# Why do cellphones work in movie theaters? **URL:** \ **Category:** Factual Questions\ **Created:** [February 27, 2003, 7:30am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841 "2003-02-27T07:30:59Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Kalt](https://avatars.discourse-cdn.com/v4/letter/k/b9e5f3/32.png) [@Kalt](https://boards.straightdope.com/u/Kalt)\ **Post date:** [February 27, 2003, 7:30am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/1 "2003-02-27T07:30:59Z")
Surely there is a simple way of blocking the signal so no phones can ring (or vibrate loudly) in a theater. Cellphones don’t work in all sorts of places, it seems so logical to me to make theaters block the signal. Is it really that hard? Thick walls? Some sort of scrambler? I was going to put this in IMHO but the annoyance of cellphones during a movie isn’t an opinion… it is a fact. If the phones couldn’t get a signal, they wouldn’t ring and annoy everyone. ---
**Author:** ![ElwoodCuse](https://avatars.discourse-cdn.com/v4/letter/e/edb3f5/32.png) [@ElwoodCuse](https://boards.straightdope.com/u/ElwoodCuse)\ **Post date:** [February 27, 2003, 7:36am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/2 "2003-02-27T07:36:23Z")
Sorry, but at least in the U.S. the jamming of cell phone signals is forbidden by the FCC. > **[What Can We Help You Find?](https://www.fcc.gov/what-can-we-help-you-find)** > > We are unable to find the Wireless Telecommunications Bureau page you were looking for. Please reference pages below or try searching for your page. “The operation of transmitters designed to jam or block wireless communications is a violation of the Communications Act of 1934, as amended (“Act”). See 47 U.S.C. Sections 301, 302a, 333. The Act prohibits any person from willfully or maliciously interfering with the radio communications of any station licensed or authorized under the Act or operated by the U.S. government. 47 U.S.C. Section 333. The manufacture, importation, sale or offer for sale, including advertising, of devices designed to block or jam wireless transmissions is prohibited.” Sounds to me from that quote that even non-electronic forms of jamming would be a no-no. ---
**Author:** ![ElwoodCuse](https://avatars.discourse-cdn.com/v4/letter/e/edb3f5/32.png) [@ElwoodCuse](https://boards.straightdope.com/u/ElwoodCuse)\ **Post date:** [February 27, 2003, 7:38am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/3 "2003-02-27T07:38:20Z")
Oops, didn’t see that first part. Transmitters, hmmm? Then can they just make every theater lined with foot-thick concrete or something? Or would the cost of simply building a signal-poor theater be prohibitive? ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [February 27, 2003, 7:44am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/4 "2003-02-27T07:44:35Z")
Cell phones are now illegal in movie theaters in NY. Or is it just NYC? And maybe the law has been passed, but is not actually in effect yet. But, the law HAS been passed. ---
**Author:** ![II\_Gyan\_II](https://avatars.discourse-cdn.com/v4/letter/i/bbe5ce/32.png) [@II\_Gyan\_II](https://boards.straightdope.com/u/II_Gyan_II)\ **Post date:** [February 27, 2003, 7:57am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/5 "2003-02-27T07:57:21Z")
> [@](#): > > \*Originally posted by John Mace \* > \*\*Cell phones are now illegal in movie theaters in NY. Or is it just NYC? And maybe the law has been passed, but is not actually in effect yet. But, the law HAS been passed. \*\* Surely, this will be selectively enforced. Or else a doctor can almost never go to a movie. ---
**Author:** ![Max\_Torque](https://avatars.discourse-cdn.com/v4/letter/m/e9bcb4/32.png) [@Max\_Torque](https://boards.straightdope.com/u/Max_Torque)\ **Post date:** [February 27, 2003, 8:31am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/6 "2003-02-27T08:31:45Z")
Designing a movie theater that can block cell phone signals would indeed be prohibitively expensive. Besides, if theater owners actually cared about whether or not their customers were annoyed, they’d stop showing commercials before the trailers. ---
**Author:** ![murphyboy99](https://avatars.discourse-cdn.com/v4/letter/m/7ba0ec/32.png) [@murphyboy99](https://boards.straightdope.com/u/murphyboy99)\ **Post date:** [February 27, 2003, 8:38am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/7 "2003-02-27T08:38:54Z")
Cell phones could be jammed by enclosing the theatre in a Faraday cage. It’s basically just a big metal cage, because the metal is a conductor, the EM radiation cannot penetrate the cage. I can’t remember any more specific physics than that I’m afraid. The problem is Faraday cages are quite expensive. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 27, 2003, 11:50am UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/8 "2003-02-27T11:50:07Z")
> [@](#): > > \*Originally posted by murphyboy99 \* > \*\*Cell phones could be jammed by enclosing the theatre in a Faraday cage. It’s basically just a big metal cage, because the metal is a conductor, the EM radiation cannot penetrate the cage. I can’t remember any more specific physics than that I’m afraid. The problem is Faraday cages are quite expensive. \*\* Wouldn’t chicken wire do it? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [February 27, 2003, 12:47pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/9 "2003-02-27T12:47:13Z")
I can’t find the cite right now, but there was at least one company that recently introduced cell-phone blocking walls. It was some type of particle board with a conductive layer, and the intended market was indeed movie theaters, restaurants and other businesses. I personally think it’s a bad idea. Many of us use cell phones for emergencies, and are responsible enough to put it in vibratation-only mode in theatres and restaurants. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [February 27, 2003, 1:30pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/10 "2003-02-27T13:30:45Z")
**Mangetout** may be on to something. In all the farms I’v been on, I can’t ever recall seeing a chicken using a cell phone. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 27, 2003, 1:47pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/11 "2003-02-27T13:47:03Z")
A few references: [Technology for blocking wireless signals spreads](http://www.cnn.com/2001/TECH/industry/03/19/blocking.wireless.idg/) (About jammers) [APCO expresses concerns about blocking cell phones in public places](http://site-mag.com/ar/telecom_apco_expresses_concerns/) (Association of Public-Safety Communications Officials complains) [Faraday cage](http://www.hollandshielding.com/pdf/faradaycage.pdf) (Existing, expensive way of shutting out pretty much everything) ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [February 27, 2003, 2:28pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/12 "2003-02-27T14:28:50Z")
Cellphones operate at ~850 or ~1850 MHz, which corresponds to wavelengths of ~34.5 or 16 cm respectively. In order to block a signal with a faraday cage, the holes in the cage need to be substantially smaller than the wavelength; the smaller the better. The first table from [this](http://www.schlegel.com/education/theory.asp) EM sheilding page, suggests that to get a 20 dB (tenfold) drop in signal, the holes, including doors and such, would have to be under 19mm or 7.5 mm respectively for the two frequencies. That’s smaller than the holes in chickenwire. Fortunately, the Canadians have identified several non-chickenwire approaches to blocking cellphones: > [@](#): > > - Jamming devices, which use radio frequency interference to prevent pagers and mobile phones from transmitting or receiving calls > - Intelligent disablers, which use a signal detection function that informs a wireless service provider that a particular mobile phone is in a quiet zone > - Intelligent beacon disablers, which instruct any compatible mobile phone to disable its ringer, turn down its volume or switch to a vibrate-signaling mode > - Direct receive and transmit jammers, which interact with the operation of local mobile phones to break or unhook the communications link -[Bevy o’ Blockers](http://www.wirelessnewsfactor.com/perl/story/8116.html) I’m not sure whatall would be legal in the US. ---
**Author:** ![Kalt](https://avatars.discourse-cdn.com/v4/letter/k/b9e5f3/32.png) [@Kalt](https://boards.straightdope.com/u/Kalt)\ **Post date:** [February 27, 2003, 2:44pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/13 "2003-02-27T14:44:12Z")
So, just some chicken wire with extra-small holes … that shouldn’t be too expensive. ---
**Author:** ![micco](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@micco](https://boards.straightdope.com/u/micco)\ **Post date:** [February 27, 2003, 2:45pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/14 "2003-02-27T14:45:43Z")
> [@](#): > > \*Originally posted by Gyan9 \* > \*\*Surely, this will be selectively enforced. Or else a doctor can almost never go to a movie. \*\* Not necessarily. A doctor (or anyone else) with even a modicum of courtesy could put his phone on silent and leave the theater if it rings. Some venues (typically symphony and live production, not movies) will also allow you to check your phone like a coat check, and they will come get you if there is a bona fide emergency. I don’t see why anyone, even someone in a time-sensitive emergency situation, has to answer a call and talk on the phone inside a theater. We have the technology to jam or block the signal, but there may be legal issues with that. Why not rely on something simple like common courtesy (enforced with fines or public humiliation for those who don’t grasp the concept)? ---
**Author:** ![Kalt](https://avatars.discourse-cdn.com/v4/letter/k/b9e5f3/32.png) [@Kalt](https://boards.straightdope.com/u/Kalt)\ **Post date:** [February 27, 2003, 2:55pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/15 "2003-02-27T14:55:59Z")
Squink I was just reading that link (bevy o’ blockers) and caught this: \*The Canadian government is seeking public input on the possibility of licensing wireless phone silencers, or “jammers,” with an eye toward eliminating the use of cell phones and other mobile devices in public places such as theaters, restaurants or libraries, Industry Canada has announced. \* If Canada does this, it will officially become the world’s most civilized country. I might have to move there. ---
**Author:** ![D\_Odds](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/d_odds/32/2882_2.png) [@D\_Odds](https://boards.straightdope.com/u/D_Odds)\ **Post date:** [February 27, 2003, 3:42pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/16 "2003-02-27T15:42:45Z")
> [@](#): > > \*Originally posted by Kalt \* > \*\*Squink I was just reading that link (bevy o’ blockers) and caught this: > > \*The Canadian government is seeking public input on the possibility of licensing wireless phone silencers, or “jammers,” with an eye toward eliminating the use of cell phones and other mobile devices in public places such as theaters, restaurants or libraries, Industry Canada has announced. \* > > If Canada does this, it will officially become the world’s most civilized country. I might have to move there. \*\* I’m going to start packing, in anticipation, when I get home. ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [February 27, 2003, 4:36pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/17 "2003-02-27T16:36:58Z")
I think I’ve heard that some university libraries already use them up here. Maybe they’re experimental. (My campus library certainly doesn’t, though. Most people are _not_ reponsible or polite enough to turn of their phones in certain places. Celphone use is as much a right, in their eyes, as smoking in doorways. Stupid spoiled kidlets. And they aren’t all freshmen, either.) My only concern would be, as someone mentioned, the unavailability of the phone if there was an emergency in the library. Any building that installed a scrambler would also have to have lots of emergency phones (maybe of the type that have no keypad or dial, and automatically dial 911 when the receiver is lifted, to avoid abuse). ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [February 27, 2003, 5:37pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/18 "2003-02-27T17:37:39Z")
> [@](#): > > \*Originally posted by micco \* > \*\*Not necessarily. A doctor (or anyone else) with even a modicum of courtesy could put his phone on silent and leave the theater if it rings. Some venues (typically symphony and live production, not movies) will also allow you to check your phone like a coat check, and they will come get you if there is a bona fide emergency. > \*\* Huh? The degree of courtesy is irrelevant. He’s saying that if they develop a system to jam cell phones in movie theaters, then doctors, or anyone who might have to receive a time-sensitive emergency call couldn’t go to a movie out of fear that they wouldn’t be able to receive that call. Putting the phone on silent doesn’t change the fact that it wouldn’t receive the signals in the first place. ---
**Author:** ![SenorBeef](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senorbeef/32/2220_2.png) [@SenorBeef](https://boards.straightdope.com/u/SenorBeef)\ **Post date:** [February 27, 2003, 5:39pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/19 "2003-02-27T17:39:29Z")
Ignore what I said. I just looked back and noticed you were replying to his post about the NY law, not about a setup that blocked cell phone signals. Almost caught it before it submitted, too… ---
**Author:** ![St.Urho](https://avatars.discourse-cdn.com/v4/letter/s/8e7dd6/32.png) [@St.Urho](https://boards.straightdope.com/u/St.Urho)\ **Post date:** [February 27, 2003, 10:59pm UTC](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841/20 "2003-02-27T22:59:50Z")
Cell phones typically operate on a frequency near that which is used by many public safety departments for communications. So there’s a good chance that a cell phone jammer would also jam, say fire department radios. I think we can all agree that this would be a bad thing. There was an article about this in a firefighting magazine recently, as soon as I find it, I’ll post a link. St. Urho EMT/Firefighter [Next page](https://boards.straightdope.com/t/why-do-cellphones-work-in-movie-theaters/157841.md?page=2)