Photo 1
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Accusound offers alternative methods of re-inforcing the acoustic sound of guitars or similar instruments with round sound holes such as cittern
1. The strip contact microphone (photo 1) provides a technique for averaging the vibrations of the sound-board. Since the pattern of these vibrations varies according to the note that is being played a ‘hot spot’ type pick up will select some of these frequencies according to its position on the sound board. The strip microphone is available in two electrical configurations. The device itself is piezo-electric and can be connected to the piezo input of any suitable amplifier. The alternative version has a FET (Field Effect Transistor) mounted in the moulding that protects theconnections to the device; since the FET requires a voltage this version will need some form of power unit/pre-amplifier. The device is attached to the sound-board by means of low-tack double-sided tape. Where especially sensitive surfaces are involved, as with some expensive violins, then a special non-invasive technique is available.
Although the contact microphone described above will reproduce the vibrations of the sound board of many instruments, e.g. piano, harp, guitar family, violin family etc., where the sound cavity plays a significant part in the acoustic sound of the instrument, then the contact microphone alone will not give an accurate representation of the instrument. This can be improved by the appropriate use of a two-band parametric tuned to the resonate frequencies of the sound-board and the sound-cavity (see relevant pamphlet available from Accusound)
2. The simplest method of reproducing the acoustic sound of any acoustic instrument is the usage of some form of microphone that receives sound impulses through the air. To avoid potential feedback and lack of separation problems the preferred technique is to develop an appropriate ‘close mic-ing’ system. In the case of the guitar Accusound has developed a method of mounting a miniature electret capsule in the sound hole directly underneath the strings (photo 2)
This method will have a tendency to preferentially reinforce the sound cavity resonance and will require frequency-selective bass roll-off. This may require the use of at least one band of parametric equalisation. This system of mounting the capsule directly under the strings has the added advantage of responding to those high frequency transients that radiate directly from the strings and not through the sound-board. (photos 3 & 4) These transients are part of the characteristic sound of each instrument and will not be part of the reinforced sound where only the vibrations of the sound-board are amplified.
3. The two devices described above can be combined and suitable power sources are available for either in-board or out-board fitting.Connections to appropriate amplifiers/main sound systems can be made via mono or stereo end -pin jacks on guitars or leads can be taken out through the sound hole where it is not appropriate to fit an end pin jack.
Any of these systems can be used with a wireless transmitter if required.
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Photo 2
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 Photo 3
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 Photo 3
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