Could I create a speaker that plays song that I can only hear with my ear to the sand? Taking the same principle of it traveling through the air but through sand grains?
Since the sand grains are loose, sound - or indeed any wave form - doesn’t travel through them very well - sand is a damper. See what happens when waves break on the beach? Well, sound is just waves of air.
There exists a type of speaker called a transducer speaker, but they work by being in contact with a solid surface (like a window or table), or some solid part of your head (normally your teeth, as in some novelty toothbrushes or candy lollipops)
I don’t see how sand is going to provide a solid enough matrix through which sound could pass, but I see that ebay has some transducer speakers for ~15$, so it’s a cheap experiment to get a base line on if you want to report back.
Since sound is a longitudinal, not transverse, wave, it can travel as easily through sand as it does through air or water, neither of which have any significant elasticity either. Here are some measurements:
These measurements, which were pretty high frequency (2-6 kHz), found for sand a speed of sound a bit less than that in air, and a fairly stiff rate of attenuation, such that it would be difficult, say, to comprehend a voice traveling though more than about 40-50 cm of sand. Interestingly, the attenuation was lower for sand than for soil in general. The attenuation is pretty strongly dependent on frequency, which is typical of complex materials, and seems to drop significantly for lower frequencies, which is kind of what you expect – if you listen to sound that travels through the earth, it loses its treble and becomes rumbly. Also, the very long waves from earthquakes have no trouble traveling enormous distance, and, on theoretical grounds we would expect that as the wavelength becomes much longer than the length scale of any inhomogeneities in the material, the attenuation should disappear.
On a purely anecdotal note, since Carl Pham covered what I wanted to write, I once recorded some ocean sounds using a Sony ECM-MS907. For its price it’s actually a very nice-sounding microphone and it’s sturdy and highly portable. It has one major fault, though, it’s extremely sensitive to contact vibrations. I have some recordings where you can actually hear my finger joints moving. That’s why I usually set this microphone on a surface. When I placed it on the beach, I later found that my otherwise very nice recordings had loud rumble below 100 Hz or so. The microphone had picked up, through contact transduction, the sound of the ocean travelling through the beach. That’s more or less what the OP is asking about.
There are of course singing sands. The “singing” part is not usually applicable, being more of a dull roar or some such. These sounds originate from activity within dunes or on the surface.
But the main thing to note is that the sound quickly passes into the air where the sound is heard. I don’t think any appreciable sound would travel thru the sand only.
It’s not going to be like a railroad line where you can hear an oncoming train by putting your ear to the rail.
Wouldn’t we expect attenuation from inelasticity in the material causing energy to be absorbed, which could happen on any frequency?