**Loudspeakertest**

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 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.