Acoustic Theory Basics for Fisheries Sampling

Summary

This presentation offers a comprehensive overview of acoustic theory in relation to fishery sampling, covering the basics of sound propagation in water, wave types, frequency, wavelength, and the key factors influencing sound in aquatic environments. It delves into the practical applications of acoustics for detecting fish, emphasizing the importance of frequency choice, timing, and pulse length in achieving accurate measurements. Key challenges such as target resolution, signal absorption, and spreading losses are explored, alongside the significance of calibration and the acoustic equation, offering insights into how various factors affect the quality and range of acoustic signals used in fishery research.

Takeaways

• 😀 Sound waves in water are longitudinal (compression) waves, where particles move back and forth in the wave direction, creating areas of compression and rarefaction.
• 😀 The speed of sound in water varies based on temperature, salinity, and pressure, affecting wavelength and frequency.
• 😀 Frequency is the number of wave peaks passing a point per unit of time, measured in Hertz (Hz), and determines the ability to resolve smaller objects.
• 😀 Lower frequencies provide deeper penetration but are less effective for high-resolution imaging, while higher frequencies are used for resolving smaller objects but have a shorter range.
• 😀 The relationship between wavelength, frequency, and the speed of sound is critical in acoustical systems used in fisheries sampling.
• 😀 Timing measurements in two-way acoustical systems are vital, as the speed of sound and distance must be known to accurately interpret signals.
• 😀 Target resolution depends on the distance between objects, the speed of sound, and the pulse length. Overlapping echoes can lead to indistinguishable targets.
• 😀 Shorter pulse lengths improve target resolution, but they also introduce challenges, such as increased noise and difficulty in converting analog signals to digital samples.
• 😀 Acoustic signals are measured in decibels (dB), where 20 log is used to express sound pressure levels. The intensity of sound is logarithmic, making it easier to handle varying magnitudes of pressure.
• 😀 Absorption and spreading losses reduce signal strength. Absorption is more significant in saltwater, while spreading loss occurs as the signal spreads out with distance from the source.
• 😀 The acoustic equation incorporates factors like source level, target strength, beam pattern, and absorption to calculate the voltage measured by an echo sounder, aiding in accurate fish size estimation.

Q & A

What is the primary focus of this presentation?

The presentation provides an overview of acoustic theory as it relates to fishery sampling, specifically how sound waves and their properties are used to detect and analyze fish in water.

What are the two types of waves discussed in the presentation?

The two types of waves discussed are longitudinal (compression) waves and transverse (shear) waves. Longitudinal waves move particles in the direction of the wave, while transverse waves move particles perpendicular to the wave's direction.

How is wavelength defined in acoustic theory?

Wavelength is defined as the distance a periodic wave covers from one point in its cycle to the next identical point, such as from peak to peak or trough to trough.

What factors influence the speed of sound in water?

The speed of sound in water is influenced by temperature, salinity, and pressure. These factors contribute to variations in sound speed at different locations.

What is the significance of frequency in acoustic systems?

Frequency is crucial because it determines how many wavelengths pass a point in one second. Higher frequencies provide greater resolution but shorter detection range, while lower frequencies offer deeper penetration but lower resolution.

What are the advantages and disadvantages of high-frequency acoustics?

Advantages of high-frequency acoustics include smaller transducer size, easier installation, and higher resolution for detecting smaller objects like individual fish. Disadvantages include reduced range and increased absorption, which lowers signal strength, and higher power requirements.

What is target resolution in acoustics, and why is it important?

Target resolution refers to the ability to distinguish between two closely spaced targets based on their return echoes. It is important for detecting individual fish or small objects instead of entire schools, and it depends on pulse length, target distance, and sound speed.

What is the shadow zone in acoustic measurements?

The shadow zone is the area at the bottom of the insonified region where the pulse's center does not overlap with the edges of the beam, causing the edges of the beam to be masked by the hard return from the center of the pulse.

How do absorption and spreading loss affect acoustic signals?

Absorption reduces signal strength due to local heating in the sound wave, particularly at higher frequencies. Spreading loss occurs because as the signal travels farther, its energy spreads out, causing the pressure to decrease. Both losses are compensated in acoustic systems, especially through time-varied gain (TVG).

What factors are included in the acoustic equation?

The acoustic equation includes factors such as the transducer source level, system gain, target strength, beam pattern factor, range, absorption, and transmission losses. These factors together determine the voltage measured by the system.