Why is there audio crackling when scratching a wire? - Page 1
Forum section: electronics / general

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 18, 2025, 01:25:19 am
content: Hello all,I am repairing an electric guitar.When I plugged the guitar into an audio amp, I noticed that when I scratch some of the internal wires, I get a crackling sound out of the speaker.The wires do not run in pairs (not sure if that makes a difference) and the are soldered to switches and pots. If it scrape the wire(s) with something that has teeth, such as the threads of a screw, that mechanical scratching cross the insulated wire(s) produces a crackling sound, almost as if the wire is acting as a piezo pickup.Any ideas why?I would really love to understand this.Thank you all.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 18, 2025, 02:12:46 am
content: Insulated wires?

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 18, 2025, 03:34:35 am
content: Yes, they are insulated with plastic.

author: amyk
author_rank: Super Contributor
timestamp: May 18, 2025, 05:41:13 am
content: The pickups on the guitar have magnets and create a magnetic field not only immediately above them but also their surroundings. Moving wires in magnetic fields induces a current in them.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 18, 2025, 03:27:00 pm
content: I think you are 100% correct.I'll do some tests to confirm your theory, just to be sure.Thanks for your reply.

author: octillion
author_rank: Contributor
timestamp: May 18, 2025, 04:59:14 pm
content: It sounds like this could be triboelectricity. Try scratching with something non-magnetic to see whether amyk's guess is correct.https://en.wikipedia.org/wiki/Triboelectric_effect

author: Analog Kid
author_rank: Super Contributor
timestamp: May 18, 2025, 09:06:18 pm
content: All of the above, plus consider this:The output of an electric guitar pickup can easily be below 100mV, a pretty small amount. This means whatever amplifier you have it plugged into is very sensitive to things like imperfect solder connections to pots and other components that could easily generate noise if wires are moved around. Basically a case of microphonics.Try just tapping the wires with something like a pencil to see if this is the case.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 18, 2025, 09:16:20 pm
content: Shielded cable acts as a capacitor, and if you change the capacitance (next sentence) it induces a voltage between the core and the screen, which then gets amplfied and becomes audible.Typically just tapping or rubbing the cable against something rough will create a tapping or rubbing sound in the loudspeakers. For it to work the cable has to be connected to a high sensitivity input.Apart from cables, I've found capacitors and vacuum tubes to be quite microphonic, especially the latter. The EF86 signal pentode on one of my amps is very microphonic.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 18, 2025, 09:47:31 pm
content: Quote from: SteveThackery on May 18, 2025, 09:16:20 pmI even had to replace a transistor once in my Allied receiver (in the phono preamp section) that had gone microphonic and was generating lots of crackling noise.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 19, 2025, 12:10:25 am
content: Quote from: octillion on May 18, 2025, 04:59:14 pmThis Triboelectric Effect is quite fascinating. I remember, when I was a kid (in Europe) the cars that had those static belts, like the Russian car in the picture on the Wikipedia page. I remember always asking my dad what that was for and never really understood it.I'll be able to test the circuit on Wednesday. I'm really interested to get to the bottom of this.Quote from: Analog Kid on May 18, 2025, 09:06:18 pmI'll definitely try that. I'll think of something with teeth that I can slide across those wires, that is not made from metal or plastic. I guess wood would be the best testing material. Perhaps I can cut a bunch of teeth along the edge of a wooden stiring stick, like those they have in coffee shops.Quote from: SteveThackery on May 18, 2025, 09:16:20 pmI was also thinking along the lines of capacitance, but wasn't sure if I was on the right track.I can definitely come up with some tests that would either conform or rule out these theories.I'll be able to work on this on Wednesday, so I'll report back my findings.Fascinating stuff.Thank you all.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 19, 2025, 01:16:45 am
content: Quote from: Frankentronics on May 19, 2025, 12:10:25 amNo, don't make a rasp: what I was suggesting was justtappingthe wires with something like a pencil, not rubbing it against the wire. That'll prove microphonics if you can hear it through the amp.

author: Frankentronics
author_rank: Regular Contributor
timestamp: May 21, 2025, 09:14:52 pm
content: OK, I have that guitar (actually it's a bass) back on the work bench.If I tap the wire(s) with a pencil or wooden stick, I hear the taps through the amp.Also, if I rub with my finger, I heat the rubbing even if I rub lightly.The portion of the wire that I am touching is about 5 inches (12mm) away from the pickup. If I approach that same portion of the wire with a strong neodimium magnet, as I am tapping and/or rubbing, the sound does not increase through the speaker.The wires are all insulated with plastic. If I rub/tap the wire(s) while not touching anything else on the guitar the sound and loudness is the same as when I am touching the ground connection of the main output jack.In this particular guitar I have on/off switches for each of the pickups. The effect is the same when pickups are turned on or off. The only difference is the pickup hum, when they are on.Does this rule out the magnetic field theory and confirms the triboelectric effect?Would there be a conclusive test that would confirm the triboelectric effect?Thank you all.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 21, 2025, 10:07:25 pm
content: Microphonics, nothing having to do with magnetism.I don't know the exact mechanism that generates the voltage when the wires are disturbed.

author: SiliconWizard
author_rank: Super Contributor
timestamp: May 21, 2025, 10:43:40 pm
content: While it could be due to a microphonic effect on the cable (which would be more noticeable if you really twist the wire pretty hard), I would rather vote for the triboelectric effect here. (Strictly speaking, a "microphonic" effect is purely mechanical, but one could argue that the triboelectric effect in a cable is microphonic in nature...)It's mostly due to the cable sleeve rubbing against the wire strands, and the wire strands rubbing against one another.https://en.wikipedia.org/wiki/Triboelectric_effect

author: BrianHG
author_rank: Super Contributor
timestamp: May 21, 2025, 11:13:48 pm
content: Quote from: Analog Kid on May 21, 2025, 10:07:25 pmThis can happen with sensitive amps if you have a strong AM radio station nearby.Rubbing 2 seemingly open air wires together can act as a shorting 2 out of phase dipole or loop antennas, making tiny unseen sparks between the wires. The resulting EMF can usually be picked up by phono and microphone pre-amps. Guitar amps with the guitar pickups would most likely be susceptible as well.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 22, 2025, 03:26:59 am
content: Nope, nothing having to do with RF pickup either. That's an entirely different animal.

author: 2N3055
author_rank: Super Contributor
timestamp: May 22, 2025, 06:42:03 am
content: Quote from: SiliconWizard on May 21, 2025, 10:43:40 pmIn practice it is all called microphonic if you can create some electrical stimulus by using mechanical energy (sound). Not important what physical effect is the reason.Same as you have different types of microphones but they are all microphones.I 've seen electrolytic capacitors both creating feedback and making a sound (acting as microphone and speaker), ceramic ones, coils etc... As long as you can hear something from speaker if you knock on it, it is microphonic..This could also be simple bad connection (bad solder or pot) that creates noise when you move wires.If you move it slowly, usually effect is much smaller, be it piezo, triboelectric or magnetic induction.When it is noise process because of bad contact somewhere, it will be audible even if you move wires slowly.

author: tooki
author_rank: Super Contributor
timestamp: May 22, 2025, 07:56:53 am
content: FYI, triboelectric noise is why good quality instrument cables (like guitar cables) use one or more layers of partially-conductive material in between the dielectric and the copper. (Usually just between dielectric and shield, but sometimes also between conductor and dielectric.)They often call it an “electrostatic shield”.Attached is a diagram of one cable that has two such layers.