Why Do Speakers Make A Weird Noise Near A Ringing Phone?

That famous "BZZ-BZZ-BZZ" buzzing right before your phone rings is the GSM buzz. On 2G GSM networks, your handset transmitted in short bursts at exactly 217 Hz (an audio-band frequency), and those RF bursts were rectified by unshielded transistor junctions in nearby speaker amplifiers, demodulating the burst envelope back into audible noise. Most 4G LTE and 5G phones don't cause this, and US carriers have now shut down their old 2G/3G networks, so the buzz is largely a historical artifact.

If you have a cell phone (and most of the world does), then you know how powerful and incredible they are, but they’re far from perfect. From cracked screens to water damage, they are still tiny machines that are susceptible to outside influences. One of the most confusing and mysterious behavior of phones, however, is their impact on nearby speakers when they ring.

You can surely remember that high-pitched screeching that rips out of the speakers, and while many people didn’t initially connect it with their ringing phones, it’s now a well known link. The question is… why does that sound occur?

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The Science Of The Screech

To put this in simple terms: every cell phone is a radio transmitter that talks to a nearby base station (the tower). On the old 2G GSM networks, each handset was assigned a 577-microsecond time slot inside a 4.615-millisecond frame, transmitting in sharp bursts at a repetition rate of about 217 Hz, which sits right in the middle of the human audio band. The wiring inside most cheap speakers is not shielded against radio-frequency pickup, and the semiconductor junctions in the amplifier (input-stage transistors and op-amp ESD diodes) act as non-linear circuit elements that rectify those RF bursts.

The non-linear element in the amplifier rectifies the radio bursts, turning the 217 Hz envelope back into audio. The result is the characteristic BZZ-BZZ-BZZ on top of whatever the speaker is already playing.

The effect was loudest on cheap powered (active) speakers and on PC sound systems with long unshielded analog input cables. Higher-end gear with shielded cables, ferrite chokes on the inputs, and properly designed PCB layouts was largely immune.

In even simpler terms: the long speaker cables and PCB traces act like an unintended antenna that picks up the burst transmissions from a nearby GSM phone. The amplifier then demodulates the envelope, producing the noise until the phone's transmission stops.

Ways To Avoid The Annoyance?

It may seem like common sense, but the best way to eliminate that irritating noise from your speakers is to simply move your phone a fair distance away (depending on the strength of the signal and the type of speaker). Usually, the range of interference is only a few feet, but can be as little as a few inches in some cases.

If you want to prevent it from ever happening, you simply need to invest in the right type of speakers – namely those that are of high enough quality that have shielded cables. Speakers that have digital inputs are also preferable and won’t react in the same way as older speakers. You can also shield your amplifier, which is located in the subwoofer in most inexpensive speakers. By wrapping the amplifier in tin foil and grounding it, you can avoid the interference noise.

Not all phones caused the buzz equally. 2G GSM phones, which used TDMA (Time-Division Multiple Access), produced it constantly because of those sharp 217 Hz on/off bursts. CDMA (Code-Division Multiple Access) phones, used by carriers like Verizon and Sprint in the US, transmitted continuously with smooth power control and so caused far less buzz. (Verizon, by the way, is a carrier, not a phone brand.) 4G LTE and 5G NR phones use OFDMA / SC-FDMA with much smoother envelopes and adaptive power control, and produce hardly any of the classic GSM buzz.

Today, this is mostly a memory. AT&T shut down its 2G GSM network on 1 January 2017. Verizon retired its CDMA 3G network on 31 December 2022. T-Mobile began winding down its remaining 2G GSM in 2024. With modern 4G LTE and 5G handsets and most 2G/3G infrastructure gone, the classic GSM buzz that defined the late 2000s and early 2010s desk-speaker experience is now a fairly rare event.

The best and easiest solution, of course, is to keep your phone in your pocket, and try not to stand next to a cheap speaker if you’re expecting a call!