A workshop facility to measure violin family acoustics

Creating an apparatus for routine quick and easy measurements while making and setting up the violin and related instruments

Acoustician, virtual maker and violinist in Birmingham, UK

This article demonstrates how to assemble a simple workshop facility for routine acoustic measurements of the violin and related instruments during their making and final set-up. The facility is based on the use of inexpensive electret microphones. One such microphone is adapted for use as an accelerometer, to measure the frequencydependent energy transmitted to the instrument when a light hammer is gently tapped at the top of the bridge. A second electret microphone measures the sound radiated inside rather than outside the hollow body of an instrument.

The illustrations used here are for a violin/viola measurement set-up. However, exactly the same system can be used for the cello and double bass, apart from scaling up the size of the hammer and microphone supports.

1a The components required

1b Electret microphones

ALL PHOTOS COLIN GOUGH

1 The components required to make the measurement include: 2 x 3.5mm stereo audio plug sockets with 3 facing terminals 1x 3.5mm audio plug socket to 2 RCA sockets 1 x twin cable connector between two 3.5mm audio plugs 1 x 10 x 10mm (diameter/height) and a 5x3mm electret microphone (1b) 2x 1microfarad tantalum capacitor 2 x 4.7k resistors 1 x 9 or 5V battery (preferable rechargeable) with connectors resin-cored solder and liquid flux A small soldering iron (60-80W), an inexpensive multimeter, odd bits of wire and heat-shrink etc.

Electret sensor connections

2 We use a length of already separated stereo cable with a single 3.5mm stereo audio jackplug at its end. This is cut from a longer length of stereo cable with audio plugs at both ends.

We start by cutting off typically 50–60cm for the violin and viola (approaching a metre for the cello and double bass), from the connected cables of a pair of stereo 3.5mm jack plugs. Using a strong pair of wire strippers, we progressively remove around 20cm of the outer protective covering. This exposes the pairs of wires to be soldered to the two electret microphones.

Soldered connections

3 With a pair of fine wire cutters, we first cut each pair of twisted copper and coloured wires to the same length. Then we pull off around 5mm of insulation from the ends of the red and white connecting wires. To achieve well-soldered connections, we initially ‘tin’ the ends of all the exposed wires with a shiny layer of solder using a hot iron and some resin-cored solder. Then, using sharp wire cutters, we cut off all but around 2–3mm beyond the insulation of the coloured wires. 

For both electrets, the short length of tinned copper wire should be soldered to the right solder pad already visibly connected to the outer metal casing. This connection becomes the 0V connection for all the devices including the attached battery. The red and white wires should be soldered to the other pad, making sure there are no electrical contacts between them.

4a End supports

4b Bent supports

4  We then mount the two electret microphones onto the ends of 15cm sections cut from 3mm wide cable ties. We use a quick-setting glue to bond the wires along the smooth side of the cable tie. The last 15mm or so is left free.

Once this is set, the 10mm electret microphone is held in a vice with its top face horizontal. It is then converted into an accelerometer by forming a shallow hemispherical cap of 2-tube epoxy resin on its top face, without removing its protective covering.

Cantilevered spring and pendulum mounting

5 The illustration shows two ways of supporting the hammer on a cantilevered support arm, which can slide up and down a vertical, circular cross-section, wooden post fixed to a heavy wooden base. The supported end of the cantilevered arm is hollowed out to match the curvature of the vertical rod. The height of the hammer head can then be adjusted to tap the top corner of the bridge at the correct position. The cantilevered arm should be sufficiently long to ensure the vertical support rod is well away from the instrument.

The support rod

6 On the left, we use a short wooden rod that passes through the arm on both sides. An O-ring (or multiturns of a strong rubber band) can then be strongly stretched between them and around the support rod. On the right, two rubber rings are fixed to the top and bottom of the cantilevered arm using strong glue. When the glue has set, the assembly can then slide up and down the support rod.

Making the dongle

7 To make the dongle (interface) between the Behrenger amplifier and the hammer and internal microphone sensors, we take two facing 3.5mm stereo audio jack plug sockets with three connecting terminals on their back plates. We then connect them together as illustrated: first by soldering a wire between the common 0V terminals, then soldering the two 1microfarad decoupling capacitors across opposite terminals, with their positive connections (indicated on the devices) soldered to the red plug connectors. Then the two raised 4K biasing resistors are soldered to the same terminals. Finally the external battery connections are soldered to the 0V ground terminal and to the free ends of the twisted-together resistors.

The finished dongle 

8 We compact the components together, making sure there are no unwanted short circuits. We then wrap a few layers of moistened paper around the inner section of the two plugs and cover the whole system with a cylinder of heat-shrink material. With a hairdryer we then gently heat the whole assembly to make a nice-looking unit. The power supply is either connected to a 9V battery or a rechargeable power supply. It is important to remember that the connection to the hammer and internal sound electrets should be inserted at the power supply end of the dongle.

For measurements, we normally mount the violin, viola and cello on foam pads (made from kitchen sponges) – under the neck joint and on the two sides of the lower bouts just below their maximum width. For stability, it is sensible to glue them to a wooden surface or large piece of card.