The physical cause of sound when bonds and materials fracture

[Kapnal Loga]

Hello, I had a hiccup in explaining how sound is produced when materials break down. For example,

A) we are tearing paper,

B) we're unwinding duct tape. There is a break in the adhesion bonds.

The breakage results in a sudden release of energy, which is converted to kinetic energy in some way, creating sound. How does sound occur? What's vibrating in there to cause it? I'm interested in physical part of production of sound in these processes.

[exchemist]

1 hour ago, Kapnal Loga said:

Hello, I had a hiccup in explaining how sound is produced when materials break down. For example,

A) we are tearing paper,

B) we're unwinding duct tape. There is a break in the adhesion bonds.

The breakage results in a sudden release of energy, which is converted to kinetic energy in some way, creating sound. How does sound occur? What's vibrating in there to cause it? I'm interested in physical part of production of sound in these processes.

Any sudden movement will disturb the adjacent air. In both the cases you mention there is a stretching action followed a break in quick succession. When the break occurs, the stored energy in the stretch is released suddenly, causing a very tiny but rapid movement of a portion of the paper or tape. This will create a sound wave. There does not have to be - and in this case there won't be - a vibration, just a single motion. This is shown in fact by the absence of any discernible pitch or tone to the sound. It's more or less white noise. If there were vibration, that would cause a sound at a particular pitch, or pitches.   

[Kapnal Loga]

16 minutes ago, exchemist said:

Any sudden movement will disturb the adjacent air. In both the cases you mention there is a stretching action followed a break in quick succession. When the break occurs, the stored energy in the stretch is released suddenly, causing a very tiny but rapid movement of a portion of the paper or tape. This will create a sound wave. There does not have to be - and in this case there won't be - a vibration, just a single motion. This is shown in fact by the absence of any discernible pitch or tone to the sound. It's more or less white noise. If there were vibration, that would cause a sound at a particular pitch, or pitches.   

I realize it's about fast motion, but then why doesn't a flying bullet squeal like crazy? That's fast motion too, what's the difference? The sudden movement of the tape, I read somewhere that the wave along the tape can move at supersonic speeds, but how does this all relate and cause this sound specifically?

[exchemist]

17 minutes ago, Kapnal Loga said:

I realize it's about fast motion, but then why doesn't a flying bullet squeal like crazy? That's fast motion too, what's the difference? The sudden movement of the tape, I read somewhere that the wave along the tape can move at supersonic speeds, but how does this all relate and cause this sound specifically?

 A bullet most definitely does make a noise as it flies through the air. Whether you hear it as a whizz or as a crack depends on whether it is sub- or supersonic as it passes. But in the case of paper and tape, you also have something else: a resonator. The surface of the paper or the tape will be made to move when the breaks take place and this will make a larger volume of air move.  

Edited May 4 by exchemist

[swansont]

1 hour ago, Kapnal Loga said:

I realize it's about fast motion, but then why doesn't a flying bullet squeal like crazy? That's fast motion too, what's the difference? The sudden movement of the tape, I read somewhere that the wave along the tape can move at supersonic speeds, but how does this all relate and cause this sound specifically?

In rough terms it’s similar to a tsunami created by an earthquake - a sudden shift in a solid, and it pushes the fluid out of the way. The impulse creates a wave pulse. For something tearing, this is happening over and over, at a microscopic level.

[sethoflagos]

15 hours ago, Kapnal Loga said:

... The breakage results in a sudden release of energy, which is converted to kinetic energy in some way, creating sound. How does sound occur? What's vibrating in there to cause it? I'm interested in physical part of production of sound in these processes.

Fracture mechanics can become very involved very quickly, but if we limit our scope to simple brittle fracture...

Both fracture propagation speed and elastic recoil shock of a fractured solid are functions of the speed of sound in that material and typically many times the speed of sound in air. Therefore a space opens up faster than that information can be passed to the surrounding air resulting in a substantial vacuum. Eventually the pressure wave information gets passed on and the air rushes in at its own sonic speed to fill the void. 

When the void is filled the air it is brought to a halt with extreme rapidity and releases it's kinetic energy primarily as acoustic shock waves (variously known as 'surge' or 'water hammer').

This is the process that produces the loud 'crack' of a bullwhip.

Now rather than one big whip imagine several thousand cellulose fibre 'whips' doing the same thing per second at a microscopic scale.

That's the sound (at least a major component of it) of paper tearing.

 

[Kapnal Loga]

3 hours ago, sethoflagos said:

Fracture mechanics can become very involved very quickly, but if we limit our scope to simple brittle fracture...

Both fracture propagation speed and elastic recoil shock of a fractured solid are functions of the speed of sound in that material and typically many times the speed of sound in air. Therefore a space opens up faster than that information can be passed to the surrounding air resulting in a substantial vacuum. Eventually the pressure wave information gets passed on and the air rushes in at its own sonic speed to fill the void. 

When the void is filled the air it is brought to a halt with extreme rapidity and releases it's kinetic energy primarily as acoustic shock waves (variously known as 'surge' or 'water hammer').

This is the process that produces the loud 'crack' of a bullwhip.

Now rather than one big whip imagine several thousand cellulose fibre 'whips' doing the same thing per second at a microscopic scale.

That's the sound (at least a major component of it) of paper tearing.

 

I also thought about the vacuum shock, but if you consider duct tape: it has a kind of vacuum under the tape and kind of vacuum shock too. BUT: there is a silent tape, without cracking, it works due to the special laying of the adhesive layer. It is viscous and there is no tearing noise. So what is the cause of the sound, is it the breaking of the bonds, the vacuum or something together?

[sethoflagos]

6 hours ago, Kapnal Loga said:

I also thought about the vacuum shock, but if you consider duct tape: it has a kind of vacuum under the tape and kind of vacuum shock too...

Define 'kind of'. What makes you think there is a vacuum under the tape? What do you understand by the term 'shock'?

7 hours ago, Kapnal Loga said:

BUT: there is a silent tape, without cracking, it works due to the special laying of the adhesive layer. It is viscous and there is no tearing noise. So what is the cause of the sound, is it the breaking of the bonds, the vacuum or something together?

If it's 'silent', then what sound are you asking about?

[sethoflagos]

On 5/5/2024 at 6:07 AM, Kapnal Loga said:

So what is the cause of the sound, is it the breaking of the bonds, the vacuum or something together?

Okay... Small steps.

As the moisture content of paper rises from bone dry to something oto 30% by weight, the 'tearing' sound gradually diminishes to practically zero. Why do you think that is?

[Kapnal Loga]

On 5/5/2024 at 3:17 PM, sethoflagos said:

Define 'kind of'. What makes you think there is a vacuum under the tape? What do you understand by the term 'shock'?

If it's 'silent', then what sound are you asking about?

— I was referring to your shock waves of air called 'surge' or 'water hammer'.

— I mean, in some industries you can buy special scotch that does not have a crackle. Therefore, I am a little confused as to how this can be explained by an air strike.

3 hours ago, sethoflagos said:

Okay... Small steps.

As the moisture content of paper rises from bone dry to something oto 30% by weight, the 'tearing' sound gradually diminishes to practically zero. Why do you think that is?

Paper fibers are soaked in water, which makes them more flexible and weaker. When you tear wet paper, the fibers tear more easily, are more inert and don't move as fast, so there is less air disturbance. This makes the process of tearing paper less audible and silent. Right? I don't quite understand, is the sound created by the sudden movement pushing the air away or is the sound created by the air pushing under the void?

[Mordred]

Try thinking of sound due to vibration for starters then consider which materials tend to vibrate more or less than other materials. Some materials such as a tuning fork has a resonant vibration to match a specific frequency.

[sethoflagos]

Just to be clear, I'm leaving discussion of resonance effects etc to others. They're two separate topics.

3 hours ago, Kapnal Loga said:

— I was referring to your shock waves of air...

In context, a shock wave signifies a pressure discontinuity - a step change in pressure level rather than the piecewise continuous waveforms of normal sounds. It indicates that something somewhere has moved faster than the speed of sound in air. It is characteristic of brittle fracture.

3 hours ago, Kapnal Loga said:

Paper fibers are soaked in water, which makes them more flexible and weaker. 

No, that isn't it. When paper is wet, the fibre-fibre bonds are almost totally replaced by fibre-water bonds and water-water bonds. When torn it is the weak water-water bonds that dislocate leaving the fibres pretty well intact. Here there is no brittle fracture and hence no shock waves. The pieces come apart through viscous shear and what little energy is released is in the form of heat rather than sound.

This is the opposite end of the spectrum to brittle fracture.

Between these two extremes are various degrees of 'ductility' of which two common types are plastic deformation and viscoelastic deformation. They are a rather diverse bunch of mechanisms and are each separate topics in their own right.

3 hours ago, Kapnal Loga said:

 I mean, in some industries you can buy special scotch that does not have a crackle. Therefore, I am a little confused as to how this can be explained by an air strike

I'm not familiar with the product. I'd guess it could be somewhere in the viscoelastic regime but specialist adhesives would be more up @exchemist's street than mine.