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I heard as long ago as 2003 that trying to call a cell phone while it's inside a microwave (with the microwave off, of course) could be used to test if the shielding of the microwave was intact. I recall trying it and that it worked (the phone did not ring). I've tried in other microwaves and it has sometimes worked.

This claim appears fairly common: See WikiAnswers, WikiHow, and ABC News.

Is this method reliable?

Why or why not? Currently, explanations mostly occur in forums and none are well-supported.


On a very related note, a similar test involves doing this with a netbook and attempting to communicate with it via wireless connectivity (e.g. to ping it). See here for an example, claiming that this works since the two frequencies are very close. Coincidentally (or not), the comments feature suggestions to use a cell phone instead...

  • Great question :) – picakhu Jun 10 '11 at 22:11
  • Aren't the waves used by both rather different? – RolandiXor Jun 10 '11 at 22:27
  • @Roland: frequency-wise, yes (if by "rather" you mean ~1,000 Hz difference). Family-wise, no; they are in the electromagnetic spectrum (compared to, say, trying to test microwave seals with sound, which are completely different type of wave). – Hendy Jun 10 '11 at 22:59
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    ofc I mean frequency wise :D (chuckle)... Yes, let me rephrase: Shouldn't the frequency distance between radio waves (used by cell phones, if I'm not mistaken (I don't remember off hand lol)) and micowaves make some shields penetrable by one and not the other? Or do the shields block/absorb anything above/below a certain range? – RolandiXor Jun 10 '11 at 23:10
  • @Roland: Not sure -- intuitively, perhaps! But, hence I'm asking :) I hoped to find out conclusively so I can stop wondering... – Hendy Jun 12 '11 at 15:55
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Microwave in the microwave oven is usually at 2.45 gigahertz

http://en.wikipedia.org/wiki/Microwave_oven#Principles

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz range

http://en.wikipedia.org/wiki/Microwave#Communication

So using Bluetooth / WiFi is a very good test for seeing if the microwave oven has a leak. The ping is a very good test, since what you really want to see is if any microwave comes out, so even if the laptop within the oven, say, can receive the request, the reply (ECHO_REPLY) should not be sent.

Cellphone frequencies vary a lot. Within GSM there are various frequencies and CDMA is different so using cellphones is not a good test for this.

On significance of frequencies / wavelengths for shielding Microwaves in specific and Electromagnetic waves in general:

The shielding is based on the wavelength of the wave which is inversely proportional to the frequency. The wavelength of a wave can be correlated to the size of it if it were a particle. Now for the shielding to work the holes in the shielding material should be smaller than the wavelength of the waves. So, while a Cell phone microwave frequency being much lesser than that of WiFi / Bluetooth, the wavelength of Cell Phone microwave is much bigger than WiFi / BlueTooth. So it is easier to block cell phone microwaves than it is to block WiFi / Bluetooth so if you find a cell phone signal to be blocked in the microwave, the smaller wavelength ( higher frequency ) WiFi / Bluetooth signals may not be blocked and hence the similar frequency microwaves in the oven may actually not shielded properly.

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    I understand the wish of the creators of this SE to have sources in every answer, but that's a bit overdone, tbh. @Randolf: 800, 900, 1800 and some other (I forgot which) – RolandiXor Jun 10 '11 at 23:12
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    @manojlds Skeptics is a bit different from the other SE sites, please provide links as a resource for those not as knowledgeable on the topic. meta.skeptics.stackexchange.com/questions/5/… Answers without appropriate references can be correct but are often incomplete. Also, apparently my microwave does not leak. – Monkey Tuesday Jun 10 '11 at 23:30
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    @manojlds usually wiki links aren't enough to stand on their own, but you seem to have enough knowledge to make your case. I would highly suggest using at least one technical source, but if wikipedia's all you can find, your case may be one of very few exceptions to the rule. I can't speak for the mods though, one of them will most likely require you to use sources other than wiki, lest a precedent be set for other people answering more controversial questions in the future. thanks for the answer. – Monkey Tuesday Jun 11 '11 at 0:02
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    Good answer. Regarding the netbook being superior because you receive an outgoing transmission -- That applies with the cell phone as well. If the tower can't receive a signal from the phone, it won't have a location to send a call to - so if you receive a call on a phone in the microwave, the phone's signal is leaking out. An alternative method - put a Bluetooth device (earpiece, for instance) inside the microwave and see if your phone / PC outside can detect it. – Saiboogu Jun 11 '11 at 1:32
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    @manojlds: thanks for the updates. I don't get the last bit; if the shielding just works like a filter, and small holes "filter" everything bigger, than holes small enough to filter 2.4GHz should also filter 800-1,800MHz, right? The gist I got from the last paragraph was that a phone probably won't ring, but if a netbook does, one has problems. Online, I'm hearing that a microwave actually won't block lower frequencies, even though it will block those freqs. in it's operating range. – Hendy Jun 11 '11 at 15:17
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While the answer from @manojlds was reasonable, I wasn't satisfied. The key question is not whether the seals on a microwave can be tested using Wifi at the same operating frequency (2.4GHz), but whether a cell phone is an accurate way of testing these same seals. In other words, will something that filters out 2.4GHz also filter out 900-1800MHz?

I did my own digging and am still not entirely satisfied, but wanted to report the results. I found a tutorial from Clemson University Vehicular Electronics Laboratory entitled "Practical Electromagnetic shielding", and tried to understand it. The key formula I thought might be helpful was this one:

microwave shielding formula

Where where f is the frequency of the field and fc is the cut-off frequency of the opening. The cut-off frequency is approximately the frequency at which the maximum height or width, a, is equal to a half-wavelength. d is the depth of the opening.

I took this to mean that if all other characteristics about the shield are kept constant, decreasing the frequency will increase the attenuation -- perhaps an effective shield for 2.45 GHz really isn't an effective shield for lower frequencies.

I emailed the contact for the site, Dr. Todd Hubing, Michelin Chair and Professor of Electrical and Computer Engineering, and got the following response:

I would be a little concerned if I was able to communicate with a cell phone in my microwave oven. Nevertheless, it could happen I suppose. Traps and seals that work well at 2.45 GHz might not work so well at lower frequencies. It is true that apertures below cutoff have a shielding effectiveness that improves as frequencies get lower. However, the absorption loss associated with shielding materials gets worse at lower frequencies. Since microwave ovens operate in a very narrow frequency band, their shielding can also be narrow band.

T. Hubing

So... not quite the definitive answer I hoped for, but reasonable.

I also posted in a forum called Microwaves101 under the Measurement and Testing sub-group. In response to my post, I got the following:

The door, specifically the surface around the cavity perimeter that it fits against, is tuned to 2.45 GHz. This surface is a 1/4 wavelength long (3 cm), so it acts as a choke specifically at that frequency. See section d of the below drawing.

See this wikipedia article on waveguide flanges:

http://en.wikipedia.org/wiki/Waveguide_flange

seal picture

I put my iPhone in the microwave and ran the WiFi sniffer app. WiFi signal (also at 2.45 GHz) was -60 dBm with the door open and -96 dBm with the door closed. 3G bars (4/5) did not change at all.

Anyway, that's 36 dB of attenuation (a 1/4000 factor) Assuming a 700W microwave, the power leaking out is 175 mW. A 19"x12" door is 1470 cm^2, so roughly, the power leaking out is 0.1 mW/cm^2. Health limit is 1 mW/cm^2, so the leakage is 1/10th of that. This is a really rough experiment, but you can see the leakage is safe.

So, there's where I've gotten. The best I can say is the following:

  • An expert seems to think that it's possible that a) the microwave seals are targeted only toward the microwave operating frequencies, and b) tha the attenuation formula indicates that seals would perform worse on lower frequencies (in other words, theory supports the idea that a cell phone might ring while indicating nothing about the efficacy of the seals)
  • An active microwave forum member conducted an experiment showing no loss in GSM bars and a reduction in microwave frequency with the door closed (practical experiment supports the theory, at least in one instance)

For reference:

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