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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
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
|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 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
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
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 µW.
The voltage across resistor R is 30mVpp (=10.6mVeff), the power in resistor R is 7.03 µW.
The efficiency is 7.03µW / 56.8µW = 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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