Air-Coupled Ultrasonic Transducers

Summary:

We have been working on air-coupled ultrasonic transducers for more than 20 years. The main result is a robust, efficient and flexible technology able to produce different transducers in terms of centre frequency, sensitivity, bandwidth and field geometry that can be adapted to different applciations that include NDT, materials characterization, surface metrology, etc.

Based on the impedance and frequency detuning optimization method developed in our group (see “Air-coupled ultrasonic transducers” in Ultrasound in food processing . ISBN 978-1-118-96418-7, Wiley-Blackwell”) and the use of extremely low impedance and low loss materials specially developed for this aplication, our transducers present unrivalled sensitivity and bandwidth response.

They are intended to be used with any electronic equipment for both transmission and reception.

The fabrication technology is flexible enough so that we can adapt the transducers to optimize sensitivity or bandwidth or both. We can produce piston-like transducers or focused transducers and adapt them to different type of excitations from very high voltage narroband tone bursts to wide band chirps and sipkes. We can also operate in through transmission or in pulse-echo. In addition, we can also adapt to different environmental conditions to produce moisture and or pressure resistant transducers. In some cases we can produce fully water proof air-coupled transducers.

So far we have used them in many different applications in the fields of NDT, materials characterization, surface metrology, etc. We are open to collaborations, in this field: in testing our transducers in new applications or in testing them with different equipments, etc.

Index:

I. Air-coupled monolithic piston-like piezoelectric transducers.

I.1. Wide-band transducers.

I.2. Narrow-band pithc-catch transducers.

I.3. NEW Narrow-band transmitters for high voltage excitation.

I.4. Water resistant transducers.

II. Air-coupled monolithic piezoelectric spherically focused transducers.

II.1. Conventional focused transducers.

II.2. NEW! Series sph75: focal distance 65 mm, frequencies: 250, 320 and 400 kHz

II.3. NEW! Series sph60: focal distance 55 mm. Frequencies 320 kHz

II.4. NEW! Series sph75WB: Large bandwidth + moderate sensitivity: Focal distance 65 mm, frequencies: 250 kHz, 400 kHz.

III. NEW! Cylindrically focused transducers

IV. Air-coupled phased array piezoelectric transducers.

IV.1 Linear array

IV.2 NEW! Sectorized array

IV.3. NEW! Cylindrically focused linear array

V. Air-coupled ferroelectret transducers.

V.1. Conventional transducers (350 kHz)

V.2 Miniaturized (9 mm diam) transducers (350 kHz)

VI. Mixed transducers: piezoelectric + ferroelectret

VII. Special configurations.

VII.1. Embedded pair of wideband transducers and sample holder for RUS of plate/layered materials.

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I. Air-coupled monolithic piezoelectric transducers.

We produce the wideband and air-coupled piezoelectric transducers that we requiere for our investigations in the fields of characterization of porous materials, NDT (CFRP, sandwich structures, etc.) and the study of vegetal tissues and food stuff, where the use of conventional ultrasonic coupling fluids is not permitted because the fluid can either contaminate or modify (or both) the properties of the material under investigation.

A main feature of our transducers is that they present a relatively large bandwidth (30-100%), while they also present an unrivalled sensitivity that ranges from -24 dB at 0.20 MHz to -40 dB at 2.00 MHz for wide badn transducers and as low as -19 dB for 250 kHz narrowband transducers.

This unique technology enable us to study wave dispersion in highly dispersive materials (foams, scaffolds, etc), to test highly attenuating materials (uncured CFRP, sandwich structures with foam or honeycomb cores) or high impedance materials (aluminium, steel, etc.) or biological samples were use of fluids is not possible: plant leaves, food stuff, etc.

Figure below shows the schemmatic design of the monolithic transducers for the frequency range 0.20-2.00 MHz

       

Figures below show the characterization of the transducers in through transmission operation mode.

I.1.WIDEBAND TRANSDUCERS

I.1.a. 200 kHz tansducers

 

I.1.b. 250 kHz transducers

  

I.1.c. 400 kHz transducers

     

I.1. d. 500 kHz transducers

   

I.1.e. 650 kHz transducers

  

I.1.f. 750 kHz transducers

    

 I.2. NARROWBAND TRANSDUCERS

I.2.a. 250 kHz transducers

I.3. WATER RESISTANT WIDEBAND AIR-COUPLED TRANSDUCERS

Unlike conventional air-coupled transducers, in this case, radiatting surface is fabricated of a completely hydrophobic material, so the transducer is water resistant: it is not affected by contact with water and does not absorb moisture.

I.3.a. 500 kHz transducers

II. Spherically focused transducers.

II.1. Conventional focused transducers

When high sensitivity is not enough and high spatial resolution is also needed (e.g. NDT applications) spherically focused transducers offer the possibility to get both.

We have developed a technology that permits to keep the sensitivity at figures similar to that shown before, and even improving them when operating in tone burst mode, while achieving a 6 dB focal spot under 5 mm.

Centre frequency         Focal distance        Focal spot (6 dB)        Sensitivity          Excitation

250 kHz                            70 mm                    5 mm                       -18 dB               tone burst (15 cycles)

400 kHz                             70 mm                   4 mm                        -18 dB              tone busrt (15 cycles)

Acoustic field of the 250 kHz focused transducer,

Transducer surface is at 22 mm at the left of the x-axis origin

II.2. NEW SERIES: sph75 and sph60 series.

The new series of reduced size transducers present a better sensitivity and a reduced size that increases transducer compatibility with different inspection machines.

They present a remarkably high sensitivity (below -25 dB). Two focal distances are available: series sph75 with 65 mm focal distance and sph65 with focal distance of 55 mm

II.3. Series sph75. NEW

This series corresppond to spherically focused transducers, with focal distance at 65 mm.  Focal spot diameter ranges between 3.0 and 4.5 mm, centre frequency between 250kHz and 400 kHz and different sensitivity and bandwidth configurations can be selected, ranging from -17 dB to -25 dB for the sensitivity, and relative -20dB bandwidth from 30 to 95%.

These transducers are designed for the most demanding air-coupled NDT problems, like the inspection of curved and asimetrical sandwich components with CFRP skins and thick honeycom core.

II.3.a. 250 kHz transducers.

This pair of transducers is intended for extremelly demanding NDT applications. They are designed to simultaneously optimize sensitivity, bandwidth and spatial resulution.

The main properties are:

6 dB focal spot: 4.0 mm; Focal distance: 65 mm

Sensitivity: -17 dB; -20dB sensitivity bandwidth: 33%

Electrical impedance: Tx: 450 ohm; Rx: 5 Kohm

Next figure shows the Tx-Rx response. Tx connected to Olympus 5077 P/R, excitation amplitude 100 V (excitation signal provided by the 5077 is a semicycle of square wave). RX connected directly to a scope (no gain). Tx-Rx separation 130 mm (confocal configuration). Sensitivity is defined as the ratio of the modulus of the FFT of the voltage signal at RX terminalt to that of the voltage signal applied to the terminals of the TX. Therefore, actual sensitivity is better than this figure as this includes the loss in the air gap between Tx and Rx

II.3.b. 320 kHz transducers.

This pair of transducers is intended for extremelly demanding NDT applications. Compared with the previous pair they do provide a larger frequency, larger bandwidth and better spatial resolution. They are designed to simultaneously optimize sensitivity, bandwidth and spatial resulution, with emphasis on the bandwidth. The main properties are:

6 dB focal spot: 3.5 mm; Focal distance: 65 mm

Sensitivity: -25 dB; -20dB sensitivity bandwidth: 95%

Electrical impedance: Tx: 250 ohm; Rx: 5 Kohm

Next figure shows the Tx-Rx response. Tx connected to Olympus 5077 P/R, excitation amplitude 100 V (excitation signal provided by the 5077 is a semicycle of square wave). RX connected directly to a scope (no gain). Tx-Rx separation 130 mm (confocal configuration). Sensitivity is defined as the ratio of the modulus of the FFT of the voltage signal at RX terminalt to that of the voltage signal applied to the terminals of the TX. Therefore, actual sensitivity is better than this figure as this includes the loss in the air gap between Tx and Rx

II.3.c. 400 kHz transducers.

This pair of transducers is intended for extremelly demanding NDT applications. Compared with the previous pair they do provide a larger frequency, narrower bandwidth and better spatial resolution and relative sensitivity. They are designed to simultaneously optimize sensitivity, bandwidth and spatial resulution, with emphasis on the sensitivity. The main properties are:

6 dB focal spot: 3.0 mm; Focal distance: 65 mm

Sensitivity:  -23dB sensitivity bandwidth: 30%

Electrical impedance: Tx: 150 ohm; Rx: 5 Kohm

Next figure shows the Tx-Rx response. Tx connected to Olympus 5077 P/R, excitation amplitude 100 V (excitation signal provided by the 5077 is a semicycle of square wave). RX connected directly to a scope (no gain). Tx-Rx separation 130 mm (confocal configuration). Sensitivity is defined as the ratio of the modulus of the FFT of the voltage signal at RX terminalt to that of the voltage signal applied to the terminals of the TX. Therefore, actual sensitivity is betterh than this figure as this includes the loss in the air gap between Tx and Rx

This series has been succesfully used for one of the most demanding NDT applications of air-coupled ultrasound: fast inspection of large non-planar and asimmetric thick sandwich components with CFRP skins and thick honeycomb cores.

 

II.4. Series sph60. NEW

This is a similar series, but it presents a shorter focal distance, that can be useful in some applications: 55 mm

II.4.1a. 320 kHz transducer:

This pair of transducers is intended for extremelly demanding NDT applications. They are designed to simultaneously optimize sensitivity, bandwidth and spatial resulution, with emphasis on the sensitivity. In addition, sensitivity can be improved at the cost of reducing bandwidth just by adding some electrical tuning network at the receiver. The main properties are:

6 dB focal spot: 3.0 mm; Focal distance: 55 mm

Sensitivity:  -13-19dB sensitivity bandwidth: 17-31%

Electrical impedance: Tx: 150 ohm; Rx: 7 Kohm

Next figure shows the Tx-Rx response. Tx connected to Olympus 5077 P/R, excitation amplitude 100 V (excitation signal provided by the 5077 is a semicycle of square wave). Tx-Rx separation 110 mm (confocal configuration). Sensitivity is defined as the ratio of the modulus of the FFT of the voltage signal at RX terminalt to that of the voltage signal applied to the terminals of the TX. Therefore, actual sensitivity is smaller than this figure as this includes the loss in the air gap between Tx and Rx

RX can be connected directly to a scope (no gain):

Alternatively, Rx can be connected to the scope through an electrical tuning network so that sensitivity is improved at the cost of bandwidth. In this case, obtained response is:

III. NEW! Cylindrically focused transducers.

250 kHz transducer. Focal distance 65 mm

IV. Air-coupled phased array piezoelectric transducers.

16 elements, 0.75 MHz array

V. Air-coupled ferroelectret transducers.

Conventional transducers (300 kHz)

These transducers are based on a ferroelectret film. Resonant frequency is 300 kHz. Compared with piezocomposite transducers they present a similar SNR though a more reduced sensitivity. An advantage is that smaller sizes can be achieved.

Miniaturized (9 mm diam) transducers (300 kHz)

Picture of the miniaturized 300 kHz ferroelectret transducers

Acoustic field of the miniaturized 300 kHz ferroelectret transducer. Receiver: flat transducer with a 0.7 mm pinhole aperture.

VI. Mixed transducers: piezoelectric + ferroelectret

 

VII. Special configurations.

VII.1. Embedded pair of wideband transducers and sample holder for RUS of plate/layered materials.

A pair of wideband tansducers (centre frequency of 0.25, 0.65 or 1.25 MHz, but a reduced size version)  are mounted on a holder specially designed to perform through transmission measurements and for the characterization of solid plates by measuring the transmission coefficient at normal incidence. This holder allows to have the right transducers location, the right separation between them and also provides a slot for an easy location of the sample to be measured (see Sensors 2013, 13(5), 5996-6013; doi:10.3390/s130505996)

Left: Picture of the Tx-Rx pair (=.65 MHz centre freq) embedded in the sample holder, Right: Picture of the same device as used to monitor variation in the thickness resonances of Vitis vinifera leaves , see Ultrasoun in Med. & Biol. 2014. http://dx.doi.org/10.1016/j.ultrasmedbio.2014.04.004