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Gadget to block local cellphone transmissions?

H

Homer J Simpson

Jan 1, 1970
0
I am really getting tired of people shouting into their phones while I sit
on a bus or train seat near them or even in an outdoor cafe. Is there any
small gadget that would create very local interference. One or two blasts
should put a flea in their ears and get them off the phone.

Anyone looking forward to a cross country (or cross ocean) trip in a plane
where passengers can use their cellphones? Noise cancelling headphones
anyone?


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Anyone looking forward to a cross country (or cross ocean) trip in a plane
where passengers can use their cellphones? Noise cancelling headphones
anyone?

If you have not personally tried these you might be surprised with
this.

Unfortunately in my experience, using three different brands of noise
cancelling headphones over the last 15 years, while they do a
surprisingly good job of whacking low frequency roar and rumble of
engines and road noise, this actually lets the conversations that you
were not previously able to hear come through loud and clear now.
Flip
the switch off and the roaring grumble of the city bus is all you can
hear, flip it on and now two dozen conversations, half on cellphones,
loud and clear is what you get to listen to.

I've asked a number of times if this is just because of the compute
power or battery life or the quality of the microphone limiting the
upper frequency that is effectively cancelled, or if there is really
deeper physics that says it would be exponentially harder for every
higher octave. I tried reading the Handbook of Noise Cancellation and
couldn't get an answer from that either.

I've never gotten what seemed like a really convincing explanation,
but
I have gotten some silly ones. The sales kid at the Bose store, who
was
1/3 my age, told me I would be putting my life at risk if they
cancelled
the speech so this would never be done, strictly them saving my
life. :)

But I have not tried the $500 Bose small plane pilot noise cancelling
headset on the bus, they didn't want to loan me a set for the
afternoon,
and you don't get to see those at your mall Bose store. Those are
supposedly playing at a completely different level from the consumer
grade Bose and Sharper Image and Philips and two other brands, one of
which I started using over 15 years ago and are now flakey. The
Philips
HN050 were particularly poor compared to others in my opinion, but
they
were cheap when closed out. They claim only 10 dB reduction between
100
and 1500 Hz. If you didn't have experience with noise cancellation I
doubt you would even notice this.

If someone finds a brand that really does cut 40 dB from 20 Hz to 6000
Hz or more and is plausibly priced let me know and I'll buy a pair.
 
G

George Kavanagh

Jan 1, 1970
0
I believe these "noise cancelling" devices work by measuring, and
cancelling, "white noise", that is, the random hiss, rumble, or roar of
engines, etc., and cannot cancel the more modulated sounds of human speech
(no matter what the frequency).

As you say, with the engine noise cancelled out, the human speech comes thru
loud & clear. Drat!

-georgeK KB1HFT
 
H

Homer J Simpson

Jan 1, 1970
0
If you have not personally tried these you might be surprised with
this.

I always take foam ear plugs with me anyway. I'll add a set of muff style
phones I think.


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I believe these "noise cancelling" devices work by measuring, and
cancelling, "white noise", that is, the random hiss, rumble, or roar of
engines, etc., and cannot cancel the more modulated sounds of human speech
(no matter what the frequency).

As you say, with the engine noise cancelled out, the human speech comes thru
loud & clear. Drat!

-georgeK KB1HFT

I've heard this theory and based on what I've read about the math
I don't believe that is the case.

For a handwaving argument why this doesn't seem to make sense,
imagine you were trying to beat me at a game where I either have
a less random "more modulated" strategy versus where I have just
randomly toss a coin heads or tails. If there is more structure it
would seem to be easier for any sort of computation to be able to
decide what to do to counter my strategy and beat me. The more
random it is the less you can do to cancel anything. But that is
just handwaving and not a boardful of mathematics.

When I spent a week studying that text on active cancellation it
was describing the process of measuring the amplitude and phase
of each frequency, introducing a suitable phase and amplitude shift
to make up for the distance between microphone and earphone and
ear and then generating approximately the needed signal to sum to
zero at the destination.

Now as the frequencies go up the sampling rates need to go up,
the precision of the sampling needs to go up, the number of the
calculations needs to go up, etc, etc, etc. But I still haven't heard
a really convincing argument whether it would be possible to have
tens of dB better cancellation over octaves more frequency range
if I were just willing to carry around three pounds of batteries for a
couple hours of cancellation calculations, instead of a couple of
silly little double-A batteries for dozens of hours of operation.
 
G

George Kavanagh

Jan 1, 1970
0
Good points. Just after posting I realized that "random" was not going to
cut it.

If more sampling is needed, and more calculations per sample to really cut
down the noise AND spoken word, one can conjecture that a laptop PC
configured with a mic, earphones, and appropriate software might do better
than the QuietComfort(r) and others, albeit at the expense of lugging along
your PC. Perhaps real work could also get done on other threads or
processors...
Does anyone know of any investigations into this possibility?

-georgeK
 
M

Michael A. Terrell

Jan 1, 1970
0
Red said:
But I already like opera. How does that help me --- oooohh, I got it. I'll
bring a radio or tape recorder along and blare it at them, and MassiveProng.
You know that guy probably has just a little worm and is exaggerating again
;-)

RF.


A 12 volt gel cell, and an old fashioned "OOGHA" horn from a couple
feet away. Tell them that you are talking to your invisible friend from
Alpha Centuri with it, while yo fake messages in Morse Code.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
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