|Making a Pipe Voicing
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
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
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
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 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!
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
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
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
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.