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I have a small jar filled with hair styling gel (or, as it calls itself: Ultra Gel-Wax). The jar is cylindrical(with the height being less than the width), has an unscreweable lid and is made of plastic. The jar is nearly fully filled with gel.

The jar however seems to have a strange property: When hit(with a mallet or simply with your finger), it does not produce a simple short 'tok' sound like one would expect from a plastic jar filled with cream or gel. Instead, it seems like the sound inside is somehow reverberating: the sound continues on for a significant longer period of time than one would expect. It is much longer than when hitting an empty jar, a jar filled with cream or other substance, or even one filled with water.

Possibly even odder, this effect still happens without the lid on.

Also, it does not matter where you hit the jar: the bottom, the lid or on the curved side. (Thus, it is not vibrating like a drum). It also does not matter if you touch the surface you hit (or even grip the whole thing in your fist), it still works equally well. (This in contrast to empty jars or jars filled with water, which might resonate a little, but only if not touching the struck surface).

I am trying to find out what the cause of this reverberation is. According to the research I did so far, reverberation in a room is caused by:

  1. The speed of sound of the medium inside: lower sound speeds make the waves travel for a longer amount of time before hitting a wall or arriving at the listener. For instance, in water, sound travels much faster(+- 1497 m/s at room temperature) than in air(+- 343 m/s at room temperature).
  2. Reflection/absorption of the walls: The more sound gets reflected each time a wall is hit, the longer there will be sound inside of the room -> the reverb will be longer. In other words: It's better to coat the inside of the room with metal than with cloth, if you want a long reverb.
  3. The attenuation of the sound: A sound gets quieter and quieter, the more distance it has traveled. In one medium sound might travel much further than in another. This also has a huge effect on the length of the Reverb: This makes Rubber, for instance, a very bad medium to use, because sound won't travel very far.

However, I did some qualitative research, and I have trouble to use these three rules to understand why the effect in my jar with gel is going on:

  • Speed of sound: It is very hard to find out what the speed of sound might be in gel, since it's such a complex mixture of different substances. However, it should be much faster than air because the gel is something between a liquid and a solid while air is a gas: Sound speeds are higher in solids than in liquids, and higher in liquids than in gases (as a general rule: this is always true if looking at one element in its different forms, but also true in general since the steps in speed between gases, liquids and solids are significantly big).

  • Reflection: Plastic is a material that does not seem to work well at reflecting: It is much easier bendable and less dense than metal, so it should not work as good as it seems to do here. Testing the response of empty plastic jars vs empty metal jars at least showed a huge difference: Plastic does not nearly keep vibrating as long as metal.

  • Attenuation: Could this be the reason that there is so much reverb in my jar of gel? I know that waves can travel much farther in water(attenuation coefficient of 0.253) than in air(137.0) (according to this source) Water is a mayor component of most gels, including this one. However, since the effect works about as good when I remove the lid of the jar, I am not sure if this alone can cause the long reverberation effect.

So, I must be missing something. I hope someone can explain why sound in the the jar of gel is behaving this way.

Listen to a sound capture of the jar here You first hear a jar filled with cream, then the jar filled with gel. Then you hear both clips in the same order again, but this time slowed down to 1/10th of the original speed.

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1 Answer 1

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Hair styling gel often contains bubbles. Mixture of gel and air is far more compressible than gel without bubbles. Speed of sound in the mixture can be much lower than speed of sound in the air. It is called Hot Chocolate Effect.

Some graphs on this topic are computed here.

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Wow... this is an amazing insight. I did know about the Hot Chocolate effect as a nice trick, but never knew the way it worked and I would never have guessed a relation. You helped me a lot! –  Qqwy Oct 27 '13 at 0:01
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