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Quote:
Originally Posted by SilverEars

Hi, I found this on a website:

"A cable becomes a transmission line when it has a length greater than λ/8 at the operating frequency"

I've calculated 1.356 Inches for 10khz signal(but this was airborne audio frequency), and headphone cable is longer than 1/8 of this.

How do you calculate the electrical signal wavelength?

You need the speed signal speed. The speed of sound in air is irrelevant. In this case, the speed of propagation of an electrical signal in an insulated copper wire is typically about 1/2 the speed of light in a vacuum. The speed of light is about 300,000,000 m/s (3*10^8). for a 20 kHz signal, the λ/8 length is (1/2) * 3*10^8 [m/s] / 20*10^3 [1/s] * (1/8) = 0.9375 kilometers

So, like i said, transmission line effects in cables are complete and utter nonsense unless you're trying to use kilometer-length cables.

Cheers

Quote:
Originally Posted by Steve Eddy

Speed of light is 3 x 10^8 meters per second in a vacuum. In a cable, it will be some percentage of that, usually around 80-90%. But even if it were 50%, the wavelength at 10kHz would be 15,000 meters or about 9 miles long.

se

Ok, that makes sense.  A headphone cable is not a transmission line at audio signals so there is no reason to take account of reflection.  Now I see why in power electronics they use the transmission line theory, since power is delivered over long distances.

I've used the calculator here and must have give me some crazy value.

http://www.mcsquared.com/wavelength.htm

Even if it was an audio signal over air waves it's should take account speed of light.

Edited by SilverEars - 5/8/14 at 10:08pm
Quote:
Originally Posted by SilverEars

I've used the calculator here and must have give me some crazy value.

http://www.mcsquared.com/wavelength.htm

Even if it was an audio signal over air waves it's should take account speed of light.

The signal in a cable is an electrical signal, not acoustic. That is why you need the speed of light (more correctly, the speed of electric field propagation in the cable) rather than the speed of sound.

The link you listed calculates the wavelength of acoustic waves in air. This is the wrong wavelength for propagating electrical signals.

Cheers

Quote:
Originally Posted by ab initio

The signal in a cable is an electrical signal, not acoustic. That is why you need the speed of light (more correctly, the speed of electric field propagation in the cable) rather than the speed of sound.

The link you listed calculates the wavelength of acoustic waves in air. This is the wrong wavelength for propagating electrical signals.

Cheers

Yes, speed of sound, my physics is a bit shaky right now.

Yeah, you get to use speed of light reference in circuits and your transducers (speakers/headphones) handle the conversion from electrical to audio for you.
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