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I've just 3d-printed what's supposed to be an ear-shatteringly loud emergency whistle (120 dB at one meter). However, it's apparently somewhat sensitive to printing conditions, because some people report their whistle doesn't make any sound at all when blown. Clearly, I need to test mine before I encounter an emergency situation.

How can I test my whistle without causing people to call Search & Rescue (or the police)?

You just need a place where blowing a whistle wouldn't be out of the ordinary.

Perhaps a city soccer/football field would work as people blow whistles there. Or if you could find an outdoor area where you know there won't be people around within hearing distance.

A distress signal is given in 3s, so one blast shouldn't be a problem.

Do note that you should definitely be using ear protection, this calculator says that if its 120 decibels at one meter then it would be 140 decibels at 4 inches and 140 decibels is more than enough to cause pain and puts you at risk of permanent hearing damage.

Very indoors, shut up tight, and at a suitable time of day.

If you live in a house (as opposed to a flat), and it has reasonably good insulation and tight-fitting windows, the high-pitched sound of a whistle will be significantly attenuated. The noise level is comparable to a burglar/car alarm siren, and they're fairly common. If you've ever heard an inside siren (only) going off from outside you'll get an idea of the attenuation.

Then if possible find a room or closet with no outside walls or windows, go in and shut the door. I'd use my downstairs cloakroom or even the cupboard under the stairs (the former has an extractor vent to outside which I know let's sound in). In such a confined space it's even more important to wear ear protection. Then a quick blow should be enough. I suggest rinsing it out in a cup of water and testing again while you're there, to ensure it works in the wet.

If your house a shares a wall with neighbours, try to do it while they're out. By this point, people in their own homes will barely notice, and it shouldn't be annoying to people outside, except right outside your house. You should still aim for a time when you won't disturb people - in the middle of the day presumably

If you're still worried, wait for lots of background noise - perhaps when the neighbours are cutting their grass. This will mask the sound as their nose is emitted outside, will make it less annoying as you're adding little to an existing noise, and will make the location of the source almost impossible to detect (a short blast remember).

It doesn't matter. Blowing a whistle is not an emergency signal: emergency signals have a certain sequence. Be sure to use your hands to close your ears (index fingers pressing ear canals closed). If the whistle works, you'll still notice.

Frankly, outdoor shops have working emergency whistles probably cheaper than what your printer materials cost. So there is not much of a point in printing one.

Inside your own car, if such is available to you. While stopped at a red light in traffic or some such, I mean. Also wear something to protect your own hearing while you're at it.