A SENSOR FOR AN ACOUSTIC INSTRUMENT FIELD OF THE INVENTION

The present invention relates to a sensor for an acoustic instrument, and in particular to a sensor for detecting vibrations in a panel of the instrument. The sensor is especially suited to the detection of vibrations in a panel, such as a front face or soundboard, of a stringed musical instrument, such as an acoustic guitar, and it will therefore be convenient to describe the invention in relation to that example application. It should be understood however that the invention is intended for broader application and use. BACKGROUND OF THE INVENTION

A stringed instrument such as an acoustic guitar includes a body or resonant cavity having (i) a front panel, often referred to as the face or soundboard, (ii) a back panel and (iii) side walls extending between the soundboard and back panel. To provide structural rigidity the panels and side walls, in particular the soundboard, are braced or strutted with thin sections of timber. A neck, carrying a finger board, projects from the body and strings are stretched between a "nut" at the head end of the finger board and a "saddle" supported by a bridge attached to the soundboard of the instrument. The strings oscillate, when plucked or strummed, between the nut and the saddle. In an acoustic guitar, these oscillations are transmitted mechanically as vibrations to the soundboard of the instrument, and hence to the resonant cavity, including the back panel and side walls. These vibrations are then transmitted to the surrounding air, predominately by the soundboard of the instrument but also by the back panel and side walls, and to some extent also by the strings directly. In the past, acoustic guitars have generally been recorded or amplified using a transducer, for example a piezoelectric transducer, situated between the bridge and the saddle of the instrument, i.e. immediately under the strings. The response achieved by such an arrangement is predominately the reproduction of vibrations of the strings according to how they are stretched between the nut and the saddle of the instrument, and of course the performance or playing of the instrument. However, as the piezoelectric transducer in this arrangement is not in intimate contact with the soundboard or face, back and side walls of the

acoustic guitar, the vibrations of these components are not reproduced to any significant extent.

There have been attempts in the past to address this problem by directly sensing the vibrations in the soundboard or face of the instrument with a transducer mounted onto the soundboard. However, such attempts have met with limited success because the transducer can only sense vibrations over the region of the soundboard in direct contact with transducer. This is due to the poor ability of vibrations to travel through the relatively soft wood from which soundboards are constructed. Attempts have been made to increase the region of the soundboard sensed by the transducer by increasing the surface area of the transducer in contact with the soundboard. However, such attempts have led to the vibrations in the soundboard being overly restrained to the detriment of the sound produced.

Accordingly it would be desirable to provide a device which overcomes or ameliorates at least one of the above described problems of the prior art.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein. SUMMARY OF THE INVENTION

Jn accordance with the present invention there is provided a sensor for detecting vibrations in a panel of an acoustic instrument. The sensor includes a brace means, for engaging the panel of the acoustic instrument, and a transducer associated with the brace means. The brace means detects vibrations in a region of the panel in engagement with the brace means and transmits the detected vibrations to the transducer. The transducer subsequently generates an electrical output signal representing vibrations detected by the transducer.

Preferably, the brace means includes one or more brace members with the transducer being mounted on one of the brace members. In addition, each brace member may include a planar elongate surface for engaging the panel.

In one embodiment, the brace means includes a plurality of brace members. At least two of the brace members may be interconnected. In a

particularly preferred embodiment, the interconnected brace members lie across one another, although this is not essential.

Each brace member may be made of relatively hard timber, aluminium, composite fibre or plastic to improve the ability of the brace members to detect and transmit vibrations detected to the transducer.

In a particularly preferred embodiment, the acoustic instrument is an acoustic guitar and the panel is the soundboard of the acoustic guitar.

It will be appreciated that a plurality of sensors may be employed with each sensor having one or more transducers on the brace means to enhance the performance of each sensor. In addition one or more sensors may be employed on other panels besides the soundboard. For example, one or more sensors may be applied to the back panel of the acoustic guitar instead of the soundboard. Alternatively, sensors may be applied to both the soundboard and the back panel.

Further, the one or more transducers used in the sensor of the present invention may be of any suitable type but are preferably of the type described in the inventors earlier Australian patent no. 632064 (application no. 43019/89). BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, which should not be considered as limiting any of the statements in the previous section. The preferred embodiment will be described with reference to the following figures in which:

Figure 1 is a perspective view of a soundboard of an acoustic guitar with a sensor according to an embodiment of the invention applied thereto;

Figures 2 - 4 illustrate a number of soundboards of an acoustic guitar at different orientations with a sensor mounted thereon in accordance with an embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENT

With reference to the accompanying drawings there is shown a soundboard 1 of an acoustic guitar with a sensor mounted thereon for detecting vibrations in the soundboard 1. The sensor includes a brace means 2 for engaging the soundboard 1 , and a transducer 3 mounted on the brace means 2.

The transducer 3 generates an electrical output signal representing vibrations

detected by the transducer 3. The electrical output signal may then be amplified and/or recorded.

The brace means 2 includes one or more brace members 5. The transducer 3 is mounted on one of the brace members 5. At least two of the brace members 5 are interconnected and intersect one another to provide a cross formation.

The brace members 5 are preferably made from a hard dense timber to improve the ability of the brace means 2 to detect and transmit the detected vibrations to the transducer 3. Each brace member 5 is substantially elongate and includes a planar elongate surface for engaging with the soundboard 1. The position of each brace member 5 with respect to the soundboard 1 may vary, however optimal performance of the sensor has been achieved by the brace members 5 extending at an angle to the direction of the grain of the timber of the soundboard 1. This positioning of the brace members 5 is also advantageous for achieving optimal structural rigidity.

It has also been found that the longer the brace member 5, the better the response provided by the sensor. This in turn enables a more realistic sound of an acoustic guitar to be produced by the amplification of the output electrical signal from the transducer 5. Each brace member 5 is mounted to the soundboard 1 with a quality adhesive suitable for timber, for example PVC or formaldehyde based adhesives, cynoacrylates or epoxies. The transducer 3 can be accommodated on the brace member 5 at different positions with the transducer 5 generating different electrical output signals, representing vibrations detected by the transducer 3, at each position. It has been found however that locating the transducer 3 on a central region of the brace member 5 provides optimal performance of the sensor.

The width of each brace member 5, at least where the transducer 3 is mounted, is slightly wider than the transducer 3 such that the transducer 3 is fully supported by the brace member 5. In addition, to achieve better performance, the width of the brace member 5 is increased approaching the position on the brace member 5 where the transducer 3 is mounted. Further each brace member 5 preferably has a wide low profile as it has been found that high narrow profiles do not provide the sensor with the same level of performance. Whilst high

density timber is the preferred material for each brace member 5, other materials such as aluminium can be used.

The transducer 3 is secured to the brace member 5 by a nut and bolt assembly, for example an M4 countersunk cap screw and nut, such that the transducer 3 is securely clamped against the brace member 5. This enables the brace member 5 to more readily transmit the detected vibrations to the transducer

3.

In an alternative embodiment (not shown), the transducer 3 may be located within a recessed portion of the brace member 5 to thereby increase the surface area of the brace member 5 in contact with the transducer 3. This arrangement also enables the brace member 5 to more readily transmit the detected vibrations to the transducer 3.

In a further embodiment (not shown), the transducer 3 may be integral with the brace member 5. In addition to the brace means 2 detecting and transmitting the detected vibrations to the transducer 3, the brace means 2 advantageously also provides structural rigidity to the sound board 1.

The sensor according to the present invention enables the transducer 3 to generate an electrical output signal more closely representing the vibrations in the soundboard 1. In addition, the brace members 5 enable a greater surface area of the soundboard 1 to be sampled by the sensor in comparison to a transducer mounted directly on a sound board.

In one embodiment, the sensor of the present invention may be used in association with a guitar having a preamplifier and an under saddle sensor as described in WO 2005/001811 , the contents of which are hereby incorporated by reference. The under saddle sensor is mounted beneath a saddle of the guitar, and the sensor of the present invention is attached to a body portion of the guitar, such as the soundboard. The preamplifier includes a mixing device for combining the output of the under saddle sensor and the sensor attached to the soundboard. The mixing device includes a first input, for receiving a signal from the under saddle sensor, and a second input, for receiving a signal from the sensor attached to the soundboard. The mixing device further includes a low pass filter, for passing signal components of the first input signal below a first frequency, and

a high pass filter for passing signal components of the second input signal above a second frequency. The mixing device further includes a mixing circuit for combining the signals passed by the low pass filter and the high pass filter to form a combined output signal. With this arrangement, the combination of (i) low frequencies from the under saddle sensor with (ii) higher frequencies from the sensor of the present invention attached to the soundboard, produces a particularly preferred, or natural, representation of the sound of an acoustic guitar.

As the present invention may be embodied in several forms without departing from the essential characteristics of the invention, it should be understood that the above described embodiment should not be considered to limit the present invention but rather should be construed broadly. Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention. Whilst the invention has been described in relation to an acoustic guitar it should not be considered as limiting the scope of the invention to only such an instrument. In this regard, the invention is also intended for other stringed musical instruments such as violins, cellos, etc.