question: Why do some materials resonate at certain frequencies when they is sound playing around them?
comment: Because they are easier to vibrate at those specific notes. Think about how your viola works. You vibrate one of the strings and if you want a higher pitched note, you shorten the length of the string by pressing the string to the fingerboard. You are using the same bow and same technique, but the string vibrates at the different frequencies(pitch). That change in strength length causes it to more easily vibrate at a different frequency. Likewise, the different shape and material of your viola versus your chair cause them to vibrate at different frequencies.
passages: BULLET::::- Second are the resonance effects, which arise from the rotations or vibrations of atoms, ions, or electrons. These processes are observed in the neighborhood of their characteristic absorption frequencies.
   The frequency of the noise depends on the nature of electromagnetic forces (quadratic or linear function of electrical field or magnetic field) and on the frequency content of the electromagnetic field (in particular if a DC component is present or not).
  
  Section::::Electromagnetic noise and vibrations in electric machines.
   These phenomena can potentially generate vibrations of the ferromagnetic, conductive parts, coils and permanent magnets of electrical, magnetic and electromechanical device, resulting in an audible sound if the frequency of vibrations lies between 20 Hz and 20 kHz, and if the sound level is high enough to be heard (e.g. large surface of radiation and large vibration levels). Vibration level is increased in case of a mechanical resonance, when electromagnetic forces match with a structural mode natural frequency of the active component (magnetic circuit, electromagnetic coil or electrical circuit) or of its enclosure. 
   There are a number of different dielectric mechanisms, connected to the way a studied medium reacts to the applied field (see the figure illustration). Each dielectric mechanism is centered around its characteristic frequency, which is the reciprocal of the characteristic time of the process. In general, dielectric mechanisms can be divided into relaxation and resonance processes. The most common, starting from high frequencies, are:
  
  Section::::Dielectric mechanisms.:Electronic polarization.
  
  This resonant process occurs in a neutral atom when the electric field displaces the electron density relative to the nucleus it surrounds.
   The main generation mechanism for metals is thermal expansion, whereas for semiconductors it is often the deformation potential mechanism. In piezoelectric materials the inverse piezoelectric effect, arising from the production of internal electric fields induced by charge separation, may dominate.
   RuBee works well in harsh environments because most steel items resonate well at the RuBee frequency of 131 kHz. As the frequency goes up over 1 MHz fewer steel items resonate. At a frequency of 10 MHz for example, nothing large made of steel can be tuned to resonance.
   Section::::Frequency selective surface based metamaterials.:AMC as an FSS band gap.
   If both materials are typical solids, the degree of reflection will be moderate, and a significant portion of the pulse will travel deeper into the sample, where it may be in part reflected by deeper material interfaces. If one of the materials is a gas such as air – as in the case with delaminations, cracks and voids – the degree of reflection at the solid-to-gas interface is near 100%, the amplitude of the reflected pulse is very high, and practically none of the pulse travels deeper into the sample.
  
  Section::::Scanning process.:Gating of the return echoes.
   Section::::Acoustic.
  
  The most familiar examples of acoustic resonators are in musical instruments. Every musical instrument has resonators. Some generate the sound directly, such as the wooden bars in a xylophone, the head of a drum, the strings in stringed instruments, and the pipes in an organ. Some modify the sound by enhancing particular frequencies, such as the sound box of a guitar or violin. Organ pipes, the bodies of woodwinds, and the sound boxes of stringed instruments are examples of acoustic cavity resonators.
  
  Section::::Acoustic.:Automobiles.
   FSS can seemingly take on a nearly infinite number of forms, depending on the application. And now FSS are being used in the development of certain classes of meta-materials.
  
  Section::::Examples.:Classification: by form or by function.
   Section::::Generation and detection.:Detection.
  
  Strain pulses returning to the surface from buried interfaces or other sub-surface acoustically inhomogeneous regions are detected as a series of echoes. For example, strain pulses propagating back and forth through a thin film produce a decaying series of echoes, from which one may derive, in particular, the film thickness, the ultrasonic attenuation or the ultrasonic dispersion.
   The resonance effect of magnetic vibration with a structural mode can be illustrated using a tuning fork made of iron. A prong of the tuning fork is wound with a coil fed by a variable frequency power supply. A variable flux density circulates between the two prongs and some dynamic magnetic forces appear between the two prongs at twice the supply frequency. When the exciting force frequency matches the fundamental mode of the tuning fork close to 400 Hz, a strong acoustic resonance occurs.
  
  Section::::External links.
   Some tangential magnetic force harmonics can directly create magnetic vibrations and acoustic noise when applied to the stator teeth: tangential forces create a bending moment of the stator teeth, resulting in radial vibrations of the yoke .
  
  Besides tangential force harmonics, Maxwell stress also includes radial force harmonics responsible for radial vibrations of the yoke, which in turn can radiate acoustic noise.
  
  Section::::Electromagnetic noise and vibrations in passive components.
  
  Section::::Electromagnetic noise and vibrations in passive components.:Inductors.
   The surface impedance is derived from the ratio of the electric field at the surface to the magnetic field at the surface, which extends far into the metal beyond the skin depth. When a texture is applied to the metal surface, the surface impedance is altered, and its surface wave properties are changed. At low frequencies, it is inductive, and supports transverse-magnetic (TM) waves. At high frequencies, it is capacitive, and supports transverse electric (TE) waves. Near the "LC" resonance frequency, the surface impedance is very high. In this region, waves are not bound to the surface. Instead, they radiate into the surrounding space.
   In other systems, such as electrical or optical, phenomena occur which are described as resonance but depend on the interaction between different aspects of the system, not on an external driver.
   The phenomenon is also called audible magnetic noise, electromagnetic acoustic noise, or electromagnetically-induced acoustic noise , or more rarely, electrical noise , "coil noise", or "coil whine", depending on the application. The term electromagnetic noise is generally avoided as the term is used in the field of electromagnetic compatibility, dealing with radio frequencies. The term electrical noise describes electrical perturbations occurring in electronic circuits, not sound. For the latter use, the terms electromagnetic vibrations or magnetic vibrations , focusing on the structural phenomenon are less ambiguous.
   Impedance is the opposition to the flow of alternating current (AC) in a complex system. A passive complex electrical system comprises both energy dissipater (resistor) and energy storage (capacitor) elements. If the system is purely resistive, then the opposition to AC or direct current (DC) is simply resistance. Materials or systems exhibiting multiple phases (such as composites or heterogeneous materials) commonly show a universal dielectric response, whereby dielectric spectroscopy reveals a power law relationship between the impedance (or the inverse term, admittance) and the frequency, ω, of the applied AC field.
   Pioneered by Georg von Békésy, a method to observe the basilar membrane in action came about in the mid 1900s. Békésy isolated the cochlea from human and animal cadavers and labeled the basilar membrane with silver flakes. This allowed strobe imaging to capture the movement of the membrane as sounds stimulated the hair cells. This led to the solidification of the idea that high frequencies excite the basal end of the cochlea and provided new information that low frequencies excite a large area of the cochlea. This new finding suggested that specialized properties are occurring for high frequency hearing and that low frequencies involve mechanisms explained in the frequency theory.
   Section::::Theory.:Quartz Rod.
   First, AMC or HIS surfaces are designed to have an allotted set of frequencies over which electromagnetic surface waves and currents will not be allowed to propagate. These materials are then both beneficial and practical as antenna ground planes, small flat signal processing filters, or filters as part of waveguide structures. For example, AMC surfaces as antenna ground planes are able to effectively attenuate undesirable wave fluctuations, or undulations, while producing good radiation patterns. This is because the material can suppress surface wave propagation within the prescribed range of forbidden frequencies.
   Section::::Analysis – first principles approaches.:Spectral domain method of moments (overview and mathematical introduction).:Background.:Floquet's principle.
   Three factors relating to the walls of a resonator will affect how it functions: the material it is made of, the thickness of its walls, and the type of surface it has. The resonance characteristics of a musical instrument obviously will vary with different materials and the amount of material used will have some effect.
   Section::::Sound modality.
  
  Section::::Sound modality.:Description.
  
  The stimulus modality for hearing is sound. Sound is created through changes in the pressure of the air. As an object vibrates, it compresses the surrounding molecules of air as it moves towards a given point and expands the molecules as it moves away from the point. Periodicity in sound waves is measured in hertz. Humans, on average, are able to detect sounds as pitched when they contain periodic or quasi-periodic variations that fall between the range of 30 to 20000 hertz.
  
  Section::::Sound modality.:Perception.
   When a resonant cavity is perturbed, e.g. by introducing a foreign object with distinct material properties into the cavity or when the shape of the cavity is changed slightly, electromagnetic fields inside the cavity change accordingly. This means that all the resonant modes (i.e. the quasinormal mode) of the unperturbed cavity slightly change. 
   Laser scanning vibrometry is an essential tool for effective NVH optimization. The vibrational characteristics of a sample is acquired full field under operational or excited conditions. The results represent the actual vibrations. No added mass is influencing the measurement, as the sensor is light itself.
  
  Section::::Investigative techniques.
  