Carbon Microphone & Loudspeaker History: Chicken/Egg?

[NTL2009]

TL;DR Summary

You need both to have a system (unless you have test equipment that I don't think was available at the time), but I've found little on early loudspeaker development.

I recently watched a YouTube video on the history of the telephone. It was fairly generic, stuff I knew, but he spent some time on the carbon microphone, which made me curious about the real details of the development of carbon microphones. I easily found lots of info with a search. But that made me even more curious - how did they know it worked? What did they hook it up to?

You would need a pretty sensitive loudspeaker, with fairly good impedance matching. But I haven't found much at all on early loudspeakers. The articles just mention them in passing, or talk about their development after the introduction of vacuum tube amplifiers.

They obviously didn't have o'scopes in the 1860/70's. I don't think they had audio frequency meters? I suppose you could set up a carbon mic with a DC meter, and see the response to pressure changes (just pushing on it with a finger), and extrapolate that to audio frequency pressure changes? And proceed with development based on that, until you had a large enough signal to drive whatever speaker they had?

For speaker testing/development, I suppose you could use a form of dynamo to generate an audio frequency - that would help with development.

Just seems like a chicken/egg scenario to me, but maybe I'm just missing the details of loudspeaker developments in the 1800's.

Side note on Carbon Element Amplifiers: I knew of this, but still find it amazing, and have thought about building one just for demo purposes. They made amplifiers back then by using a loudspeaker element directly driving a carbon element 'microphone'. Since you can put a larger current on the carbon element, you can actually amplify the power of the signal in this way. It would be a "Class A" amplifier configuration, and some were water cooled.

 

[Borek]

My bet is they tried and it worked a bit so they refined it by trial and error. Surprisingly many things were successfully designed and made by people having no idea about theory behind

 

[sophiecentaur]

Summary:: You need both to have a system (unless you have test equipment that I don't think was available at the time), but I've found little on early loudspeaker development.

until you had a large enough signal to drive whatever speaker they had?

You don't mention earphone technology. Earphones would have been used for telegraphy long before speech was transmitted ( I would have thought) much more sensitive than the standard galvanometer (or loudspeaker). It is not hard to make (at home in the garage) an earphone with a coil, an iron horseshoe and a light iron diaphragm. Placed right next to the ear, a very small audio signal can be detected. That was how people listened to the (later) crystal set, with much less than a mW of received RF signal levels.
Earphones could be tested with mechanically generated AC tones.

 

[NTL2009]

You don't mention earphone technology. Earphones would have been used for telegraphy long before speech was transmitted ( I would have thought) much more sensitive than the standard galvanometer (or loudspeaker). It is not hard to make (at home in the garage) an earphone with a coil, an iron horseshoe and a light iron diaphragm. Placed right next to the ear, a very small audio signal can be detected. That was how people listened to the (later) crystal set, with much less than a mW of received RF signal levels.
Earphones could be tested with mechanically generated AC tones.

That's a great example, a crystal set with no external power.

I didn't find any references to anything like that back in the carbon mic development days, but I imagine it was something like that. I'll look some more, I'd really like to find some historic reference documents. Just a matter of curiosity, I find it interesting to review how these people went about things, w/o the knowledge we have today. Like @Borek says, some/much of it was just trial error.

 

[gneill]

Some history of loudspeaker technology:

https://edisontechcenter.org/speakers.html

 

[Baluncore]

Side note on Carbon Element Amplifiers: I knew of this, but still find it amazing, and have thought about building one just for demo purposes.

It was called a "Brown’s Relay".
It is mentioned in; A History of Telegraphy, by K.G. Beauchamp.
The IET History of Technology Series. Volume 26.

 

[sophiecentaur]

Some history of loudspeaker technology:

https://edisontechcenter.org/speakers.html

Interesting and entertaining link and it makes my point that the moving coil Loudspeaker (1920's) must have been superseded by an alternative form of transducer because the history of the Telephone goes back to before 1880. The link has a picture of a postage stamp with a telephone receiver shown which has the form of the well known diaphragm type Earphone design from WW2 and later. There is a timeline of the development of the telephone system at the bottom of the link.

 

[anorlunda]

Did you find the amusing bit of history about Thomas Edison's chalk telephone receiver?

https://www.thehenryford.org/collections-and-research/digital-collections/artifact/359181

In an Edison biography, I read that the chalk receiver was invented because Bell's receiver had such low volume that it was hard o use. Edison's invention produced so much volume that it caused privacy problems. Everyone in the house could hear the phone conversation.

 

[tech99]

The first loudspeaker was, of course, Bell's telephone receiver, which used a permanent magnet wound with a coil which attracted a soft iron diaphragm. I think Bell's transmitter was initially the same device. Development of the earphone idea was rapid as telephone networks started appearing. When a loudspeaker was required, I think the first method was to fit a large horn to the telephone receiver. When broadcasting started in the 1920s, it first used crystal (diode detection) receivers with very sensitive earphones, but then tube amplifiers became more common and loudspeakers were possible. They were initially the moving iron and moving coil types, but the latter had less distortion and eventually won. You can make quite a good loudspeaker using the top 4 inches of a big cola bottle as the cone. Wind a coil of a hundred turns or so on the threaded top of the bottle, then insert a stack of supermagnets (button magnets) into, but not touching, the neck. These can be supported on a cardboard "spider".
The permanent magnet in the earphone is required to avoid the diaphragm being attracted every half cycle instead of every cycle, and it increases sensitivity very greatly.

 

[sophiecentaur]

The first loudspeaker was, of course, Bell's telephone receiver, which used a permanent magnet wound with a coil which attracted a soft iron diaphragm. I think Bell's transmitter was initially the same device.

When I was a lad, I used that arrangement to talk to my next door neighbour's boy (around 50m of wire between our bedrooms). I also tried a moving coil loudspeaker as the transmitter. It produced a fair output when you shouted into it. It had great 'sound gathering power' which seemed to offset any impedance mismatch.

 

[tech99]

Thank you for reminding me about that. I will try it at with our pupils' outdoor camp next week!

 

[sophiecentaur]

Thank you for reminding me about that. I will try it at with our pupils' outdoor camp next week!

Don’t forget the string and cans version and the speaking tube too.
I guess you don’t teach them semaphore.

 

[Keith_McClary]

chalk telephone

I never heard of that. I found an explanation.

 

[anorlunda]

I never heard of that. I found an explanation.

See Edison His Life and Inventions. Edison was financed by Western Union to make the telephone commercially successful. (Bell's wasn't.) In addition to the chalk receiver and the carbon microphone, he did much more.

His first inventions in the art [of the telephone], made in 1875-76, continue through many later years, including all kinds of carbon instruments —the water telephone, electrostatic telephone, condenser telephone, chemical telephone, various magneto telephones, inertia telephone, mercury telephone, voltaic pile telephone, musical transmitter, and the electromotograph. All were actually made and tested.

The book has a whole chapter on Edison's telephone inventions.

In typical Edison fashion, he didn't stop with the basic chalk invention:

I made six of these [chalk] receivers and sent them in charge of an expert on the first steamer. They were welcomed and tested, and shortly afterward I shipped a hundred more. At the same time I was ordered to send twenty young men, after teaching them to become expert. I set up an exchange, around the laboratory, of ten instruments. I would then go out and get each one out of order in every conceivable way, cutting the wires of one, short-circuiting another, destroying the adjustment of a third, putting dirt between the electrodes of a fourth, and so on. A man would be sent to each to find out the trouble. When he could find the trouble ten consecutive times, using five minutes each, he was sent to London.

Full disclosure. I am a big Edison fan and I love telling about him.

 

[Keith_McClary]

the water telephone, electrostatic telephone, condenser telephone, chemical telephone, various magneto telephones, inertia telephone, mercury telephone, voltaic pile telephone, musical transmitter, and the electromotograph.

My cell phone has all those features.

 

[tech99]

My cell phone has all those features.

But Bell's telephone did not require a battery, unlike a cell phone.

 

[NTL2009]

OP here, just catching up - thanks for all the replies. Never heard of the chalk speaker, crazy! Makes me wonder if a modern implementation of that would be practical. Audio transducers have very low efficiency (well, at least ones with decent fidelity), I wonder if modulation of a force from a motor (which are pretty high efficiency) might have some merit? Or maybe most of the inefficiency is due to the practicality of matching mechanical impedance to free air?

I'll read in more detail later, but I guess the answer ro my OP is just a basic permanent magnet dynamic speaker, much the same as today. The sources I had found just were not clear about it.

 

[sophiecentaur]

But Bell's telephone did not require a battery, unlike a cell phone.

Just a wire from the wall.
On that matter, My 11 yr old granddaughter asked me what did we carry with us, in our 'youth', which did the same as her iPhone. She was really flummoxed by my answer about wires and phone boxes.

 

[Nik_2213]

May I direct you to the Museum of RetroTechnology ??

http://www.douglas-self.com/MUSEUM/museum.htm

Care: Dangerously addictive site...

 

[tech99]

Or maybe most of the inefficiency is due to the practicality of matching mechanical impedance to free air?

The inefficiency of a modern loudspeaker is due to its small size. The original cinema speakers were giant horns having dimensions commensurate with the lowest frequency. For 50Hz, the wavelength is 6m. The amplifier needed only 2 watts or so for an auditorium. The horn provided good matching from the diaphagm to the air. The radiation from the back of a speaker cancels that from the front, so for low frequencies a long path is need from front to back in order to introduce a further 180 degree phase shift. For 6m wavelength, this means a baffle board of radius 1.5m. A big box makes a speaker efficient. The alternative method, now popular and providing a very small enclosure, is to apply sound absorption to the rearward radiation by having a sealed box with cotton wool inside. However, this is very inefficient and will require, maybe, ten times the amplifier power. It is rather like having a very small antenna with a resistor in parallel to improve the matching.

 

[anorlunda]

There was a recent thread praising cell phone speaker audio quality. Power efficiency is a different thing.

https://www.physicsforums.com/threads/modern-speaker-technology.979639/post-6253869