Ummm, “soft” plastic bags can be handled and make no noise at all, but the sort that you get at supermarket checkouts, or worse, the kind that contain potato crisps and the like, make the ubiquitous crinkly-plastic-bag sound.
Usually I seem to encounter this noise in cinemas during the quiet bits, or in other places where I just don’t want to hear annoying things.
So the question is, just how come they make this sound? What is happening on the surface of the bag to propogate these annoying soundwaves…?
The answer for the grocery bags is that they are two different types of plastic.
The “soft” bags are low density polyethylene.
The crinkly ones are high density polyethylene.
There are pros and cons to each type. With the high-density ones, you can use a thinner plastic to get the same strength as a comparable low-density one. High density bags are thinner and cheaper. This makes them ideal for an operation like a supermarket which uses large numbers of bags. Even if cost per bag wasn’t an issue, the thinner plastic (more bags per bundle) allows for greater efficiency.
This comes at a price, though. High density bags are much more brittle and prone to splitting and breaking. Low density ones are stretchier. This is why Hefty bags and the like are usually low-density–they can resist punctures and tears much better than high-density bags, and if they are punctured, the cut won’t necessarily spread. Plus, as you noted, the low density bags have a much better “hand.”
About the potato chip bags, I don’t know, but I’ll guess it has something to do with gas permeability.
And apparently tastier, too, since for some weird reason my cats LOVE to chew on them. Fortunately, the noisiness of this kind of bag alerts me that the cats have located one, and I can get it away from them before they aspirate a piece.
Thanks for the responses so far, but I’m still at a bit of a loss.
It’s just that I keep thinking of a whip cracking. My understanding is that the characteristic “snapping” sound is a result of the tip of the whip exceeding the speed of sound.
I also appreciate that the pitch of any sound is dependent on the frequency of vibration of whatever is making the sound.
But surely a bag swinging idly by the side of someone in a lift, for example, has surfaces that are neither moving faster than the speed of sound, or vibrating at a sufficiently high frequency to cause that annoying noise.
So what’s the bag actually doing that generates the sound waves? Ain’t it just hanging there?
[WAG]In looking closely at the surface of a crinkly plastic bag while crinkling I’d hazard a guess that the crinkling represents the accumulated noise of thousands of micro-second long high frequency surface “snaps and pops” as the tensioned sections released energy (as noise) as they are re-oriented by handling. Looking at the surface of a bag like this the surface is fractured into millions of wrinkled sections that will crack when the surface is realigned. The “crackle” is all these little tiny microseconds snaps in concert.[/WAG]
I think that astro is saying that it’s like if you take a piece of paper at each end and pull it tight, it will make a snapping sound.
Probably the chips and whatnot in the bag push out in various point creating tension points, so when the bag flexes some number of those will pull a spot tight and it will make the snapping noise.
I would imagine that low-density bags don’t do this since they’re more rubbery. You pull them tight and they just stretch further.