Why do cell phones interfere with Microphones?
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Have you ever been in a meeting and hear a buzz in the speaker caused by a cell phone? I looked up the issue and have 2 results below.
How do I control Blackberry or cellphone interference in MB microphones?
My microphones are getting interference from Blackberry devices. Do you carry a filter that will eliminate or reduce Blackberry interference in audio?
Reply: Blackberries, cell phones and other wireless devices radiate powerful electromagnetic fields that can cause interference and noise in microphone signals. Blackberry devices cause serious interference for all manufacturers’ microphones. The radio signal goes through the mic grille into the mic capsule and circuitry. The FET gate lead inside the mic capsule is particularly vulnerable to RFI, but it is inaccessible, so filtering does not help. Two solutions are to keep the Blackberry devices at least arm’s length away from the microphones, or shield the Blackberry devices from the mics.
For shielding, take a 24″ piece of aluminum foil and tape it to a sheet of cardboard. Bend it 90 degrees at 6 inches from each end to form a U, and stand it up on the table as shown below. It will be 12″ high. Put the Blackberry inside the U-shaped foil shield and see if the interference is reduced. You might try grounding this shield via a wire to the mic mixer chassis. Experiment with shorter shield heights to determine the smallest size that is still effective. You could try tightly woven window screen instead of foil for improved appearance.
As another experiment, try wrapping one mic in tightly woven window screen that is grounded by a heavy wire to the mic mixer chassis. Temporarily turn off all the other mics and operate the Blackberry to see how effective the screen was. The acoustic performance of the mic will be degraded slightly by the screen, but it might be your only option.
The following paraphrased quote is from Gary Gunn, Market Development Manager for Installed and Touring Sound, AKG Acoustics, U.S.:
“The past two years have brought a whirlwind of engineering nightmares with Blackberry and other wireless units creating problems in wired and wireless microphones. The variable power on every Blackberry call makes research on the subject even harder. AKG has made a number of engineering changes to protect against the interference, but currently I know of no one who had successfully engineered a 100% RF protected microphone without drastically changing the frequency response.
We feel that our Discreet Acoustic Modular series is as RF free as that of any other manufacturer and maintains AKG’s full frequency response . In my opinion at this point, the best protection against RF interference is to use the new RF-beaded XLR connectors built by Neutrik. This, combined with our engineering, works quite well.
We, as all mic manufacturers, continue to work on the problem and look forward to an RF-interference-free microphone in the near future.”
Q: What causes interference between digital cell phones and hearing aids?
A: When using a digital cell phone, the telephone conversation is transmitted over a wireless network using radio waves. The radio waves emitted by the cell phone are referred to as radio-frequency (RF) emissions. The RF emissions create an electromagnetic (EM) field around the phone’s antenna. This EM field has a pulsing pattern. It is this pulsing energy that may potentially be picked up by the hearing aid’s microphone or telecoil circuitry and perceived by the hearing aid wearer as a buzzing sound.
To complicate matters, the technology for transmitting calls over a wireless network differs depending on the carrier or service provider. For example, Verizon Wireless and Sprint PCS use CDMA technology, Nextel uses iDEN technology, and AT&T Wireless, Cingular Wireless and T-Mobile use GSM technology. The interference generated by these various technologies has different characteristics, some of which may cause more annoying interference for hearing aid users than others.
Telecoil users may experience another form of interference referred to as “baseband, magnetic interference,” which originates from the cell phone’s electronics (e.g., backlighting, display, keypad, battery and circuit board). Unfortunately, baseband magnetic interference occurs in addition to the RF-interference potentially increasing the interference perceived by the hearing aid user.
The amount of interference experienced by hearing-aid users depends on the degree of RF emissions produced by a particular digital cell phone, and how immune his/her particular hearing aids are to these emissions. This is also true for hearing-aid telecoil users, but in addition, baseband emissions from the cell phone and the hearing aids’ immunity to baseband emissions must be considered.