TECHNICAL FIELD

The present document relates to the absorption of sound waves, e.g. for providing soundproof rooms and/or headphones.

BACKGROUND

The absorption of sound waves is of great industrial interest. Applications include the reduction of leakage of sound and/or noise to and/or from adjacent rooms or outdoors, the acoustic treatment of listening rooms and sound studios, the reduction of external noise when using headphones, etc. The phenomenon of sound absorption is intrinsically frequency dependent, and has led to a vast field of design strategies to provide materials, which are assembled in different patterns and geometries, and which are typically dedicated at performing sound absorption at different ranges of the audible sound spectrum. Typically, relatively thick layers of different materials are interleaved in order to obtain a desired degree of soundproofing over a relatively large frequency range. For example, by interleaving a number of layers of acoustic foam, concrete and plaster board adjacent apartments in a building or different rooms of a multiplex cinema may be acoustically isolated. For anechoic rooms or high-quality listening rooms air gaps may be used as a layer for sound isolation.

To further improve the acoustic properties of a room, sound absorbing materials may be combined in different patterns and topologies to perform sound diffusion on the fraction of incident sound energy that is not absorbed by the materials. US 2015/017360 Al discloses an insulated component (1) of a household appliance, in particular of a dishwasher, comprising a substrate (10) and an insulation structure (12) applied onto a surface (13) of the substrate (10); wherein the insulation structure (12) comprises one or more layers (14) made of one or more sprayable filled polyurethane materials, wherein said one or more layers (14) comprise a layer (14a, 14d) made of elastomeric polyurethane material applied directly onto the surface (13) of the component (1) and having the following properties: a specific density comprised between 1 and 3 g/cm3; a tan delta at 20° C. and at 100-300 Hz comprised between about 0.4 and 1.6; and a tan delta at 60° C. and at 100-300 Hz comprised between about 0.5 and 2.2. The present document is directed at the technical problem of providing a sound absorbing material and/or a sound absorbing system comprising such a material, wherein the sound absorbing material exhibits beneficial sound absorption characteristics, such as a relatively high acoustic absorption coefficient. As the same time, the sound absorbing material and/or system should have a compact size.

SUMMARY

According to an aspect, a sound absorption system comprising at least one layer of material is described. The material is such that it exhibits a phase transition at a change-of-phase- temperature (CPT). In addition, the material is configured to absorb acoustic energy during said phase transition. Furthermore, the sound absorption system is such that the temperature of the material is (e.g. maintained) at the CPT. According to another aspect, the use of a phase transition of a material for performing sound absorption is described.

According to a further aspect, a method for improving acoustic isolation of a physical structure (e.g. of a wall) is described. The method comprises providing a layer of material on or within the physical structure, wherein the material exhibits a phase transition at a change- of-phase-temperature (CPT). Furthermore, the method comprises setting a temperature of the material to the CPT.

According to another aspect, an absorption system for absorbing energy of a wave is described. The absorption system comprises a layer of material, wherein the material exhibits a phase transition at a change-of-phase-temperature (CPT). Furthermore, the absorption system comprises a temperature setting unit configured to provide thermal energy to and/or to draw thermal energy from the layer of material, for setting a temperature of the layer of material to the CPT.

It should be noted that the methods and systems including its preferred embodiments as outlined in the present patent application may be used stand-alone or in combination with the other methods and systems disclosed in this document. Furthermore, all aspects of the methods and systems outlined in the present patent application may be arbitrarily combined. In particular, the features of the claims may be combined with one another in an arbitrary manner. SHORT DESCRIPTION OF THE FIGURES

The invention is explained below in an exemplary manner with reference to the

accompanying drawings, wherein

Fig. 1 shows an example temperature-energy curve for an example material;

Fig. 2 shows a block diagram of an example sound absorption system;

Fig. 3 shows an example control scheme for a sound absorption system;

Figs. 4a and 4b show example applications for a sound absorption system; and

Fig. 5 shows a flow chart of an example method for absorbing sound.

DETAILED DESCRIPTION

As outlined above, the present document is directed at the technical problem of sound absorption. Sound absorption may be achieved using various different types of materials. Although the materials which may be used for sound absorption can be very different in nature and implementation, these materials are typically based upon the same microscopic principle: The incident sound wave interacts with the molecules of the material by converting part of its acoustic energy into vibrations of the material molecules. These vibrations travel across the material, exciting other molecules along their path. In some cases, these excitations correspond to collective motion of entire layers and/or parts of the material. By way of example, this phenomenon may be observed when an incident acoustic wave hits a soft curtain. Typically, part of the energy of the sound wave is reflected back, meaning that the material's molecule excitations re-excite air molecules, thereby generating a reflected acoustic wave. Part of the energy fully travels through the material and is converted into a sound wave on the other side, thereby generating a transmitted wave. The above mentioned processes are typically not lossless. Part of the original sound wave's energy may be dissipated into heat and friction of the material's molecules, thereby partially absorbing the sound wave.

It has been observed that a substantial amount of acoustic energy may be absorbed during the phase transition of a material. When heat is supplied to a material, the temperature of the material rises. This is illustrated in Fig. 1, where it can be seen that the temperature 102 of a material rises continuously with the amount of thermal energy 101 that is provided to the material. Materials typically exhibit a so-called Change-of-Phase Temperature (CPT) 104. At the CPT 104, all the heat (i.e. all the thermal energy 101) which is supplied to the material is used to change the internal structure of the material. In particular, the thermal energy 101 is used for braking up and/or changing intermolecular bonds between the molecules of the material. Hence, at the CPT 104 the thermal energy 101 is used to perform a phase transition of the material (e.g. from solid to liquid, or from liquid to vapor).

When the entire material sample has undergone the phase transition, newly supplied thermal energy 101 again leads to a rise of temperature 101. The properties of the material within the new phase are typically very different from the properties of the material in the initial phase. Examples of such phase transitions are water- liquid or liquid- vapor transitions of H ₂ 0 molecules, or ferromagnetic phase transitions in magnets. At the CPT 104, both phases may co-exist, in proportions which depend on the amount of thermal energy 101 which is supplied to the material.

The aforementioned properties of materials at the CPT 104 may be used for absorbing the energy of acoustic waves. In this context, different materials may be used which undergo phase transitions at different temperatures, including typical room temperatures (around 20°C). It has been observed that during the change of a phase of a material, a sound wave, when hitting the material in the form of acoustic energy, is not partially absorbed by means of the standard mechanical phenomenon mentioned above. Indeed, a significant fraction of the energy of the sound wave, instead of being converted into kinetic motion of the material's molecules, is used to break/weaken the intermolecular bounds of the material. In other words, the energy of an acoustic wave is used to speed up the change of phase. More generally it has been observed that, if a material undergoes a phase change of a particular physical property (e.g. a change of its ferromagnetic properties), a significant fraction of a sound waves' energy is used to perform the phase change of a particular physical property. As a result of this, only a relatively small fraction of the sound waves' energy is reflected back or fully transmitted through. Hence, materials at their CPT 104 may be used for efficient sound absorption.

The energy in a typical acoustic field is many orders of magnitude lower than the energy which is required by a typical material sample to fully undergo a phase transition. Hence, sound absorption may be achieved with relatively low amounts of material. As such, space efficient sound absorption systems may be provided using a layer of material at the CPT 104 of that material. Fig. 2 shows a block diagram of an example sound absorption system 200. The sound absorption system 200 comprises a receptacle 202 which is configured to hold a sound absorbing material 205. The sound absorbing material 205 exhibits a phase transition from a first phase to a second phase at a change-of-phase temperature 104. The material 205 has different physical properties within the first phase and within the second phase. By way of example, the material 205 may be within different states (solid (e.g. crystal), liquid, gas, plasma) in the first and in the second phase. The receptacle 202 may be configured to carry the material 205 within the first phase and within the second phase. By way of example, the receptacle 202 may be flexible in order to cope with a change of volume of the material 205 during the phase transition. At the same time, the receptacle 202 may be sufficiently stable and/or rigid, in order to maintain the material 205 in a layered shape.

The material 205 within the sound absorption system 200 may be at the CPT 104 of the material 205. This may be achieved by using the sound absorption system 200 within an environment (e.g. within a room) which naturally is at the CPT 104 of the material 205.

Alternatively or in addition, the sound absorption system 200 may comprise a temperature setting unit 203 which is configured to provide thermal energy to and/or to draw thermal energy from the material 205. In other words, the temperature setting unit 203 may be configured to heat and/or to cool the material 205. As such, the temperature setting unit 203 may be used to set the temperature 102 of the material 205 to the CPT 104.

Furthermore, the sound absorption system 200 may comprise a temperature sensor 204 which is configured to provide an indication of the actual temperature 102 of the material 205 within the layer of material 205. The temperature setting unit 203 may be controlled in dependence of the indication of the actual temperature 102. In particular, the sound absorption system 200 may comprise a control unit 201 which is configured to control the temperature setting unit 203, possibly in dependence of the indication of the actual temperature 102. Fig. 3 shows an example control scheme which may be implemented by the control unit 201. The CPT 104 of the material 205 may be used to set the reference signal 304 of the control scheme, and the actual temperature 102 of the material 205 may provide the actual signal 303 of the control scheme. Using an error determination unit 313, the control error 301 may be determined (as the difference between the reference signal 204 and the actual signal 301). The control error 301 is input into a controller 311 (e.g. a P(roportional), I(ntegral) and/or D(ifferential) controller) to provide the system input 302 which is used control the temperature setting unit 203. Furthermore, Fig. 3 shows the actual system 312, which is to be controlled by the control scheme.

By using a control scheme, the material 205 may be maintained at the CPT 104, thereby ensuring that the material 205 is in a condition to absorb a substantial amount of energy of a sound wave 211 emitted by a sound source 210. A material 205 having a CPT 104 at typical room temperature may be obtained directly from nature, or may be engineered. If the material 205 is in liquid or vapor state in at least one its two relevant phases, the material 205 may be embodied inside a receptacle 202 (e.g. a cover) which is aimed at confining the material 205 within a desired layer. The material 205 may be set to its CPT 104, by temperature setting means 203 (comprising e.g. a set of resistors and/or a cooling system) or as a natural consequence of activities taking place in the vicinity of the material 205.

An example for the use of a sound absorption system 200 is soundproofing of a sound recording studio 400 (see Fig. 4a). The material 205 used within such an application may be designed to undergo a solid- liquid phase transition at a CPT 104 of Tc=20°C. The material 205 may be covered by a thin plastic shell as a receptacle 202, and the receptacle 202 comprising the material 205 may be located on top of or within the walls of the studio 400. The receptacle 202 prevents the material 205 from spreading, when transitioning to its liquid phase.

During night time, when the studio 400 is not used, its temperature naturally falls below Tc, so that in the morning, the material 205 may be entirely in solid state. When engineers enter the studio 400, start switching on electronic equipment, and conduct their normal activities, the room temperature naturally rises some degrees above Tc. When Tc is reached, the material 205 begins its change-of-phase without raising its temperature 102. This transition phase lasts for as long as the material 205 comprises solid parts. Hence, the duration of the transition phase may be set by changing the amount of material 205 comprised within the receptacle 202. Alternatively or in addition, a temperature setting unit 203 (notably a cooling system) may be used to maintain the material 205 at its CPT 104.

When the material 205 is at Tc (i.e. at the CPT 104), a significant fraction of the energy of sound waves that are generated inside (or outside) the studio 400 is absorbed and used to change the physical properties of the material 205, when the sound waves reach the layer of the material 205. In particular, the energy of sound waves is used to turn remaining solid parts of the material 205 into liquid. As long as the material 205 is at its CPT 104, and as long as some solid fraction of the material 205 remains, the studio 400 is substantially sound- isolated.

In another example, a layer of material 205 and/or a sound absorption system 200 may be used for isolating headphones 410 (see Fig. 4b). The headphones 410 comprise pads 411 which carry a speaker 412 and which comprise a layer of material 205 at CPT 104 (e.g. at Tc=25°). The material 205 may be kept at CPT 104 by the ambient temperature and/or by a temperature setting unit 203. By doing this, a substantial isolation from outside noise may be provided.

A layer of material 205 at CPT 104 and/or a sound absorption system 200 may be fitted to any kinds of walls and/or furniture. By doing this, wideband acoustic absorption may be provided for decreasing room reflections and reverberation time. Furthermore, a layer of material 205 at CPT 104 and/or a sound absorption system 200 may be provided as part of standard office equipment to provide improved isolation between adjacent desks, and/or within restaurants to isolate adjacent tables.

Experiments with various different materials 205 and various types of phase transition have shown that sound absorption in the range between 0.1 and 10 dBs (per centimeter of material 205 used within a layer) within different frequency bands of the audible spectrum may be achieved. Phase transitions may be classified into first-order phase transitions and second-order phase transitions. First-order phase transitions are transitions that involve latent heat (i.e. that involve the release or the absorption of energy at a constant temperature 102). During such a first-order phase transition, a material 205 absorbs or releases a fixed amount of energy per volume of the material 205. During the phase transition, the temperature 102 of the material 205 stays constant as heat 101 is added. During the phase transition, the material 205 is in a "mixed-phase regime" in which some parts of the material 205 have completed the transition and other parts of the material 205 have not completed the transition. Examples for first-order phase transitions are the melting of ice or the boiling of water.

Second-order phase transitions may also be referred to as continuous phase transitions.

Second-order phase transitions are characterized by a relatively large (up to infinite) correlation length. As a result of a relatively large correlation length, a material 205 which performs a second-order phase transition does not exhibit a "mixed-phase regime" during the phase transition. In contrast to a first-order phase transition, the complete layer of material 205 transitions from an initial phase to a target phase in a (substantially) single step.

Examples of second-order phase transitions are the ferromagnetic transition, superconducting transition and the superfluid transition. The sound absorption properties described in the present document are not limited to first- order phase transitions. Further benefits may be observed for materials 205 that undergo second-order phase transitions. Due to the relatively large correlation length a relatively large portion of the material 205 at the CPT 104 reacts to a given local perturbation of the material 205. This allows for the design of absorbing layers of material 205 that are particularly efficient at low acoustic frequencies. Typically, low-frequency acoustic absorption is attained by using very thick and massive layers of sound absorbing material. The use of materials 205 which exhibit a second-order phase transition allow for the use of relatively thin layers of material 205 (as long as the overall area of available material 205 is relatively large). The reason for this behavior is that upon incidence of a low- frequency (and hence, long wavelength) sound wave onto a relatively small portion of a layer of material 205, a relatively large fraction of the entire layer of material 205 responds to the local excitation (due to the relatively large correlation length) by performing a change-of-phase. Consequently, low- frequency acoustic waves may be absorbed well by relatively thin and extended layers of a material 205 which is at the CPT 104 of a second-order phase transition.

It should be noted that the absorption system 200 may also be used for absorbing the energy of other types of waves such as electromagnetic and/or mechanical waves.

As such, the present document describes a sound absorption system 200 which comprises a layer of material 205. The material 205 exhibits a phase transition at a change-of-phase- temperature 104, referred to as CPT. As outlined above, the material 205 is configured to absorb acoustic energy during said phase transition. Furthermore, the temperature 102 of the material 205 is at the CPT 104. In particular, the sound absorption system 200 may be configured to maintain the temperature 102 of the material 205 at the CPT 104. By providing a layer of material 205 at the CPT 104, a substantial fraction of energy of a sound wave may be absorbed by the layer of material 205, thereby providing an efficient sound absorption system.

The sound absorption system 200 may comprise a temperature setting unit 203 which is configured to provide thermal energy to and/or to draw thermal energy from the layer of material 205. In particular, the temperature setting unit 203 may comprise means for heating up and/or for cooling down the layer of material 205. The temperature setting unit 203 may be used for setting the temperature 102 of the layer of material 205 to the CPT 104. By doing this, it may be ensured that the sound absorption system 200 provides effective sound absorption for long periods of time. The sound absorption system 200 may further comprise a temperature sensor 204 which is configured to provide an indication of the temperature 102 of the layer of material 205. In particular, the temperature sensor 204 may be configured to measure the temperature 102 of the layer of material 205. Furthermore, the sound absorption system 200 may comprise a control unit 201 which is configured to control the temperature setting unit 203 in dependence of the indication of the temperature 102 of the layer of material 205. By providing feedback regarding the actual temperature 102 of the layer of material 205, a reliable sound absorption system 200 may be provided. The control unit 201 may be configured to determine a temperature deviation 301 (e.g. a control error) based on the CPT 104 (as a reference signal 304) and based on the indication of the temperature 102 of the layer of material 205 (as a feedback signal or actual signal 303). Furthermore, the control unit 201 may be configured to control the temperature setting unit 203 in dependence of the temperature deviation 301. In particular, the temperature setting unit 203 may be controlled such that the temperature 102 of the layer of material 205 corresponds to the CPT 104. By regulating the temperature 102 of the layer of material 205 to the CPT 104, a reliable sound absorption system 200 may be provided. During the phase transition, the material 205 transitions from a first phase to a second phase. The material 205 exhibits different physical properties in the first phase and in the second phase. In particular, the material 205 may exhibit at the CPT 104 a phase transition between a solid state (e.g. the first phase) and a liquid state (e.g. the second phase), or between a liquid state (e.g. the first phase) and a gas state (e.g. the second phase).

The sound absorption system 200 may comprise a receptacle 202 which is configured to accommodate the layer of material 205 in the first phase and in the second phase. In particular, the receptacle 202 may be such that it sustains the physical properties of the material 205 in the first phase and in the second phase. Furthermore, the receptacle 202 may encapsulate the layer of material 205 in the first phase and in the second phase. In addition, the receptacle 202 may be configured to maintain the layer of material 205 within a predetermined shape and/or at a pre-determined location in the first phase and in the second phase. By arranging the layer of material 205 within an appropriate receptacle 202, a reliable and stable sound absorption system 200 may be provided.

The material 205 may exhibit at the CPT 104 a first-order phase transition. Examples for such first-order phase transitions are transitions between a solid state and a liquid state, or between a liquid state and a gas state. Such phase transitions are particularly beneficial for absorbing sound in a medium to high frequency range (e.g. from 100Hz to 20kHz).

Alternatively or in addition, the material 205 may exhibit at the CPT 104 a second-order phase transition. Examples for such second-order phase transitions are ferromagnetic transitions or superconducting transitions. Such phase transitions are particularly beneficial for absorbing sound in a low frequency range (e.g. from 20Hz to 100Hz).

By using appropriate materials 205, the sound absorption system 200 may be adapted to different sound absorption requirements. In particular, by providing a sound absorption system 200 with a first layer of a first material having a first-order phase transition and a second layer of a second material having a second-order phase transition (and possibly appropriate temperature setting units 203 for the first and the second layer), a sound absorption system 200 for the complete frequency spectrum audible by a human being may be provided.

The second layer of the second material may exhibit a phase transition at a second CPT 104, wherein the second CPT 104 may be different from the first CPT 104. The temperature 102 of the second material may be set to the second CPT 104. For this purpose, the sound absorption system 200 may comprise a second temperature setting unit 203 and possibly a second temperature sensor 204. By doing this, optimal sound absorption properties may be provided for the layers of first and second material.

By varying the relative amount of the first material and the second material, different absorption coefficients in different frequency ranges may be achieved. This may be used for designing the acoustics of a room 400. In particular, by varying the amount of different materials 205, a pre-determined sound absorption curve as a function of frequency may be designed.

The material 205 may be such that the CPT 104 is in the range of 15°C to 30°C. By doing this, an energy efficient sound absorption system 200 for room temperature may be provided.

The CPT 104 of a material 205 typically depends on one or more properties of a (direct) environment of the material 205. The one or more properties may comprise an air pressure at which the material 205 is maintained within the layer of material 205. Other possible properties of the environment are, for example, air or fluid density and magnetic field. The latter property effective on ferromagnetic - paramagnetic transitions of the material. The sound absorption system 200 may comprise one or more sensors configured to provide sensor data regarding one or more properties of the environment of the material 205 (e.g. a pressure sensor or barometer). Other possible sensors are density meters for sensing the density of fluids, e.g. gravity-based where they compare the density of a fluid against a known density object, for example by seeing how the apparent weight of the object changes when immersed in the fluid. Various sensors for measuring magnetic fields (magnetometers) are known to the skilled person, for example MEM sensors. By taking into account one or more properties of the environment within which the material 205 is maintained, the CPT 104 may be determined in a precise manner, thereby increasing the quality of sound absorption.

The temperature setting unit 203 may be configured to provide thermal energy to and/or to draw thermal energy from the layer of material 205, in dependence of the sensor data. In particular, the temperature setting unit 203 may be configured to set the temperature of the layer of material 205 to the correct CPT 104, which is applicable for the specific

environmental conditions (i.e. for the current one or more properties of the environment). The sound absorption system 200 may comprise a CPT sensor which is configured to determine CPT data that is indicative of whether the material 205 within the layer of material 205 is at the CPT 104. In particular, the CPT sensor may be configured to detect directly that the layer of material 205 comprises a mixture of material 205 within two different phases. By way of example, the CPT sensor may be configured to detect whether the layer of material 205 comprises a mixture of solid and liquid or a mixture of liquid and vaporous material 205. In other words, the CPT data may be indicative of the fact that the layer of material 205 comprises a mixture of solid and liquid or a mixture of liquid and vaporous material 205.

The sound absorption system 200 may be configured to regulate a temperature of the material 205 based on the CPT data. In particular, the temperature setting unit 203 may be configured to provide thermal energy to and/or to draw thermal energy from the layer of material 205, in dependence of the CPT data. In particular, the temperature setting unit 203 may be configured to set the temperature of the layer of material 205 such that the CPT data indicates that the material 205 within the layer of material 205 is at the CPT 104.

Furthermore, a room 400 comprising a wall (e.g. a recording studio or a cinema) is described. The room 400 is such that at least part of the wall comprises the sound absorption system 200 described in the present document.

Furthermore, headphones 410 comprising a pad 411 with a speaker 412 are described. The pad 411 comprises the sound absorption system 200 described in the present document.

In addition, the use of a phase transition of a material 205 for performing sound absorption is described.

Fig. 5 shows a flow chart of a method 500 for improving acoustic isolation of a physical structure 400, 411 (e.g. of a wall or a planar structure). The method 500 comprises providing 501 a layer of material 205 on or within the physical structure 400, 411. The material 205 is such that it exhibits a phase transition at a change-of-phase-temperature (CPT) 104.

Furthermore, the method 500 comprises setting 502 a temperature 102 of the material 205 to the CPT 104.

Furthermore, an absorption system 200 for absorbing energy of a wave 211 is described. In particular, the absorption system is configured to absorb (at least partially) the energy of an acoustic wave, of an electromagnetic wave and/or of a mechanical wave. The absorption system 200 comprises a layer of material 205, wherein the material 205 exhibits a phase transition at a change-of-phase-temperature (CPT) 104. Furthermore, the absorption system 200 comprises a temperature setting unit 203 configured to provide thermal energy to and/or to draw thermal energy from the layer of material 203, for setting a temperature 102 of the layer of material 205 to the CPT 104. It should be noted that the features which are described in the context of the sound absorption system 200 are also applicable to the generic absorption system 200.

The methods and systems described in the present document may be implemented partially as software, firmware and/or hardware. Certain components may e.g. be implemented as software running on a digital signal processor or microprocessor. Other components may e.g. be implemented as hardware and or as application specific integrated circuits.

Various aspects of the present invention may be appreciated from the following enumerated example embodiments (EEEs): EEE 1. A sound absorption system (200) comprising a layer of material (205), wherein

- the material (205) exhibits a phase transition at a change-of-phase-temperature (104), referred to as CPT;

- the material (205) is configured to absorb acoustic energy during said phase

transition; and

- the material (205) is at the CPT (104).

EEE 2. The sound absorption system (200) of EEE 1, further comprising a temperature setting unit (203) configured to provide thermal energy to and/or to draw thermal energy from the layer of material (205), for setting a temperature (102) of the layer of material (205) to the CPT (104).

EEE 3. The sound absorption system (200) of EEE 2, wherein the temperature setting unit (203) comprises means for heating up and/or for cooling down the layer of material (205).

EEE 4. The sound absorption system (200) of any of EEEs 2 to 3, further comprising

- a temperature sensor (204) configured to provide an indication of the temperature (102) of the layer of material (205); and

- a control unit (201) configured to control the temperature setting unit (203) in dependence of the indication of the temperature (102) of the layer of material (205).

EEE 5. The sound absorption system (200) of EEE 4, wherein the control unit (201) is

configured to

- determine a temperature deviation (301) based on the CPT (104) and based on the indication of the temperature (102) of the layer of material (205); and

- control the temperature setting unit (203) in dependence of the temperature

deviation (301).

EEE 6. The sound absorption system (200) of EEE 4, wherein the control unit (201) is

configured to control the temperature setting unit (203) such that the temperature (102) of the layer of material (205) corresponds to the CPT (104).

EEE 7. The sound absorption system (200) of any previous EEE, wherein

- during the phase transition the material (205) transitions from a first phase to a second phase; and

- the material (205) exhibits different physical properties in the first phase and in the second phase.

EEE 8. The sound absorption system (200) of EEE 7, further comprising a receptacle (202) which is configured to accommodate the layer of material (205) in the first phase and in the second phase.

EEE 9. The sound absorption system (200) of EEE 8, wherein the receptacle (202)

- encapsulates the layer of material (205) in the first phase and in the second phase; and/or

- is configured to maintain the layer of material (205) within a pre-determined shape and/or at a pre-determined location in the first phase and in the second phase.

EEE 10. The sound absorption system (200) of any previous EEE, wherein the material (205) exhibits at the CPT (104) a phase transition

- between a solid state and a liquid state; or

- between a liquid state and a gas state.

EEE 11. The sound absorption system (200) of any previous EEE, wherein the material (205) exhibits at the CPT (104) a first-order phase transition or a second-order phase transition.

EEE 12. The sound absorption system (200) of any previous EEE, wherein the CPT (104) is in the range of 15°C to 30°C.

EEE 13. The sound absorption system (200) of any previous EEE, further comprising a second layer of a second material, wherein the second material exhibits a phase transition at a second CPT (104), and wherein the second material is at the second CPT (104).

EEE 14. The sound absorption system (200) of EEE 13, wherein

- the material (205) exhibits a first-order phase transition; and

- the second material exhibits a second-order phase transition.

EEE 15. The sound absorption system (200) of any previous EEE, wherein

- the CPT (104) of the material (205) depends on one or more properties of an

environment of the material (205); and

- the sound absorption system (200) comprises one or more sensors configured to provide sensor data regarding one or more properties of the environment of the material (205).

EEE 16. The sound absorption system (200) of EEE 15 referring back to EEE 2,

wherein the temperature setting unit (203) is configured to provide thermal energy to and/or to draw thermal energy from the layer of material (205), in dependence of the sensor data.

EEE 17. The sound absorption system (200) of any previous EEE, wherein the sound absorption system (200) comprises a CPT sensor configured to determine CPT data which is indicative of whether the material (205) is at the CPT (104).

EEE 18. A room (400) comprising a wall, wherein at least part of the wall comprises the sound absorption system (200) of any previous EEE.

EEE 19. Headphones (410) comprising a pad (411) with a speaker (412), wherein the pad (411) comprises the sound absorption system (200) of any of EEEs 1 to 17.

EEE 20. Use of a phase transition of a material (205) for performing sound absorption.

EEE 21. A method (500) for improving acoustic isolation of a physical structure (400, 411), the method (500) comprising - providing (501) a layer of material (205) on or in the physical structure (400, 411), wherein the material (205) exhibits a phase transition at a change-of-phase- temperature (104), referred to as CPT; and

- setting (502) a temperature (102) of the material (205) to the CPT (104).

EEE 22. An absorption system (200) for absorbing energy of a wave, wherein the

absorption system (200) comprises

- a layer of material (205), wherein the material (205) exhibits a phase transition at a change-of-phase-temperature (104), referred to as CPT; and

- a temperature setting unit (203) configured to provide thermal energy to and/or to draw thermal energy from the layer of material (205), for setting a temperature (102) of the layer of material (205) to the CPT (104).

EEE 23. The absorption system (200) of EEE 21, wherein the absorption system (200) is configured to absorb the energy of an acoustic wave, of an electromagnetic wave and/or of a mechanical wave.