the songs of quartz crystal that echo through your body

A good deal of fantasy depends on magical properties of crystals.  It's easy to see where that comes from.  They're often physically beautiful and can do wonderful things to light but there's no magic.  On the other hand some allow us to do what was considered fantastic. .  Quartz for example..  Squeeze a crystal and it deforms a tiny amount generating electricity in the process.   Tun electricity through the crystal and it mechanically deforms.   The piezoelectric effect (squeezing electricity).

 

Quartz happens to be one of the most common minerals on the Earth's crust.   It's a regular arrangement of silicon and oxygen atoms (SiO₂) that forms a repeating shape in three dimensions  forming the larger crystal.  In most crystals (metals for example)  the repeating structure is arranged symmetrically, but piezoelectrics have asymmetric structures.   A quartz crystal just sitting on your desk is electrically neutral.  Although the arrangement of atoms isn't symmetrical, the distances and angles are just such that all the charges balance out: a non-symmetric neutrality.  Squeeze it and you ruin this perfect arrangement creating a net electric charge.  It takes mechanical energy and turns it directly into electrical energy.  It works in reverse too.   Apply current to a crystal and the resulting change imbalance causes the whole crystal to deform just a bit.  

 

This is extremely useful and a good deal of technology is based on it.

 

Attach a quartz crystal to a needle and place the needle on a revolving phonograph record,  and you get an electric current that dances to the tiny wiggles the needle traces.  You can speak at or or sing to it and produce a current that can be transmitted or recorded.   The same crystal sings and listens. Put a piezoelectric on something that vibrates and you can harvest a bit of power -- potentially important for tiny wireless Internet of Things devices where batteries are impractical.  There are other technologies that can be applied, but quartz crystals can be very effective sound transducers when you move up to million of deformations per second - the land of ultrasound.

 

Light or sound waves at the boundary of dissimilar materials can reflect as well as transmit. They can change angle and even scatter into a number of directions.  Materials can be identified by observing what continues through or what is returned.  Animals are good at this.   Our brains process information from our eyes and ears to sort out a tiny bit of the reality that surrounds us.   (silly us - we consider that reality when, in fact, our brain is construction a good enough virtual reality).   If it's dark we can use a flashlight (or torch for those of you in the UK), but some things are frustratingly out of sight

 

Imagine feeling yourself and finding a subsurface lump.  What is it?  Is it just your body messing with your mind or is it something serious?    It may only be an inch or so below the surface but X-rays may not good enough to diagnose and physically accessing it isn't exactly fun or safe.  You need a safe "flashlight" that lets you see what's below.  

 

Medical ultrasound is a flashlight that can be effective in soft tissue and even bone.  A burst of ultrasound travels through you and does something when it encounters anything different.  The busts are repeated regularly bounding comparatively long periods of silence when the crystal that generated the sound is listening.  A clock starts when the sound is emitted and the round trip time for each bit of sound reported by the crystal, along with it's strength, is recorded.    A distance for each echo is calculated - just half of the time of flight times the speed of sound in the tissue.¹  A digital map is made and  usually displayed in real time on a video scree in a form more useful for diagnosis. Processing is usually done on a PC.

 

A critical bit hasn't been described.  How do you make an image with more than one dimension?  In the old days the crystal was moved mechanically.  Getting this right and reliable was expensive and the scanning heads weren't easy to handle.  The innovation was to use a number of crystals in a phased array.  

 

Dip two fingers in still water at the same time you'll see expanding waves moving out from contact with interesting interference patterns forming where the waves intersect.  If you have a lot of equally spaced fingers you'll find that the interference conspire to form linear waves.

 

An ultrasound array many have fifty or a hundred crystals in a line.  Here the transmitter (Tx) generates the power to make the crystals sound off and the little antenna thingies are the quartz crystals. If the crystals all sing at the same time the flat wave from moves off to the right.  But if you apply a bit of delay to each crystal increasing it as you move to it's neighbor, you can control the angle of the wavefront.  Smoothly change the angle and you sweep out a beam.  

 

Make it a two dimensional array and you can scan out an area with the time of flight depth information adding the third.  You can even get clever with the phase delay at each crystal and focus the beam.  Some ultrasound machines only use a linear array.  It's application and cost dependent. 

 

Resolution improves with increasing frequency, but range decreases.²  A range of frequencies are used depending on the task at hand.  I was given a tour as reward for telling the operator what one of the display algorithms was doing.  We looked at several parts of me with a $150,000 brand new machine and used frequencies ranging from 2.5 to 18 megahertz.   A six and a half octave range ...  most pianos are seven.

 

A singing quartz crystal flashlight that augments the reality you perceive.  

 

Oh - and the special lubrication they use.  It has a speed of sound similar to human tissue to minimize reflections.  Trapped air would produce nasty reflections coming and going - it's all about minimizing noise and maximizing signal,.   You can buy expensive ultrasound gel, but I'm told most people find K-Y Jelly is an inexpensive substitute.

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¹ This varies with tissue type and increases with rigidity.  Bone has a much higher speed of sound than soft tissue and breasts tend to be a bit slower than muscle.  The average speed for tissue is about 1540 meters per second.  

² Specifically resolved size goes as the inverse of frequency.  At the commonly used 3 megahertz resolution in tissue is about a half millimeter.  That tells it it is overkill hoping to reliably see smaller detail.