AMITIAE - Thursday 28 March 2013

Silent Pocket Signal Blocking Case: Testing in the Lab

apple and chopsticks


By Graham K. Rogers

Silent Pocket

When I attended a meeting a few weeks ago, I handed my iPhone to the secretary as I went into the room and collected it when the session was over. One of my colleagues saw this and was amazed. Why? he asked. My reply was simple: I was in a meeting, I did not need it.


I may be unique here as in meetings, while at restaurants, even (as was seen not so long ago here) during a classical music concert, most people feel that they are unable to exist without checking their phones. I am often infuriated when, in the middle of a conversation, the phone rings and the person I am chatting to regards that as the priority; or even resorts to messages instead of focussing on the business of the moment.

With the increase in ways that governments and commercial organizations are able to track people, for whatever purpose, for some people it may be useful to disappear. While I will often turn my phone off for specific events (cinema, concert) and check messages later, there is a chance that some phones will continue to transmit data.

We could of course lock our phones in lead-lined boxes, but that would not be too convenient (effective though). An alternative solution is provided by Silent Pocket who have a range of wallets and cases that are lined with RF shielding material. This affects the electric and magnetic fields: incoming signals are reflected and outgoing high-frequency electromagnetic radiation is trapped inside.

Silent Pocket

The products were announced late last year. When they became available for international sales in February this year, Silent Pocket sent me an email, but I was initially unable to make an order. The company fixed the problem in double quick time and I ordered one of the smaller ones - the Suit Pocket - specifically for my iPhone.

The outside is of leather while the inside is lined with cloth. The top of the pouch snaps shut with a magnetic clasp. As well as the Silent Pocket label inside, there is another: "Made in China." It arrived a few weeks ago and I have been trying it out since then.

Silent Pocket

It is easy to see that as the iPhone is slid into the case, even with the top open, the Wi-Fi signal reduces. When I attended one of the meetings recently, I sealed the iPhone in the pouch and was undisturbed. Later, back in my office, I removed the phone and the screen showed "No signal". Within a couple of seconds, however, I was back online and with a link to the carrier.


I could have written a review right away, but as some of my students have access to testing equipment, I waited until their projects were done. A couple of students - Nachatpong and Sirun - who had just completed a project on electromagnetic radiation walked me through a series of tests with the Silent Pocket pouch using my iPhone 4S and a couple of other students' phones.

Silent Pocket

The device that we used was a GTEM (Gigahertz Transverse ElectroMagnetic) Cell. This had been made three years ago by a group of students in the Electrical Engineering Department of Mahidol University (where I work) and has been used for student project work and other research since then. This has the effect of shielding whatever is inside - one project tested seed growth - from electromagnetic radiation so that there are no interfering signals.

Silent Pocket

The outside is like an aluminium pyramid on its side while the inside is lined with signal absorbing foam material which protects devices being tested from reflection. This is a considerably smaller version of Apple's anechoic chambers that were in the news when there were reported to be problems with the antenna of the iPhone 4.

Silent Pocket

The students set the cell up with an Agilent N9320B Spectrum Analyzer and I asked them to check the effectiveness of the pouch: signals in and signals out. We ended up testing three devices. While there were small signal variations with each, the pouch did what it was supposed to do.

  • The students first checked the signals from the iPhone 4S that I use and we saw that these registered on the spectrum analyzer.

  • The iPhone was then placed in the pouch which was shut at the top and we saw there was no signal from the iPhone, even when one student tried to make a call. He was unable to make a connection, and the phone did not ring.

  • When the phone was removed from the GTEM cell (and the pouch), the student again called the number and a connection was made as normal.

Silent Pocket

As all three of us are iPhone users, we asked around and came up with a couple of other phones. First was a Samsung, but not a smartphone.

  • As before, the students first checked the signals from the Samsung, which was also using the same DTAC carrier as my iPhone, and saw that these registered on the spectrum analyzer. However, they noted that the signal coming from the Samsung phone was slightly higher.

  • The phone was then placed in the pouch as before and again there was no signal, even when a student tried to make a call. He was unable to make a connection, and the phone did not ring.

  • When the phone was removed from the GTEM cell (and the pouch), the student again called the number and a connection was made as normal.

The third phone used was a Sony Ericsson Xperia (carrier - True). I was concerned that because of its length it would not fit in the pouch, but Nachatpong was able to seal the top and the test was carried out as before:

  • The signals from this phone were checked. These registered on the spectrum analyzer. They noted that the signal coming from the phone was slightly higher than the iPhone but lower that that from the Samsung.

  • The phone was then placed in the pouch. Again there was no signal, even when the student tried to make a call. He could not make a connection, and the phone did not ring.

  • When the phone was removed from the GTEM cell (and the pouch), the student again called the number and a connection was made as normal.


By testing, I mean that the students noted the readings on the spectrum analyser (I do know what this is), pointed at the screen for me, and told me. These guys were really good and understood how this was working. While the tests were going on, a small group collected and they were all discussing signal strengths.

Note that there are signals being transmitted by any phone that is on, even when the user is not actively using it. As had been evident in casual use, the tests showed that the pouch did prevent signals coming from the phones and that signals were unable to be picked up by them either.

Silent Pocket has available a number of cases and wallets, ranging from smaller pocket sized versions up to larger ones into which an iPad could be slipped. The Suit Pocket version which I bought cost me $69.95 (2,050 baht) and there was a small charge for mailing, bringing the charge on my credit card statement to 2,556.20 baht. The airmail package took about a week to arrive.

As to the cleansing properties claimed for the Silent Pocket products, I was not in a position to test this. However, an online search provided many sources of useful information. There are, for example, several reports of silver use as an anti-bacterial agent going back as far as Hippocrates. More recently, a study into the use of silver as an anti-bacterial agent - available online via the Nursing Times - concluded that Silver-treated materials can reduce levels of bacteria contaminating healthcare settings (Lesley Taylor, Paul Phillips and Richard Hastings).

The Suit Pocket that I have is quite well made and looks neat enough when carried. The students who helped me in testing were interested and asked for more information, but the price was a little high for them: they are just graduating and do not yet have jobs.

While I will use the pouch when in meetings and in class, the chances of bringing my colleagues onto the same path are somewhat limited with the phone-over-all culture that exists. I can request, but I cannot compel.

Graham K. Rogers teaches at the Faculty of Engineering, Mahidol University in Thailand. He wrote in the Bangkok Post, Database supplement on IT subjects. For the last seven years of Database he wrote a column on Apple and Macs.



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