Making a Pipe Voicing Machine

by Melvyn Wright

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Making a Pipe Voicing Machine
A pipe voicing machine is simply a box containing a stable supply of compressed air at a specified pressure. The box has a row of holes on the top onto which organ pipes can be fitted. In front of the holes is a row of keys, and pressing a key allows air into the associated pipe so that it can be worked upon and the sound and tuning adjusted.  The machine will feature a means of adjusting the air pressure to suit the pipes being voiced.

Why do you need a voicing machine?  Well you don't!  If you are building something simple like a John Smith Busker Organ with only 20 pipes, you can voice the pipes quite easily on the organ (although you may need to get somebody else to turn the handle!).  But if you are building something with more pipes (like the Universal or Topsy) a voicing machine greatly assists in setting up the pipes correctly. You can experiment with different pressures; set up the pipes on the workbench instead of having to fumble about inside the organ; set up multiple pipes together by comparing one pipe with another; set the stoppers to the correct position, and do various other fine adjustments to voice the pipes, before the organ is even finished.  Then when the organ is finished, all you have to do is to mount the pipes inside it, knowing that they have all been perfectly set up beforehand.

In some cases it is not even possible to build the organ without knowing how big the pipes are going to be, so you have to make the pipes first, but you've got no organ to voice them on! So you need a voicing machine to enable you to finish all the pipes and decide how they are going to be mounted before you can build the organ.  A voicing machine also provides a steady supply of variable wind pressure to allow you to experiment with valves and other pneumatic mechanisms like drum motors, etc.

The Finished Voicing Machine
Note the extra adapter on the bench, to provide for different pipe connections

The voicing machine described here is just a simple one, but it will be adequate for the needs of the amateur organ builder and experimenter.  The source of compressed air is a bit of a dilemma for a small device such as this.  One choice is to use conventional bellows operated by a foot pedal, but this makes the machine very cumbersome.  An electrical blower would be more convenient, but a suitable one is not easy to find.  Bouncy castle blowers, hair dryers, tyre inflators, Bontempi blowers, old vacuum cleaners and fan heaters have all been suggested in the past.  I decided to use one of the small 230volt air blowers that is used to inflate airbeds and other inflatable toys.  There are plenty of these for sale on ebay from around £5 - £25, just search for 'Air pump'.  The amount of air supplied by these blowers is plentiful, but I had no idea of the pressure, or whether they would be suitable. So I decided to get a mid-price one which cost me around £15.  The photograph below shows the one I bought.  When I received it I measured the output pressure, and was very pleased to see that it was just over 20 inches of water ("H2O).  The maximum pressure we are likely to need is around 10" H2O, so this should do the job nicely.

This mid-range 'air pump' is mains operated and came with an air hose and a selection of hose adapters

The idea is to mount the pump inside an airtight box, with the pipe(s) standing on top of the box.  Unfortunately, it is not as simple as that because we also need to make a reservoir in order to adjust the pressure and to provide a safety valve.  We also need to provide a valve underneath each pipe, operated by a row of keys, to switch the air supply to the pipes on and off.

Start by making a box big enough to house the pump you have bought. You can make it out of plywood, MDF or chipboard, around 1/2" thick.  The box needs to be about double the size of the blower. Mine turned out to be 12" x 8" (300 x 200mm) which was dictated by the scrap materials I had to hand!  Simple glued joints will suffice. You can seal the inside of the box with diluted PVA glue.  Fix the blower into the box, with the input connection poking through a hole in the side of the box. These blowers supply plenty of wind so absolute airtightness is not necessary, but any leaks will reduce the output pressure slightly.  As you can see from the above photo, the input connection to my pump was on the top, and the photo below shows the hole in the box. You will also see that I wired in a switch to switch the pump on and off, as access to the original on/off switch would be impossible once the box was closed!

Pump mounted inside the box. I made a wooden cradle for the pump to sit on, and secured the pump in position by means of a simple cable tie!  You can also see the on/off switch on the side of the box

The pressure box is divided into two.  The first half is the pressure box which contains the pump.  The other half is the windchest which the pipes sit on.  The reservoir is mounted on top of the box and is connected to both chambers by holes in the lid of the box. Construct the dividing baffle and glue it in position. Then make the box lid and fix it in position with screws around the outside.

Showing the two halves of the box. The windchest on the left and the pressure box on the right. The dividing baffle does not need to be absolutely airtight. It is only there to prevent the blower from blowing air directly into the windchest and causing turbulence and localised high-pressure spots

Remove the lid and drill holes in it to allow the air to pass through to the pressure box and also to the windchest. I drilled 6 holes of 16mm diameter (see below).  Next drill the holes for the pipes. I drilled a row of five holes of 12mm diameter as that is the largest size of pipe foot I am likely to need.  Alongside the holes for the pipes, you will need to drill a second row of holes for the push buttons.  I used M4 bolts for the push buttons and drilled a row of 4.5mm holes to accommodate them. These holes need to be counter-bored underneath the lid so that the heads of the bolts lie below the surface.

Top of lid showing the push buttons which operate the pallet valves underneath the lid.  The 2 screws sticking up are for mounting the interchangeable pipe adapters

Underneath view of lid, showing the pallet valves and springs. Three of the large holes go to the pressure box, the other three go to the windchest

The Pallet Valves

The pallets are made from thin strips of MDF faced with leather.  These are then positioned over the pipe holes and the leather hinges glued to the underside of the lid.  A strip of wood is then glued across all of the hinges, to hold the springs. The springs are simply made by bending a length of piano wire around a 6mm dowel or bolt (a 6mm drill would do). I used 0.7mm wire. Insert the springs into holes drilled in the wooden strip (see the photo below for the details).  The springs do not need to be heavy, as the air pressure inside the box will keep the valves closed.  The push buttons should sit on the pallets, so that when you operate the buttons they open and close the valves underneath.  You can now screw the lid back onto the box.

Close up of one pallet valve. The spring is made from piano wire and inserted into a hole in the wood.  Note the spring guide on the pallet to keep the spring in place. This is just 2 small pieces of MDF.

The Reservoir
The reservoir is made in the conventional manner. I made it out of two MDF sheets hinged together. Make it as large as possible to fit in the space available on the box lid.  The front opening is about 2.5" wide. Use blackout cloth for covering the reservoir, I don't think the expense of leather is justified for this purpose.  Before covering the bellows, drill 6 holes in the base to match the 6 holes in the box lid, and mount your favourite type of relief valve on the top board.  The relief valve must be large enough to vent all the air from the blower otherwise the bellows will blow out.  I made the hole about 35mm square, which is about double the area of the three input holes.  This proved to be just about right. I fitted an external relief valve, operated by a cord, but any type could be used.  When the reservoir is finished, glue it down onto the box lid, making sure there is plenty of glue around the six air holes.
Making a simple Relief Valve.

The advantage of an external relief valve is that it is accessible for adjustment, and it doesn't take up any room inside the reservoir

The pressure spring and its supporting framework can clearly be seen here. Pressure adjustment can be made by inserting the pin into different holes in the aluminium strip

The Pressure Spring

The pressure spring determines the air pressure delivered to the pipes, and it has to be easily and finely adjustable.  I used a conventional bent wire spring of the type used in the John Smith organs, mainly because it is easy to make, and you can make it as strong or weak as you like.  I used a length of 2.5mm piano wire bent around an 8mm former, but this is not critical. Just make the spring to provide the tension you require. If you don't have a spring bender, it would be easier to make two weaker springs instead of one strong one. In all cases, take extreme care when making and handling these kind of springs, as they can cause severe injuries if released uncontrollably. The spring is held in position by a strong framework built over the top of the machine.  It would probably be better to construct this first before making the spring, so that you can test the spring in situ. My framework is quite easily seen from the photos, and consists of two wooden uprights which support a square wooden batten. This batten is free to swivel on the fixing screws.  Fixed to this batten is another horizontal member, which holds down the spring. The other end of this piece is supported by a vertical aluminium strip bolted to the back of the machine.  The aluminium strip is drilled with a series of holes into which a pin is inserted to provide a range of adjustment for the spring mounting. Further adjustment is made possible by moving the spring backwards and forwards along the reservoir. An aluminium strip is fixed to the reservoir top board, and this is drilled with a series of holes into which the end of the spring sits. An identical strip is fixed to the horizontal strip above the spring.  Positioning the spring in different holes will adjust the pressure. Make sure your framework is strong enough to hold the spring down, as any failure could be dangerous.

This photo shows the upper and lower spring-retaining strips drilled with holes, to locate the ends of the spring. Moving the spring to the hinge end of the reservoir reduces the pressure

When the spring is positioned towards the hinge end of the reservoir, it can be mounted sideways to avoid fouling the framework

Adjustment of the hinged hold-down strip, together with the positioning of the spring gives a very large range of pressure adjustment, in my case this was around 1" H2O up to 10" H2O. You can also make a strong spring and a weak spring to allow even finer adjustment. The hinged hold-down strip also allows easy spring adjustment as it can be lifted up to remove the spring, and then pressed down again after the spring has been re-positioned.  You don't have to struggle to compress the spring with your bare hands. It also forms a convenient carrying handle!
Pipe Connections
The output connections to the pipes are made via interchangeable adapters. These are simply strips of MDF drilled with suitable holes to fit the pipes you intend to voice. Any number of these can be made to suit your purpose. They fit over two M4 bolts and are held down with wingnuts. I made two adapters, which can be seen in the top photo: One with a series of holes to suit the 5 different sizes of pipe feet; and the other fitted with the equivalent sizes of brass tube to which plastic hoses can be connected.  If you are intending to voice several pipes together, you would make an adapter with equal size holes to take the pipes concerned.  One hole could also go to a pressure gauge or manometer to measure the pressure.

Before switching the blower on, you must adjust the relief valve to open when the reservoir is around 3/4 full. Failure to do this will damage the reservoir. Remove the pressure spring, make sure all the pallet valves are closed, and switch the blower on. If the relief valve is working correctly, all the wind from the blower should be escaping through the relief valve and the top board should still have some movement left. Adjust the relief valve as necessary to achieve this.  Open one or more of the pallet valves so that the reservoir drops slightly, and check that the relief valve is closing properly and not leaking. Now all that remains to be done is to insert the pressure spring above the reservoir and adjust it to get the pressure(s) you require. Using a pressure gauge or manometer, you could mark the various spring positions that give 3", 4", 5", 6" of water, etc.  Or if you have a convenient pressure gauge like the one shown below, you could simply measure the pressure each time you use it.

The machine being used with a digital manometer (pressure gauge) to set the pressure to 8" of water.
Details of this useful gauge can be found here

Additional Notes

The blower is fairly noisy, but the pipes can still be heard clearly above the noise! The fact that the blower is enclosed in an airtight box cuts down the noise considerably, but it may cause the motor to overheat if used continuously for a long time. This does not affect the intended purpose, as a pipe can easily be voiced before the motor bursts into flames! This is just a warning not to walk away and leave the blower switched on.  I have run my blower for 15 minutes with no problems.  Of course, all blowers will behave differently and there's generally no way of finding out how noisy they are except by buying one.  I am quite happy with the blower I got, as it does the job with plenty of wind to spare.

I did think of using a 12volt blower instead of a mains one, as it would be easier to control the speed and possibly slow it down to make it quieter, as well as providing another level of adjustment. I decided that a 230volt blower would be more likely to provide more air, and I didn't want the extra hassle and expense of providing a 12volt power supply and a motor controller, which would have made the whole machine bigger. And I didn't want to have to run it on batteries! I may purchase a 12volt blower in the future to experiment with, but I don't expect it to be as effective as this 230volt one.

Having made this voicing machine I have learnt quite a lot about how air pressure and movement are inter-related, and how one affects the other, sometimes in unexpected ways.  I've also discovered several interesting voicing and tuning techniques!  I intend to do some experiments with pneumatic valve actions in the near future. This is the type of machine that makes you wonder how you ever managed without it!

Showing the finished voicing machine with a pipe to be voiced

Voicing in Practice
You should be aware that the pressure that you voice your pipes on will be lower than the pressure at the organ reservoir. If you have set the pressure spring on your organ to give (say) 6" H2O, the pressure at the foot of the pipes may only be 4" H2O. Even if there are no leaks, the pressure at the pipes will always be lower whilst air is flowing.  So you should voice the pipes at the pressure required to make them speak properly, and then adjust the pressure spring on the organ to give that pressure at the pipes.

Short demonstration video

Due to the automatic gain control of the camera, this video gives a false impression of the noise of the blower at the start and end of the video. You will notice that the blower is much quieter when the pipes are sounding!  Observant viewers will also notice that the reservoir drops like a stone when the blower is switched off. This is not because it leaks! When the blower stops producing pressure, all the air in the reservoir escapes back through the blower because the reservoir is not fitted with a one-way valve.


DISCLAIMER: As I have no control over how you use these ideas, or how proficient you are at this kind of work, no responsibility can be accepted for any accident or injury howsoever caused by following these instructions. This article is simply a record of how I made the machine, if you decide to make one yourself you are responsible for your own safety. The utmost care must be used when forming or handling the spring. You are advised to wear suitable gloves and safety goggles.  

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