Measurement of the Insertion Loss

The insertion loss characterize the sensitivity of a transducer, and is given by :

In order to measure the insertion loss, an ultrasonic burst is emitted by a transducer. The same transducer receives an echo issued from a plane perpendicular target, which acoustic impedance differs very much from the liquid used as a transmitting media.

The voltage ratio between the echo and the emitted burst, expressed in logarithm, is called Insertion Loss (IL).

Signal Processing's transducers have an IL between 10 and 20 dB, depending on the type of transducer.

To measure the Insertion Loss of your transducer, you need the following equipment :

- 1 burst generator
- 1 oscilloscope
- 2 coaxial cable (50 W) + 1 T
- 1 water container, with a transducer holder
- 1 target (stainless steel preferred)

Set up

Burst generator :

Fburst = Ftransducer
Amp = 10 Vpp / 50 W
Nb burst = 4
Space between burst = 10000 Cycles

Connect the transducer, the burst generator and the oscilloscope as shown in the figure below.
You can trig the Echo signal by using the TTL Output signal coming from the burst generator (if available).

Warning

To prevent any damage to the burst generator, we recommend to check the specifications of the instrument in order to verify that it can support the connection of a very low impedance (<50 ohm) during a long time.

Measuring procedure

1. Be sure that the contact between the front surface of the transducer and the water is good (no air bubbles).
2. Be sure that the transducer is perpendicular to the target (max echo voltage).
3. Measure the voltage of the emitted burst.
4. Measure the voltage of the echo.
5. Compute the Insertion Loss.

Important notices

The distance between the transducer and the target must be enough in order to avoid the disturbances generated by the ringings, but not too long to be sure to collect all the emitted energy (divergence of the ultrasonic beam).

The thickness of the target must also be considered.
To avoid any interference generated by second echo, coming from the far wall of the target, a minimum thickness is required.
As the amount of the energy that penetrates inside the target depends on the difference in acoustic impedance between the target and the transmitting media. It is recommended to use a target with an acoustic impedance as high as possible.

The following table gives recommended values for the thickness of the target (Tk) and the distance between the transducer and the target (D) for different transducers.

For D : 4 cycle burst, time between the emitted burst and the received echo of 40 ms, and a sound velocity of 1500 m/s (water).