1. Why Fireworks Must Be Lit Seconds Before the Beat
When you see a firework burst at the exact moment a drum hits, it feels like one event. But fireworks do not explode the instant they are lit. A shell shoots out of a tube, rises into the air, and then opens. For a 10-gou shell, the time from ignition to opening is estimated at about 6.5 seconds. [1] This is roughly the time it takes to count "one, two, three..." up to six, with a little extra wait.
Therefore, to match the explosion to the music's beat, the ignition must happen about 6.5 seconds earlier. The target is the moment the shell opens, not the moment the fuse is lit. But is this wait time the same for every firework?
2. Why Larger Firework Shells Take Longer to Open
Firework size is measured in "gou." According to data from one fireworks company, the estimated time from ignition to opening varies by size: 3.0 seconds for a 3-gou shell, 4.0 seconds for a 5-gou shell, 6.5 seconds for a 10-gou shell, 10.0 seconds for a 20-gou shell, and 12.0 seconds for a 30-gou shell. [1] In these examples, larger shells take longer to open.
The fuse helps create this delay. It acts as a timing device, and the fuse is longer for larger shells. [2] A 10-gou shell is said to open to about 300 meters across. Its ignition-to-opening time is about 6.5 seconds. [7] Interestingly, the numbers do not scale evenly. A 30-gou shell is ten times the size number of a 3-gou shell, but its wait time is only four times longer (12.0 seconds versus 3.0 seconds). To sync with music, organizers must calculate this specific wait time for each shell and adjust the ignition time accordingly. Do humans decide these times while watching a clock?
3. How Computers Remember Each Firework's Ignition Time
In large fireworks festivals in the 2020s, electric ignition is common. In systems where tubes are connected by cables, a switch can be triggered from a distance, and the launch timing is programmed into a computer in advance. [2] Shows designed to synchronize fireworks with music are often called "Music Starmines." These do not just play background music; a computer program ignites the fireworks in sync with the audio track. [4]
One article describes the precision of these programs. It states that everything from launch to explosion and disappearance is calculated in units of 1/30th of a second. [5] If you count the 6.5-second wait in these small units, it equals 195 frames (6.5 × 30). A company called Nihonbashi Marutamaya claims to have held the first show in Japan to sync fireworks and music with a computer, in 1995. [6] This "first in Japan" is the company's own explanation, so it should be read as one perspective. But does the time it takes for the electric signal to travel cause delays?
4. Why Electric Ignition Is Much Faster Than the Shell's Wait
How fast is the electricity for ignition? In an example from a foreign patent, a device sends about 12 volts and up to 5 amps of electricity for only about 12.5 milliseconds. The computer can switch to the next ignition in less than 1 millisecond. [8] One millisecond is 1/1000th of a second.
12.5 milliseconds is 0.0125 seconds. If you divide the 6.5-second wait of a 10-gou shell by this, you get about 520 times (6.5 ÷ 0.0125). The time electricity flows to ignite the shell is much shorter than the time the shell takes to rise and open. [8] This example suggests the wait is mostly on the shell side. However, the delay for the signal to reach the shell is not shown in these numbers alone. Also, this is just one example from a foreign patent and may not match the equipment used in Japanese festivals. Is the shell's wait time always exactly the same?
5. How Timing Is Adjusted During the Show Because Shells Vary
Foreign patents explain that while the launching device controls the launch, it does not control the exact moment of opening. The time from launch to opening is nearly constant, but it can vary depending on the type of shell and the manufacturing batch. [3]
Because of this variation, the design shown in this patent allows an operator to slightly speed up or slow down a reference clock during the show to adjust the overall timing. [3] This means a human makes the final adjustments. Another method is sharing the music's timing. Systems using audio effects and production timecodes (SMPTE timecode) can synchronize multiple launch sites far apart. [8] This can synchronize music and fireworks across multiple launch sites. Note that patents describe one design and may not reflect how all Japanese festivals operate. So, the explosion matches the music. But what about the sound reaching our ears?
6. Why You See Firework Light Before You Hear the Sound
It can take 2 to 3 seconds for the sound of a firework to reach you after you see the light, depending on where you are standing. [2] There is a natural gap between the light and the sound of the firework itself. It is important to distinguish this from the synchronization with music. The "sync" discussed earlier is about placing the explosion on the musical beat. The delay between seeing the light and hearing the boom is a separate physical phenomenon.
Music-synced fireworks shows aim for the moment the shell opens in the sky. To place that moment on the beat, ignition starts seconds early, the estimated wait time for each shell is taken into account, electricity sends the fire, and clocks are fine-tuned during the event.
7. How to Count the Light-to-Sound Gap and Try the Lead Time
You can check this light-and-sound gap yourself. If you go to a fireworks festival, when you see the flash, count "one, two, three" in your head. Stop when you hear the "boom." It is said this can take 2 to 3 seconds. [2] Always be aware of your surroundings and stay in a safe place, especially in crowded areas.
You can also try a small version of this calculation at home. Pick a song and choose one moment where the music gets loud. Use a stopwatch to measure the time from the start of the song to that loud moment. Then, start the song and the stopwatch together. When the stopwatch shows "the loud moment minus 6 seconds," tap the table lightly and count "one, two..." up to six. If the loud moment is at 20 seconds, tap at 14 seconds. Does the end of your count match the loud moment? Since the estimate for a 10-gou shell is 6.5 seconds, using 6 seconds leaves a difference of about 0.5 seconds. Seeing which one comes first helps you understand why clocks are adjusted during real shows.
For more reading, the Honda Kids fireworks page and the Mitsubishi Electric column are available in Japanese. [2] [4]