The modern family of brass instruments can be broken into valved brass instruments (trumpet, horn, euphonium, tuba) and slide brass instruments (trombone). Brass instruments could also be broken up into Cylindrical bore (constant diameter tubing like the trumpet and trombone) and Conical bore (increasing diameter tubing like the horn, euphonium, and tuba).
The tubing on the brass instruments is curved to get a long tube into a short length.
The size of the tubing remains the same throughout the length of the instrument.
to make up the length for the sound
A cylindrical brass instrument is like the trumpet. It's not literally cylinder-shaped, but the pipe width stays constant for a while before flaring out as the bell. Its shape is the reason for its brassier sound. A conical instrument is more like the French horn. Again, it's not really a cone, but its pipes are constantly growing wider throughout the instrument. That's why it sounds so full and rich.
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waves travel in more cylindrical patterns. If the whole in stringed instruments was square it may actually hinder the sound by disrupting the sound waves.
I can't say why it persists, but tubing sizes are called "french" because they were invented by a French man who made surgical instruments.
The tubing wraps around the way it does because if it was all one straight line, the horns would be too long to play.
Other than shape and or dimensions, not a whole bunch. Both materials are most likely rolled from the same billet or raw material (a long, rectangular or cylindrical unfinished bar of iron or steel) in the same or similar rolling mills. These types of materials are known simply as mild carbon steel or merchant bar. This is assuming you mean HSS (Hollow Structural Steel) also known as square, or rectangular tubing. Most cylindrical pipe (sometimes called tubing) is mild carbon steel as well. There are seamless, or heavy wall pipes that may be of higher tensile. These are for specialized application and are less common.
Alembic started making its famous bass instruments in 1972 for Jack Casady. This bass has state variable filtering capacity and has pickups mounted on brass tubing so that their positions could be adjusted.