| David Satz wrote on Thu, 10 February 2005 14:52 |
Dan,
It was great to see/hear you at the New York AES meeting earlier this week. Thank you for coming in and speaking with us.
--Regarding skin effect, the reference numbers for the K factor that you gave three messages back were for a single, solid conductor, no? In stranded wire, skin effect is lessened even further because it really applies to each strand individually. So the use of stranded wire not only helps to prevent breakage when a wire is bent, but it also brings the wire's resistance at high frequencies considerably closer to its DC resistance.
Stranded wire is what nearly everybody uses for audio cables. So if those K numbers were for solid conductors, then in practice, skin effect at audio frequencies matters even less than they would indicate. Maybe that 100-foot "zip cord" speaker wire isn't so bad after all!
--best regards
|
quote title=David Satz wrote on Thu, 10 February 2005 14:52]Dan,
It was great to see/hear you at the New York AES meeting earlier this week. Thank you for coming in and speaking with us.
--Regarding skin effect, the reference numbers for the K factor that you gave three messages back were for a single, solid conductor, no? In stranded wire, skin effect is lessened even further because it really applies to each strand individually. So the use of stranded wire not only helps to prevent breakage when a wire is bent, but it also brings the wire's resistance at high frequencies considerably closer to its DC resistance.
Stranded wire is what nearly everybody uses for audio cables. So if those K numbers were for solid conductors, then in practice, skin effect at audio frequencies matters even less than they would indicate. Maybe that 100-foot "zip cord" speaker wire isn't so bad after all!
--best regards[/quote]
Hello David,
It was great to be in NY, and I lucked out missing the cold weather.
Yes I was talking about a single solid wire, and the concept of "skin depth" is also for a single wire.
There are at least 2 reasons for the use of stranded wires instead of solid:
The first one is most obvious - mechanical flexibility (such as for the AC chords we use at home to light the lamps, power the TV......) the 60Hz AC application has nothing to do with skin effect.
The second reason is for carrying high frequency signals, way faster then audio.
If we take a circular cross sectional area of a given diameter, and fill it with solid copper, the circumference is well known, and so is the surface of that wire. It is also correct to say that one may fill the same cross sectional area with a tight fit of many smaller diameter wires and the combined surface will be much higher which will reduce the skin effect.
However, it is a common misconception that the skin effect improvement is proportional to the combined surface area packed into the available space. There are 2 reasons why the relationship is not linear:
1. Each wire strand must have enough diameters to take advantage of the "skin depth" concept and in practical terms, each strand must be at least 3-4 times the skin depth, or else the electrons will not "be concentrated on the skin".
2. The resistance of say 10 wire strands is not always one tenth of a single strand because when you "bunch wires together" the electrons on one wire interact with electrons from other wire strands with forces that counteract the multi wire improvement.
So yes, stranded wire has lower skin effect, but a wire design can be optimized electrically for say 10MHz, for 100Mhz, for 1GHz all calling for different dimensions, and there is of course a whole other set of requirements namely mechanical considerations...
Having said all that, skin effect is of no consequence for audio frequencies. It is a marketing hype!
Regards
Dan Lavry
