#march25posts

Sound

I've already posted a few videos and text explanations from others about what sound is, so I'm not going to go too in detail again here on this post, but a few terms for you to start off the month:

Sound can be defined in a variety of different ways using different terms. It is often described as a mechanical wave, a pressure wave, as longitudinal waves that travel through a medium (though sound can also take the form of shear or surface waves), or as an energy, and is all of these things. (Ultrasounds and infrasounds are simply sounds the average human can't hear, with infrasounds being lower in hertz and ultrasounds being higher in hertz than the human range, where hertz are a measure of the frequency of sound waves.)

Acoustics is the science of sound, its "production, control, transmission, reception, and effects", and so on.

Here are a few resources if you're interested in diving more into exactly what sound is, beyond those posts already shared:

NASA

Iowa State University, Center for Nondestructive Evaluation

Brigham Young University

Stanford University

Sound Attenuation and Impedance

Just as waves of light can be reflected, refracted, absorbed, etc., so to can waves of sound. Even just traveling through the air results in a gradual loss of sound as the atoms and molecules that make up the air interact with the energy. Acoustic attenuation is the measurement of the energy that is lost during the propagation of sound through a medium. As energy can never be created or destroyed, part of the sound energy that is lost is converted into heat energy. The energy of sounds can also cause deformation in materials.

In general, soft and porous materials (including liquids and gases) tend to absorb more sound energy, while harder and denser materials, such as metals, absorb less. The angle at which sound interacts with an object, and it's frequency, also has an important effect. The degree of a material's attenuation is measured by its attenuation coefficient; the higher the value, the more sound energy is lost, resulting in quieter sounds at a distance. It is often measured in units of decibels per meter. In materials characterization methods that rely on acoustics, the attenuation of a material is significant, affecting the depth of penetration of sound waves.

Acoustic impedance, meanwhile, is the measure of a material's opposition to the movement of sound waves through that medium. It is typically given in units of pascal second per cubic meter. Impedance is directly affected by (and can be calculated with) the density of a material.

Speed of Sound (Part 2)

Because the speed of sound is directly effect by materials properties (see part 1 about the relevant equations), if the speed is known it can be directly used to measure these properties in addition to other factors!

I've posted a little bit about this before with this post, which is about one of the different techniques for measuring distance. This is the principle behind sonar, primarily used underwater, as the speed of sound underwater is faster than in air. Read more about sonar from NOAA here. Again, the material properties matter here! When sonar is used in the ocean, both the temperature and the salinity of the water, among other factors, can affect the speed of sound. Here's a resource on underwater sound.

The speed of surface waves in solids has also been used to measure the elasticity of materials, as posted about on this blog about this research from 2022. Other ways the speed of sound has been used to characterize materials is to determine the temperature of the ocean, as discussed in this research from 2020, or this research from 2017 that characterized wood rot in trees.

More information about the ways that sound can be used in materials characterization will be posted later this month!

Speed of Sound (Part 1)

An equation for the speed of sound can be typically generalized as the square root of the elastic properties (typically represented as bulk modulus) of the medium the sound is being transmitted through over the medium's density. I say generalized, because you'll find different versions of this equation, some depending on the phase of the medium, as well as expanded versions of this equation.

Because these properties are affected by composition and external factors such as temperature and pressure, there is no one set speed of sound, not even for a single material. For example, the speed of sound in air is often given as 343m/s, but this is the speed at 20°C; at a higher temperature of around 40°C, the speed of sound is faster, at 355m/s. Altitude also affects this number.

Separate from the properties of the medium specifically, the speed of sound can also be represented in equation form as the frequency of the sound times its wavelength.

Also generalizing, sound typically travels fastest in solids, then liquids, then gases.

Ultrasonic Welding

Sound can also be used in a variety of materials processing and manufacturing methods, including ultrasonic welding. Ultrasonic welding is a process that uses ultrasound to join parts together. These parts can be metals or polymers, but there are limitations. Typically only chemically compatible thermoplastics can be welded together and regardless of the material, the depth of the ultrasonic weld is minimal, best used for thinner parts. Ductile metals are better suited than brittle ones, in the form of thin sheets or wire.

Ultrasonic welding is considered a solid-state welding process when applied to metals. It does not melt the pieces involved, does not require a filler material, and relies on atomic diffusion to fuse pieces together. While elevated temperatures can result from the energy used in the process the heat generated is typically less than 50% of the melting point when metals are welded together. With plastics, however, local melting does occur, but it is distinct from the welding process. Two plastics may melt together under ultrasonic welding but not be truly compatible; compatible plastics will form molecular bonds when welded properly.

Advantages of the process include its speed, cleanliness, low thermal impact, and the lack of a need for a filler. Disadvantages include the limited materials it can be used to join, the small depth of the weld, and its inability to be used on large parts. Given all these factors, ultrasonic welding is used in a wide variety of industries, including the electronic (circuits and wires), automotive (a smooth finish when welding thermoplastics), biomedical (for its cleanliness), aerospace (for welding thin aluminum parts), and many others.