Interesting question!
I found the following email exchange buried in my email inbox recently. It’s from a couple of years ago, but I thought I would share it with my readers.It has to do with whether or not speaker cable should be shielded. Obviously, conventional wisdom would say shielding is not necessary for speaker cables, as has been the installation practice of professional installers and hi-fi aficionados alike since audio advanced past the Victrola.
But, the question was posed by a client who was convinced that shielding was indeed necessary.
Hi Steve,
I am a sound system installer who has used Belden mic/line level and speaker cable products for years. I hope you can offer some clarification to the follow.
While we all know to use shielded wire for mic/line cables, but I have never used large gauge (12ga.) shielded cable for speaker runs. And, never had any problems. I understand low impedance and capacitance issues. I am not a white lab coat guy. I just know what works from wiring super expensive studios to larger PA rigs, and troubleshooting existing sound systems. That’s my background.
This week I had a client get a hold of some (I believe) misguided information and claimed I wired their stage wrong two years ago, even though they have never had any issues or ill effects of any kind. He said I should have used shielded speaker cable when used around 120v power. I called Belden yesterday and asked if they even made a shielded 12 gauge speaker cable. The sales person said they had never heard of any Belden speaker cable like that. After searching the internet I did find the following product.
http://www.showmecables.com/ Product/Belden-2-Conductor-12- Awg-Stranded-Shielded-Cable- Per-FT.aspx
So, this begs the question: If shielded speaker cable can cause capacitance issues that degrade sound quality, and worse, cause oscillation problems that can potentially damage some amplifier and speaker equipment, why is the product offered, and is it possible this guy I referred to is right? If so, I would want to make adjustments to my design installs in the future.
As a standard install practice, we avoid running high voltage and low voltage cable together parallel whenever possible. We will also run cable through an EMT conduit if necessary, but even in those cases we did so, not because we experienced audible RFI or EMI issues, but because it seemed a better install practice.
I can see maybe in extreme cases, like running speaker cable parallel with highly amplified radio antenna wire, or literally right next to a 480v transformer, where maybe shielded speaker cable might be worth the negative trade-offs. Or, small diameter low voltage commercial building installs where all the cables, power or otherwise, are bundled together for long distances.
I have purchased Belden through Pacific Radio in Burbank for years. They don’t even carry, or have even heard of people requesting large diameter shielded speaker wire, and they sell thousands of miles of cable per year. The other source I have purchased cable through said he has had maybe three requests for shielded speaker wire in his career.
The Belden sales rep I spoke to yesterday said to email you, and that you were the Belden guru! 🙂
Thanks for you time and response.
Darren Bowls
BESA-Bowls Engineering Sound & Acoustics
1059 Kensington Dr.
Redlands CA 92374
(909) 644-5588
This is Steve Lampen’s response. Bold fonts are mine:
Darren-
The foil shield in shielded cables (including 12 AWG shielded pairs) doesn’t begin to work until 10 MHz, and really is not effective until 50 MHz. This is really an RF shield. Its effectiveness at audio frequencies is very low. You get more noise rejection running a twisted pair as a balanced line, such as microphone cables, than any shield you can put around it. (This is why the vast majority of data cables are UNSHIELDED twisted pairs.)
The effectiveness of foil shields at 60 Hz is probably close to unmeasurable. Even solid steel conduit (perfectly installed) has about 30 dB of RF rejection at 60 Hz, not great. Besides, where are you going to terminate your foil shield ? One of the speaker terminals is already ground, so by attaching the foil shield to the chassis of the amp, you are effectively sending the noise back into the amplifier. (Speaker outputs are NOT balanced lines except in some very rare cases. So twisting the wires, much less shielding them, gets you nothing extra in performance.)
By the way, we certainly DO make 12 AWG shielded cables. Just check out Belden 8718 or Belden 5000FE. (Now I see you did hit one of those two with your internet search.) These designs are to prevent RF from getting back into a device through speaker wires, DC power wires or other applications that need big wires. Yes, shielding increases the capacitance between wires, but I could make a good case that capacitance at very low source impedance is fairly close to meaningless. The output of a power amp is not 8 ohm or 4 ohms, it is actually much, much lower (often less than 1 ohm). The lower the source impedance, the more energy is transferred to the load (i.e. speaker) and less is used up in the cable itself.
The National Electric Code requires that power wiring and any other wiring (like speaker wiring) are separate for SAFETY reasons. And keeping them apart reduces any effect between them Since neither power wiring or speaker wiring are balanced lines, it is quite possible to transfer 60 Hz from the power wiring into the speaker wiring (and any foil shields on either would have NO EFFECT.) The greater the distance, the less the effect. Inverse square law says ‘double the distance and you get 1/4th the effect’.
Who did you talk to in customer service? I should talk to them.
By the way, not to confuse you, but Belden is now producing some high-end expensive speaker cables. But the designer is one of our senior cable design engineers. They are currently carried by only one dealer, Hansen Audio in Dayton, Ohio. Contact them if you’re interested.
Your letter would make a great blog. (I write a weekly blog.) Is this OK? What if we just put your initials? Just “reply to all” with your answer, and any further comments.
Thanks for your interesting email.
Steve Lampen
Multimedia Technology Manager
Product Line Manager – Entertainment Products
Belden
steve.lampen@belden.com
http://www.belden.com
Anyway, I hope you found this informative. I did! It’s nice to know the technical “whys” to the assumptions we often work by.
Take care, and be blessed!
Darren Bowls
BESA-Bowls Engineering Sound & Acoustics 