hi,
The intercoms I worked on in the 1960's mostly used the same speaker as a microphone, press to talk, worked very well for its era.

 

At a gas station when I use the cheeeep intercom, the person at the cash register cannot understand what I say and I also cannot understand what he says because he doesn't speak English. (Edited by moderator)
Even if he speaks with perfect English I don't know what he says because the sound from the intercom is so bad.

 

Intercom speakers are commonly 45Ω for their higher output as a microphone. You can also use small audio transformer to boost output voltage because hum and noise are a greater problem with the low signal. Maybe try an 8Ω-1kΩ which will boost signal voltage.
The vestibule panel is usually many feet away from the amplifier and needs shielded cable.

 

@prairiemystic I guess I had to know about it to find it.. Here is a schematic of speaker/no-mic intercom where an impedance matching X-former is used at speaker-out. Validates your point. See attached image. Source:
https://www.electroschematics.com/intercom/

Intercom speakers are commonly 45Ω for their higher output as a microphone. You can also use small audio transformer to boost output voltage because hum and noise are a greater problem with the low signal. Maybe try an 8Ω-1kΩ which will boost signal voltage.
The vestibule panel is usually many feet away from the amplifier and needs shielded cable.

 

You'd have to measure how many mV you get from the loudspeaker as a microphone. I just remember 45Ω as being a standard intercom speaker impedance, and you could get them waterproof.
That circuit looks good, I think the two IC's give you a max. voltage gain of 550 and then the transformer gives some as well.
The LM380 circuit uses a transformer with turns ratio of 25:1 along with the LM380 voltage gain of 50, giving a max. gain of 1,250.
The LM388 circuit uses no transformer with voltage gain set to 300. I think it would be a bit quiet, same for LM386 which has max. voltage gain of 200.

 

That seems counter-intuitive.
A larger cone area should cause a larger deflection, and thus a higher voltage, for a given sound pressure.
I'm quite sure that going down to say, a 1-inch speaker, would not further increase the speak deflection and voltage.

The reason the large cone speaker does not work well is because the mass of the larger cone takes much more energy to move rapidly. The larger cones will not work well for voice frequencies. Look at an actual microphone and see that with dynamic microphones the diaphram is very thin and extremely light. The big speaker cone is perhaps a thousand times heavier,
What does make a fairly effective microphone is one of those 40 ohm earphones that have the better frequency response. Of course you still need quite a bit of voltage gain, but that is cheap and easy if perfection is not required. The intercoms that do use a single speaker as a mic do provide more gain, and often use a higher efficiency speaker as well. But not everybody has figured that out yet.

 

The intercoms that do use a single speaker as a mic do provide more gain, and often use a higher efficiency speaker as well. But not everybody has figured that out yet.

But with Electret microphones available for about $0.25 in quantity, and the fact you cannot duplex the communication without a separate microphone, I don't think anyone needs to figure out how to use a speaker as a microphone anymore. I couldn't imagine the Amazon reviews of an intercom that does not allow duplex communication. Oh, wait, what does anyone need an intercom for when you cannot Text with it - lol.

 

Recall those "Sound Powered " phones from the distant past? They may still be used for emergency communication in some buildings. The sound into the microphone generates enough power to drive the earphone at the far end. They work fairly well. They require no other power besides speaking into them, so they are reliable.

 

You mean the ones made from 2 tin cans and a piece of string?

 

NO!!!! The technical description is variable reluctance audio transducers. The newer versions were more effective than the older versions.