Ultrasonic Modulation

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Ultrasonic signals, modulation

General

Unlike signal transmission methods, the modulation of ultrasonic signals is passive in nature. It is caused by the acoustic excitation of material inhomogeneities (defects), which can then act as secondary sound generators. In addition, a suitable excitation signal can be used to amplify the effect of material-specific defects on the measurement signal.

Principle

Since the dawn of communications technology, signal modulation has been a necessary process for achieving signal transfer with as little loss as possible. It consists of changing a (carrier) signal in terms of amplitude, frequency, and/or phase by means of the information-carrying signal to be transported.

Due to the inherently lossy transmission of sound or electromagnetic waves through a medium, the signal is attenuated as it travels through the transmission medium and thus loses energy (electrical energy, sound energy). The initial signal amplitude A₀ decreases according to Eq. (1),

$A=A_{0}\cdot e^{i\omega t}\cdot e^{-\alpha t}$

(1)

where the factor e^–αt represents the attenuation factor and α represents the sound attenuation coefficient (Fig. 1).

Fig. 1:

Schematic representation of a damped oscillation with decay function

If this signal is superimposed with another (information) signal, usually a low-frequency one, the information is preserved over the transfer distance. The higher-frequency carrier signal is attenuated more strongly (δ ~ ω²). This means that the length of the transfer distance essentially depends only on the amplification of the carrier signal.

There are three different types of modulation for electrical signals and ultrasonic signals: amplitude modulation, frequency modulation, and phase modulation, with the latter two playing a greater role in ultrasound testing. In principle, modulation with digital signal structures is also possible, which allows for easier detection in the echo signals. Fig. 2 shows an example of a high-frequency carrier signal modulated with a low-frequency signal.

Fig. 2:

Carrier signal modulated with a transmission signal (sine wave)

Signal modulation in ultrasound testing modulation

In materials testing, ultrasonic signals are passively modulated by inhomogeneities in the material, which alter the sound wave [1]. The need to evaluate modulated ultrasonic signals arises particularly in ultrasonic testing of plastics, which have a high sound attenuation coefficient and are very heterogeneous due to reinforcing fibres. Figure 3 below illustrates the principle of ultrasonic modulation using a practical example. The HF-scan was recorded using transmission technology. Only response signals 1–3 are shown, labelled as signals 1 to 3 in Fig. 3.

Fog. 3:

Example of an amplitude-modulated ultrasonic signal through a defect in a plastic plate

Fig. 4:

Signal response from an amplitude-modulated excited glass sample (a) and the corresponding modulation spectrum (b) [2]

In addition to its sound-absorbing and sound-scattering properties, the material under investigation influences the signal structure by exciting material defects. These are reflected in the response signal primarily through modulation of frequency and phase. Therefore, various numerical evaluation algorithms – such as undersampling of a modulated signal or Hilbert transformation – are used to demodulate the response signal and better characterise the material state with ultrasound.

Figure 4a shows the frequency spectrum of a glass sample with and without monofrequency excitation by a second, low-frequency ultrasonic sensor. The frequency spectrum in Fig. 4b corresponds to the demodulated signal from Fig. 4a.

See also

• Imaging ultrasonic testing
• Non-destrictive testing (NDT)
• Non-destrucrive polymer testing
• Ultrasonic direct coupling

References

[1]	Berktay, H. O.: Possible Exploitation of Non-Linear Acoustics in Underwater Transmitting Applications. J. of Sound and Vibration 2 (4), (1965) 435–461
[2]	Moussatov, A., Castagnede, B.: Ultrasonic Defectoscopy of Damaged Materials by Modulation Transfer Method: Nonlinear Pump-Probe Interaction. WCU 2003, Paris, September, 7–10 (2003)