When you put the thermos bottle next to your ear, it will make a buzzing sound. What mysterious substance is inside it?

When I was a kid, I went to the mall with my parents to pick out thermos bottles. I had this experience: I blew into the inner liner of the thermos and put the liner next to my ear. If there was a very obvious buzzing sound, it meant that the liner had good heat preservation properties.

This method is very practical, but have you ever thought about why this is the case? What is the scientific reason behind it?

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The generation of sound waves and resonance phenomenon

To understand this question, we must first understand how sound waves are generated and what is the resonance phenomenon?

Sound waves are mechanical waves that are produced by the vibration of an object . Sound travels through a medium (such as air, water, or solids), and when the sound source vibrates, it creates areas of compression and rarefaction in the medium, which produces sound waves. The frequency of the sound wave determines the pitch we hear, while the amplitude of the sound wave determines the loudness of the sound.

Tuning fork resonance

**Resonance refers to the phenomenon that when an object is subjected to external vibrations of the same or close frequency to its natural frequency, it will vibrate with a large amplitude. **Resonance is very common in daily life. For example, when we pluck a string of a guitar, not only will the string vibrate, but other strings tuned to the string will also resonate, thus making a sound. Similarly, the buzzing sound of a thermos is also the result of resonance.

The cause of the buzzing sound of the thermos

As we know, the basic structure of a thermos bottle includes the outer shell, the bottle liner and the bottle cap. The bottle liner is the core part of the thermos bottle, usually made of double-layer glass with a vacuum layer in the middle. This design can effectively block heat conduction and heat convection, thereby maintaining the temperature of the liquid in the bottle. The double-layer structure of the bottle liner can not only keep warm, but also isolate external noise.

The basic structure of a thermos

First, let's discuss why you hear a continuous buzzing sound when you blow into a thermos and hold it close to your ear? This is because when we blow, the air inside the bottle vibrates, and this vibration forms a standing wave with a specific frequency under the specific spatial conditions of the thermos, and this standing wave is the source of the sound. In fact, any container containing an "air column" can make a sound in this way , and many wind instruments make sounds based on this principle.

So why does a liner with good insulation make a louder and longer-lasting sound? This requires us to compare the difference between thermos bottles with good insulation and those with poor insulation. The insulation effect of a thermos mainly depends on isolating heat conduction: the liner is composed of two layers, an inner layer and an outer layer, and a vacuum state is formed between the two layers as much as possible. The higher the vacuum degree, the better the insulation effect . In this way, the phenomenon of "loud and long-lasting sound" is easy to understand, because the vacuum between the inner and outer layers not only blocks heat conduction, but also greatly reduces the energy dissipation of the standing waves in the "air column" . Standing waves with less energy dissipation will naturally make louder and longer-lasting sounds.

Now, let's go back to the original question: Why can we hear a buzzing sound when a thermos with good heat preservation performance is placed next to our ears even without blowing air? This is actually because the air column in the thermos selects the "white noise" in the surrounding air. The air around us is actually vibrating and producing sound all the time, but this vibration has no fixed frequency and is a "white noise" that does not carry information. The air column in the thermos has a filtering effect on this white noise , and can select the sound that matches the natural frequency of the "air column" . According to the previous introduction, we know that when an object is subjected to external vibrations with the same or close natural frequency as its own, resonance will occur, so the buzzing sound we hear is the effect of resonance . Here, white noise plays a role similar to "blowing air" . However, because the sound selected by the "air column" is very weak, we can only hear this weak sound in thermos bottles with slower energy dissipation and better heat preservation performance.

Other resonance phenomena in life

When traveling to the seaside as a child, I believe many of my friends have heard their parents say: put a conch to your ear and you can hear the sound of the sea. So why do the shells on the seashore emit the sound of the ocean? Where do these sounds come from?

In fact, this phenomenon is called " shell resonance ", and its principle is the same as the principle of the buzzing sound of a thermos. To put it simply in one sentence, the noise around the ear is amplified by the shell . We are usually surrounded by various noises: the sound of the wind around us, the hum of electronic devices, the sound of passing cars, etc. All these sounds around us add up to form a noise with a fixed frequency - white noise.

Since white noise is everywhere, our brain will automatically filter it out and our ears will automatically adapt, so we can't hear most of the noise. When the frequency of the vibration or sound from outside the shell is the same as the inherent frequency of the shell cavity, resonance will occur and amplify the sound.

When we put a shell over our ears, a resonance cavity is formed. Depending on the shape and size of the shell, certain frequencies of sound will be amplified . The shell collects the sound waves around the ear and resonates inside the shell, so that we can hear the noise around us, or amplify the surrounding noise. Because the air inside the shell vibrates up and down, this resonant sound is similar to the sound of waves, so people have a beautiful illusion that it is the sound of the sea. In fact, the sound of the ocean in the shell has nothing to do with the sea.

Planning and production

Source丨Popular Science of Physical Science (ID: lxcp_cstam)

Editor: Zhong Yanping

Proofread by Xu Lailinlin

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