How do fireworks whistle?
Short synopsis: Fireworks can be made to whistle by using a hollow tube and a special “whistle powder”. These hollow tubes cause the burning “whistle powder” to resonate and create loud, piercing noises. As the powder quickly burns away, the sound dramatically changes tone as the hollow area becomes larger.
A whistle is essentially a tube device, and inside of this tube is a compressed pyrotechnic composition of a specific quality. Generally, the tube is made out of cardboard, and clay is most often used to close off one of the two ends (although sometimes caps are made out of cardboard or other materials). The composition in the tube itself must be well packed to eliminate any air pockets. Air pockets can be dangerous because they may act as a kind of mini-combustion chamber that might cause the device to explode. Colorful labels mask the simple inner workings within these tubes, but anyone who thinks that a firework whistle behaves anything like an ordinary whistle is completely mistaken.
How the fireworks whistle actually works
The pyrotechnic composition to create these loud novelties has to be very specific because it needs to combust at a certain pulsating frequency. In other words, when this pyrotechnic composition burns, there will be what is best described as a very high frequency “stutter” locked deeply within its chemistry. When this compound is compressed within a tube, the surface of this pyrotechnic composition creates a pulsation. These pulses generate a standing wave inside the tube. Thus, the empty tube—closed at one end—allows the resonance (link: https://en.wikipedia.org/wiki/Acoustic_resonance#Resonance_of_a_tube_of_air) to occur. The longer the resonant tube, the lower the pitch of the sound.
Is it possible to make a colored flame that whistle at the same time?
Yes, this is technically possible. This can be done by adding chemicals such as those containing strontium into the composition, in combination with a suitable chlorine donor. Although it is nearly impossible to generate fireworks whistles conjointly with chemicals that create deeply saturated colors like greens or blues, but, yellows and whites—since they are natural burn colors—can be easily made to whistle simultaneously. As most pyrotechnic chemists already know, you can create a whistle with a silver tail simply by adding the correct mix of titanium. This inventive creation dates all the way back to the early 70s.
[Link to patent] http://www.google.com/patents/US3712223
What about the safety of whistle manufacturing?
The manufacturing of whistles has gotten a rather bad reputation in recent years. There are probably a couple of good reasons this reputation is somewhat deserved. In the old days, people would ram their whistles by hand, using a type of ramming rod used by sculptors to pound or chisel into wood or stone. Of course, this is no longer the method recommended (because it is not considered to be safe), and today hydraulic or pneumatic presses are used instead. To make it even safer, these presses cerate the whistles and the presses are operated remotely.
Another reason whistle manufacturing is considered to be rather unsafe is because most whistle composition used during the manufacturing process is at one stage an uncompressed or granulated powder. In that condition, whistle composition can aggressively burn, similar to flash powder. This is the main reason that whistle manufacturing is so often involved in pyrotechnic accidents—explosions can happen very rapidly.
How do you get ‘raspy’ whistles?
The tone of a whistle, like I mentioned earlier, is primarily the function of the tube’s length. The shape of the compression whistle and the composition also affect the sound. The German Zink Feuerwerk (link http://www.zink-feuerwerk.com )Company is famous throughout Europe for making (among its many other unique products) raspy whistles. They have made these raspy wonders these for a few decades.
At a PGI convention in 1994, held in New Castle, Pennsylvania, Fred Ryan and his team created all the accompanying tones for the song, “Stars and Stripes Forever” simply by using variously sized whistles cut at different lengths.
Interesting whistle applications
Some interesting applications for the use of pyrotechnic whistles you may not have ever thought about, but are regularly in demand are: whistles used to scare birds at airports and to scare birds and other animals who feed on agricultural crops/products. Some older types of whistles were once actually wired into car alarms, and some are still used as artillery simulators for military use.
Does a strobe work the same as a whistle?
Yes, in a sense. That is because a strobe also oscillates. But the strobe phenomenon is usually around 0.5 Hz to about 20 Hz. Also, strobes generate mainly a light pulse and produce less residual gasses so their sound is more like a “puff” than a whistle.