Loud Car Stereo

Comments on Cecil's Columns/Staff Reports
#1

I know the keyword is permanently silence. How about discussing temporary measures? For example, my neighbors ham radio often came in though our tv. Back in the early 1980’s, our church’s PA system would pick up CB radios in cars driving by.

A noisy car radio could easily be handled with a very weak transmitted signal. Most people would turn off a radio that suddenly had a shrill pitch blasting through it. The range wouldn’t be over 25 feet. That’s not enough to cause problems with the FCC. The ideal sound would be to sweep frequencies between 100 Hz and 5 khz. You get a very obnoxious low to high sound that wouldn’t damage anything.

Once in awhile, even cd players can pick up signals through unshielded speaker wire. :slight_smile: There might be ways to target them too.

Seriously, something has to be done. My old office was in a single story bldg on a corner with a 4 way stop. I was in an interior office on the opposite side of the building. There were times, that I could hear cars at the 4 way stop. The thumping of music would vibrate my walls. :rolleyes:

#2

Some comedian on TV was saying the same thing… “I’m stopped at light and a car pulls up beside be playing this song. Now it’s stuck in my head. Does anyone recognize it? It goes like this… thump-thump-thump-thump-thump…”

First, electromagnetic interference is frowned upon by the FCC. What may drown out car stereos will definitely interfere with quite a few other devices in the neighborhood. Sooner or later someone with enough IQ will drive by and think - “every time I get near this corner, I pick up interference”. You want your device to be intermittent or sound-triggered.

Second, unfortunately most of those obnoxious guys whose volume goes several dozen decibels beyond their IQ, are using CD (or mp3) players today. I heard of some college physics professor who had a device for getting rid of obnoxious radios at the beach, back in the days of transistor radios. That was easy. CD’s are designed as much as possible not to pick up outside interference.

Third, most electromagnetic power is not easily focussed into beams, except at thehigher (microwave and above) levels. One again, aiming high doses of microwaevs at people or out into the world in general is an FCC offense. For regular radio power, think of the EMP pulse signal as an expanding bubble. Therefore, at twice the distance, the surface area of the bubble is four times larger. 3x gives 9x, etc. Square law; so to generate a decent signal that can zap a device at say, 50 feet, you need a source signal that’s pretty darned strong, even if it isn’t a thermonuclear event. To zap a 500W speaker you would need one heckuva zap. On teh plus side, the newer electronics like CD or MP3 players use much more sensitive IC transistor electronics.

As a side note, there are “Cell Phone Range extender” devices for that odd corner of the building where the cell signal is too weak. they work by acting as a dumb repeater amplifying signals in and out of a pair of antennas. I’ve always wondered if you could create a cell phone jammer by making the function only go one way - boost the signal coming in from outdoors but not outward…

the Israelis were rumored to have a device to shut down electronic (electric) devices at a distance, that was used as part of the Entebbe raid. Of course, this rumor was back when the Israelis could do anything; the last Lebanon imbroglio has brought their reputation down to earth.

#3

Most interference picked up is by poorly shielded, or unshielded equipment. The most common scenario is if there is a length of unshielded wire going to an amplfier. It will act like an antenna and pick up signal. The amplifier will demodulate and amplify the signal just like regular audio signals. If you are trying to bleed a signal in to a wire after the amplification stage, it will have to be a very powerful signal to be audible

Due to microelectronics, most amplifiers are located in the same case as the stereo these days. Car stereos are in a grounded metal case, and well shielded. In other words, you will have a hard time bleeding in significantly.

Home electronics are another matter entirely. Any receiver with poor quality parts may have inadequate shielding and signal rejection. You can get bleeding in from a significanty strong transmitter. This is the fault of the cheaply built reciever and not the transmitter, which is a common misconception. That is what the FCC label on your electronics means. In other words, if the ham radio guy next door is operating legally and your TV reception is getting screwed up, it’s your fault for buying cheap equipment with inadequate shielding and rejection. A very unpopular rule when it’s discovered.

Antennas are the first tuned component. Antennas pick up all signals, but are tuned to a certain center frequency. The further away you go from the center frequency, the greater the reactance and signal attenuation. You will eventually hit a harmonic and the signal will start coming through again. The tuner itself adjusts the capacitance and inductance of the circuit to reject all but a very narrow range of frequencies. This filtered signal is then sent to the amplifier stage where it is amplified and sent to the speakers.