How a Pipe Organ Makes Sound

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One of the most fantastic looking and sounding instruments ever created by human beings has to be the pipe organ. A pipe organ at its most simplest level is a device which forces air selectively through one or more of a variety of tubes of different sizes in order to produce sonic resonance at a particular tone within the selected tube(s). 

Evidence of its existence as a musical instrument dates back almost 2,000 years. 

Modern pipe organs are just as amazing to behold as they are to hear, sometimes having hundreds or even thousands of individual pipes in the largest versions.  

The pipe organ in Saint-Germain l’Auxerrois, Paris, France.

Individual pipes or sets of pipes are activated by using a combination of fingered keys, as on a typical organ or piano keyboard, as well as foot operated keys on larger models. A pump or bellows system is required to force air into the pipes according to the keys selected by the person playing the instrument.

There are often sets of valve knobs known as stops, which can be turned on or off to automatically combine sets of pipes together or separate them individually for a desired quality, or timbre, of sound being played.

The console of the organ in Salem Minster in Salem, Germany.

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So how exactly do the tubes, or pipes, in a pipe organ produce sound?  

In order to get any sound from the pipes in a pipe organ, air must be forced through them. This is generally done through a mechanical fan or bellows system that pushes the air into any of the tubes that happen to be open, which is determined by pressing the keys, foot pedals, and stop knobs. 

When air is blown into the tube, it creates a ripple effect within the air molecules already existing inside the tube. This “rippling” of the air is what we hear as sound; alternating pressurized and unpressurized regions of air molecules moving through the air and eventually reaching our ears which convert it into signals our brain interprets as different noises and tones.  

Air flowing through a tube will create a light whooshing sound, but it will also simultaneously ring out with a noticeable resonating tone. This tone is due to the particular physical characteristics of the tube reinforcing the portion of flowing air that happens to be moving in just the right way to match the tube’s dimensions. Only one particular tone will resonate clear and loud, due to its strong reinforcement by the air moving in just the right way to resonate within that particular tube.  

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How are the pipes tuned to resonate precisely at the desired notes?  

This has to do with the physics of the way air moves within a pipe or tube.  While the exact nature of this motion is dependent upon the specific construction of the tube, in general it has to do with where the air pressure is allowed to build up strongly within the pipe and where it maintains free movement inside which doesn’t build up any extra pressure. 

Any of the air moving inside the tube that is moving in such a way that it matches the exact conditions of pressurization and free motion that best fit that particular tube’s physical characteristics will be reinforced and build up into a loud resonance within the tube, producing sound waves bouncing around at that one frequency, or tone, at which it resonates.

Any of the air that we forced into the tube which ripples the air molecules inside the tube in a manner that compresses the air molecules the most at the very bottom of the tube, which is the location where the greatest pressure could be, and moves the air molecules around most actively at the very top of the tube, which is the location where the freest motion of air molecules could be, is “resonating” with that tube’s physical characteristics. 

That motion of air molecules is greatly reinforced and continues to build up more strongly as we push more air into the tube. Meanwhile any other type of air motion in the tube quickly dies out as its motion is not reinforced by the physical characteristics of our tube. You can think of this in a similar manner as jumping up and down on a trampoline.  

If you’ve ever jumped on a trampoline, you know that if you time your jumps just right you can bounce very high with surprisingly little effort. In that case, the timing of your up/down motion happens to fit the physical characteristics of that trampoline perfectly and the trampoline itself reinforces your motion, boosting the height you reach each time.

If, on the other hand, you try to jump too slowly or too quickly, you don’t move much at all.

This is because these other speeds of motion are not reinforced by the physical characteristics of that trampoline. Each trampoline is different, depending on its size and the materials it is made of, and so each will have its own particular timing of motion at which it will resonate.

Just like a trampoline, the resonance of a tube that is open at one end and closed at the other will depend on its physical characteristics. Namely, how long it is and what material is inside it that the rippling is moving through. In the case of a pipe organ pipe, the material inside the tube carrying the “ripples” is always air. And so it is only the length of the pipe itself that determines the conditions for resonance.

Sound waves have a wavelength, which is the distance between its maximum pressurization peaks as it ripples along through the air. This is very similar to how ocean waves have a wavelength which is the distance between adjacent wave crests as they move through the ocean water. 

Sound waves also have a frequency, which is how often the peaks in its wavelength repeatedly pass us. Just as ocean waves have a frequency, which is how often its wave crests pass you or crash onto the beach. Longer wavelengths take longer to pass, so their crests occur less frequently, and shorter wavelengths pass by quicker so their crests occur more frequently.

The frequency of a sound wave is the “tone” of sound that we hear. Or more accurately, our brain interprets air pressure fluctuations entering our ears at a particular frequency as being a particular tone. Higher frequency sound wave pressure fluctuations our brain interprets as higher pitched tones (treble) and lower frequency sound wave pressure fluctuations it interprets as lower pitched tones (bass).

A pipe organ pipe that is closed at one end and open at the other produces resonance for sound waves that have a maximum pressure buildup at the closed end of the pipe and remain at the same pressure as the rest of the atmosphere at its open end, which happens over 1/4 of a wavelength. 

We know this because, as can be seen in the example of an ocean wave, the distance between the top of a wave crest and its nearest “calm sea level” position is 1/4 of the total distance to the next wave crest, or 1/4 of its wavelength. 

Since sound travels through the air at a well-known speed, it is thus possible to calculate how frequently the pressure peaks would pass our ears for a given wavelength of sound. Since the wavelength of sound within the pipe organ pipes that is resonating within the pipe must be four times as long as the pipe length, its frequency can be calculated. That resonant frequency correlates to a particular pitch our ears hear. 

And so all that is needed is to adjust the length of the tube such that its resonating wavelength correlates to a frequency that precisely matches one of the notes on a keyboard. Since lower tones are related to lower frequencies and longer wavelengths, this means longer organ pipes will resonate at lower tones. Conversely, shorter organ pipes will resonate at higher frequencies, meaning higher tones.

A mechanical system can be designed to direct a flow of air into a single pipe, or set of pipes, whenever its associated keys are pressed. 

In larger pipe organs, different tubes that produce the same note but in different octaves, meaning different frequencies can be activated through turning the stop knobs on or off to produce a fuller, richer, louder sound whenever a single key is pressed on the keyboard.

Other instruments that use tubes or pipes in their design follow a similar principle as well, though perhaps not in such a straightforward way as the pipe organ design with an individual tube of a particular length chosen for each desired note. 

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