Every organist has the experience of noticing, more likely sooner than later, that organ pipes are like every other wind instrument -- their pitch varies with temperature.
Pipes will sound FLAT WHEN THE AIR IS COOL, and sound SHARP WHEN THE AIR IS WARM; the reason is, the air in the pipe is less dense when it is warmer and therefore oscillates faster.
Expansion or contraction of the pipe metal itself (photo), as it turns out, is negligible.
This is important for an organist to know and understand:
When a metal organ pipe is heated its length will expand a miniscule amount which, if nothing else were happening, would cause it to go flat -- however the air inside it will heat up at the same time, thus increasing the speed of sound in that air which will work the opposite way and sharpen the pitch.
Temperature indeed is a condition which affects the speed of sound.
Sound, like heat, is a form of kinetic energy; molecules at higher temperatures have more energy, thus they can vibrate faster; and, since they vibrate faster, sound waves can travel more quickly.
One way to think of this is: hotter air is lighter; the gas molecules in hot air are spaced farther apart -- colder air is heavier; the gas molecules in cold air are closer together -- thus, a pipe has less molecules of air in it when the room is warmer than when it's cooler, which affects the pitch that same pipe will produce.
If, let's say, a pipe is tuned at 70 degrees Fahrenheit, it will come back to that pitch at which it was tuned every time the temperature comes back to 70 degrees.
If, after that same pipe is tuned, the temperature increases to 75 degrees, the pitch of the pipe will begin to go sharp -- and if the temperature cools to 65 degrees, it will begin to go flat.
This isn't a problem for organs in environments where the temperature stays within 5 degrees plus or minus of the tuning temperature; it IS a problem if the temperature excursion is greater, which explains why pipes installed in attic locations where the roof of the building is also the ceiling of the pipe chamber need to be tuned more often -- why -- because attic temperatures shift widely back and forth, uncertainly.
Seasonally, the temperature typically fluctuates so much that most pipes won't hold pitch throughout the whole year, and, at the height of summer or in the dead of winter when the ambient air temperature in the room is, let's say, 87 degrees or 62 degrees, respectively, good tuning is simply impossible.
The speed of sound in room temperature air has been measured at 346 meters per second; this is faster than 331 meters per second, which is its speed in air at freezing temperatures, thus sound travels faster as temperature increases, and this increase in speed tends to increase the frequency we humans hear.
The speed of sound is also affected by other factors such as humidity and air pressure, but this effect of hotter air being inside the pipe on the frequency of the pipe is always greater than the miniscule expansion, or lengthening, that same hotter air creates in the length of the pipe -- and so, we find the former always overriding the latter and that the frequency of a metal organ pipe always INCREASES AS TEMPERATURE RISES.
Conversely, the more dense air inside a colder pipe oscillates slower, and this effect is always greater than the miniscule amount of contraction, or shortening, that same cold air creates in the length of the pipe, thus we find that the frequency of a metal pipe always DECREASES AS TEMPERATURE FALLS.
This can be demonstrated in a simple experiment using a cold open metal organ pipe and 2 tuning forks:
If we assume, for example, that the frequency of the cold organ pipe is 400Hz and a beat of 4Hz is heard when a tuning fork is sounded, the tuning fork could have one of two frequencies: 396Hz or 404Hz.
If the tuning fork has a frequency of 396Hz and the temperature rises, the beat frequency will increase because of the increase in frequency of the pipe.
If on the other hand the tuning fork has a frequency of 404Hz and the temperature rises, the beat frequency will decrease because of the increase in frequency of the pipe.
A high enough temperature could result in no beat, and an even higher temperature could eventually result in the beat frequency increasing.
Thankfully an organ will quickly come back into tune when the room's tuning temperature is reached regardless of how raucous it sounds when it's extremely hot or cold in the building; temperature extremes however are not as big a concern as are excessive seasonal variations in humidity, which can cause issues with certain wooden components in the organ.
Wood contracts, expands, and twists, for example, as humidity rises and falls, causing wood movement.
Good organ design can compensate for most of these problems, but serious wood cracking can develop from the extreme dryness caused by continuous winter heating for several days at a time.
The humidity gauge placed inside the organ chamber should generally stay above 30 per cent in the winter and below 80 per cent during the summer; prolonged humidity readings below 30 percent generally means that a humidifier is indicated, but it should never be placed near the blower intake of the organ; great care also must be taken to ensure that water cannot drip on any organ parts -- malfunctioning humidifiers and overflowing dehumidifiers can severely damage organ parts and windchests.
If the summer humidity readings are above 80 per cent a dehumidifier may be needed in the duct line to help the primary compressor remove enough moisture in the wind (pressurized air).
When the heating/cooling system of the building is turned on, stable temperature usually can be achieved inside the organ 3-6 hours afterwards; if outside temperatures are extreme, or if air does not circulate freely through all parts of the instrument, additional time should be allowed.
As long as the humidity remains below 80 per cent, the organ components should be fine even though the temperature in the chamber may rise as high as 90 degrees during the hottest weather.
If the pipes are enclosed and situated behind a tone grille, then keeping the swell shades open and removing the grille cloth wherever feasible will promote better air circulation within the pipe chamber.