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This is a test for comparing the sensitivity of several loudspeakers and headphones.
With this test I want to determine wich speaker needs the least inputpower to produce a hearable sound.

The testcircuit is shown below.

G= audio signal generator

R1 =10 k.Ohm.
R2 =1 Ohm
V =Voltmeter which gives a accurate reading at 1 kHz and low voltage (100mV).
The voltmeter measures the effective voltage (RMS value) from the audio generator.

Instead of a voltmeter we can also use a oscilloscope, in this case divide the peak-peak reading by 2.828 to become the effective voltage.

The tested speaker is connected in parallel with resistor R2.
R1 and R2 form a 1 / 10000 voltage divider.
Because R2 has a low value, the voltage across R2 almost won't change when a speaker is connected.


The measurement goes as follows:

-Set the generator to 1 kHz sine wave output, some volt amplitude.

-Listen to the speaker, lower the generator voltage until the tone can just be heard.

-Measure the effective voltage from the generator (U1).

-Calculate the voltage over the speaker in Volt; U2=U1/10000

-Calculate the power into the speaker in Watt; P=(U2 X U2) / Z
Z =the speaker impedance in Ohm

The minimal power needed to hear the tone is in the range of pico-Watt's.
1 pico-Watt = 1pW = 1.10^-12 Watt = 10 to the power minus 12= 0.000,000,000,001 Watt.

In the following table you see the tested speakers and the minimal power.

Headphone Sennheiser Model HD433 9.6 pW
Headphone Sennheiser Model HD330 0.78 pW
2x 2000 Ohm headphone Telefunken EH333 0.022pW
2x 2000 Ohm headphone Omega 0.033 pW
Cristal earplug "Taiwan" 0.13 pW
Driver unit Adastra Model: 952-207 0.0078 pW

The crystal earplug is in crystal receivers often used with a resistor of 100 k.Ohm parallel, in the calculation I used a impedance of 100 k.Ohm for the earplug.
The crystal earplug was tested with a 1800 Hz tone because at that frequency it was most sensitive.

Both 2x2000 ohm headphones have maximal sensitivity at about 1 kHz, at other frequency's the sensitivity is lower.
With these headphones the impedance at 1 kHz is much higher than 2x2000 Ohm.
In the calculation of the inputpower, we must use this impedance at 1 kHz.

The driver unit from Adastra is the most sensitive speaker tested.
More information about driver unit's you will find here.

For listening to a normal audio signal, we need more power, with the driver unit I needed a input power of 0.3 pW to recognize speech or music.

Efficiency measurement on a loudspeaker

In this measurement I try to measure the efficiency of driver unit 952-207.
To do this, I used two of these driver unit's, one on top of the other, with the sound openings towards each other.
Speaker 1 is connected to a signal generator.
The sound of speaker 1 (SP1) goes to speaker 2 (SP2) wich works as a microphone, across resistor R we can now measure a voltage.

The efficiency of this setup is the power in resistor R divided by the input power of SP1. 


The impedance of the driver unit's is 16 Ohm, load resistor R is also 16 Ohm.

SP1 is connected to 1 kHz 100mVpp (=35mVeff.).
I measured the input impedance of SP1, this was 22 Ohm, the input power of SP1 is 56.8 W.

The voltage across resistor R is 30mVpp (=10.6mVeff), the power in resistor R is 7.03 W.

The efficiency is 7.03W / 56.8W = 0.123 or a loss of 9 dB.

Because there are two speakers in this test, I assume both are responsable for 4.5 dB loss in the conversion electric to acoustic and back.

The efficiency is quite varying with different frequencies, at 1 kHz there was a peak.
At other frequencies the efficiency is lower.
This can be caused by resonances in the speakers, because this situation with two speakers connected is quite different from the normal use.

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