I'm turning a Western Electric 500 into a mobile phone using a FONA Feather.
I see the Feather want/expects an 8Ω speaker, but the speaker in the handset measures 44Ω.
I presume if I try to drive the handset speaker, the FONA won't have enough power and I'll barely be able to hear. Is that assumption correct?
What would be the best way to work around this? An amp in between the FONA and the headset? Or might it work OK without? Or another method?
Actually the Simcom data sheet for the SIM800H found on that board assumes a 32 ohm element in its output figures, pragmatically speaking your 44 ohms (if that is indeed an accurate figure at audio frequency - doubt has been raised) will probably work, though you could also replace it with a different element, since the earpiece cup readily unscrews. Given the SIM800H data sheet, if you do go with a replacement you might look at a 32 ohm element, say from unpowered personal audio speakers that plug into a headphone jack.
(If you want to figure out why Adafruit listed an 8-ohm speaker on their product page you could try asking on their own forums - it's entirely possible that was what they had on hand or in stock, they tried it, and it seemed to work)
“Enough power” is not the issue. If we ignore the efficiency of the transducer itself, the electrical power in is the acoustic power coming out.
What a too-high load impedance does is it doesn't draw enough current to produce the output power that the source is capable of. On the other hand, a too-low impedance draws too much current, resulting in audio distortion or damage to the source.
Whether or not the specifications you have say so, in practice there must be a range of acceptable impedances. 4 Ω and 8 Ω are common full-size speaker impedances and they can be considered fairly low. Headphones can have a very wide range of impedances, commonly 25 to 600 Ω, but in general higher than typical speaker impedances.
Additionally, simply for acoustic reasons, headphones/headsets/earpieces always need much less power than a speaker, in the sense of something that you don't hold directly to your ear, requires. Thus, a particularly high impedance is not necessarily a bad thing.
All this together means that it is highly likely that an output that says it can drive an 8 Ω speaker will almost certainly be just fine — possibly even too loud! — when used with a 44 Ω transducer.
If you hook it up and it turns out it doesn't work well, and you confirm that it is for impedance reasons, then the solution is to use a transformer with a suitable ratio and frequency response.
The earpieces used in old telephones were driven directly by the line signal, so they had to be efficient. However the impedance at voice frequencies is probably much higher than 44Ω.
It would just try it direct, and if it is too soft then replace it with a modern element (which should have significantly better fidelity).
