The invention relates to a sound control device for controlling sound transmitted through and/or reflected from a barrier member.

The isol ation and absorption of low frequency acoustic noise has been a recognised problem for many years. In situations such as vehi cle cabins and ships engine rooms , high levels of low frequency acoustic noise are generated which may be transferred to the rest of the vehicle/building structure throuoi partition walls such as single leaf walls. The sound transmission of a single leaf wall can be approximated by the mass law which gives a transmission loss which is proportional to frequency and the mass per unit area of the partition. Good isolation at low frequency requires a high density wall . Iπprove ents can be made by providing multiple layout isolation structures, but these result in increased coπplexity, cost and volume/weight for the final appl ication. Conventional porous absorber materials also tend to be poor at deal ing with low frequencies. It is possible to use resonant lossy plates as low frequency absorbers , but these need to

be large and mounted away from a rigid support structure. In many instances, such as vehicle cabs, and exteriors of ships hulls, their use is impractical.

A low volume/low mass system capable of producing significant acoustic isolation and/or absorption would be of significant use in vehicle cabins, workshop 's , engine rooms and radiated noise of ships, and the l ike where reduction of noise is desired.

An active noise reduction system is known to be very effective at low frequencies where wavelengths are long, so that little phase difference occurs between the generated and sensed sounds. Thus, this type of system has a good attenuation at low frequencies and can complement a passive system (good attenuation at hϊoi frequencies) to achieve a good overall reduction across a wide frequency range.

Active noise control systems have been used in the past to demonstrate control of reflection coefficient but have used discrete transducers for example conventional moving coil transducers such as loudspeakers.

The use of discrete anti-noise sources limits the physical spacing of separate generators and so l imits the upper frequency of operation for non-normal incidence of the sound. The use of discrete actuators also produces a massive structure with complex mounting arrangements. Similar cons derations apply when such a system is appl ied underwater.

According to the invention there is provided a sound control device for controll ing sound transmitted through and/or reflected from a barrier member, the device comprising sensor means to sense said transmitted and/or reflected sound and to produce a signal representing the transmitted and/or reflected sound, an actuator mounted on said barrier member and arranged to conform to at least one surface of the barrier member and control means responsive to said signal to provide a control signal to the actuator.

The present invention uses an active method to control the radiated and/or reflected noise from the barrier member by the use of an active system comprising the acoustic sensor, control electronics and the distributed actuator as a conformal layer or coating arranged on at least one surface of the support member or barrier member. The principal advantage is that the acoustic impedance match to the surrounding medium can either be iπproved which reduces the acoustic reflection or decreased which increases the decoupl ing between the two surfaces.

The actuator may be made of a flexible piezo-electri c material , such as a piezo-ceramic/polymer composite. The composite includes ceramic materials such as PT (Lead Titanate) or PZT (Lead Zirconate Titanate) dispersed in a polymer. The piezo-electric material may be a polymer of vinyl idene fluoride or a copolymer of vinyl idene fluoride and trifluoroethylene. The actuator may be in the form of a flexible layer or coating provided on at least one surface of said barrier member.

Preferably the actuator and sensor are integrated into a single structure arranged to conform to the surface of said barrier member. The actuator, sensor and control means may be integrated into a combined unit. The control means may provide a phased control signal to the acuator so as to cancel the signal sensed by said sensor means.

The invention wil l now be described further by way of example with reference to the accompanying drawings in which:

Figure 1 il lustrates a noise control device embodying the present invention; and

Figure 2 il lustrates another embodiment of the noise control device of the present invention.

The noise control device as shown in Figures 1 and 2 is arranged on one side of the support member or barrier member 10. The device includes an actuator 11 of a flexible piezoelectric material for example piezoceramic material embedded in polymer matrix. The flexible layer of piezoelectric material is arranged

to conform to a surface 14 of the barrier member 10 and a pressure sensor 11 is arranged over the actuator 12 as shown in Figure 1. In the embodiment shown in Figure 2 the sensor 11 and the actuator 12 are formed as an integral unit in which the sensor 11 is surrounded by the actuator 12 and the flexible integral unit is arranged to conform to the surface 14 of the barrier member 10. The sensor 11 in both embodiments is connected to a control device or processing electronic 13. The sensor 11 produces a signal representing the noise detected by the sensor. The acoustic noise detected by the sensor may be a noise transmitted through the barrier member to the surface 14 or it may be the noise reflected from the surface 14 of the barrier member 10. The control device, in response to the detected signal received from the sensor, may provide an inverted phase signal to the actuator to cancel the acoustic noise signal received from the sensor. By using a conformal actuator as part of the noise control system, it is possible to produce large area noise control systems as illustrated schematically in Figures 1 and 2. The output from the pressure sensor which may be mechanically decoupled or directly coupled to the actuator is electrically filtered and amplified to provide drive to the actuator material. Dependent upon whether it is desired to produce a system which reduces reflected sound, or reduces transmitted sound, the system may feeback phase inverted or same phase signals to the actuator compared to the sensor output. Dependent upon the phase of the electronic control loop, it is possible to produce a material interface with an apparently improved acoustic impedance match to the surrounding medium (water or air), and thus reduce acoustic reflection from the interface. Alternatively the system can increase the decoupling between the two surfaces by decreasing the acoustic impedance match.

The use of a large area transducer which is conformal allows the system to be applied to non-planar surfaces and greatly simplifies the implementation of the system. The control electronics may be physically remote from the sensor/actuator

combi nat on or may be integrated into the combined unit to provide a so-cal led 'smart ' material . Distributed actuator materials which offer particular advantage are dispersions of piezoelectric ceramic in a polymer carrier (so-cal led '0-3' composite) , ceramic rods in polymer with or without a glass reinforcement phase and pure piezoelectric polymers. The use of a large area transducer material allows tranducers elements to be defined by electrode definition and even allows sensor and actuator functions to be integrated into the same material . The noise control layer (sensor/actuator) may be appl ied as a conformal coating to the required structure where it can then control the reflected and/or radiated sound.