Cell Phone Interference

[keypounder]

Hi everybody hoping this is the right forum for this.

 

I've been doing a piano bar gig for several years now --the kind where people actually sit around my baby grand as I play (sadly a rarity these days I know). As time has gone on, cell phone use at my piano has increased (which of course is no problem if they're using them to video me;)) --the problem as of late is though, since my piano is itself miked with 2 PZM's on the underside of the lid, my sound system is picking up some sort of interference from their phones which makes itself known in the form of a rumbling sound in my speakers. The problem is of course exacerbated when they actually lay their phones ON the lid.

 

Anyone have any ideas with a situation like this? (and good luck trying to get them to shut them off!)

 

Thanks!!

[abzurd]

You could try a magnetic choke core to help block the EMI.

 

http://www.amazon.com/stardrift-10-Pack-Diameter-Ferrite-Filter/dp/B015RAZTIA/ref=sr_1_1?ie=UTF8&qid=1461447008&sr=8-1&keywords=magnetic+choke+core

 

 

[wesg]

Are you using balanced (eg XLR) cables throughout your infrastructure? Are the wires properly twisted and shielded underneath the lid?

 

BTW, this is a tough nut to crack, it affects Digital Piano users also.

 

Wes

[agedhorse]

Exactly which "PZM's" are you using?

[keypounder]

Thanks for the input --I am using balanced XLR cables throughput and the PZM's are a pair of Crown PZM-30D's

[wesg]

Are you in a situation where you can audition a pair of Audio Technica U851Rs? I have a sneaking feeling that the RFI is getting into the electronics in the mic capsule. Reading specs, I SUSPECT the AT U-series is much better shielded against RFI than the PZM-30D. But I don't actually KNOW. What can happen inside the mic body is that a short length of wire connected to the FET (transistor) gate acts a radio antenna, and this injects interference right into the signal at the source - so no choke etc can get rid of it. The U-series advertises RFI shielding and appears to have a metallic body, which is presumably grounded to the shield conductor of the XLR cable.

 

If I am right (a big IF) about the source of the interference, and you can't change microphones, fixing this problem gets a lot more difficult. One possible way to shield PZMs inside a piano:

- glue (contact cement) a piece of aluminium foil (the thicker the better) to the underside of the lid, at least 17" (43cm) in diameter. Best to use one large piece of foil, but if you have to stitch together multiple pieces, they should be connected electrically by overlapping them, and screwing through the overlap in several places with unpainted metal screws and washers.

- mount the mic to the aluminium foil; if screws are necessary, use unpainted metal screws.

- Tent the mic with a piece of metal window screen material, you want the holes around 1/8" to 3/16" .... small enough to stop radio waves but big enough to not interfere with high-frequency audio. "Hardware cloth" would work even better if you can find some with a small enough screen. My local hardware store carries 1/8" hardware cloth, this would be my first choice.

- Make sure the screen doesn't touch the mic, and screw it with several unpainted metal screws and washers, through the aluminium foil

- Make up a custom XLR cable which has a shield conductor leaving the back of the connector, terminated with a ring terminal

- Pass the XLR cable through the screen with as small a hole as possible, as far away from the mic as possible

- Connect the ring terminal to the foil with an unpainted metal screw; use a star washer between the foil and the terminal.

 

This would create a sort of Faraday cage around the microphone; the idea is that the cellphone interference would get picked up by the foil/screen and shunted to ground via the shield conductor of your XLR cable, rather than getting into the electronics.

 

Note that I do not do this for a living. I am just a hobbyist throwing ideas out there. For the EEs out there, I figure he is having trouble with wavelengths between 12 and 43cm (700MHz to 2.5GHz) based on my knowledge of cellphone frequency allocation in the US. The biggest offenders are probably the 850 and 1900 MHz bands.

 

Hopefully if you have to do this that you play with the lid closed......................................

 

Wes