I was looking at music stands and a number of them do not have solid backs, rather, they have holes through them, like shown. Are these just for aesthetic purposes, or do they have a practical purpose?
They save weight. They don't save a large percentage of the overall weight of the stand, but they make it less top-heavy and therefore more stable when raised high or angled back a long way.
Just to expand on the question of "why not just make it thinner?" - the reason for that is because a thicker sheet with holes will be stiffer than a thinner sheet with no holes with the same amount of metal in. It won't necessarily be stronger in every loading mode - indeed it may be weaker in some - but with a structure like a music stand, wobbliness becomes a problem long before breakage does.
A loose page settles faster on the stand (and is easier to remove) when the air behind it can escape. That's at least the reasoning behind the namesake "vent hole". In practice, this is of relevance at best when you are playing from one large scotched-together wobbly monster sheet. In that case both the settling time gain as well as the removal time may be noticeable: if the monster sheet keels over and peels off the stand, you want it do so while you are still having your hand nearby.
As soon as several loose sheets stack up, the difference is unlikely to be noticeable unless you have static electricity involved.
I run the sound system at my church, and I have found that the music stands with holes make it easier for the musicians and vocalists to hear themselves in the stage monitors. The solid music stands can block a lot of the sound from the monitors. Of course a thick book on the stand with holes, also blocks the sound. But the sound easily passes through a few sheets of paper.
I agree that weight is probably the reason, but that can't be the whole story – you could always achieve a lower mass by just making the stand out of thinner sheet metal.
The problem with that is that it would lack the stability one expects from a proper orchestral music stand, for two reasons:
- Strength of elements that are much thinner than wide scales with the square of thickness (while mass increases only linearly). Therefore, if you use just a little bit thicker metal sheet than would be needed as such, you can already cut out a whole lot of weight in form of these holes (which also reduces strength only linearly).
This is actually the same reason that the stand stands on metal tubing, rather than heavy solid pillars – at the same mass (where you'd rather call the wires), these would be much weaker!
- Not all parts of the surface need to have the same strength. In fact most of the forces hinge on the center, where it's attached, and on the edges. That's why there are no holes in the lower center part, and the edges are folding back.
@topo morto is correct when he talks about weight. It most likely lowers the centre of gravity of the stand, stopping it from falling over. Another major reason is that it would reduce the weight of a shipment of these stands. The weight saving on one stand might only be a few grams, but the weight saving on a container full off these would be significant enough to save on shipping costs.
I joined this StackExchange site just to post this. Not sure why I care.
Sam is correct when referring to shipping costs, but has missed out a bit. The less material used in manufacturing, the lower the cost. Also, the metal that would have been in the holes would be used to make more stands from the original metal. Result, Holes mean more manufactured units with a lower unit cost.