Comparing Acoustic Measurements of Underwater Materials in Pressure Tanks Using a Calibration Panel Scott E Kasprzak 1 Naval Surface Warfare Center, Carderock Division 9500 MacArthur Blvd Bethesda, MD 20817 USA Matthew A Craun 2 Naval Surface Warfare Center, Carderock Division 9500 MacArthur Blvd Bethesda, MD 20817 USA Stephen P Robinson 3 National Physical Laboratory Hampton Rd Teddington TW11 0LW, UK

ABSTRACT Researchers in the United Kingdom and the United States worked together to compare acoustic panel measurement methods and equipment using a calibration test panel in two temperature- and pressure-controlled tank facilities. Underwater testing can be used to calibrate various transducers or to measure acoustic properties of materials. The calibration panel used in this study consisted of a urethane material. Tests were initially conducted in the Acoustic Pressure Vessel (APV) at the National Physical Laboratory (NPL) in Teddington, UK and were subsequently performed in the Acoustic Pressure Tank Facility (APTF) at the Naval Undersea Warfare Center (NUWC) in Newport, Rhode Island, USA. The UK tank is a smaller-scale version of the US tank. However, the measurement approaches developed by the two groups of researchers differs. The UK utilizes a parametric source with a baffle and a planar hydrophone array for reflection and transmission measurements. The US uses a spherical source, a linear pseudo-array along with coherent subtraction for reflection, and a single hydrophone for transmission measurements. Results show that the measurement methods generally agreed on the panel’s acoustic response, but differences varied based on environmental conditions and as a function of frequency. Potential sources of error were enumerated and are discussed.

1. INTRODUCTION

The acoustic behavior of materials in water are of interest in numerous maritime applications such as sonar, noise control, and underwater communications. Underwater acoustic measurements of materials can be used to extract standard material properties [1] at various temperatures and pressures, as well as the dilatational wave speed [2] and the influence of the backing substrate [3]. But rather

1 scott.e.kasprzak.civ@us.navy.mil 2 matthew.a.craun.civ@us.navy.mil 3 stephen.robinson@npl.co.uk

than study the material properties of any material of interest, the purpose of this investigation was to compare and contrast the measurement equipment and methods developed separately by US and UK investigators. A calibration/reference panel was fabricated by the UK and tested at both facilities. The simplest method of capturing the energy incident upon, reflected from, and transmitted by the test panel is via a single hydrophone placed on either side of the test object [4]. The measurement quantity being investigated during this set of experiments was the reflection loss for the material. The reflection loss, RL, is defined as:

𝒑 𝒓𝒆𝒇𝒍𝒆𝒄𝒕𝒆𝒅

𝒑 𝒊𝒏𝒄𝒊𝒅𝒆𝒏𝒕 ) , (1)

𝑹𝑳= −𝟐𝟎∗𝐥𝐨𝐠 𝟏𝟎 (

where p is the reflected or incident pressure, as measured at the same source-side reflection hydrophone. The measurement of the reflection loss is particularly challenging in pressure tanks and thus was the focus of the experimental comparisons. The high sound speeds in the medium and the comparatively small dimensions of the tank limit the separation time between desired direct arrival and undesired wall reflections at the measurement point. Additional complications exist due to edge scattering from the panel itself and the rigging that supports the panel. The need to separate the incident and reflected pressures at the hydrophone(s), combined with signal contamination sources and arrival times, creates an experimental challenge especially at low kilohertz frequencies. It is, therefore, necessary to subtract from the reflection time history an incident signal recorded at the same point and under the same conditions without the test panel present [5]. Coherent subtraction, judicious timegating, proper window selection, and advanced signal processing are used to minimize the impacts of the undesired factors on the measurement. Of particular interest to the researchers at both locations was the level of the panel’s response as well as the frequency and amplitude of any peaks or valleys.

The results of this paper focus on the reflection characteristics of the panel, but the techniques can be extended to transmission behavior. The transmission loss, TL, is defined as:

𝒑 𝒕𝒓𝒂𝒏𝒔𝒎𝒊𝒕𝒕𝒆𝒅

𝒑 𝒊𝒏𝒄𝒊𝒅𝒆𝒏𝒕 ) , (2)

𝑻𝑳= −𝟐𝟎∗𝐥𝐨𝐠 𝟏𝟎 (

where 𝑝 is the transmitted or incident pressure, as measured at the behind-panel transmission hydrophone. Typically the transmission hydrophone is placed as close behind the panel as practicable to minimize the impact of diffracted waves on the measurement. When measuring the transmission loss, it is not possible to capture the incident signal at the measurement point behind the test panel for reference, and for this reason a separate measurement must be made to capture the incident signal without the test panel present.

2. TEST ARTICLE AND MEASUREMENT CONDITIONS

Measurements were conducted on a test panel designed to have suitable performance in the frequency range 1 kHz to 15 kHz. The panel was 800 mm wide, 900 mm high, and 125 mm thick. The test panel was designed and manufactured by QinetiQ, Ltd. and consisted of a two-layer acoustic absorber fabricated from polyurethane, to which Expancel spheres had been added as a filler material. The panel is shown in Figure 1.

Figure 1: Test panel used for measurement comparisons.

Conditions of measurement:

 Frequency range: 1 kHz – 15 kHz  Pressures: 0.1 MPa, 0.7 MPa, 1.4 MPa, and 2.8 MPa,  Temperatures: 8 °C & 20 °C

3. TESTING FACILITIES AND EQUIPMENT

3.1. United States

The Underwater Sound Reference Division (USRD) was established in the early 1940s under the Office of Scientific Research and Development (OSRD), created in response to the need for sonar development in World War II [6]. Following the war, the USRD consolidated testing facilities in Orlando, Florida in 1948 to allow for open-water testing year-round. In 1997, as a result of base realignment and closing (BRAC), the USRD became a part of the Naval Undersea Warfare Center (NUWC) in Newport, Rhode Island, and some facilities and equipment were moved or sold - in particular, the large and small acoustic pressure testing vessels. The smaller vessel was sold to the United Kingdom and is in use at the National Physical Laboratory (NPL) in Teddington, UK; the larger vessel was barged to the NUWC facility in Newport. Today, the USRD continues to perform the equivalent to the National Institute of Standards and Technology (NIST) in underwater acoustics with traceability to NIST electrical voltage and current standards. The Acoustic Pressure Tank Facility (APTF) [7] provides acoustic testing over a wide range of simulated ocean environments. The facility can provide test temperatures from 2 degrees Celsius to 35 degrees Celsius and hydrostatic pressures up to 18.6 megapascals (2700 pounds per square inch), corresponding to depths up to 1860 meters.

The APTF consists of a large, water-filled tank equipped with a three-carriage positioning system, two independent rotators, and a fully automated data acquisition system. The facility can provide measurements of the acoustic characteristics of sonar system components and other underwater acoustic devices and materials, including: transducers, hydrophones, baffles, and windows. The tank itself (Figure 2) is a thermally insulated, vibration isolated, closed steel tank 3.81 meters in diameter and 11.1 meters in length with two access ports. The larger port, used for hydrophone and panel loading, as well as panel rigging, is 1.85 meters in diameter. An underwater rotator located below the large port allows devices and/or material samples of up to 2700 kilograms to be rigged and supports the panel during testing. The smaller port used for loading the acoustic source is 0.84 meters

in diameter and one of the three carriages held the source during testing, roughly 2 m from the face of the test panel as seen in Figure 2. The second carriage was not used, while the third carriage was used to traverse a quad of hydrophones during testing. The tank is lined on the small-lid end with sound absorbing wedges to reduce reverberation and allow the use of high pulse repetition rates.

Figure 2. Experimental setup and schematic of Acoustic Pressure Tank in Newport, Rhode Island (experiment components spaced for clarity and not to scale)

An 8-channel Transducer and Hydrophone Acoustic Measurement and Evaluation System (THAMES) was used for testing and data acquisition (DAQ) at 200 kHz sample rate. THAMES integrates control and measurement software with Hewlett Packard (HP) VXI data-gathering modules and all required supporting instruments and controls. The system generated digital timeseries records including sufficient time to capture the waveform voltage and to observe the panel and tank response. Testing was performed using a Type F56 acoustic source and five Type H52 hydrophones – one for transmission measurement and the other four for reflection measurement. 3.2. United Kingdom

The basic experimental set up for the work undertaken at NPL has been described in previous scientific publications [4], [8]. The NPL Acoustic Pressure Vessel (APV) consists of a cylindrical tank of external dimensions 7.6 m long by 2.5 m in diameter [4], [9], [10]. The tank may be pressurized to simulate increased water depth up to a maximum hydrostatic pressure of 68 bar. The facility also allows the water temperature to be controlled in the range from 2 °C to 35 °C. There are two access ports, the centres of which are 2.4 m apart. A diagram of the arrangement inside the vessel is shown in Figure 3.

A mounting arrangement enables the receiving hydrophones to be positioned at any preferred distance from the panel face in the range from 0 m to 0.4 m. Typically, when using a single hydrophone receiver, the hydrophone is mounted either in front or behind the test panel (for reflection or transmission, respectively) with one side of the panel measured at a time. The parametric array consists of the 300 kHz piezoelectric source transducer placed at the end of the vessel, with array truncation provided by the acoustic filter (a 35 mm thick Expancel-filled polyurethane sheet). The separation between the source transducer and the panel under test is 2.75 m and the acoustic filter is normally 1.88 m from the transducer [4].

Figure 3: Schematic diagram of the measurement configuration in the APV, showing location of transducer, acoustic filter, test panel and receive array. 4. TEST METHODS

4.1. United States

The US testing method was designed to isolate the incident, reflected, and transmitted signals by using FM pulse compression, combined with a synthetic endfire array and coherent subtraction of measurements with and without the test panel installed. The test method ensonified the panel with multiple three-second full-band FM chirps, which were amplitude-corrected to account for the transmit voltage response (TVR) of the source. Measurements were first taken without the panel present to establish background noise levels and receive levels at the hydrophones. These measurements also established time-of-arrival of tank and fixture reflections. These data were stored for post-processing by subtracting them from equivalent measurements with the panel installed, following methods similar to [5], to minimize the effects of these reflections as well as overlap between incident and reflected signals.

Hydrophones were installed as shown in Figure 2. To measure the acoustic reflections, four hydrophones were installed on the trolley system as shown. These included two hydrophones on the panel centerline and two off-centered, in near and far standoffs. By repeating measurements with the trolley repositioned at incremental distances from the panel, a synthetic line array was formed, which had an end-fire response centered on the panel. This further aided in rejecting tank and fixture reflections as well as potential edge diffraction from the panel. For acoustic transmission, a single hydrophone was placed on centerline behind the panel. A synthetic array such as done for reflections could also have been implemented for transmission, however it was determined that the acoustic shadow of the panel, as well as the short time of arrival of this signal, resulted in minimal interfering reflections at this location.

souce Small Port ta Pon Heesoreneee Resigg Region ‘Transducer Interaction Acoustic Test Panel Hydrophone Region Filter Array

Figure 4. APTF testing hydrophone layout

All the hydrophones were mounted at the same vertical height, which aligns with the source and panel center along the centerline of the tank. Though the hydrophones are omni-directional, convention held that the reference point on each hydrophone was roughly aligned to face the panel. The reflection hydrophones were mounted on a rectangular baseplate with rods to maintain the spacing during the experiment. The distances between each hydrophone were measured and recorded. Once the hydrophone fixture was inserted into the tank on the third carriage, the carriage was positioned so that reference length gages barely touch the front panel surface as optically observed via underwater camera monitoring. The relative position at which each gage touched the surface was used to better indicate any additional yawing / twisting of the hydrophone rig within the carriage. Absolute positions of the hydrophones relative to the panel were made using classic multidimensional scaling techniques based on intra-hydrophone spacing and rig yaw observed by touch points of the temporary gages. From there, all additional positions were estimated using the unidirectional motion increments of the third carriage.

All waveforms were transmitted and collected with the reflection hydrophones in the nearest position relative to the panel face at all pressures. Subsequently, the trolley system was used to move the hydrophones approximately 4cm further from the panel face, and all waveforms repeated at all pressures. This continued for a total of ten hydrophone trolley positions, with the furthest position resulting in Hydrophones 4 and 5 being approximately 76 cm from the panel face, as shown in Figure 4.

The post-processing for the reflection phone data followed these steps: 1) Perform pulse compression via cross-correlation of the responses with the FM chirp waveform. 2) Measured signals from the no-panel case were subtracted from the panel measurement for each

hydrophone location 3) Using the positional data for the hydrophones, the signals were range-corrected using spherical

spreading from the monopole source 4) The data was windowed to remove incident (direct path) signal from the source and returns

from the rigging and tank walls 5) The timeseries data for reflected signal were time aligned based on zero-crossings 6) The data were averaged – arithmetic mean 7) The result was fast Fourier transformed (FFT) into frequency space 8) Reflection loss, RL, was calculated at each frequency via Equation 1

4.2. United Kingdom

The experimental method developed and utilized by UK researchers is well-documented in previous publications [4], [8], [11], [12], and relevant details for comparison to US facilities and methods are provided below. A parametric array is used as the acoustic source for the work described here [13]. Such a source uses the non-linear propagation of primary wavefields to generate additional lower frequency (secondary) components that are then used to insonify the test panel. Such a source has the advantage of producing a more directional sound field than would be possible from a linear source of similar size. In the NPL arrangement, the primary transducer is driven with a short pulse of 300 kHz carrier frequency, with a raised cosine bell envelope. The low-frequency secondary waveform generated on axis can be shown to be proportional to the second derivative, with respect to time, of the square of the transmitted pulse envelope [13]. The generated waveform shape and spectrum is then easily modified by altering the envelope function. In practice, for parametric array measurements in confined spaces, it is necessary to limit the length of the interaction region of the primary beams, which is achieved by placing a panel of absorbing material, known as the acoustic filter, across the field at some distance from the source transducer to absorb the high-frequency primary beams and transmit only the low-frequency beam [13].

The acoustic pressure wave generated by the truncated parametric array can be readily acquired and time resolved by a single hydrophone under free-field conditions. However, as noted in the introduction, measurements in the APV are not true free-field measurements, particularly when a test panel is present; the benefit obtained by additional directivity from the parametric array source is substantially reduced at frequencies below 10 kHz. Since the signals of interest are plane waves travelling in directions orthogonal to the panel surface, and the contaminating signals are predominately travelling in directions across the panel at significant angles (from panel edges or vessel walls), it is possible to utilise a directional planar array to select the plane wave components of interest [8], [11], [12].

The planar array used is constructed from eight Neptune Sonar T293 spherical hydrophones of approximately 20 mm diameter, which are mounted using a mount which is acoustically transparent at frequencies below 10 kHz (a thin 0.6 mm diameter cord and a 550 mm square aluminium frame). Figure 5 shows the experimental arrangement. The configuration is a pseudo-random, 2D arrangement covering an area of approximately 0.4 m 2 .

Figure 5. Hydrophone array consisting of eight hydrophones in the measurement position for the test panel.

Measurements are performed by capturing the hydrophone signals using simultaneous capture on an etec B2008 eight channel charge amplifier and a National Instruments multifunction DAQ using bespoke LabView software. In the analysis, an average is initially calculated across the eight channels and the resulting waveforms averaged over a number of pulses. Because the reflected signal has travelled further than the incident signal (twice the separation between the measuring hydrophone and the test panel), it is necessary to correct the reflection loss for the reduction in signal amplitude due to the extra distance travelled [4]. Spectral analysis is then performed on the time histories, from which the reflection and transmission losses can be calculated [4] from the incident, reflected, and transmitted pressure waves by Equations 1 and 2.

5. RESULTS

The results of testing the panel at both facilities are shown in Figure 6 and Figure 7 for test temperatures of 8°C and 20°C, respectively. For the 8°C testing, there is excellent agreement between the two facilities for frequencies between 1-13 kHz and all pressures except 0.7 MPa. Of particular note is that the peak locations and heights agree well between the two measurements; the 0.7 MPa measurement agrees in peak location if not height. There is a peak detected by the US at 1.4 MPa and 12.8 kHz that was not captured in the UK tank. Above 13 kHz, the chirp used in the US method has diminishing energy and so SNR has dropped considerably.

The measurements at 20°C are not as well aligned as those at 8°C, but they are nonetheless in quite good agreement. The peak heights and locations match well between the two measurements. Again, the midpressure of 0.7 MPa shows the largest difference between the two. At the highest pressure, the US measurement indicates a much flatter panel response than that from the UK measurement.

Figure 6. Reflection Loss measurements of test panel at US and UK acoustic pressure tanks, 8°C

Figure 7. Reflection Loss measurements of test panel at US and UK acoustic pressure tanks, 20°C

6. CONCLUSIONS

6.1. Error Sources

The measurement of acoustic properties of a material in water as a function of frequency, temperature, and hydrostatic pressure involves numerous potential sources of variability that could contribute to discrepancies between two independent measurements. Possible mechanisms for discrepancies can be divided into those associated with the panel conditions, the test environmental conditions, and factors associated with the measurements and instrumentation.

Elastomeric materials can exhibit time-varying acoustic properties associated with phenomenon such as creep, hysteresis, aging, and water uptake (molecular absorption). The pressure, temperature, and water exposure history of a rubber-like material (both long and short term) can result in variability in acoustic properties. In particular, for these types of measurements, it was important that the pressure cycling regimen and hold times be as similar as possible between the two test facilities. Pre- test investigations showed large repeatability errors for this specimen at the mid-pressure regime depending on whether preceding testing had been conducted at low or high pressures, for example. Regarding temperature, holding the specimen at temperature at a sufficient time to allow thermal equilibrium was also important. Sensitivity of the panel to temperature and pressure is self-evident from both test results, for example the panel response at 8 kHz changes approximately 10 dB over the temperature range and more than 5 dB over the tested pressures.

More difficult to control were any changes to the panel between testing at the two facilities. Exposure to heat, humidity, UV, etc. during shipment and storage prior to retesting was difficult to control and document. Even under ideal conditions, the panel would exhibit relaxation after returning to ambient air pressure after high pressure exposure. It is postulated that errors due to these effects were minimal due to care taken in packaging and shipping, but this cannot be quantified.

Other sources of error relate to the different tank facilities. Accuracy and stability of pressure and temperature control systems at the two facilities was a potential source of error with regards to panel behavior. Tank size and local internal structures such as hatches, fixtures, and acoustic treatment such as internal anechoic materials and the parametric array baffles can all contribute to differences in the acoustic response of the tank. Water chemistry, dissolved air content, particulates, etc. can also affect acoustic reverberation characteristics in large water tanks [14]. Ensuring air bubbles were removed from the hydrophones and panel surfaces was also a concern mitigated by best efforts on both teams. Background noises of the facilities differed, and can influence results, particularly during the late- time portion of an echo where SNR drops as the signal decays.

Difference in instrumentation could have had significant contributions to differences in results. The use of a parametric array versus a spherical source affects the wave-front curvature impinging on the panel. Investigations of this effect have been studied previously. Spatial locations of hydrophones, particularly for the reflected field, could certainly influence results due to the expected complexity of scattered nearfields. It should be noted that hydrophone calibration, which is critical for many underwater acoustic measurements, is not of much concern for this comparison since reflection and transmission measurements are dimensionless quantities based on ratios of amplitude levels, usually measured on the same hydrophones.

Lastly, signal processing parameters such as time windowing, signal alignment and subtraction, waveform type, and averaging regimens differed between the two teams. These are anticipated to also contribute to potential differences in results. 6.2. Assessment

The measurement comparisons presented here were intended to mutually inform the community on reasonable bounds on measurement uncertainty based on practical test considerations. No particular standardization was enforced, such as using the same instrumentation or test waveforms. Certainly, more controlled testing to quantify error sources could have been undertaken, and is warranted based on observed differences. However, the authors feel that particular insight was gained

from am unconstrained ‘blind’ comparison between two test teams whose approaches to the same measurement were allowed to follow their own best practices using locally available resources. The resulting comparisons raise important questions which can be pursued in subsequent work to further improve such measurement comparisons.

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