Sensitivity tests loudspeakers

I have a few earpieces and mikrophone pieces from a soundspowered decktalker and soundpowered handsets. For some years now I used a Dynalec H-200/U
decktalker chestset for my crystal radio. Mikrophone piece not used and the two earpieces connected in parallel.
I measured the sensitivity of my loudspeakers so I could look wich one was best to use in my crystal radio. I`ve made a small tool using two BNC female connectors with a voltage divider in between. I used on 100K and one 1-ohm resistor for that one to create a 1:100,000 divider. High enough to get low voltages using my function generator. The lowest voltage generated was still a bit to high for a 1:10,000 divider.

Another amateur used the 1:10K divider, further the same method to measure. see here.

Beware if you have better hearing then I have, measured voltages could be lower,...or higher if you hear worse then I do.

Also you need more power as in the sensitivity test to identify speak or music. The test is only a single tone.

Tools used: Function generator, oscilloscope instead of DVM, LCR meter.
I used the sweep function of the generator, 500-1000Hz, sweeptime 1second. Better to hear the variations in sound.
Connect everything as in the schematic above with the correct settings. Put earpiece inside earmuff, and put on your head. Adjust the voltage coming from the function generator to the lowest voltage you can hear sound coming from the earpiece. View on the scope the peak-peak voltage and write it down.
Disconnect earpiece and measure it`s impedance at 1Khz while on your head (caution,.can be loud. Alternative block using your hand/fingers.)
Write down the value.

-Now you can calculate the effective voltage coming from the function generator by dividing the measured voltage through 2.828.
-Calculate the voltage over the speaker Uspeaker = Ugenerator / 100000
-Calculate power into the speaker P= (Usp X Usp) / Z
-Z is the impedance of the speaker at 1Khz you measured.

I made a excel sheet to do the calculations, just fill in the values you measured.

PP-voltages below are measured from the function generator, has to be divided to calculate voltage over speaker.

Measured impedances can be different from factory specs.
For example: The Monacor driver unit is rated 16Ω at 1000Hz, this is measured the same. But when holding to your ear or block the output with
your hand, the impedance goes up to approx. 30Ω.
These are the impedances i used with my calculations.

On Ebay a seller is selling these handsets as seen at the right. These were sold in pairs. According to the text on the handset these are coming from France. With two handsets and a few miles of wire only you could communicate with eachother.

 Sensitivity results:
Speaker piece: 0.015pW,  Z= 290Ω
Mikrophone piece: 0.027pW, Z= 270Ω


Monacor KU-516 Driver unit for 100V systems.
Sensitivity: 0.003pW, Z=30Ω

This type is the most sensitive speaker you can use with a crystal set, but creating a
headset will give you an extra weight of 5Kilograms on your head. 2½ each.

Dynalec H-200/U headset only. Connected two in parallel.
Sensitivity: 0.0156pW,  Z= 293Ω

I used this one since about 2007/2008
Microphone piece gone.


Stromberg-Carlson H-200/U headset. Only one side measured.
Sensitivity: 0.0155pW, Z= 750Ω)

Microphone piece, as is, not disassembled. PTT activated.
Sensitivity: 1645pW,  Z=0.73Ω)

Must be something more inside then the microphone element. Impedance very low and without pushing PTT same resistance.

Clansman Screened earphone Transducer, 783-5393
Sensitivity: 0.08pW,  Z= 130Ω

This one was sold by Reinhöfer Elektronic as a crystal radio piece a few years ago.

Dictograph R1 headset from approx 1925 I think.
2x 1500 ohm. Measured Z= 7KΩ.
Sensitivity: 138pW (pp-voltage 8.8volt @ Z= 7KΩ

This was the first headset in use in 2005 with my crystal radio setup.
But most stations were boomers back then.

Kapsch horn speaker, age approx 1920`s
Sensitivity: 1.7Watt (pp-voltage 7.71mV @ 4.2KΩ.
(voltage direct from function generator because of very low sensitivity)

Sensitivity/volume is adjustable by changing diaphragm distance from magnet.