transducer and locating signal there. Once signal is found, move gradually further away from
transducer (aligning hydrophone as necessary on the way) until desired field position is
achieved.
▪Noise. Where there is poor signal to noise ratio (SNR), it can sometimes be difficult to locate
signal.
oIf you believe hydrophone position is correct, use time domain averaging to improve
SNR. If signal is buried in noise, it should reveal itself.
oIf averaging is not available on oscilloscope, try moving hydrophone slightly backwards
and forwards along the acoustic axis of the transducer. This should cause the signal to
move on the screen of the DAQ and the human eye can sometimes recognise a moving
pattern easier than a stationary one.
CONVERTING HYDROPHONE VOLTAGE TO ULTRASONIC PRESSURE
A hydrophone produces an electrical output signal in response to the surface integral of the acoustic
pressure received over its active element. There may not be a linear relationship between the
acoustic pressure stimulus and the voltage response. In fact, the relationship is almost always
frequency dependent and a conversion process will be required to obtain the acoustic pressure
signal from the hydrophone output voltage. A detailed description of this process can be found within
references [1] [2] and [3]. When the source of ultrasound is only operating at one frequency (e.g.
continuous wave) or with very narrow bandwidth (e.g. sinusoidal tone burst) the relevant IEC standard
[4] states that the pressure signal, p(t), can be calculated from the measured hydrophone voltage,
v(t), according to the relationship
where M(fawf)= sensitivity of a hydrophone at the acoustic working frequency of the source. More
generally however, and especially if the acoustic signal is broadband, the pressure signal should be
calculated according to the relationship
where and are the Fourier and inverse Fourier transforms and M(f) = sensitivity of a hydrophone
FINDING THE BEAM MAXIMUM
Many acoustic measurements require the maximum value of the acoustic pressure field to be located.
The following two steps are required to maximise the hydrophone signal.
•Correct for directional response of hydrophone –consider the hydrophone active element as
lying in the X-Y plane (Z direction coming normally out of active element). There will be a
directivity pattern associated with both the X and the Y axes (although hopefully the patterns
are the same). First rotate about the X-axis to obtain maximum signal, and then repeat around
the Y Axis.
