SLAA732A February   2017  – April 2021 PGA460 , PGA460-Q1

 

  1. 1Trademarks
  2. 2Overview
  3. 3External Performance Factors
    1. 3.1 Range Requirements
    2. 3.2 Detectable Target and Objects
    3. 3.3 Ambient Environment
  4. 4Component Selection
    1. 4.1 Sonar Configuration
    2. 4.2 Transducer Selection
    3. 4.3 Driver Selection
    4. 4.4 Passive Tuning
      1. 4.4.1 Impedance Gain-Phase Analyzer
      2. 4.4.2 Tuning Capacitor
      3. 4.4.3 Damping Resistor
      4. 4.4.4 Tunable Transformer
  5. 5PGA460 Parameters
    1. 5.1 Center Frequency
    2. 5.2 Pulse Count
    3. 5.3 Current Limit
    4. 5.4 Time-Varying Gain and Digital Gain
    5. 5.5 Threshold
  6. 6End-of-Line Calibration
    1. 6.1 Transducer Parameters
      1. 6.1.1 Optimal Frequency and Sound Pressure Level Measurements
        1. 6.1.1.1 Frequency Diagnostic Feature of PGA460
        2. 6.1.1.2 External Microphone
  7. 7Revision History

3.2 Detectable Target and Objects

The type of target from which the ultrasonic echo reflects from will impact the returning echo strength. For example, a large, flat steel wall provides a greater return echo compared to a narrow tree. This difference is because a combination of the acoustic impedance, surface coarseness, orientation, and maximum cross section of the target.

Acoustic impedance is based on the density and acoustic velocity of a given material, and is important to determine the amount of reflection that occurs at the boundary of two materials having different acoustic impedances. The acoustic impedance of air is four orders of magnitude less than that of most liquids or solids; therefore, the majority of ultrasonic energy is reflected to the transducer based on the difference in reflection coefficients. However, lighter materials with low densities or significant amount of air gaps, such as sponge, foams, and loosely woven fabrics, tend to absorb more ultrasonic energy. Table 3-1 shows an example listing of characteristics of various material types as they relate to air-coupled ultrasonic absorption.

Table 3-1 Acoustic Impedance Of Various Materials
MaterialDensity (kgm–3)Speed of Sound (ms–1)Acoustic Impedance
(kgm–2s–1 x 106)
Air1.33300.000429
Sponge1007500.075
Fat92514501.38
Water100014501.45
Soft tissue105015001.58
Muscle107515901.70
Aluminum2700632017.1
Steel7800590046.02
Iron7700590045.43
Gold19320324062.6

A flat or smoother surface results in the strongest reflections, while a coarse or ridged surface causes the ultrasonic echo to scatter in multiple directions, reducing the return strength in the direction of the transducer. The amount of surface area at a right angle to the transducer provides maximum returns. This surface area is defined as the maximum cross section (σ), which measures of the ability of the target to reflect sonar signals in direction of the sonar receiver, in m2, and applies to both ultrasonic sonar and radar applications. Table 3-2 provides a description of how the sonar cross section of certain targets impacts performance.

Table 3-2 Sonar Cross Sectional Comparison
TargetMaximum Sonar Cross SectionAdvantageDisadvantage
Sphereσmax = π × r2NonspecularLowest RCS for size; radiates isotopically
Cylinderσmax = (2 × π × r × h2) / λNonspecular along radial axisLow RCS for size; specular along axis
Flat rectangular plateσmax = (4 × π × l2 × w2) / λ2Largest RCS for sizeSpecular along both axes; difficult to align

Depending on the target, the sonar cross section can be averaged based on size and orientation to determine the reflected portion of incident power in units of sound pressure. Table 3-3 lists example targets in relation to sonar cross section as they equate to point-like targets to show the effects of target strength.

Table 3-3 Sonar Cross Sectional Comparison
TargetSonar Cross Section (dB)
Rodent–20
Human0
Automobile20
Truck25
Corner reflector40