I used to have a subwoofer that would get shortwave broadcasts. Pretty cool, but if you consider that a lot of transistors are dual-purpose, it's not that far-fetched. I've actually modified cheap speakers to become shortwave radios.
I'd try to move it, and you might get rid of it. Try powering them from a different outlet. Try unplugging the input audio cable, try using better, thicker, heavier cables. If that fails, open them up, and add some shielding, or just buy some new ones. But remember that the FCC has regulations about these things:
FCC Part 15; Subpart A; Section 15.17 said:
Susceptibility to interference.
(a) Parties responsible for equipment
compliance are advised to consider the
proximity and the high power of non-
Government licensed radio stations,
such as broadcast, amateur, land mo-
bile, and non-geostationary mobile sat-
ellite feeder link earth stations, and of
U.S. Government radio stations, which
could include high-powered radar sys-
tems, when choosing operating fre-
quencies during the design of their
equipment so as to reduce the suscepti-
bility for receiving harmful inter-
ference.
FCC Part 15; Subpart A; Section 15.19 said:
(3) All other devices shall bear the following statement in a conspicuous location on the device:
This device complies with part 15 of the FCC Rules. Operation is subject to the following two conditions: (1) This device may not cause harmful interference, and (2) this device must accept any interference received, including interference that may cause undesired operation.
It's been trimmed a little bit, but the meaning is there. Basicly, it's the manufacturer's responsibility to ensure that their product(s) are within the criterea. Complain to the radio station nearby about the interference you've been receiving, that's probably the source. If you want, you can look up the nearby operators of shortwave and high-power stations and give them a call. Just complain, and they'll try to work with you to fix the problem. They have to.
The volume thing... the radio broadcast will go away, because there's a different load on the op-amp and that'll change the resonate frequency, try messing with bass and treble knobs too.
Btw, I think you're using possessed the wrong way, it is the right word, just the way you are using it implies that the speakers are possessed by an evil spirit; when in actuality, they are possessed by the 'Holy Ghost' (or Reverend Jackson, either one.)
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Here's what you can do, as advised by the FCC:
FCC Part 15; Subpart B; Section 15.105 said:
Information to the user.
(a) For a Class A digital device or peripheral, the instructions furnished the user shall include the following or similar statement, placed in a prominent location in the text of the manual:
This equipment has been tested and found to comply with the limits for a Class A digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instruction manual, may cause harmful interference to radio communications. Operation of this equipment in a residential area is likely to cause harmful interference in which case the user will be required to correct the interference at his own expense.
(b) For a Class B digital device or peripheral, the instructions furnished the user shall include the following or similar statement, placed in a prominent location in the text of the manual:
This equipment has been tested and found to comply with the limits for a Class B digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference in a residential installation. This equipment generates, uses and can radiate radio frequency energy and, if not installed and used in accordance with the instructions, may cause harmful interference to radio communications. However, there is no guarantee that interference will not occur in a particular installation. If this equipment does cause harmful interference to radio or television reception, which can be determined by turning the equipment off and on, the user is encouraged to try to correct the interference by one or more of the following measures:
• Reorient or relocate the receiving antenna.
• Increase the separation between the equipment and receiver.
• Connect the equipment into an outlet on a circuit different from that to which the receiver is connected.
• Consult the dealer or an experienced radio/TV technician for help
I'm betting you have a Class A device. You can try the things I suggested above, if that doesn't help, looks like SOL.