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Monochord Assembly Instructions

Module by: Julius Smith. E-mail the author

Summary: This module describes assembly of the RealSimple monochord. The monochord consists of a steel guitar string, rigidly terminated and stretched to a variable tension. Both optical and magnetic pickups are used. Subsequent laboratory exercises are concerned with making elementary measurements and observations of monochord vibration and decay.

Introduction

Figure 1 shows how the different components of the monochord fit together. The longitudinal supports C1C1 and C2C2 and the transverse supports B1B1 and B2B2 are attached to the the bottom plate AA. Part DD houses the string sensors.

Figure 1: Diagram of Monochord
Figure 1 (mono_schem.png)

Parts List

  1. One plank 93 mm wide and 20 mm thick from which is sawn, Bottom plate, 600 mm long (A), 2 transverse supports 93 × 60 mm (B1, B2), 2 longitudinal supports 100 × 60 mm (C1, C2), Magnet support 93 × 100 mm high (D). and with a cutout 14 mm wide and 55 mm deep.
  2. In the magnet support, two magnets stick to two bolts M6×40 mm.
  3. Violin steel G string (196 Hz).
  4. A tuning screw (tuning peg) for guitar.
  5. 2 nuts for guitar (the part, at the top of the fingerboard, over which the string runs through a groove).

Optional optical sensors (typically less than $10 each–in the US, see Mouser Electronicshttp://www.mouser.com/):

  • OPB815 W with 9.5 mm gap (Optek)
  • ELITR8102 with 3 mm gap (Everlight)
  • ELITR8402 with 6 mm gap (Everlight)

Neodymium magnet suppliers:

  • In Sweden: Elfa. Suitable magnets are part number 37-522-68.
  • In the U.S.: K&J Magneticshttp://www.kjmagnetics.com. Suitable magnets are part number B824.

Parts Description

Longitudinal support C1 (where the string is anchored)

  1. 100 mm long, 20 mm thick and 60 mm high.
  2. A hole of 2 mm diameter, 7 mm from the upper edge and 25 mm from one end, through which to pass and anchor the string.
Figure 2: Longitudinal support showing string anchor
Figure 2 (long_sup_c1.png)

Longitudinal support C2 (for the tuning screw)

  1. 100 mm long, 20 mm thick and 60 mm high.
  2. A hole for the axle of the tuning screw, 20 mm below the top edge and 40 mm from the end. Modify for the chosen screw if necessary.
Figure 3: Longitudinal support for tuning screw
Figure 3 (long_sup_c2.png)

Transverse supports B1 and B2 (boundary condition for the string)

  • 93 mm long, 20 mm thick, and 60 mm high.
  • Make a recess for the guitar nut along the 60mm top edge, 3 mm wide and 8 mm high. Modify for the chosen nut if necessary.
Figure 4: Transverse supports B1 and B2
Figure 4 (trans_sup.png)

Magnet Holder D

Figure 5
NOTE: unable to translate the contents of this figure.
  1. 93 wide, 100 mm high.
  2. A recess from the upper edge 55 mm tall and 14 mm wide.
  3. On both sides a 5.5 mm hole, 55 mm from the upper edge, into which 40 mm M6 bolts are screwed in. The tip of each bolt needs to be filed flat.
  4. Two rectangular neodymium magnets (8×5×3 mm³) are held in place magnetically to the bolt tips.

For a horizontal magnetic field, the Magnet Holder is held vertically by two small clamps (see photographs). It can easily be moved along the string, and can also be rotated 90 degrees for a vertical magnetic field.

Assembly

  1. Acquire a plank of solid wood 93 mm wide by 20 mm thick of about 2 metres length.
  2. Cut off the 600 mm long bottom plate (A).
  3. Cut off two blocks 93 mm long (transverse supports B1, B2).
  4. Cut off two blocks 100 mm long (longitudinal supports C1, C2).
  5. Cut the blocks to 60 mm width.
  6. Make a recess 3 mm wide and 8 mm deep along one upper edge of the transverse supports B (see below).
  7. In C1, drill a 2 mm hole through which to pass and fasten the string.
  8. In C2, drill holes as necessary for the chosen tuning screw.
  9. Transverse supports B1 and B2 are glued with the recesses away from each other and with their inner faces 140 mm from the ends of bottom plate A.
  10. Fit the guitar nuts into the recesses in B1 and B2. Secure the nuts snugly with plumber's putty or epoxy.
  11. C1 and C2 are glued so as to support B1 and B2 respectively.

Magnetic Pickup Amplifier

The signal levels are low. Using a 9V battery supply reduces possible problems with hum and earth (ground) loops. The output signal is proportional to the string velocity.

Figure 6: Magnetic pickup amplifier circuit schematic
Figure 6 (mag_pick.png)

Optical Transducers

Each transducer is a LED/phototransistor pair that are mounted facing each other, in what is henceforth called a "fork." The fork must be carefully positioned such that the string at rest partially obstructs the light passing from LED to the phototransistor. The string does not need to be an electrical conductor. Optical transducers measure the string's displacement. With two of them mounted at right angles, both horizontal and vertical deflection can be measured. Optical transducers are best mounted near one end of the string.

Mounting

  1. Make a small wooden block that can be clamped onto support B1.
  2. Mount the fork on the small block, such that the light from the LED is shadowed by the string before reaching the phototransistor, see http://www.speech.kth.se/music/acviguit4/http://www.speech.kth.se/music/acviguit4/, chapter 9, Figure 9.14.

Drive circuit and pickup circuit for optical sensor elements, O1 and O2

Figure 7: Optical sensor circuit
Figure 7 (opt_drive.png)

Positioning the Optical Sensor

The optical transducer can be correctly positioned using only a voltmeter, measuring the output voltage of the circuit shown above.

Test the fork by measuring the output voltage both for unobstructed light and when the light is blocked by an opaque object. This will give the working range. An example is shown in the table.

Note that stray light in the room should not significantly affect the result, due to the filter on the phototransistor that passes only infrared light.

Table 1: Unblocked and blocked voltages for different fork gaps
Fork gap Unblocked [V] Blocked [V]
9.5 mm 5.63 3.15
6 mm 5.3 3.0
3 mm 5.3 2.9

Select the resting point as the voltage midpoint between the two extremes. Adjust the position of the fork so that the midpoint voltage is reached. Start from the unobstructed position and move the fork slowly.

For example, with a violin G string of .75 mm diameter, we might have: Fork gap 9.5 mm, unblocked 4.8 V, blocked 3.4 V, giving a midpoint voltage of 4.1 V.

The position must be accurate to a few tenths of a millimeter. When acquiring the string vibrations, the oscilloscope and/or sound card should be AC-coupled. Sound cards are almost always AC-coupled.

The optical method requires 10 times higher accuracy for positioning than does the magnetic method. The magnetic velocity signal can be time-integrated to give displacement, which is physically more intuitive.

Block Diagram for the Optical and Magnetic Sensor Components

Figure 8 shows how the power supply connections for the optical and magnetic sensor components should be connected together. The sentinel LED indicates whether the circuits are on or off to help avoid inadvertently draining the battery.

Figure 8: Block diagram for the two optical sensor components O1 and O2 and magnet preamplifier M
Figure 8 (opt_bd.png)

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