This invention relates to a protective device for protection from blast explosions, in particular to a wearable device for protecting a wearer's brain from injury.

Blast panels are provided in shields, walls or helmets, e.g. for military use, to protect against injury from explosions. In particular, when a blast panel is used in a helmet in order to help prevent the wearer of the helmet from sustaining a traumatic brain injury, such blast panels aim to reflect or attenuate the Shockwave incident from explosions, as well as to prevent the penetration of debris from explosions. In the case of helmets, in order to do this in a manner that safely protects the wearer from sustaining traumatic brain injuries, conventional helmets containing blast panels may be bulky, heavy and uncomfortable. This is particularly apparent when a blast panel capable of dissipating the energy of a Shockwave is placed in the visor of a helmet, which can restrict both the head movement and the visibility of the wearer.

The present invention seeks to provide an improved protective device for protection from blast explosions.

When viewed from a first aspect the invention provides a protective device comprising a first portion extending over a first area of the protective device, a second portion extending over a second area of the protective device, wherein the first area is adjacent to the second area;

wherein the first and second portions are arranged such that when a Shockwave is incident upon the protective device, the first portion delays the propagation of the Shockwave therethrough relative to the propagation of the Shockwave through the second portion, such that the incident Shockwave is transformed into a more concave or less convex Shockwave once it has propagated through the first and second portions; or

wherein the first and second portions are arranged such that when a Shockwave is incident upon the protective device, the second portion delays the propagation of the Shockwave therethrough relative to the propagation of the Shockwave through the first portion, such that the incident Shockwave is transformed into a less concave or more convex Shockwave once it has propagated through the first and second portions.

The present invention provides a protective device for protecting a user of the device from injury from a Shockwave, e.g. a blast from an explosion. The protective device includes first and second portions that cover adjacent first and second areas of the protective device respectively.

The first and second portions of the protective device are arranged such that the propagation of an incident Shockwave through the first portion is slower than the propagation of the same Shockwave through the second portion (or vice versa), thus delaying the propagation of the part of the Shockwave that passes through the first portion relative to the part of the Shockwave that passes through the second portion (or vice versa). This results in a Shockwave passing through the protective device being converted into a more concave or less convex Shockwave (viewed from inside the protective device) when it exits from its propagation through the protective device, or the Shockwave passing through the protective device being converted into a more concave or less convex Shockwave (viewed from inside the protective device) when it exits from its propagation through the protective device. For example, when a planar Shockwave is incident upon the protective device, the first portion delays the propagation of the Shockwave therethrough relative to the propagation of the Shockwave through the second portion, such that the incident planar Shockwave is transformed into a concave Shockwave once it has propagated through the first and second portions.

In the alternative configuration, for example, the second portion delays the propagation of the Shockwave therethrough relative to the propagation of the Shockwave through the first portion, such that an incident planar Shockwave is transformed into a convex Shockwave once it has propagated through the first and second portions.

The particular design of the protective device, e.g. whether to delay the propagation of the Shockwave through the first portion relative to the second portion, or vice versa, in order to shape the incident Shockwave (e.g. that then enters the user's head), may be chosen depending on a number of factors, e.g. the particular type of reduction in stress concentration, reduction in energy transmission, etc..

It will be seen that the protective device, in converting an incident Shockwave into a more concave or less convex (or a more convex or less concave) Shockwave, means that the Shockwave that is incident upon the user (e.g. the head of the wearer) of the protective device is a more concave or less convex (or a more convex or less concave) Shockwave. The Applicant has appreciated that a planar Shockwave incident upon a head has the potential to cause a traumatic brain injury. First, owing to the roughly spherical shape of the head when such a Shockwave enters the head, a planar Shockwave is focussed towards the centre of the brain (i.e. the head acts as a parabolic system which concentrates both pressure and shear waves to the centre of the head). Thus, the highest concentration of energy from the Shockwave occurs in the centre of the brain.

Second, owing to the position of the ventricles at the centre of the brain (which contain cerebrospinal fluid and are surrounded by white matter in the corpus callosum) and the surrounding brain tissue, this area of the brain is particularly susceptible to injury from Shockwaves. This is because of the abrupt changes of material properties (e.g. of the shear modulus of the ventricles and the brain tissue) in this part of the brain and thus the interface effect becoming important when a Shockwave is incident.

Similar causes and effects may result in other areas of the body being susceptible to injury from Shockwaves from blast explosions.

By modifying the shape of a Shockwave as it passes through the protective device, the protective device transforms an incident wave into a more concave or less convex wave (or into a more convex or less concave) which is then incident upon the (e.g. head of the) user of the protective device. For example, a concave wave when it enters a head (after passing through the protective device) can be converted back into a planar wave as it propagates through the brain. It will be appreciated that such a wave has the potential to reduce the concentration of energy dissipated by the wave and thus reduces the damage, e.g. to the brain as the Shockwave is not focussed onto any particular part of the brain. In particular, the modulation of the shape of the Shockwave as described above can help to avoid a concentration of stress, e.g. in the centre of the brain, thus helping to reduce the potential for injury to the ventricles and the surrounding tissue at the centre of the brain.

In other conditions, for example, having a more convex or less concave wave incident upon the head, after passing through the protective device, may be beneficial in helping to reduce the damage to a user's brain, e.g. owing to a reduction in localised energy transmitted into certain parts of the brain.

While some devices (e.g. helmets) aim to maximise the deflection or absorption of the energy of a Shockwave from or into a helmet, the Applicant has appreciated that in some circumstances (e.g. the type of explosions that a user of the device may be exposed to), the shape of the incident Shockwave can also be modified in order to help reduce localised concentration of its intensity, e.g. as it passes through the brain. Reducing the local concentration of the Shockwave helps to decrease the accumulation of stress, e.g. in the brain from the Shockwave, which may thus help to protect vulnerable areas of the brain (e.g. the ventricles and the surrounding tissue) from damage.

Thus, the modulation of the shape of the Shockwave by the protective device of the present invention may allow a protective device to be provided which is more practical than conventional devices, because it may not be necessary to provide a bulky blast panel in order to provide effective protection for the user, e.g. from Shockwaves and flying debris.

The protective device of the present invention could be any suitable and desired device, e.g. a shield, a panel (e.g. of a vehicle), a wall (e.g. of a building), a head guard, a face mask, etc.. In a preferred embodiment the protective device comprises a wearable protective device, i.e. designed to protect the wearer from incident Shockwaves.

In one embodiment, the wearable protective device is shaped to be worn on the head of a wearer. In a preferred embodiment, the wearable protective device comprises a helmet. Thus preferably the wearable protective device is shaped to extend substantially all the way around and over the head of a wearer. Preferably the wearable protective device comprises a hemisphere, e.g. the wearable protective device subtends a solid angle of at least 2Trfrom the centre of the wearable protective device (e.g. a point at the middle of the widest part of the wearable protective device).

The first and second portions of the (e.g. wearable) protective device could be any suitable and desired shape. For example, one or both of the first and second portions could be flat. However, in a preferred embodiment the first and second portions are shaped to match the shape of the (e.g. wearable) protective device. Preferably the first and second portions are curved (e.g. convex (from the perspective of the incident Shockwave)), e.g. to fit in a helmet. Thus preferably the protective device is curved (e.g. convex). The first and second portions may be provided in any suitable and desired part of the (e.g. wearable) protective device. For example, the (e.g. wearable) protective device may comprise a face shield, wherein the face shield comprises at least the first portion (e.g. the face shield may also comprise at least a part of the second portion). Furthermore, the face shield may comprise a (e.g. at least partially) transparent (e.g. glass) window, wherein the transparent window of the face shield comprises at least the first portion or the first portion comprises the (e.g.

transparent window of) face shield. It will be appreciated that such a face shield or a transparent window may be provided in a protective device that is not necessarily wearable, e.g. as a window in a wall or panel.

The Applicant has appreciated that the protective device may provide better protection from an incident Shockwave when the Shockwave is first incident upon the first portion and subsequently incident upon the second portion. Thus, preferably the protective device is arranged such that the Shockwave is first incident upon the first portion and subsequently incident upon the second portion.

The first and second areas, over which the first and second portions of the (e.g. wearable) protective device extend respectively, are adjacent to each other. The first and second portions may be arranged in any suitable and desired way in the (e.g. wearable) protective device such that the first and second areas are adjacent to each other. For example, the first and second portions may be arranged depending on how the shape of the incident Shockwave is desired to be modified as it passes through the (e.g. wearable) protective device, e.g. based on which part of the user (and, e.g., their brain) it is desired to maximise protection for, as will be discussed below.

In one embodiment the first area is contiguous with the second area (i.e. there is no gap between the first and second portions). Preferably, the first area does not overlap with the second area. Between them the first and second areas may cover substantially the whole of the (e.g. wearable) protective device or first and second areas may together cover only a part of the (e.g. wearable) protective device.

Preferably, the second portion extends either side of the first portion. Thus in one embodiment the second area surrounds the first area. For example, the first area may comprise a circular area or a rectangular (e.g. square) area, and the second area may surround this area. In another embodiment, the first area forms a strip having the second area on both sides of the strip, and thus the second area may not fully surround the first area. In a preferred embodiment the ratio of the first area to the second area is approximately 1 :2, i.e. preferably the second area is twice as large as the first area. Thus, when the second area extends either side of the first area, preferably each part of the second area either side of the first area is of approximately equal size and of approximately equal size to the first area. Thus, in this embodiment, the area of the (e.g. wearable) protective device over which the first and second areas extend has a third of its area covered by the one half of the second area, a third of its area covered by the first area and a third of its area covered by the other half of the second area. (The first and second portions may not necessarily extend all the way across or around the protective device, e.g. the protective device may comprise further portions which could be located on a side of the protective device upon which a Shockwave is not designed to be incident.)

The relative shapes and/or sizes of the first and second areas may be chosen in any suitable and desired manner. For example, different designs (e.g. shapes and/or configurations) of the (e.g. wearable) protective device may be provided for different situations, e.g. depending on the part of the user's brain that is desired to protect and/or the type of Shockwave that the user may be exposed to (which may itself be of greater risk to a certain part of a user's brain).

Preferably the first portion is positioned in the protective device such that it is arranged to shield the part of the user's brain that the (e.g. wearable) protective device is designed to protect. Thus, preferably the first portion is positioned in the protective device to be between an incident Shockwave and the part of the user's brain that the protective device is designed to protect.

Thus, when the (e.g. wearable) protective device is designed to protect the central region of a user's brain (e.g. around the ventricles), preferably the first portion is positioned in the centre of the protective device. It will be appreciated that in this embodiment, the (e.g. wearable) protective device will afford protection to other regions of the brain; however, the combination of the mechanical properties of the tissue and the geometry of the ventricles (and the surrounding brain tissue) has been found to be particularly susceptible to damage from incident Shockwaves.

When the wearable protective device is designed to protect the wearer's ear or eye channels, preferably the first portion is positioned to cover the ear or eye channels. Thus, the first portion may form a band (in the wearable protective device) around the head which covers the ear and/or eye channels.

The first and second portions of the (e.g. wearable) protective device may be arranged to delay the propagation of an incident Shockwave through the first portion relative to the second portion (or to delay the propagation of an incident Shockwave through the second portion relative to the first portion) in any suitable and desired way. In a preferred embodiment, the first and second portions are arranged to have different respective acoustic impedances and/or wave speeds such that the different wave speeds of the first and second portions causes the propagation of a Shockwave to be delayed through the first portion relative to the second portion (or to be delayed through the second portion relative to the first portion) and/or the different acoustic impedances of the first and second portions causes the stress amplitude of a Shockwave to be decreased. Thus, the acoustic impedance and/or the wave speed of the first and second portions may be tuned to cause the propagation of a Shockwave through the first portion to be delayed relative to its propagation through the second portion and to be reduced in intensity, e.g. owing to the difference in wave speeds between the first and second portions and the acoustic impedances of both portions.

Preferably the acoustic impedance of the first portion and the acoustic impedance of the second portion are as high as possible, given the material constraints desired for the first and second portion, e.g. such that they transform the incident

Shockwave according to the present invention. When the protective device is arranged to delay the propagation of an incident Shockwave through the first portion relative to the second portion, preferably the wave speed of the first portion is less than the wave speed of the second portion, e.g. at least two times less, e.g. at least five times less, e.g. at least ten times less. When the protective device is arranged to delay the propagation of an incident Shockwave through the second portion relative to the first portion, preferably the wave speed of the second portion is less than the wave speed of the first portion, e.g. at least two times less, e.g. at least five times less, e.g. at least ten times less. When the first portion is at least partially transparent, the wave speed of the first portion may be limited by this requirement, i.e. to allow the user of the protective device to see through the first portion.

(It will be appreciated that the acoustic impedance and the wave speed of a material are not independent of each other, owing to their dependence on both the stiffness (e.g. Young's modulus) and the density of the material. It should also be noted that the wave speed (in the case of uniaxial deformation of the material) is preferably calculated from the Young's modulus and the density of the material only (as is outlined below). However, all wave speeds (e.g., longitudinal, shear, bulk) will obey the same paradigm by involving the appropriate respective stiffness, instead of the Young's modulus (which is a function in general of both the Young's modulus and Poisson's ratio).)

The (e.g. wearable) protective device preferably has a variation (e.g. a gradient) of acoustic impedance and/or a variation (e.g. a gradient) wave speed (speed of sound through the material) between the first and second portions, such that a Shockwave propagating through the (e.g. wearable) protective device is delayed by the first portion more than the second portion (or such that a Shockwave

propagating through the protective device is delayed by the second portion more than the first portion), thus converting an incident (e.g. planar) Shockwave into a more concave or less convex (or a less concave or more convex) Shockwave profile. Preferably, the variation in acoustic impedance and/or wave speed between the first and second portions comprises an in plane variation of the acoustic impedance and/or wave speed, i.e. in a direction perpendicular to the direction through the thickness of the first and second portions.

It will be appreciated that the variations in the acoustic impedance ( /Ep) of the first and second portions, where E is the Young's modulus of the respective portions and p is the density of the respective portions, help to control (e.g. reduce) the attenuation of the stress amplitude from the Shockwave incident upon the (e.g. wearable) protective device.

Similarly, the variations in the wave speed (e.g. jE/p for uniaxial deformation) of the first and second portions, where E is the Young's modulus of the respective portions and p is the density of the respective portions, help to control the delay of the Shockwave propagating through the first portion relative to the second portion. In addition, variations in the density (or mass) of the first and second portions help to control (e.g. reduce) the impulse transmitted from the Shockwave incident upon the (e.g. wearable) protective device. In a preferred embodiment the first and second portions comprise one or more material (e.g. mechanical) properties that vary (e.g. have a gradient) between the first and second portions (or sub-sections thereof), such that a Shockwave propagating through the (e.g. wearable) protective device is delayed by the first portion more than the second portion (or is delayed by the second portion more than the first portion). Thus preferably the variation in the material properties (are chosen to) determine the variation in the wave speed and/or acoustic impedance of the first and second portions, e.g. in the manner outlined above. It will be appreciated that the materials of the first and second portions may, for example, be chosen to have particular material properties in order to tune (e.g. optimise) the relative wave speeds and/or acoustic impedances of the first and second portions, e.g. such that the incident Shockwave is transformed to reduce the damage on the user. In one set of embodiments, the material (e.g. mechanical) properties of the first and/or second portions are uniform (e.g. homogeneous) across each of the first and/or second areas respectively (but with a variation therebetween). In this embodiment, preferably there is a discontinuity between the material properties of the first and second portions (i.e. at the boundary between the first and second areas). Thus in one embodiment the first and second portions comprise discrete (e.g. modular) portions, e.g. having a defined boundary between the first and second areas (where the material properties of the first and second portions change abruptly). For example, the first portion may comprise an (e.g. removable) insert of the (e.g. wearable) protective device.

In another set of embodiments, the material properties of the first and/or second portions vary (e.g. continuously) across the first and/or second areas (or subsections thereof) respectively. Thus in one embodiment there is no defined boundary between the first and second areas (e.g. the first and second portions are part of the same piece of material), and the material properties of the first and second portions are arranged such that the propagation of the Shockwave is delayed through the first portion relative to the second portion. Preferably the variation in the material properties between the first and second portions comprises an in plane variation, i.e. in a direction perpendicular to the direction through the thickness of the first and second portions.

Thus, for example, the first and/or second portions may have material properties (e.g. the acoustic impedance and/or the wave speed) that exhibit a graded (e.g. continuous) change across the first and/or second areas respectively. In these embodiments, the first and/or second portions preferably comprise a (e.g. linear or non-linear) gradient of change of their respective material properties (e.g. stiffness or the density, and thus the wave speed and the acoustic impedance) which determines the change (e.g. delay or decrease of the stress amplitude) in the propagation of an incident Shockwave through the first portion relative to the second portion. The delay of the incident Shockwave through the first (or second) portion relative to its propagation through the second (or first) portion may be determined (e.g. set) by any suitable material property of the first and second portions (e.g. to determine the acoustic impedance and/or the wave speed of the first and second portions). For example, one or more of the: type of materials, material properties (e.g. stiffness ((e.g. uniaxial) Young's modulus, shear modulus and bulk modulus) and/or density) of the first and second portions may be chosen to determine the delay of the incident Shockwave through the first (or second) portion relative to its propagation through the second (or first) portion.

Additionally or alternatively to variations in the material properties of the first and second portions, in one embodiment, the thickness of the material of the first and second portions could be different from, or varied between, each other to determine the delay of the incident Shockwave through the first (or second) portion relative to its propagation through the second (or first) portion.

The time taken for a Shockwave to propagate through a piece of material is proportional to the thickness of the material. Therefore, if the thickness of the material of the first and second portions is different from each other, the time taken for a Shockwave to propagate through the first and second portions will be different (for the same type of material). Preferably, the thickness of the first (or second) portion is greater than the thickness of the second (or first) portion. The type of material of the first and second portions (e.g. instead of or in addition to the thickness) could be different from, or varied between, each other (e.g. to determine the relative wave speeds of the first and second portions) to determine the delay of the incident Shockwave through the first (or second) portion relative to its propagation through the second (or first) portion. It will be appreciated that the time taken for a Shockwave to propagate through a piece of material is inversely proportional to the wave speed of the material. Therefore, if the first and second portions comprise different materials having different wave speeds, the time taken for a Shockwave to propagate through the first and second portions will be different (for the same thickness of material). In one embodiment, the first portion is made from polycarbonate. In one

embodiment, the second portion is made from metal. It will be appreciated that polycarbonate has a lower wave speed and lower acoustic impedance than metal. It may be that the material of one of the portions is dictated by its function, e.g. a visor that has to be polycarbonate for its transparency. In such embodiments, the material of the other portion is chosen to have the appropriate material properties (e.g. to give the desired ratio of the wave speed between the first and second portions and/or to maximise the acoustic impedance of the first and second portions) such that between them the first and second portions act to modify the shape of the incident Shockwave in the desired manner.

When the protective device is arranged to delay the propagation of an incident Shockwave through the first portion relative to the second portion, preferably the Young's (and, e.g., bulk and/or shear) modulus of the first portion is less than the Young's modulus of the second portion (e.g. for the same density). This may help to make the wave speed of the first portion less than the wave speed of the second portion. When the protective device is arranged to delay the propagation of an incident Shockwave through the second portion relative to the first portion, preferably the Young's (and, e.g., bulk and/or shear) modulus of the second portion is less than the Young's modulus of the first portion. This may help to make the wave speed of the second portion less than the wave speed of the first portion (e.g. for the same density).

As will be appreciated by those skilled in the art, this aspect of the present invention can and preferably does include any one or more or all of the preferred and optional features of the invention described herein, as appropriate. Thus preferably the protective device of this aspect of the invention has the same material properties as the protective device of the first aspect and embodiments of the invention, except that the first and second portions are reversed.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows schematically an idealised helmet;

Figure 2 shows schematically a helmet according to an embodiment of the present invention; and Figure 3 shows schematically a helmet according to another embodiment of the present invention.

Helmets are used, for example, in military situations, to protect the wearer's head against injury from explosions. Such helmets may include blast panels that act to prevent traumatic brain injury, e.g. by reflecting or attenuating Shockwaves that are incident on the helmet from explosions, as well as by preventing the penetration of debris from explosions through the helmet. Figure 1 shows schematically an idealised helmet 10 made of a single material. The helmet 10 is shown on a head 11 that is modelled to be spherical and includes a central region of cerebrospinal fluid and ventricles 12 of the brain. Figure 1 also shows a planar Shockwave 13 from an explosion that is incident upon the helmet 10.

The helmet 10 comprises a shell 14 that has uniform material properties over the shell 14. Thus the wave speed (e.g. jE/p) is uniform for the whole of the helmet 10. When the planar Shockwave 13 from an explosion is incident upon the shell 14 of the helmet 10, it will be seen from Figure 1 that as the Shockwave 13 propagates through the shell 14 and enters the brain of the head 1 1 , owing to the greater wave speed of the brain compared to the wave speed of the air outside of the helmet 10 and the head 1 1 , the incident planar Shockwave 13 is transformed into a convex Shockwave 15 inside the brain of the head 11. This transformation of the incident planar Shockwave 13 (as well as other interactions, such as the reflection of the Shockwave inside the head 1 1), result in the energy of the Shockwave being concentrated towards the centre of the brain at the location of the ventricles 12 of the brain.

An embodiment of the present invention will now be described with reference to Figure 2. Figure 2 shows schematically a helmet 20 that can help to prevent injury to the cerebrospinal fluid and ventricles of the brain.

The helmet 20 shown in Figure 2 is similar to the helmet 10 shown in Figure 1 in that it comprises a shell 24 arranged to protect the head 21 from an incident Shockwave 23. However, instead of the shell having uniform material properties, the shell 24 of the helmet 20 shown in Figure 2 includes a first, central portion 26 (e.g. having a wave speed ^E ₁ /p ₁ ) that is surrounded on each side by a second, peripheral portion 27 (e.g. having a wave speed ^E ₂ /p ₂ )- The first portion 26 covers a third of the area of the helmet 20 and the second portion 27 covers the remaining area of the helmet 20, with the second portion 27 being divided equally between each side of the first portion 26. In one embodiment the first portion 26 is a visor made of polycarbonate and the second portion 27 is made of metal.

The wave speed of the first portion 26 is significantly less than the wave speed of the second portion 27. Thus, when a planar Shockwave 23 is incident upon the helmet 20 shown in Figure 2, as the Shockwave propagates through the shell 24 of the helmet 20, the Shockwave takes longer to pass through the first portion 26 than the second portion 27, i.e. the first portion 26 introduces a time delay for the propagation of the Shockwave. Thus, when the Shockwave enters the brain of the head 21 of the wearer of the helmet 20, the Shockwave 25 inside the brain is shaped differently to the corresponding Shockwave shown inside the head shown in Figure 1 .

Owing to the different wave speeds of the first and second portions 26, 27 of the helmet 20 shown in Figure 2, the incident planar Shockwave 23 is transformed into a concave Shockwave 25 once it has propagated through the first and second portions 26, 27.

From simulations performed, the Applicant has found the helmet 20 shown in Figure 2 to have the potential to reduce traumatic brain injury depending on the chosen set of trauma criteria.

Furthermore, it will be appreciated that the protective device may be provided in which the first and second portions are inverted, such that the Shockwave is delayed through the second portion instead, e.g. for use when the wave speed of the material through which the Shockwave propagates to be incident upon the protective device is higher than the wave speed of the material on the inside of the protective device. Another embodiment of the present invention will now be described with reference to Figure 3. Figure 3 shows schematically a helmet 30 that can help to prevent injury to the brain. The helmet 30 shown in Figure 3 is similar to the helmet 20 shown in Figure 2 in that it comprises a shell 34 arranged to protect the head 31 from an incident Shockwave 33. The shell 34 of the helmet 30 shown includes a first, central portion 36 (e.g. having a wave speed ^E ₁ /p ₁ ) that is surrounded on each side by a second, peripheral portion 37 (e.g. having a wave speed ^E ₂ /p ₂ )- The first portion 36 covers a third of the area of the helmet 30 and the second portion 37 covers the remaining area of the helmet 30, with the second portion 37 being divided equally between each side of the first portion 36. In one embodiment the first portion 36 is a visor made of polycarbonate and the second portion 37 is made of metal. The wave speed of the first portion 36 is significantly greater than the wave speed of the second portion 37. Thus, when a planar Shockwave 33 is incident upon the helmet 30 shown in Figure 3, as the Shockwave propagates through the shell 34 of the helmet 30, the Shockwave takes longer to pass through the second portion 36 than the first portion 37, i.e. the second portion 36 introduces a time delay for the propagation of the Shockwave. Thus, when the Shockwave enters the brain of the head 31 of the wearer of the helmet 30, the Shockwave 35 inside the brain is shaped differently to the corresponding Shockwave shown inside the head shown in Figure 1 . Owing to the different wave speeds of the first and second portions 36, 37 of the helmet 30 shown in Figure 3, the incident planar Shockwave 33 is transformed into a convex Shockwave 35 once it has propagated through the first and second portions 36, 37. From simulations performed, the Applicant has found the helmet 30 shown in Figure 3 to have the potential to reduce traumatic brain injury depending on the chosen set of trauma criteria. I can be seen from the above that in at least preferred embodiments the present invention provides a wearable protective device (e.g. a helmet) that causes an incident (e.g. planar) Shockwave to be transformed into a more concave (or less convex) Shockwave (or into a more convex or less concave Shockwave) after propagating through the first and second portions of the device. Such a modulation of the incident Shockwave may help to reduce traumatic brain injury through reducing the energy that is dissipated from the Shockwave in vulnerable regions of the brain. It will be appreciated that although the embodiments of the invention described above with reference to Figures 2 and 3 have discrete first and second portions, e.g. having uniform material properties over each portion, wearable protective devices according to other embodiments of the present invention may comprise first and second portions in which the material properties vary (e.g. continuously) over the first and second portions to provide the desired modulation of the incident Shockwave.