Eli5 – Why are drums round? Is it something to do with the shape that determines the ability to hit something and get a percussive sound out of stretched materials?
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With a round shape, you can maintain consistent tension in all directions from the centre at the same time as consistent distance from the center. This will make the residence frequency of the shape clear. For objects under tension, like a drum head or a guitar string the residence. Frequency formula is dictated by length, tension and properties of the material. Assuming that you were comparing equivalent, drum head materials from a round drum to a square drum the round drum resonance frequency will be dictated by its diameter and tension. With a square drum head, you would not have the same distance in all directions therefore the residence frequency may not ring as clear.
That’s what the theory suggests. I have not actually tested a square drum head with a frequency analyzer yet.
The circle is ideal. It puts tension across the drum skin evenly which makes the sound more even. . You can do a square or triangle drum but it would not be as easy to tune the whole skin evenly.
Drums are mostly round because:
1. The material used to make the earliest drums tended to be round – hollowed logs and gourds. That’s why when you see pictures or photos of drum artifacts or still-used traditional designs, they are not completely cylindrical, or they look like spheres with a flat side cut out for the head to sit on.
2. It has nothing to do about the ability to hit something and get percussive sound. Percussive sound is simply (and literally) that: the sound made when something is struck. Specifically about stretched materials, well, they will make sound when struck.
3. It is however about two things today: resonance and practicality.
– a round drum allows both the membrane and the sound waves bouncing around in the cylinder to resonate as evenly as possible. Because there are no corners, it is much easier for generate purer sound waves with less harmonic distortion. The cylinder on the other hand allows the waves to propagate with less distortion as well, and also for longer.
– a round drum facilitates so much ease in installing the membrane and get even tuning/tension. With corners, there is a lot of stress where the membrane meets them (i.e. the corner pokes through). Extra care is taken to ensure premature wear does not happen, and there is not much range in terms of tension and tuning.
4. Not to say drums have to be round. As others have already mentioned, the cajon is a box, and quite literally so. There is also for example the pandero jarocho, an octagonal frame drum. What you get from these polygon- shaped drums is more attack and shorter resonance. The closer to round you get, the more resonant the instrument.
The cajon being a rectangular prism allows some resonance but it is mostly due to hitting the middle and allowing air to resonate in the body of the instrument. But most of the sounds are sharp and not reverberant.
The pandero jarocho on the other hand has more resonance. Due to the membrane made of skin and the octagonal shape of the frame, there is more resonance to the skin. However you will hear the pitch go down after a hit, or you will hear distinct high and low notes depending on where the player hits the skin. This is because you have different lengths of skin depending on where you measure it (side to side is shorter compared to corner to corner).
For comparison, the Rototoms are extremely resonant despite having no cylinder at all. The round shape of the head and its design that allows it to be tuned very evenly gives the membrane extreme resonance and projection that it also allows the metal bracing of the instrument to act as its resonator.
The roundness makes it louder. So it is more efficient, cheaper and practical. Sound travels bouncing, so if there were corners then the corners kind of eat the sound making it quieter. Thats why all loudening devices are round too, guitars echo hole, the end of a trumpet/saxophone, the ball in a whistle, stereos etc.
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