[Lowfer] 8820 Hz question - maybe OT

[Chuck Hays]

> From: N8OOU 

> I'm thinking I remember it being said that at some magic frequency above 
> the audio range a signal starts to radiate.  Obviously that conclusion 
> is not true.

Basically an "audio" signal is an electromagnetic signal that is converted
into sound through the use of a transducer, or what we usually call a
"speaker."

What we hear as audio is pressure variation in air, or soundwaves. Those
can be created entirely mechanically -- by clapping one's hands, say -- or
they begin as electromagnetic signals -- such as the output of a receiver --
that are transduced from electrical to sound pressure variations.

Plucking a really long wire with your finger can induce resonant vibrations
which are entirely sound. Feeding the same frequency into the wire as an
electromagnetic signal will generally cause some radio signal emission, 
assuming the conditions are set up to allow that. 

Tesla's early research into "wireless transmission of intelligence" was most
often conducted at frequencies below 20 kHz because of the equipment he
had developed up to that point, primarily multiphase alternators driven by
steam engines. His early receivers (pre-1899) employed crude mixers to
heterodyne a locally-generated IF with the received signal. He used a short
wire under both mechanical and electrical tension as his audio transducer.

(Some of his experiments used remotely-generated IF transmitted with the
main signal to "unlock" the receiver. An early version of spread-spectrum
transmission.)

I have no idea what his IF frequency was, but he was able to "get the note"
or hear his transmitter signal. Post 1899 in his Colorado Springs work he
relied more often on a telegraph sounder or relay inker for reception. It
was at this time he received VLF signals in regular patterns (series of two and
three "dots") that he thought might be coming from Mars. After all, so far
as he knew he had the only radio on the planet! Many years later we observe
that Jupiter emits regular pulses of RF energy that frequently appear as
short pulse trains.

Point being: if it makes pressure waves in air it's sound (or infrasound or
ultrasound; we don't have to hear it for it to be sound). If it makes electro-
magnetic waves it's radio (up to the point where we start to call it "light").

Cheers,

Chuck Hays VE7PJR/WB7PJR
CO90ux