No__83___How does a Pipe Organ work?

[Peter Anderson]

I happen to be currently posting a series on the Pipe Organ, and decided to run this subject in parallel.
Here I describe, over 6 sessions, the fundamental workings of the organ.

You may want to read about Basic organ stops explained, which you will find in Peters Pearls No 81, and you can do so by opening the subject in a separate window by clicking this link:

http://www.ar-group.org/smforum/index.php?topic=2976.0


How does a Pipe Organ work?

In order for it to work, an organ requires wind, and lots of it!

Nowadays, the wind in the bellows is supplied by electric blowers but in the olden days, bellows had to be pumped by hand.   Many electrically powered organs still have hand bellows built in.     At times of a power cut, I have pumped a church organ by hand during the service.   There is a telltale, like plumb bob on a string, that you have to maintain between a high and low mark on the scale.    When the organist requires more wind, you simply pump more quickly, keeping your eye on the 'gauge' or else the sound withers and dies away, to everybody's consternation.

The wind from the bellows is controlled by the action, that determines which pipes are allowed to speak, as well which pipes are to remain silent. 
Since it is the length of a pipe that determines the pitch of the sound it makes, there must be at least one pipe for every note on the keyboard.    Furthermore, to get the different colours of sound and varying degrees of loudness, there needs to be a great number of pipes.

There are many, many different types of organ pipe and each type has its own timbre and name.  For example Stopped Diapason, Lieblich Flute, Dulciana and Principal, to name a few examples.     

Such a vast number of varying designs has come about through experimentation and a desire to emulate other musical instruments like the Trumpet, Oboe, Flute or Violin.  The result is that the organ is both the loudest and the quietest of all instruments. 

However, all organ pipes belong to one of two families.       These are the Flue pipes and the Reed pipes. 
An example of the sound produced by a reed pipe is the Trumpet.  An example of the sound produced by a flue pipe is the Flute.
       
There are many other variables that control the sound other than the design of pipe.  For example the wind pressure; the materials used in construction and even the fabric of the building in which the organ is installed.
A pipe made from wood will have a different timbre to one of similar dimensions made from metal.   

Metal pipes are generally made from Tin, Copper, Zinc and Lead.

The greater amount of lead in a pipe gives its sound a softer tone.
The greater amount of Tin in a pipe gives its sound a harder tone.

Wooden pipes will often have metal in their mouths.
Brass is commonly used in Reed pipes.
The choice of wood, metal, or alloy is at the discretion of the organ builder as he endeavours to meet the requirements of his customer.

Peter

[Peter Anderson]

In the next Reply, we show how organ pipes work.

First let me remind you that one of our members, Don Wherly, made organ pipes commercially and still retains some equipment that he used to manufacture them.   His knowledge and expertise in this area is way beyond mine, but if you have specific questions, he would be prepared to help answer them.

If you have such questions, then it might be a good idea to post them here, under this subject, so that we can all get involved in the discussion, as Don's replies would also appear here.

Peter

[Peter Anderson]

How do organ pipes work?

As mentioned at the beginning of this subject, there are two main families of organ pipes, which are called, Flue pipes and Reed pipes. 

Here is a schematic cross-section of a simple Flue pipe, showing the names of their different parts.   
(Drawing not to scale)




Wind enters the pipe at the Tip A and fills the Foot of the pipe B.   

Since it is under pressure, the wind is forced through the narrow gap called the Flue or Wind-way D between the Languid E and the Lower Lip C.   

The thin sheet of air which leaves the Flue, rushes across the Mouth of the pipe F and flows past the front of the Upper Lip G into the room. 
This flow of air across the Mouth pulls air from inside the Body of the pipe H along with it, out into the room and so the column of air inside the Body of the pipe, which until now has been at rest, begins to travel downwards. 
At this point, there is a large amount of air leaving the Mouth of the pipe.  There is the air coming from the Foot B together with the air from the Body H.

The air does not continue behaving in this fashion for very long because as the air column moves downwards, it creates a depletion of air molecules at the top of the pipe.    When the force of the depletion becomes greater than the force of the air flow from the Flue, the column of air inside the Body stops moving downwards and begins ascending. 
As the air column moves back up the pipe, a depletion of air molecules now develops at the Mouth and so the air coming from the Flue, which until now has been flowing across the Mouth and into the room, is sucked back into the Body of the pipe to maintain an even air pressure.  At this point, there is no air leaving the Mouth of the pipe, in fact some air from the room may even be drawn in through the Mouth.  And so we return to the beginning of the cycle. 
It is the movement (oscillation) of air molecules up and down the Body of the pipe that produces the sound waves.

Peter

[Peter Anderson]

Length of pipes

The pitch of the sound produced is determined by the length of the body of the pipe.   

This is just as the pitch of a note made by a stringed instrument is determined by the length of the string hit or plucked. 
So just as a short string will create a higher pitch than a long one, so a short pipe will create a higher pitch than a long pipe.

Therefore, the longer the pipe, the lower the note and equally the shorter the pipe, the higher the note.

Pitch can be expressed in terms of the frequency of the vibrating column of air in the pipe.
 
Middle C on a piano has a frequency of about 256 Hertz (= cycles per second) {Wikipeadia quotes the standard as 261.62 Hertz} and to produce the same pitch on an organ, a 2 foot organ pipe would be required.  As it happens, if the length of a pipe is doubled, the frequency is halved and the pitch goes down an octave. 

So the pitch of a 4 foot pipe will be Tenor C and an 8 foot pipe will produce a Bass C.  Going the other way, a 1 foot pipe will produce a Treble C and a ½ foot pipe will produce a Top C and the octave above that is a 3 inch pipe.

The longest pipe in an organ is usually in the pedal department and on most organs is 16 foot in length. 
Cathedral organs usually have one or two 32 foot ranks, and such 32 foot pipes have a frequency of just 16 Hertz! 
The longest pipes in existence are 64 foot long, but these enormous pipes are few and far between. 

As far as I know, there are only two organs in the world having pipes that are actually 64 feet in length.   For comparison, this is about the height of a 7 story building!    The famous Hill Organ in Sydney Town Hall which has a 64 foot Contra Trombone and the colossal Midmer-Losh organ in Atlantic City which has a 64 foot Diaphone-Dulzian.

Just to confuse things a little, some pipes are stopped which means that the top is blocked off by a stopper. 
On some organs you may see Gedeckt on a stop, and that means the same thing i.e.stopped.
This has an harmonic effect and the sound heard from a stopped pipe is an octave lower than its open counterpart.   

For example, when it says Stopped Diapason 8' on the Stop Knob, the longest pipe is actually only 4 foot long.

Peter

[Peter Anderson]

What are Stops?

        The Stops are used to stop the air from flowing through the pipes.  The Stops are part of the Action that controls which pipes are allowed to speak and those which are to remain silent.

We cover Action in a later Reply, but at this point, you need to realise that the Stop Action links the Stop Knobs in the Console which is the area from where the organ is played, which holds the manuals (aka keyboards), pedals, and stop controls, to the Sliders in the Soundboard. 
 
Here is an example of a rather grand Organ Console:




In electric-action organs, the console is often movable, as demonstrated in the picture above..

The actual Slider is a piece of wood with holes drilled in it to line up with the feet of the pipes.  The Slider can move either in or out.      When it is in, the holes don't line up with the pipes and the air is stopped from getting through.    When it is pulled out, the holes do line up and air is allowed to pass through to the pipes.

There has to be one Stop for each Rank of pipes. 
 
Note: We explain this term Rank in the next Reply.

The Soundboard is that part of the pipe organ on which the pipes stand.   It also contains the Pallets and the stops that control the air flow.

Without organ Stops, whenever a key is pressed on the keyboard, every pipe above that note on the Soundboard would speak!   Before organ Stops were invented, that is exactly what happened.   This chorus of pipes for every note on the keyboard is called Blockwerk.

Organ Stops are identified at the Console by the names of the pipes that they control.  So the Stop that controls the Open Diapason rank has "Open Diapason" written on the Stop Knob.  Also inscribed is a number that corresponds to the length in feet of the longest pipe within that rank, so that the organist has some idea of the pitch. 

If the pipes are stopped, the longest pipe won't actually be as long as it says!  It will be half the length, because a stopped pipe sounds an octave lower than its open counterpart.  However, the length inscribed on the Stop Knob is always the equivalent open length.

Many people use the term Stops to identify the different sounds produced by an organ or to describe a particular set of pipes.   For example, "The Principal Stop is out of tune again!", or "That's a nice Stop, what's it called?"  This is not strictly correct because it is the pipes that make the sound, the Stops merely control which pipes are allowed to speak.

As previously mentioned, in the next Reply we look at Ranks.

Peter

[Peter Anderson]

What are Ranks?

        The pipes in an organ are grouped together in rows called Ranks according to the particular sound that the pipes make.  Each rank has enough pipes for every note on the keyboard and so there are usually 61 pipes in a rank, because this is the standard length of an organ Keyboard.

        Ranks are identified by the length of the longest pipe in the rank.  For example, a rank of Open Diapasons (8 foot) will hold pipes varying in length from a couple of inches to 8 feet.  In this example, the lowest note is produced by the 8 foot pipe and so this rank is referred to as the 8 foot Open Diapason rank.  As already mentioned, it is this name that appears on the Stop Knob in the Console.   Similarly, a Stop Knob with "Principal 4" inscribed, controls the 4 foot Principal rank.

        Sometimes pipes are named according to the interval between them and the 8 foot pitch.  For example, a 2 foot Flageolet produces a note that is fifteen notes higher up the scale than an 8 foot pipe (an interval of a fifteenth).  This means that when a middle C on the keyboard is pressed, with only the Flageolet Stop out, the note that is heard is actually a top C (two octaves higher).  So it is sometimes labelled a Fifteenth (15th).

        The Nazard pipes sound a twelfth higher than the note that is being played and the inscription on the Stop Knob will read Nazard or Twelfth.  If a length is inscribed, it would be 2 2/3 because that's the length of the longest pipe in the rank (2 foot 8 inches).  If a Nazard Middle C is pressed on the keyboard, a treble G is what we would actually hear! 
         The Nazard is referred to as a Mutation Stop which means that it could sound odd when played on its own.  However, when played together with another rank, it can enhance the colour of sound.
In the final Reply we explain the different types of Action.

Peter

[Peter Anderson]

Different types of Action
       
The action is the system that is used to allow the organist to control the instrument.  There are two parts of an organ that are controlled by the action. 

The first part controls the Stops and the second part controls the Pallets.

Pallets are a component of the Soundboard and can be thought of as valves that are normally closed.     There is one pallet for every key on the on the Key (or Pedal) Board.   
When a key is pressed the corresponding Pallet is opened and, depending on the position of the Slides, wind is admitted to the pipe(s).

In some organs the type of action used is the same for everything and in other organs there might be one type of action for the Stops and another type of action for the Pallets.

The pipes are arranged over the wind chest and that in turn, is connected to the keys via a set of pallets, or valves, and fed with a supply of air by electrically or mechanically activated bellows.

Each rank is brought into action by a stop that is connected either by levers, or electrically, to a slider.

To bring a pipe into ‘speech’ the organist must first draw a stop to bring the holes in the slider into alignment with the foot of the pipes on the toe board.     Then by pressing a key, the pallet under that pipe is opened, allowing air to travel along a narrow channel, through the slider hole, and into the pipe.

Therefore ensuring that there is enough wind (or driven air) to generate the required sound and volume, requires the knowledge of an expert.

Tracker action
        Tracker Action, or Mechanical Action as it is often called, is the oldest type of action.  It does not require any electrical apparatus for it to operate because the link between the organist and the Pallets is completely mechanical.   

A series of levers and connecting rods transfers the movement from the keys to the Pallets.   
The diagram below shows you an example of the type of mechanical leverage which may be used and how it relates to the pipes that are energised to speak.

Electropneumatic action
       
This type of action uses electric current between the manuals and the Soundboard and then pneumatic motors to actually open the Pallets.   

Pressing a key on the keyboard completes an electrical circuit and the corresponding solenoid is energised.  The solenoid releases air from a pneumatic motor which opens the Pallet for that note.
Here is a drawing of how the stops and the pallets (operated by the keys) align to produce sound from a particular key, with one or more stops selected, to supply that rank of pipes.



Peter

[Peter Anderson]

Please feel free to add your comments about this Pearl, as a Reply to this posting.


P.S.    I already know that in the diagram above, where the note is labelled H, we know it as B!     
Congratulations if you spotted this, it means you were concentrating.

Peter

[Peter Anderson]

We will endeavour to respond to any Questions you have on this subject, and willingly converse about any Comments that you may have.

Peter

[Peter Anderson]

Not sure where this was taken, or when, but it shows the size of some organ pipes and the consequent problem of moving them and installing them.



Some of the larger organ pipes weigh close to a ton!

Peter