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I remember that years ago when I had a classic mobile phone (the small one without touchscreen) when I was about to receive a call from someone and my phone was near a speaker, the speaker would start to vibrate quite hard before 2-3 seconds of receiving the call.

I don't have a way to experiment that now since I don't own a small mobile phone anymore and with modern smartphones it doesn't work. Is there any particular reason why?

How is the phenomenon produced after all? I also tried to google about it, but I had no idea how it is called.

Honestly I have no idea where I should've put this question, but searching trough all SE sites I ended up here, but if this isn't the right place please tell me.

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    This question likely belongs in the electronics stackexchange , or maybe the (ham) amateur radio one – hotpaw2 Apr 13 '19 at 00:25

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A little background... How a speaker works: Speakers are controlled by driving small diaphrams with a magnet and that magnet is controlled by wires that take isolated 'wired' signals and convert them into sound.

How your cell phone works: Your cell phone has antennas in it. They work by sending and receiving transmissions of electromagnetic waves. These waves are wireless signals that are picked up by antennas and then processed later. Your phone is transmitting/receiving those electromagnetic signals.

What is happening between the speaker and the phone: The wires in your speakers are inappropriately acting as antennas and are picking up your phones transmissions. Note, that it only is happening when the phone is transmitting because it requires significant power to induce a current in your speaker's wires. Also some consideration must be taken to understand the frequencies that control your speakers but that is more detail than necessary to understand the concept of what is happening.

Read this reference here if you'd like to hear more.

Dr. Dan
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    You forgot the most important part, the amplifier, which is mentioned in the article you linked. A mobile phone signal is too weak to influence just the speaker coil or the wires going to it. – Justme Apr 12 '19 at 17:32
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    @Justme : no amplifier required. An old mobile phone GSM transmitter could cause an unplugged AM radio to emit vibrating noises – hotpaw2 Apr 13 '19 at 00:20
  • @Justme Adding an amplifier will of course effect things and is a good point. It adds one more detail that if the interested person wants could look into how an amplifier can change things. Thank you for your comment. The basics include the transmission of the cellphone signal and receiption of that emission in the circuitry of the speaker. No need to get into all the details of specific circuits. – Dr. Dan Apr 13 '19 at 18:04
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    Now that you mentioned circuits, I might have caused some confusion here. So I was not talking about a separate devices what people call as amplifiers and connects to passive speakers. I am talking about the amplifier circuitry inside devices that have speakers in them, and this amplifier is made of ICs / transistors / valves etc and amplifies weak signals to be able to drive the speaker. So most of these devices with speakers and amplifiers embedded in them (radios, electric pianos, etc) were susceptible to amplifying the otherwise weak GSM burst tones to be heard amplified via speakers. – Justme Apr 13 '19 at 19:09
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The Global System for Mobile Communications (GSM) is a 2G (2nd generation) technology which uses the time division multiple access (TDMA) method. With this method, each phone transmits within its own repeating time slot. During the audio interference in question, the phone's radio transmits over a 888 ms / 1625 = 0.546 ms slot (excluding a guard period) that repeats every 60 ms / 13 = 4.62 ms. This means that the power drain by the radio is a 217 Hz rectangular pulse wave with a 12 % duty cycle. The audio-frequency (or from radio perspective: extremely low frequency, ELF) waveform of the DC current between the phone's battery and radio is radiated as a magnetic field and may be picked up as interference by a high-impedance input of an audio amplifier. Ground loops may play a role. Also, the modulated 900 MHz and 1800 MHz GSM radio-frequency (RF) transmission may be rectified by non-linear elements within the audio amplifier giving the same pulse waveform. Speaker cables and loudspeaker wiring may act as an antenna that receives the RF interference, providing an RF path to the audio amplifier.

Newer audio equipment may be more immune to interference. However, in my experience, even older equipment are immune to interference from phones using 3G and later technologies. In 3G, code-division multiple access (CDMA) is used allowing the radios to transmit continuously, eliminating the periodic radio on/off switching waveform. But even the single switching transients in the beginning and at the end of a transmission could result in audible clicks. Perhaps the transients are not audible with phones employing CDMA because they better moderate transmitting radio power, possibly to comply with newer wireless telecommunications technology and electromagnetic compatibility (EMC) standards. In my experience, newer phones don't seem to erase magnetic cards either.

References:

Olli Niemitalo
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Well, two things are different. Back in the day cell mobile phones only used lower frequency bands (900 MHz but depends on country), phones these days use higher frequency bands (like 1800 MHz) by default and fall back to lower frequency bands if they even exist any more. Also other electronic devices back then may not have been designed having a mobile phone near them so as mobile phones became more common also other devices need to be protected better from nearby RF transmitters. The tone is just RF interference affecting amplifier circuitry that drives the speakers.

Justme
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