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We just bought an ultrasonic humidifier. While researching if I needed to use demineralised water, I came across a website (this one, in German) saying that ultrasonic humidifiers are harmful/annoying for pets, as it emits a sound humans cannot hear but animals can. I just turned it on a few minutes ago, to see what their reaction is. They have been very curious, but it's hard to tell if they find it annoying (or will do so if they hear it for a longer time). I haven't been able to find any other sources regarding ultrasonic humidifiers and pets. Does anyone know if I can safely use it?

Edited to add: I had the brilliant idea to do another search, this time in English and not in German. I found a few websites saying that it depends on the frequency - cats can hear frequencies from 55 Hz up to 79 kHz, any ultrasonic device should use a higher frequency than that. This prompted me to email the producer of the humidifier with the question which frequency this specific humidifier uses. Nonetheless I would be curious if there's anyone with experience regarding this topic.

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Considering the lack of data on ultrasonic cat repellants, I doubt a google search will give you much information regarding ultrasonic humidifiers. Instead let's look at what we can find and try to be deductive.

Here is an abstract (full article behind paywall) detailing the results of a study on one brand of cat repellants.

The abstract shows an efficacy rate below 35% for a short trial, and the efficacy rate dropped in the second trial.

The data seems to show that ultrasonic noise is irritating, but your question asks if the noise is harmful. While not a totally credible source (non-academic), this article leads me to believe that we can assume these noises are obnoxious, but not dangerous.

But if you look at most ultrasonic cat repellants, there is a 'direct to air' directional component. By this, I mean the sound vibrates air without any medium between the air and your cat's ears. Yikes, I bet that's annoying.

Mechanics of Humidifier

This animated GIF, from this site, shows how an ultrasonic humidifier functions. The water is covering the ultrasonic plate entirely. The energy generated by the ultrasonic vibrations is transferred into the liquid (water) and the water then expels this energy by becoming a mist.

In general, a higher frequency will become distorted easier than a lower frequency when traveling through space. This is why long range radio uses lower bands in the radio spectrum, and microwaves and X-Rays use higher frequency but require closer range. The same should apply here. The ultrasonic plate immediately hits water and the energy is either absorbed into the water creating small waves or cavities (see .gif) or if the energy breaks surface tension a mist is output.

Therefore, I doubt a humidifier will 'harm' the cat for two reasons.

  1. Ultrasonic noise is obnoxious but not harmful.
  2. The 'noise' is not noise at all, it will be transferred into water to generate mist.
  • Actually, most of this doesn't really answer my question as your answer focuses on ultrasonic cat repellant devices, not on ultrasonic humidifiers. Yes, both use ultrasonic technology, but the cat repellant device will use a frequency designed to scare away cats, while a humidifier could be set to a frequency that cats won't be able to hear (mine happens to use 1.7 MHz, which is well out of the hearing range of most animals). Also, even obnoxious noise can be harmful if you're exposed to it long term (it would drive me batshit, to be honest). That very last point is interesting though. – Little Ms Whoops Feb 17 '15 at 21:56
  • Hey Matt. Did you read everything? Humidifiers transfer the energy into water instead of air. This makes mist instead of noise. And ultrasonic noise is not harmful. It's just annoying. I use a Roolen humidifier and Muji Aroma Diffuser and my cats never seem harmed, bothered, or concerned. – Julian Wan Feb 18 '15 at 19:06
  • I suppose you mean me, not Matt (Matt simply edited your answer). Yes, I did read everything. The first five paragraphs address ultrasonic cat repellant devices, and that part simply isn't relevant to this question. I also addressed your point that something can be obnoxious/annoying but not harmful, and still be undesirable. – Little Ms Whoops Feb 18 '15 at 21:37
  • Actually, let me add to that last point: even if harmless in the short term, something obnoxious/annoying can be harmful long term, if you cannot escape it. My appartment is small, my cats are indoors-only cats, so even if it were merely annoying, it probably wouldn't be a good idea to use the humidifier, if the humidifier actually emitted ultrasonic noise. That it doesn't only becomes clear at the very end of your answer. – Little Ms Whoops Feb 18 '15 at 22:14
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I have a non-ultrasonic humidifier, which I have used and never got even a curious glance at it from my cat. Recently, I purchased a new fancy ultrasonic one, which from the moment I turned it on, (making no discernably irritating noise to my human ears) attracted the curiosity of my cat such that I have never seen before! She spends a lot of time staring at it after I turn it on, tries to jump on the nightstand to investigate it, and I came in the room to find that she had knocked it off the nightstand onto the floor.

Could it just be a coincidence? Possibly. Probably. Who knows, I'm not a scientist. But I'm pretty sure it must be making some noise that I can't hear that is driving her (destructive) curiosity.

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