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Why, if sound waves travel faster through water, will the pitch of a metal gong go down when you immerse it?

topic posted Mon, November 5, 2007 - 12:57 AM by  Rick
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Actually, the pitch of any resonant object goes down when immersed in water?
.....at least to the person who is listening to it through the air outside the water.

I can't figure out the physics of this phenomenae...........any fluid physicists out there to
help me explain this to my students?
posted by:
Rick
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  • as several people have pointed out, the speed sound travels, and frequency, are different domains.. so, there isn't a relationship.. but, in understanding why sound moves more quickly thru water, wikipedia can help us.. :)

    en.wikipedia.org/wiki/Speed_of_sound

    "The transmission of sound can be explained using a toy model consisting of an array of balls interconnected by springs. For a real material the balls represent molecules and the springs represent the bonds between them. Sound passes through the model by compressing and expanding the springs, transmitting energy to neighboring balls, which transmit energy to their springs, and so on. The speed of sound through the model depends on the stiffness of the springs (stiffer springs transmit energy more quickly). Effects like dispersion and reflection can also be understood using this model.

    In a real material, the stiffness of the springs is called the elastic modulus, and the mass corresponds to the density. All other things being equal, sound will travel more slowly in denser materials, and faster in stiffer ones. For instance, sound will travel faster in iron than uranium, and faster in hydrogen than nitrogen, due to the lower density of the first material of each set. At the same time, sound will travel faster in iron than hydrogen, because the internal bonds in a solid like iron are much stronger than the gaseous bonds between hydrogen molecules. In general, solids will have a higher speed of sound than liquids, and liquids will have a higher speed of sound than gases.

    Some textbooks mistakenly state that the speed of sound increases with increasing density. This is usually illustrated by presenting data for three materials, such as air, water and steel. With only these three examples it indeed appears that speed is correlated to density, yet including only a few more examples would show this assumption to be incorrect."

    peace
    -cpr
  • Unsu...
     
    I am an architect and Cymatics researcher who would like to give our question a shot.

    If something exists, it vibrates from which we can conclude that all things visible and invisible, audible and inaudible vibrate. Using a chemical reaction as a comparison, when we combine two or more vibrating things, a new harmonic results. This is true for tangible objects and intangible sounds. Some are harmonious and some are not but there is always more than one measure which is why this gets complicated in relation to our standard human scale.

    There is always a planetary and even astrological vibes impacting any object as well, but these are generally stable and predictable so we can treat these as constants. The physical world is relativistic so there is harmony in relation to the cosmos and the Earth as well there is local harmony. A chord on the piano might replicate cosmic harmonies and the fact that it exists demonstrate local harmonies while discordant sound is locally harmonious though cosmically discordant. Being clear on our measure must become part of our language. I see these as different scales of reality and occurring simultaneously and do not underestimate beliefs which impact what we allow ourselves to understand and experience. The 2012 phenomena is a rare exception and very very important to understand but not the point of this response.

    The pitch of the metal is actually the pitch of the air vibrating around the metal. The pitch of water is a slower and a more dense frequency akin to the Shumann Resonance of the Earth. The harmonic of the metal in the water creates a new vibe that is slower than vibrating air and faster than 7.8 Hz (and rising). The sound you hear which was in the air (the ear is an evolved mechanism to hear in the air as opposed to whales and dolphins that hear in the water up to 80 octaves I understand) but is now in the water and is therefore a slower (lower) frequency due to the combined vibe and your particular orientation.

    Getting smaller....sound (phonons) do not bend like light (photons). At the subquantum level, photons are contiguous and continuous contrary to what we see at our scale of phenomenal reality. Imagine all objects and the air alight with electrical activity! That is what is actually going on! Inside the photon, an energy balance goes on which has it's own internal structure and is therefore electromagnetic. A phonon on the other hand is a quantum of vibration that will mechanically oscillate any matter in it's path so it's presence is dependent on photonic matter. The phonon is a vibration within matter or sound energy and is carried within a photon. When combined these are called Gluons. Sound organizes and integrates all matter so it can be thought of as the grammar of presence while photons can be thought of as the words. We hear energy in relation to movement but never as it exists as such except in rare moments of clarity or what seems like an advanced state of perception that some people have achieved.

    Gluons surround and protect protons and neutrons and true to it's name, it's the binding glue within which the memory of the cell's journey is recorded. The subparticle reminds the gluon which energy level it should be counting (jumping between scales of reality). Bringing this back up the the human scale of things, the sound we hear depends upon two sources; the memory of the gluon inside the submerged metal and the memory of the cells in our body having been previously vibrated by the metal in the air. We do not ever experience life directly for this reason but rather the distinct hybrid of ever new expressions of matter. the water 'hears' this sound differently than you do.

    Now, explain what I just said to me!
  • High frequencies are far more weak than low frequencies. Since water is more dense than air it poses more resistance to the sound wave specifically to the higher frequencies dissipating them as heat ( a sound wave can heat water molecules ex. a microwave oven) Therefore low freq. travel longer distances through water. And if on top of that the listener is out of the water well the wave must transfer from a dense medium to a less dense one Ex. air loosing even more energy. Think about this: Picture that a sound wave is nothing but suspended molecules bouncing against each other in a cycle that repeats itself. That movement is called compression and rarefaction (push and pull, positive and negative) Water molecules being closer to each other creates an easier medium fro the wave to travel in therefore sound not only travels faster in water but it also travels longer distances. Temperature also plays an important role. The colder the temperature the close the molecules are to each other and the hotter the temp. the farther away they re from each other making the wave spend more energy.
    • Well as stated above, it really boils down to the fact that pitch and sound conductivity are two completely different things in the same situation. Pitch is governed by the length of the wave created by a vibrating object. When an object is immersed in water, it can't vibrate as fast so the pitch decreases. For instance, flap your arms above water and then flap them underwater. They flap much slower underwater, and that is due to friction of water on your arms.

      Sound waves, however, are CONDUCTED by water, which is a completely different principle. Sound is a mechanical wave that is energy transferred from one molecule to an adjacent molecule within a given medium. This means that because water molecules are closer together than air molecules it can transfer that wave at a much greater amplitude over a much greater distance because there is greater energy maintained within the wave.

      So in the end...

      Object vibration (source of sound wave)
      In air = Faster (higher pitch)
      In water = Slower (lower pitch)

      Sound wave (transfer of energy)
      In air = Less dense (shorter range)
      In water = More dense (longer range)

      Two completely different principles acting on the same perception of a sound. Hope this helps! =)
      • well, that's the very clearest explanation , yet, for me, and clears up what I thought was a contradiction that I got
        viscerally, but not intellectually..........thanks, I think I finally get it.
        • Hi, I racked my brain trying to figure this out last night and gave up. There is no reason for the frequency to change when crossing to another medium, assuming that the sourse sounds the same in the first place.

          That is not the case here. If an object is struck it doesn't vibrate freely, but is dampened by the medium it's in. Just as a speaker is dampened by the air in its enclosure, altering its resonant frequency, so is your resonant object. It resonates at a different frequency depending on the pressure of the medium.

          You can perform this experiment in class, by placing a clock or something that emits sound in a belljar, and start remiving the air with a pump. The pitch will start rising as the pressure drops and will eventually stop when there is no more air to conduct the sound.

          Something similar happens when you immerse the sourse in water, the water changes the resonant frequency of the object, the frequency you hear is not different than that of the sourse. ...unless it is physically moving, the dopler effect.

          Note: you would also hear differently when under water, because the water pressure would dampen your ear-drum.

          Furthermore, pitch continues to lover as you descend deeper in water, due to increasing pressure.

          The same effect can be observed when you inhale gasses of different molecular mass, eg. Heilium is lighter - higher voice pitch, if you inhale something heavier than air your voice would be deeper.


          I took a course called the Physics of high-fi sound reproduction, it was fun and a great example of physics applications in audio.
          I am also a physics majour and love it.
  • One of your posters had pretty much stated why. Air is lighter than water. The air is a medium for which vibrations travel, but so is everything else to one degree or another.

    As such water too is a medium for vibrations to travel however, water is far more dense than air and therefore will damp the vibrations as the gong enters the water, thus slowing its pitch / frequency of the sound.

    Dipping the gong in a pool of oil should lower the pitch further still.

    No phenomena at all when you think about.

    Now what should be a phenomena is that supposedly sound (which is vibrations) travels faster in water than it does in air. In fact, I think sound travels at 4500 ft per second in water.

    But then again, water is more dense than air and hence closer to a solid and vibrations travel faster thru a solid. So the more dense the faster the travel.

    I think the real phenomena lies with how one accurately measures the speed of sound pitch in air and then the exact same sound pitch in water (since water changes the pitch).
    • Another interesting thing to note here is that not all frequencies are effected by the same amount when entering another medium.
      There will be some distortion as the different frequencies get out of phase. It's analogous to what happens to white light, getting split up by a glass prism into its different colours.

      So all waves seem to depend on the properties of the medium, light being a wave of the electromagnetic field also changes its speed, while sound can be thought of as a pressure-density wave.

      I suspect the speed of propagation there depends on the nature of the chemical bonds between molecules in a solid, the density of a fluid like water or oil, and the pressure and density of a gas.

      If you think of what happens in a gas, for the wave to pass, one molecule must travel some distance before hitting another and transmitting the wave energy.

      In a liquid the distances are smaller between molecules, there are weak bonds, hydrogen bonds in water, polar or other weak bonds in oil, depending on what it's made of the speed of a wave would vary.

      In solids the distances between atoms are much smaller and the atoms are "fixed", unlike in a fluid, they are all bonded to one another by strong, shorter bonds, metallic, covalent or ionic bonds, which transmit the sound wave much faster.

      I think the structure of the solid must also have an effect, if it is crystalline or amorphous, any imperfections would distort he wave, creating interference inside the medium, some of the wave energy would be lost as heat.


      There are many interesting phenomena happening all the time, we just don't notice them. I think what makes a great scientist is paying attention and noticing what others habitually miss. It takes a kind of curiosity and love for discovering the mystery of the world.

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