How a phone turns your voice into electricity
A telephone turns your voice into an electric current that rises and falls with it, sends the current along a wire, and turns it back into sound at the other end. Two parts in the handset do the work. The microphone turns your voice into electricity, and the earpiece turns electricity back into sound. A desk phone today still has both, and turns your voice into numbers before it leaves the phone. Alexander Graham Bell got the first patent for the telephone in 1876, though Antonio Meucci and Elisha Gray were working on the same idea.
Purple: your voice as electricityKey
- Purple: your voice as electricity
The handset is lifted · the line is ready
Press a button, or click the mouthpiece or the earpiece to look inside it.Press a button, or tap the mouthpiece or the earpiece to look inside it.
Switch to 1951 for the Bell System's 500-type telephone. The steps are the same; what changed is inside the microphone, and what leaves the phone.
Anatomy of a telephone: one call through an office desk phone today and through the Bell System's 500-type set of 1951. The drawing is not to scale.
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Follow one call
Press Play to watch the call, or scroll down and the drawing follows you one step at a time. Try it, under the drawing, makes your own call.Press Play to watch the call, or Next to go one step at a time. Try it, under the drawing, makes your own call.
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Step 1 of 6
You lift the handset
You pick up the handset to make a call. Under it, the hook switch springs up and tells the phone that a call is starting. On the 1951 phone, it let a steady current from the line flow through the phone. Your voice will ride on that current.
Why does the phone need a steady current?
Because your voice does not make electricity by itself here. It changes a current that is already flowing. That is how the US Supreme Court described Bell's method in 1888. A continuous current flows in a closed circuit, and its strength changes with the changes in the air made by the voice.
The 1951 set took that current from the line. The Bell System called it a common battery set, and it adjusted itself to how much current reached it, since a short line brings more than a long one.
Sources: US Supreme Court: The Telephone Cases, 126 U.S. 1 (1888); Bell System Technical Journal: an improved telephone set (1951)
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Step 2 of 6
Your voice pushes a diaphragm
You speak into the mouthpiece. Your voice squeezes the air in front of your mouth and lets it go, very fast. Behind the holes of the mouthpiece sits the microphone, and in it a thin diaphragm that moves in and out with the air. Look inside it below.
Look inside The microphone
- 1Metal can
- 2Charged foil
- 3Spacer ring
- 4Back plate
- 5Transistor
10 to 20 dB of boost is what this microphone needs from the phone, because it gives far less than carbon did.
- 1Mouthpiece
- 2Diaphragm
- 3Front contact
- 4Carbon grains
- 5Back contact
- 6Plastic cup
No amplifier in the phone: carbon was so sensitive that the phones it was made for had none.
How much of your voice does a call carry?
A standard call carries sounds from 300 to 3,400 vibrations a second, written Hz. Your voice has higher sounds than that, and the call leaves them out. High-definition voice, or HD voice, reaches 7,000 Hz.
The 1951 microphone was built to give its most between about 2,500 and 3,500 Hz. Above that, its output drops off.
Sources: ITU-T G.712: transmission performance of PCM channels (2001); ITU-T G.722: 7 kHz audio coding within 64 kbit/s (2012); Bell System Technical Journal: an improved telephone set (1951)
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Step 3 of 6
The push becomes a changing current
The moving diaphragm changes how much electricity gets through the microphone. In 1951 it squeezed a chamber of carbon grains, and squeezed grains let more current through. Today it moves a charged foil near a metal plate, and the voltage between them changes. Either way, the current that runs down the cord now rises and falls with your voice.
What does it mean that the current follows your voice?
Bell's patent of 1876 calls it electrical undulations, similar in form to the vibrations of the air made by the voice. Draw the air pressure and the current as two wavy lines, and they have the same shape.
That shape is the whole trick. Everything after the microphone only has to keep it, all the way to the far earpiece.
Sources: US Patent 174,465: Bell, improvement in telegraphy (1876)
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Step 4 of 6
The line carries your voice away
The current runs down the cord into the phone, out of the back and along the cable to the socket in the wall. In 1951 it went on as a current, on a pair of copper wires to the telephone exchange. A desk phone today first measures it 8,000 times a second and sends the measurements on as numbers.
How many numbers is that?
Each measurement is stored as 8 bits, ones and zeros. So a standard call needs 64,000 bits a second in each direction.
This way of sending a call is a 1988 standard of the International Telecommunication Union. A European standard says every internet desk phone must be able to use it.
The pair of wires goes back to 1881, when Bell patented the two-wire circuit. From 1891 the two wires were twisted round each other, to cut the interference from the lines beside them.
Sources: ITU-T G.711: pulse code modulation of voice frequencies (1988); ETSI ES 202 737: transmission requirements for narrowband VoIP terminals (2020); IEEE History Center: telephone transmission
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Step 5 of 6
The earpiece turns it back into sound
Your friend answers, and their voice comes back the same way, as a current up the cord to the earpiece. There it flows round a coil beside a magnet. The pull changes with the current, so a thin diaphragm moves and pushes the air in your ear. You hear their voice.
Look inside The earpiece
- 1Grille
- 2Diaphragm
- 3Seal
- 4Coil
- 5Magnets
- 6Housing
1876 is when Bell patented this idea: a current in a coil, a magnet and a diaphragm that copies the sound.
- 1Earcap
- 2Dome
- 3Iron ring
- 4Pole ring
- 5Coil
- 6Ring magnet
5 dB louder than the earpiece before it, for the same current. The Bell System called the new design a ring armature.
Is this how Bell's telephone worked?
Yes. In Bell's 1876 patent the current flows through an electromagnet, which makes its armature, the moving part, copy the movement of the armature at the other end.
The 1951 earpiece pulled on a ring of magnetic metal round a light dome, and it was 5 dB louder than the one before it for the same current. A phone receiver in a 2019 patent application still uses a coil, magnets and a diaphragm partly made of magnetic metal.
Sources: US Patent 174,465: Bell, improvement in telegraphy (1876); Bell System Technical Journal: an improved telephone set (1951); European Patent Office: EP 3 648 472, thin receiver (2020)
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Step 6 of 6
You hear yourself, a little
While you speak, a little of your own voice goes straight from the microphone, through the phone, to your earpiece. It has a name, sidetone, and engineers put it there on purpose. A circuit inside the phone keeps it quiet, so you hear your friend over yourself.
Why not cut it out completely?
Too much sidetone, and people lower their voices without noticing, so the other end hears them less well, Bell System engineers wrote in 1953. Cutting it out completely was not what they wanted either. They aimed to keep it at a low level.
The rule still holds. A European standard for internet desk phones sets how loud sidetone should be, and says it may reach your ear at most 5 thousandths of a second late.
Sources: Bell System Technical Journal: an improved circuit for the telephone set (1953); ETSI ES 202 737: transmission requirements for narrowband VoIP terminals (2020)
Look inside: 1951 and today
The microphone changed completely, and so did what leaves the phone. The earpiece still works the way Bell's did in 1876: a current, a magnet and a diaphragm.
The same two parts, 70 years apart, taken apart along the way the sound goes. Point at a part, or tap it, to see what it does.
500-type set, 1951
Desk phone, today
The microphone
- 1Mouthpiece
- 2Diaphragm
- 3Front contact
- 4Carbon grains
- 5Back contact
- 6Plastic cup
No amplifier in the phone: carbon was so sensitive that the phones it was made for had none.
The microphone
- 1Metal can
- 2Charged foil
- 3Spacer ring
- 4Back plate
- 5Transistor
10 to 20 dB of boost is what this microphone needs from the phone, because it gives far less than carbon did.
The microphoneScience Museum Group; Bell System Technical Journal; US Patent 3,118,022; US Patent 5,946,392
Carbon
Grains that let more current through when squeezed
Electret
A charged foil beside a metal plate
Amplifier for the microphoneUS Patent 5,946,392
None
Carbon is sensitive enough on its own
10 to 20 dB
Of boost, because an electret gives far less
The earpieceBell System Technical Journal; European Patent Office; US Patent 174,465
Ring armature
A magnet pulls on an iron ring round a dome
Coil and magnets
Bell's idea of 1876, made small
What leaves the phoneUS Patent 174,465; ITU-T G.711; ETSI ES 202 737
A current
That rises and falls with your voice
Numbers
8,000 measurements a second, 8 bits each
Highest sound carriedBell System Technical Journal; ITU-T G.712; ITU-T G.722
About 3,500 Hz
Above that, the microphone gives less and less
3,400 Hz
On a standard call. HD voice reaches 7,000 Hz
The earpiece
- 1Earcap
- 2Dome
- 3Iron ring
- 4Pole ring
- 5Coil
- 6Ring magnet
5 dB louder than the earpiece before it, for the same current. The Bell System called the new design a ring armature.
The earpiece
- 1Grille
- 2Diaphragm
- 3Seal
- 4Coil
- 5Magnets
- 6Housing
1876 is when Bell patented this idea: a current in a coil, a magnet and a diaphragm that copies the sound.
How it came to be
- 1860s
A telefon in Germany
Johann Philipp Reis, a physics teacher, builds a device that carries speech, if badly, and calls it a telefon. He shows it widely but never seeks a patent.
Library of Congress: who is credited with inventing the telephone? (2022)
- 1871
Meucci's notice
Antonio Meucci files a caveat, a notice of an invention to come, for his talking telegraph. He cannot pay to renew it after 1874. In 2002 the US House of Representatives honours his work on the telephone.
US House of Representatives: Resolution 269 on Antonio Meucci (2002)
- 1876
Two filings in one day
On 14 February, Bell's patent application is the 5th entry of the day at the US Patent Office. A few hours later Elisha Gray files a caveat for a very similar telephone, the 39th. The Patent Office grants Bell's patent on 7 March, and the courts later rule in his favour.
Library of Congress: who is credited with inventing the telephone? (2022)
- 1876
"Mr. Watson, come here"
On 10 March, in their laboratory in Boston, Bell calls his assistant Thomas Watson through the telephone: "Mr. Watson, come here! I want to see you." It is the first time a wire carries a sentence anyone can understand.
IEEE: Milestone, first intelligible voice transmission over electric wire (2006)
- 1878
Carbon in the mouthpiece
Western Union had asked Thomas Edison to improve how speech is sent. His carbon transmitter puts a button of carbon powder behind an iron diaphragm, very like the microphones of David Edward Hughes.
Science Museum Group: Edison carbon button transmitter (1878)
- 1951
A new phone for every home
The Bell System brings in the 500-type set, shaped with the designer Henry Dreyfuss. Its handset is about a quarter lighter than the last one, and you can turn its ringer down.
Bell System Technical Journal: an improved telephone set (1951)
- 1962
A microphone without carbon
At Bell Labs, Gerhard Sessler and James West file their patent for the electret microphone. Its thin foil keeps its own electric charge for a long time, so no outside voltage has to charge it.
US Patent 3,118,022: Sessler and West, electroacoustic transducer (1964)
Where Voxbi fits
Voxbi, a cloud phone system hosted in the European Union, works with the desk phones on your desks and with apps on your computer and your mobile.
See it in Voxbi