The Propagation of Sound

Understanding the acoustic impedance of sound waves.

Let's take a trip back some couple hundred years ago. Sir John William Strutt, 3rd Baron Rayleigh (1842 - 1919) was an English physicist who co-discovered the element Argon and was awarded a Nobel Prize for the discovery in 1904. ​

However, Rayleigh was also well known for his important works and contributions in the fields of quantum physics, optics, and sound theory. Appointed as the Cavendish Professor of Experimental Physics in 1879, his first publication was published 2 years prior in 1877. ​This new theory clarified how sound is transmitted and emitted from varying mediums. ​He referenced and experimented with stretched strings, bells, metal plates, and membranes to not only show but prove how sound waves are transmitted through different mediums by using mathematical models and discussions about the experiment’s findings. ​​​

Sir John William Strutt, 3rd Baron Rayleigh

In his publication, Rayleigh states that the “Very cursory observation often suffices to show that sounding bodies are in a state of vibration, and that the phenomena of sound and vibration are closely connected". Additionally,“When a vibrating bell or string is touched by the finger, the sound ceases at the same moment that the vibration is damped. But, in order to affect the sense of hearing, it is not enough to have a vibrating instrument; there must also be an uninterrupted communication between the instrument and the ear.”

What produces ultrasound?

Many things will produce ultrasound, and it’s the job of the CBM Analyst to identify which ultrasonic emissions are occurring in the area they will be working, prior to collecting data and sound waves.​

5 common categories of ultrasound emissions are sometimes referred to as FITEM: Friction, Impacting, Turbulence, Environmental, and Man-made. Friction, impacting, and turbulence are often directly related to a fluid/electrical/mechanical issue, and would therefore be the very things you're aiming to detect. On the other hand, environmental and man-made ultrasonic emissions are "antagonistic", meaning that they compete and conflict with the target issue's frequencies.

• Friction: Can be generated in many ways, such as when two surfaces slide against each other, roll over one another, or when an object moves through a gas or liquid.

• Impacting: Higher-frequency, shorter bursts of energy caused by surfaces striking each other.

• Turbulence: Only occurs in gases or liquids when they interact with various surfaces.
  

• Environmental: Natural occurrences of ultrasound are not uncommon even in industrial settings, like wasp nests and bat roosts.
  

• Man-made: Motion sensors, pest repellent, industrial cleaners, display panels, monitors, and many other human-made objects can emit ultrasonic frequencies as a by-product of their intended function.
  

As an ultrasound inspector, it's important to remember that the sound wave is a vibration of the medium that it is travelling through. That medium must be elastic so it can transfer its energy through, and with no interference between it and the inspector's tool/module.​

Inverse Square Law

For every doubling of distance from the sound's source, the sound intensity will diminish by 6 dB. This knowledge is crucial to understand when detecting ultrasonic emissions, as the distance that you are from the source will directly affect your readings. Assuming that we're starting 1 foot from the source with no dB loss, there would be a 6 dB loss from 2 feet away, and a 32 dB loss from 30 feet away.

Now we know in great detail how the acoustic energy attenuates over a distance in a gaseous environment, and how atmospheric influence can affect how a sound wave travels through said medium. However, the Inverse Square Law (and the respective measurements above) assume that the sound is travelling in a "free field" situation, unaffected by other external factors. In the real world, this isn't always the case. See below for 6 of the most common Sound Energy Impedances.

Acoustic Impedances - Air

• Transmission: Ultrasound transmissions travelling through the medium of an atmosphere. We need to take into consideration how the sound travels at a slower speed through a gaseous atmosphere, versus a solid or liquid medium. ​ For example, a train can be heard from miles away by placing your ear to a train track (solid - highly dense particles). Whales can communicate to each other from miles apart underwater with minimal delay (liquid - moderately dense particles). The sound of thunder can sometimes take several seconds to be heard after a lightning strike occurs (gas - less dense particles). All of these are sound transmissions.

• Reflection: Occurs when a sound wave bounces off of a flat surface. The point of reflection (also known as the Angle of Incidence) can often be mistaken for the original ultrasound source. For example, if you've pinpointed an ultrasonic emission using an ultrasonic detector, but the identified location doesn't have any possible sources of ultrasound, you may have located a reflection point that the original ultrasonic sound wave has bounced off of.

• Refraction: Similar to reflection, refraction refers to when a sound wave passes from one medium into another. Where as reflection happens on a solid surface though, refraction generally takes place in liquid (a state of matter that allows sound energy to pass through). The refracted sound wave will now experience the influence of the new medium's elasticity and density (also known as its refractive index). This can be used to calculate the effect of the material on the speed of the energy of the new sound as it travels through the new medium vs that of the original medium using Snell’s Law.
  

• Diffraction: The redirection of a sound wave by bending around, or spreading/squeezing through, a space. Diffraction can often occur when a sound wave passes through a very small opening. Hearing someone speak from around a corner is also a common example of diffraction, because the sound wave is bending around and through the space to the listener's ear.
  

• Absorption: Acoustic absorption refers to the ability of a medium to capture a portion of received sound energy. Solid, porous materials such as foam or fibreglass are common examples of sound-absorbing matter. The absorbed portion of the original sound wave is transferred into an entirely new form of energy (usually heat) and is transmitted out of the material. The remaining portion of the sound wave that was not absorbed is instead reflected.​
  

• Scattering: The process of a sound wave interacting with a particularly rough surface, resulting in multiple new sound waves with reduced energy. These resulting sound waves have unpredictable directions and amplitudes, making scattering a difficult impedance to counter when performing ultrasonic detection.

Acoustic Impedances - Density's Effect:

In addition to the factors above, the actual speed of a mechanical (sound) wave traveling through a medium is directly proportional to the temperature, elasticity, and density of that medium. ​Different mediums will cause a different amount of delay due to their absorption and reemission properties. ​

The warmer and denser the material, the closer together the atoms are and the faster the sound will travel through that medium. ​This is why mechanical waves, such as sound, can't exist at all in space. Mechanical waves require a physical medium to travel through. Space is a vacuum, where the atoms are spread so far apart that sound propagation cannot occur. See below for sound velocities of various common materials.

From Sound Propagation to Ultrasonic Leak Detection

Now that we've learned more about external and environmental factors and impedances, there's some general tips and best practices that should be utilized when performing ultrasonic detection. The industrial settings that ultrasonic issues are commonly found in are often loud, chaotic, and filled with sound waves ranging throughout the acoustic spectrum. It's important to know how to counter antagonistic ultrasound and competing noises and gather accurate data.

1. Sound the room before beginning the detection process to get a better initial understanding of the space, and to determine the approximate location of any ultrasonic anomalies.
   

2.  If you're dealing with a compressed air leak, there are a few common tips to assist in identifying the source:

   1. Confirm that the systems are fully pressurized. 

   2. Follow the flow of the piping from the compressor.

   3. Confirm that all relevant valves are open.
      

3. If you're using the PROSARIS OL2 Puck to centralize on the source location, it's important to implement the following best practices:

   1. Physically reduce the distance from the source. Once you begin to narrow in on the source, the obvious next step is to move closer to obtain a more accurate location. However, remember to adjust the Distance slider in the PROSARIS App on your tablet once you're within the 6-foot range. This will ensure that the various record calculations are correct.

   2. Scan in all directions around the source of the emission. It's not uncommon that other leaks may be nearby. These leaks could be mistaken for the main leak unless you get close enough to confirm that multiple sources are present. 
      

4. If you're having trouble pinpointing the leak - whether it's due to other leaks or general antagonistic ultrasound, try implementing a shielding technique. The simplest way to do this is to position yourself in between the target source and the other undesired ultrasonic emissions.

   1. In a pinch, you can even use a piece of cardboard or a clipboard to temporarily block any remaining antagonistic ultrasound sources.

   2. Tying a shop rag over a leak also helps to dampen its acoustic profile in the area. This allows for easier isolation - especially for lower-level leaks.

Knowledge is one of the most valuable items in your toolkit when it comes to completing successful ultrasonic detection work. Understanding the science behind the sound will always inform you of which techniques, strategies, and tools are best suited for the job.