and rubber using Scopus search engine and of the patents registered and granted using Espacenet and Google Patents search engines on noise reduction in buildings from the years 1996 to 2023. The 18% and 68% (out of 32 articles) relevant articles, respectively, were written on cotton and rubber and other recycled natural material e.g., consumed teabags, rubber crumb, indicating a clear vision for experimentation and modeling of natural materials e.g., to investigate their sound absorption coefficients at lower frequencies (below 250 Hz) for (recycled) cotton sound absorbing materials and for producing ultralight density with high sound absorption materials (0.56) using recycled rubber material. There were found 31 patents registered and granted, in total. Natural materials, namely, cotton, rubber, glass wool and rock wool were applied in 29% of patents registered and granted from 1996 to 2022. Yet, 93% of patents registered and granted lacked data on sound absorption index of materials they invented. LDA models estimated 29% of Espacenet and Google Patents patents registered and granted having an increased trend of patent innovations for example, in method, structure and building materials in the last decade.

Keywords

Buildings cotton insulation materials noise patents rubber

Introduction

Noise is perceived as an unwanted sound, and, today, the environmental noise is labeled as “the new secondhand smoke”.¹ Noise can be measured. High levels of noise exceeding WHO guidelines² are measured in urban areas in different parts of the world for example, in Europe,³ India.^4,5 Traditional approaches such as building façade shielding were built-in to reduce environmental noise (e.g., traffic noise) entering in building façade openings like windows acting as entry point of noise.⁶ Building materials insulating or absorbing sounds are used to reduce the unwanted sounds for example, from entering from outdoors. Materials insulating sounds block sounds from entering or leaving a space; materials absorbing sounds, which are classified into porous and resonator material types, improve sound quality preventing indoor unwanted reverberation by transforming absorbed sound energy into heat,⁷ yet, without blocking sounds. Sound absorbing materials present techniques for reducing passive noise reduction.⁸ Combined porous and resonant absorbing sounds materials, respectively, can perform well at high and low frequencies.⁹

The thirteen thermal insulator materials, like stone wool slab, glass wool slab, cellulose slab, wood fiber slab available in the year 2021, were used for acoustic insulation in building interiors as walls, floors, doors, roofs, duct wrappings, and these installations in buildings played an important role in the acoustic properties of the structural application¹⁰ resulting to either air and or impact noise reduction. Today, insulator materials (e.g., rock wool, polystyrene) are commercially available and more in use compared to innovated natural and recycled insulation materials (e.g., rice, cotton, oil palm fiber, glass foam, plastics, recycled textile fibers) that are scarcely commercially developed or not still available,¹¹ and yet, glass and rock wools coming from natural source processes are high energy-demanding building insulation materials.¹¹ While, petrochemicals based porous synthetic materials (e.g., rockwool, glasswool, polyurethane, polyester) sound insulation materials may cause adverse effects on human health and the environment, there is an increasing demand for environmentally-friendly insulation materials.¹²

Nowadays, there are strong reasons for developing sound and or thermal natural based insulation materials. Natural materials and or recycling materials support a circular economy (e.g. Aygun et al.¹³; Martínez et al.¹⁴), present a solution for global environmental conservation and CO₂ reduction,¹⁵ are environmentally friendly,^13,16 and replace porous synthetic materials like glasswool, rock wool and polyurethane that are associated with human health and environmental problems.^17–19 Low-frequency sounds are not efficiently absorbed by natural materials^9,12 emerging new natural fibers with porous structure for noise reduction in building construction like rubber crumb that may significantly absorb low-frequency sounds by increasing the bulk density, flow resistivity of the composite, posing fewer health risks to humans and the environment.¹⁷ New high acoustic resistance materials were produced by combining natural fibers with non-natural fibers,²⁰ like multilayers composed of polylactic acid inter-layers that were combined with natural material layers for example, cotton layer to increase the sound absorption of the composite,²¹ or sustainable thermoplastic sandwich composite panels that were tested using three types of recycled nonwoven fabrics, namely, jute, polyester and hybrid jute-polyester (80%:20%) with polypropylene composites reinforcement, respectively, showing impact sound reduction for jute composites and for hybrid jute-polyester composites in the mid and at high frequencies.²²

Recycled materials are increasingly tested to be used as potential new sound absorbing materials, for they are having new unique features (e.g. light weight, wide frequency range i.e. absorbing low-frequency sounds, and high sound-absorption capacity) for example rubber aerogel produced by recycling waste tire fibers,²³ mussel shell out of canning industry converted into a loose-fill material²⁴ similarly to other bio-based insulation materials, recycled natural fibers coming from agriculture or from other production cycles^8,25 presenting a solution to the environmental pollution and waste. Green materials are also being experimented as sound absorbing materials to replace traditional synthetic materials in the fields of acoustic treatments and energy saving.²⁶ Here, green walls facilitated acoustic insulation, indicating that green walls had a significant potential as a sound (absorbing) insulation tool for buildings.²⁷

This review presented 1- an overview of studies and of the patents registered and granted, respectively, selected by using Scopus search engine, Espacenet and Google Patents search engines on sustainable natural materials (preferably, (recycled) cotton, (recycled) rubber), and their combination with non-natural materials) applied for noise reduction in buildings, and 2-the trends of patent innovations of noise reduction in buildings by using LDAShiny open source application²⁸ that applied Bayesian Probabilistic Model and machine learning tools.

Methods

Preferred Items for SystematicReviews and Meta-Analyses (PRISMA)²⁹ was applied to identify, select and then analyze the literature using Scopus search engine. PRISMA is a well-used framework applied for literature review for example, in building acoustics³⁰ and environment.³¹ A set of formulated questions help identify, select and analyze³² research collected.

Scopus search engine was used for searching relevant studies on noise reduction in buildings addressing questions as follows: (1) Are there studies on cotton and rubber and their recycled materials strictly used for sound absorbing and or insulation in buildings? (2) -What are the acoustic parameters measured (preferably sound absorbing coefficients, and in what frequency ranges)? The inclusion criteria were as follows: (1) -a study tested, experimented, modeled natural materials and or their waste as noise absorption/absorbing material or noise insulation material applied only in buildings, focusing on sound absorbing coefficients and frequencies (when available), (2) researchers preferably used cotton, rubber, recycled cotton, recycled rubber and or their combinations with non-natural materials, (3) either review or articles were written in English language from the years 1996 to 2023. Open access publications were strongly preferred. Dissertations, chapters of books or books and any research articles and reviews on noise reduction of either natural or non-natural materials not focused on buildings, but in vehicles, aircrafts, and energy conservation were excluded from the analysis. Studies of Building Acoustics Journal were manually searched for meeting the inclusion criterion (1–3). Search terms were “noise reduction” AND “buildings” or “cotton” OR “rubber” OR glass wool” OR “rock wool.”

Espacenet and Google Patents search engines were separately used to select patents registered and granted for either air and/or impact noise reduction in buildings in the English language for all the time, respectively obtaining Espacenet and Google Patents patents registered and granted. Google Patents search terms were “noise reduction in buildings” OR “noise attenuation in buildings” OR “noise absorption material in buildings” NOT “engines” NOT “aircraft” NOT “machines”. Espacenet search terms were “noise reduction” OR “noise attenuation” AND “buildings” NOT “engines.” The inclusion criteria were as follows: (1) patents registered and granted were strictly concerned about noise reduction or noise attenuation or noise absorption/absorbing material or noise insulation material applications in buildings (not in engines, aircrafts, machines), (2) patents registered and granted worldwide were to be written and or translated in English language. Patents registered meeting inclusion criteria, but that were withdrawn and or pending, were excluded from the analysis. Two Espacenet natural materials noise reduction patents registered (CN112627371A, CN110616649A), but with pending status (in Google Patents), were included into the analysis. Patents registered and granted for noise reduction in engines, aircrafts, machines (including vehicles) were excluded from the analysis. Abstracts of registered and granted patents, which met inclusion criteria, were respectively manually added into Espacenet and Google Patents patents registered and granted. This study analyzed the natural and recycled materials (see Asdrubali et al.¹¹) of patents registered and granted, and the trends of innovations of noise reduction in buildings by separately using LDAShiny open source application for Espacenet and Google Patents patents registered and granted. The patents registered were downloaded as a csv file to upload at LDAShiny open source application. The title (as ID document), abstract (as document vector), year of patent publication were preprocessed.²⁸ There were removed words as “step,” “provided,” “based,” “primary,” “however,” “comprises,” “relates,” “arranged,” “side,” “provided,” “connecting,” “wt,” “jp,” “us,” “ca,” “ru,” “kr,” “cn,” “de,” “fr,” “first,” “cold,” “hot,” “present,” “weight,” “left,” “making,” “aggregates” with Sparsity of 99.5% allowing terms that were present in more than 0.5% from the preprocessing analysis. An optimal number of topics was defined by the parameter of k that was estimated by Coherence metric (see De la Hoz et al.²⁸). This k parameter defined the α (by 50/k)³³ to fit-in in the LDA model. The LDA model estimated the probabilities of each patent registered and granted for “noise reduction” topics (theta matrix). “Allocation of documents to the topics,” helped to select the most relevant patents (see De la Hoz et al.²⁸) registered and granted on the topics of “noise reduction” and “building.” The LDA model estimated the trends of innovations of noise reduction in buildings.

Results

Studies

In total, 32 studies were identified using the Scopus searching engine. Yet, 10 articles were not relevant to three inclusion criteria (see Methods) and were excluded from the analysis. The six studies were obtained on natural materials of cotton, rubber, glass wool, rock wool, and polystyrene. Key findings of the four most recent studies (from the years 2020 to 2023) on natural materials, namely, kenaf, teabags, banana fibers were summarized, yet, meeting inclusion criterion one and three (see Methods). A summary of four studies and or reviews (from the years 2014, 2015, 2022) on green walls and or green roofs were summarized.

Natural fibers were categorized as genuinely sustainable and green sound absorbers for buildings using environmentally friendly materials, which were either obtained from agricultural products or from other production cycles, mostly, as by-products.⁸ Taban et al.⁸ studied the acoustic properties of porous absorbers made of natural Kenaf fibers by testing samples of sound absorber with thicknesses of between 10 and 40 mm at two different bulk densities of 150 and 200 kg/m³, and found that their sound absorption coefficient (SAC) at low, mid, and high frequencies increased significantly with increasing the bulk density having values of SAC of 0.95, 0.85, and 0.7 for frequencies above 1250 Hz for samples of 40, 30, and 20 mm thickness, respectively.

Aygun et al.¹³ tested potential applications of consumed teabags as sound attenuation materials for example, in built environment using circular teabag panels in an impedance tube using a transfer function method to determine their sound absorption coefficient and transmission loss obtaining a SAC value of higher than 0.8 for frequencies between 400 and 1600 Hz for 75 mm thick circular panels, and found up to 9.8 dB sound transmission loss of circular panels at higher frequencies.

Singh et al.³⁴ tested acoustic properties for example, sound absorption and absorption, sound insulation, and transmission loss with fibrous structures made of banana fibers with samples of areal density of 1500, 3000, and 4500 gsm tested in frequencies from 150 to 6600 Hz (see also Figure 5 in Singh et al.³⁴). Areal density samples of 4500 gsm showed a higher and more consistent sound absorption coefficient, for 4500 gsm samples had a higher areal density, were thicker and had a larger number of air cavities (compared to 1500 gsm, 3000 gsm samples) resulting to a higher sound absorption coefficient. Lengthy tortuous path of 4500 gsm samples resulted in more heat energy converted from sound wave energy compared to samples of 1500 and 3000 gsm. The highest and the most consistent, (with maximum noise reduction coefficient value of 0.78 and transmission loss up to 23 dB), was obtained for areal density of 4500 gsm of banana fibers (Table 1).

Table 1.

Sound absorption coefficients from and above 0.56 measured at low frequencies (125, 200 Hz), mid and high frequencies for cotton, rubber, and their recycled materials, and from and above 0.78 starting from the frequency of 150 Hz and higher for kenaf, consumed teabags and banana fibers tested as sound absorbing materials in buildings by publications between 2020 and 2023. NA* refers to no available information in section snippets. NA** no figure was available about sound absorbing coefficient, though, it was indicating that the innovation may reduce sounds by 25–36% higher than common materials.

Study/type of natural material	Sound absorption coefficient	Thickness	Density	Frequency	References
Cotton (carbonized)	0.626	1 mm	NA*	200–6000 Hz	9
Cotton (stalks)	0.65	50 mm	NA	>500 Hz	35
Cotton (recycled)/denim (recycled)	0.95	89 mm	25–45 kg/m³	125 Hz	11
Rubber (aerogels)	0.56	3–8 mm	91 mg/cm³	NA*	23
Kenaf fibers	0.95	40 mm	150, 200 kg/m³	>1250 Hz	8
Teabags (consumed)	0.80	75 mm	208.5 kg/m³	400–1600 Hz	13
Banana fibers	0.78	36 mm	4500 gsm	150–6600 Hz	34
Patent//Type of natural material	Sound absorption coefficient	Thickness	Density	Frequency	References
Cotton	NA**	20 mm	NA	NA	41
Rubber	23–35 dB (transmission loss)	NA	0.6–1.1 g/cm³	100–2500 KHz	42
Cotton	NA	20 mm	NA	NA	39
Rubber	NA	50 mm	NA	NA	39

Bousshine et al.¹⁸ studied several natural material coming from vegetative, agriculture and animal wastes, namely, date palm (trunk, petiole, pinnate leaves, bunch, and fiber mesh), reed, esparto, olive tree, fig tree, wood sawdust, chicken feathers, and sheep wool resulting to high sound absorbing coefficients SAC and thermal insulation values for esparto (0.9, 0.065 W/m.K), petiole (0.88, 0.072 W/m.K), chicken feathers (0.94, 0.045 W/m.K), and sheep wool (0.95, 0.044 W/m.K) that were comparable with glass wool (0.95, 0.044 W/m.K) presenting potential alternatives for replacing synthetic materials.

Dong et al.⁹ tested carbonized cotton (1 mm thickness, and included 1.3 mm diameter perforation holes that were hexagonally arranged 4.7 mm apart) with honeycomb pore structure that was made using aramid paper honeycomb with 50 mm thickness and 5.5 mm side length to increase the sound-absorbing performance of the structure at low frequency with no increase of the structure weight, and found that sound absorption coefficient resulted to an average sound absorption coefficient of 0.626 ± 0.0280 for frequencies in the range between 200 and 6000 Hz.

Aly et al.²² tested three types of recycled nonwoven fabrics jute 100%, polyester (PET) 100% and hybrid jute-polyester (80%:20%), respectively weighting of 800, 800, and 1200 g/m², with polypropylene (Polypropylene [PP] with weight of 40 g/m²) as the matrix in composites reinforcement evaluating the acoustic insulation performance of sandwich composite panels in terms of impact sound attenuation. All composite samples recorded an increased impact sound reduction at low frequencies (below 200 Hz), jute and hybrid jute-polyester composites showed a decrease in impact sounds reduction at high frequencies. The impact sound reduction index ΔLw ranged between 12 and 21 dB for all composites. The thermal conductivity for three types of recycled nonwoven fabrics were between 0.03549 and 0.04912 W/mK for PP/Jute samples, between 0.03795 and 0.04990 W/mK for the PP/PET and between 0.03873 and 0.05371 W/mK for the hybrid Jute-PET samples.

Thai et al.²³ converted rubber fibers from tire waste into aerogels, having low density, high porosity, largely elastic, low thermal conductivity, and a high sound absorption coefficient using freeze-drying process. Rubber aerogel with a robust mechanical performance, had a Young’s modulus up to 965.6 kPa and density 91 mg/cm³, had a porosity above 90%, a ultralight density of between 20 and 91 mg/cm³, and a noise reduction coefficient of 0.56 indicating a high sound absorption efficiency, and a low thermal conductivity between 0.035 and 0.049 W/mK.

Asdrubali et al.¹¹ presented a state of art of sustainable natural materials, namely, unconventional natural materials: reeds, bagasse, cattail, corn cob, cotton, date palm, durian, oil palm fiber, pineapple leaves, rice, sansevieria fiber, sunflower, straw bale and recycled materials, namely, glass foam, plastics, textile fibers and their main acoustic, and thermal properties, which were not necessarily developed and present in the market. It was found that recycled cotton insulation had density (25–45 kg/m³) and thermal conductivity (0.039–0.044 W/mK) comparable to expanded polystyrene (EPS) (density 15–35 kg/m³; thermal conductivity 0.031–0.038 W/mK), extruded polystyrene (XPS) (32–40 kg/m³; thermal conductivity 0.032–0.037 W/mK) and sheep wool (10–25 kg/m³; thermal conductivity 0.038–0.054 W/mK). Recycled denim materials showed high measured sound absorption of (NRC = 1.15), reed-based materials had density (130–190 kg/m³) and specific heat (1.2 kJ/kgK) similar to rock wool (density 40–200 kg/m³; specific heat 0.8–1.0 kJ/kgK), and had a higher thermal conductivity value (0.045–0.056 W/mK) than rock wool (0.033–0.040 W/mK), though. Recycled textile sound absorption material had a sound absorption coefficient higher than 0.85 for frequencies above 500 Hz, and recycled denim had high values of sound absorption of 0.95 at low frequencies of 125 Hz.

Asdrubali et al.³⁵ presented an updated survey on the acoustical properties of sustainable materials, and of composite materials and systems, like green roofs and green walls that used recycled waste and plants incorporating recycled fibers and granulates in the form of porous substrate used for plants to grow. There were various studies that were conducted, particularly between the years 2008 and 2012 indicating that green roofs and green walls had positive effects to noise attenuation, noise reduction with sound absorbing coefficients reaching up to 10 dB and transmission loss up to 13 dB (for frequencies between 50 and 2000 Hz for green roofs), and sound absorption coefficient of 0.5 and between 7 and 10 dB insertion loss for frequencies between 500 and 10,000 Hz (for green walls).

Bakker et al.³⁶ showed that unless air cavities or resonators were introduced, vertical greenery effectively reduced mid and high frequency noise acting as porous absorbers for sounds frequencies between 125 and 8000 Hz with a sound absorption coefficients ranging from 0 to approximately 0.5 at frequency of 125 Hz, ranging from approximately 0.1 to 1.2 at frequency 250 Hz and approximately 0.9 at frequency of 8000 Hz (referring to Figure 15 of Bakker et al.³⁶). Yet, several studies suggested a cavity between (the supporting) wall and the substrate to increase the sound absorption at low frequency that is, below 250 Hz.

Azkorra et al.²⁷ measured the acoustic characteristics in laboratory conditions of a module-based green wall to better understand the contribution of vertical greenery systems to noise reduction in buildings, noting: the (1) the small number of studies on green walls used as an acoustic insulation, (2) very different methodologies used by studies and (30 no consistent conclusions obtained, when comparing the study methodologies. Azkorra et al.²⁷ used recycled polyethylene modules (a closed box filled with coconut fiber) of 600 mm wide by 400 mm high and 80 mm thick, and resistant to UV radiation measuring airborne sound insulation at frequencies 125, 250, 500 Hz, 1000, 2000, and 4000 kHz, and measured the sound absorption in a reverberation room using building materials as coarse concrete block, unglazed brick, glass, wood (10 mm thick), fiberglass board (25 mm thick). Azkorra et al.²⁷ concluded that the green wall showed a close or better acoustic absorption coefficient value than building materials, and green walls absorbed more low frequency sounds than some other sound absorbent materials in use. Green wall sound absorption coefficients were approximately between 0.4 and 0.5 for frequencies 125, 250, 500, 1000, 2000, and 4000 Hz, which were higher than nine building materials (unglazed and painted brick, block-painted concrete, smooth on tile/brick plaster, 3/8″ plywood panel wood, unglazed brick, coarse concrete block, glass, marble/tile, and fiberglass board [25mm(1″) thick]) at frequencies 125 and 250 Hz and higher than eight building materials for frequencies 500, 1000, 2000, and 4000 Hz, besides fiberglass board (25mm(1″) thick) (referring to Figure 12 of Azkorra et al.²⁷).

Recent reviews on green walls showed that green walls can efficiently absorb sounds with frequencies between 800 and 5000 Hz, but hardly sounds of lower frequencies than 800 Hz³⁷. Oquendo Di-Cosola et al.³⁸ reviewed studies on green walls focusing on plant species or substrates and found that green walls had a positive impact on noise by reducing it by 80% thanks to the substrate with a specific contribution of vegetation, particularly dense vegetation that can increase the sound absorption coefficient by 0.2–0.3.

Patents

There were respectively 14 and 20 Espacenet and Google Patents search engines patents registered and granted for noise reduction in buildings. Three patents, which were withdrawn and pending, were removed from Google Patent records resulting in 17 Google Patent patents registered and granted from 1996 to 2022 (Figure 1).

Figure 1.

The Espacenet and Google patents number and year of publication of patents registered and granted from 1996 to 2022.

Natural and or recycled (cotton, rubber, glasswool, rockwool) noise reduction patents, respectively, resulted in three Espacenet (numbered two, eight, nine in Supplemental Table A1) and seven Google Patents patents registered (four, six in Supplemental Table A1) and granted or respectively, 9.7% and 19.3% (Figure 2) out of 31 Espacenet and Google Patents patents registered and granted. Combined natural material types with non-natural materials resulted in three Espacenet patents registered. Expanded polystyrene, foamed polypropylene materials were three Espacenet and one Google Patents patents registered and granted, respectively. Natural or recycled noise reduction material types (glasswool, rock wool, cotton and rock wool, and cotton and or rubber, combined natural material types with non-natural materials and expanded polystyrene, foamed polypropylene) resulted in 51% (out of 31) Espacenet and one Google Patents patents registered. Cotton and or rubber, and glasswool were 56.2% out of sixteen natural and or recycled noise reduction patents, which resulted in Espacenet (9) and Google Patents (7) patents registered, Figure 2. Category “Others” are non-natural materials noise reduction patents combined with natural materials like recycled rubber and green plants mounted in non-natural materials sound barriers, and expanded polystyrene, foamed polypropylene. There were respectively one Google Patent and six Espacenet patents registered in the category “Others,” Figure 2.

Figure 2.

The type of natural material noise reduction patents resulted in Espacenet shown in outer ring (numbered 2, 8, 9 in Supplemental Table A1) and seven Google Patents shown in inner ring patents registered (4, 6 in Supplemental Table A1) and granted from 1996 to 2022, in percentage. Category “Others” are non-natural materials (e.g. metal) noise reduction patents that used natural material like (recycled, waste) rubber (numbered 23 and 13), green plants (numbered 29), and expanded polystyrene (numbered 3) in Supplemental Table A1. Fields of technology applied by patents registered and granted selected by LDA models are in Supplemental Table A1.

Yao et al.³⁹ presented a utility model for damping sound insulation of laboratory structure constructed for animal, which applied sound absorbing cotton and rubber in different parts of a novel laboratory construction for example, on top surface, on the ground, on walls (cotton) and on the floor (rubber and plastic) reducing noise and vibrations in the laboratory construction. The construction was a simple, low-cost, effective structure reducing sounds and vibration, less affecting laboratory animals. No sound absorbing coefficient figure was given by this patent.

Guo et al.⁴⁰ presented a new low density polymer-based sound insulation and noise reduction material sound. The low density polymer-based sound insulation and noise reduction material was based on two types of different macromolecular materials or polymer-based composite having different density and modulus; density ratios and modular ratio were above 1.2. Rubber was in the macromolecule matrix, because experiments showed that rubber influenced the composite sound insulation capacity. Overall, transmission loss varied from 23 to 35 dB for five embodiments. No sound absorbing coefficient figure was provided.

There were no patents registered and granted applying rice, oil palm fibers, pineapple leaves and any other natural (see Methods) and or recycled glass, plastics, textile materials. A patent was found on sound barrier structure covered by green plants (CN110616649A). No patents were found on green walls and or green roofs.

The optimal number of topics, which was defined by the parameter of k, and estimated by Coherence metric was, respectively, ten and eight for Espacenet and Google Patents patents registered and granted. Using terms of “noise reduction” in “Allocation of documents to the topics,” resulted in ten Espacenet and two Google Patents patents registered and granted, shown in Supplemental Table A1. “Building” terms allocated three Espacenet (3, 4, 6 in Supplemental Table A1) and two Google Patents (4, 6 in Supplemental Table A1) patents registered and granted.

There were eight Espacenet and one Google Patents patents registered that showed an increasing trend of innovations of noise reduction in buildings shown in Supplemental Table A1.

Conclusions

This study concluded that natural materials like cotton and rubber were obviously researched and patented for noise reduction in buildings. Cotton was either studied as a primary noise reduction material in building (e.g. Dong et al.⁹) or was combined as a cotton layer increasing the sound absorption by 30% of multilayer structures consisting of for example, glass fiber cloth reinforced polylactic acid inter-layers.²¹ Experiments of natural materials were clearly showing that natural materials for example, kenaf, wood, hemp were less efficient in absorbing sounds of low frequencies for example, 125, 250 Hz,¹² yet sound absorption of low frequencies could be increased by making for example, a sandwich-type material.¹⁴ Materials produced from either recycled cotton or recycled textile fibers (e.g. Asdrubali et al.¹¹) or by recycling waste tire fibers (e.g. Thai et al.²³) were interesting recycled materials in that they may be used as sound absorbing and or sound insulating materials indicating a promising opportunity to tackle environmental pollution and waste, supporting a circular economy.

There was sound absorbing cotton used in a plate for noise reduction in Zhang⁴¹, and noise reduction was 25% to 36% higher compared to common noise reduction materials (Table 1). There was a detailed description including sound insulation in dB of materials used in Guo et al.⁴² and those were compared with traditional ones. The 93% out of 31 patents registered and granted did not provide any data on sound absorption index of the noise reduction materials they claimed. This absence of a sound absorption index of materials used for noise reduction is not new. For example, Hongisto et al.¹⁰ measured the acoustic performance of commercially available of building thermal insulators identifying differences in acoustic performance of building thermal insulators, thus, concluding that six acoustic quantities (namely, sound reduction index of bare insulator, sound reduction index of encapsulated insulator, sound absorption coefficient, airflow resistivity, dynamic stiffness, and reduction of impact sound pressure level) were to be considered by manufacturers when designed.

Natural cotton and rubber were applied in Espacenet patents and granted numbered 8, 9 and cotton in Espacenet patents and granted numbered 2 and Google Patents patents and granted numbered 4 and 6 in Supplemental Table A1. There were 29% of patents Espacenet and Google Patents registered and granted applying natural materials such as cotton, rubber, glass wool and rock wool, excluding the category of “others” (Figure 2), from 1996 to 2022.^10,11,35 concerned the acoustic properties of natural and recycled building insulator materials and of commercially available thermal insulators. For example, Asdrubali et al.¹¹ reported the state of art of building insulation products made of natural and or recycled environmentally-friendly materials that were at their early state of development or were scarcely developed as well as data of acoustic performance of natural and recycled materials. These natural and recycled materials as cotton were not commonly in use like polystyrene that was a petrochemical made material,¹¹ though, polystyrene resulted in a higher impact noise reduction (30 dB) compared to some natural materials like sheep wool (18 dB) (see Asdrubali et al³⁵). Yet, Bousshine et al.¹⁸ found competitive natural materials to glass wool in terms of sound absorbing coefficients and thermal conductivity, and Shaid Sujon et al.⁴³ stated that experiments had shown a higher acoustic absorption of cotton fibers in the polymer matrix composites (PMC) fiber assemblies.

Here, approximately 9% out of 31 patents registered and granted applied glass wool, rock wool^44–46 that were patented between 2002 and 2005. Cotton, rubber, natural materials were applied in about 22% (out of 31 patents) patents published in 1996⁴⁷ and between 2020 and 2021, for example, Espacenet patents numbered 2, 8, 9 and Google Patents patents numbered 4 and 6 in Supplemental Table A1. Data on airborne and or impact noise reduction and or as sound absorption index were not provided in dB in a large number of patents registered and granted (29 out of 31). Overall, 29% of Espacenet and Google Patents patents registered and granted with the highest estimated probabilities by LDA models showed increased trends of noise reduction in method, structure and building materials and in a device for noise reduction and energy-saving in buildings between 2012 and 2022 (Supplemental Table A1).

There were found no patents registered and granted selected by LDA models applying any innovation about water and sound absorption for natural materials. D’Alessandro at al.⁴⁸ showed by experiments that a correlation existed between water content and sound absorption for natural made thermo-acoustic materials used in buildings.

Three Espacenet patents registered showed two methods and a utility method applied in constructions and buildings combining recycled/waste rubber and green plants. Here, patent numbered KR20180025420A⁴⁹ newly showed a method using recycled rubber, expanded polystyrene and concrete to reduce interlayer noise of a reinforced concrete apartment house having a sound absorbing performance the same as of a recycled rubber, while the patent registered CN110616649A⁵⁰ showed a new utility model for a sound insulation and noise reduction structure (barrier) composed of two sound barrier structure of metal perforated plate where the second barrier was covered with green plants.

Green walls or green roofs used, as a passive noise reduction application in buildings were not aimed in this study, though, considering the positive effects to carbon storage ability of plants, green roofs, green walls, and façades presented an opportunity for applying vertical greenery in urban constructions indoors and outdoors and for climate change (urban heat, carbon storage) in urban areas. Green walls were one of the most promising building greenery systems, yet, few studies focusing on green wall sound insulation potential were conducted (Azkorra et al.²⁷), for a large number of studies on green facades and roofs were likely focusing on thermal and energy performances, stormwater management, water treatment, and use.³¹ Here, it was found that green walls using dense vegetation (plants) effected the acoustic absorption capacity.³⁸ Here, wild plants that are native and or naturalized can be studied for a potential use as sound absorption and or sound insulation plants in green walls as they may require less water (e.g. Benvenuti et al.⁵¹), which might be essential during water shortage in summer, for example, in Mediterranean countries. Recycled natural materials, like straw, had acoustic performance similar to commercial materials already in use, and could be applied on wall structures contributing to lower greenhouse gas emissions,⁵² to reduce noise,^27,35 for example, of mid and high frequencies,²⁷ in new and old buildings, where acoustic insulations of old buildings were in need in Europe as well as worldwide.⁵³

Supplemental Material

sj-pdf-1-bua-10.1177_1351010X241268280 – Supplemental material for An overview of patents of noise reduction in buildings

Supplemental material, sj-pdf-1-bua-10.1177_1351010X241268280 for An overview of patents of noise reduction in buildings by Kuenda Laze in Building Acoustics

Footnotes

Annexes

(Table A1).

The Espacenet and Google Patents title and number of patent registered and granted, LDA model estimated probability of patent registered and granted, theta, increasing trends (↟) and decreasing trends (↡) for natural materials (including expanded polystyrene) shown in green color, increasing trends (↟) and decreasing trends (↡) for non-natural material shown in dark yellow color, increasing trends (↟) and decreasing trends (↡) for non-natural materials also used natural material like (recycled, waste) rubber (citation numbered 56) and green plants (citation numbered 61) shown in light yellow color.

Declaration of conflicting interests

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Kuenda Laze

Supplemental material

Supplemental material for this article is available online.

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