- #1
trueacoustics
- 18
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First, I find the term "resonance" and its relatives to be used quite loosely. What is the actual definition according to physics/acoustics.
An example of its used:
The port of a Helmholtz Resonator resonates at the system's resonant frequency.
The port itself is not that thing resonating correct? It is rather the air within the port.
Second, how does the Helmholtz resonator actually work. It has been defined as follows by multiple sources:
"The vibration here is due to the 'springiness' of air: when you compress it, its pressure increases and it tends to expand back to its original volume. Consider a 'lump' of air at the neck of the bottle. The air jet can force this lump of air a little way down the neck, thereby compressing the air inside. That pressure now drives the 'lump' of air out but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring. The jet of air from your lips is capable of deflecting alternately into the bottle and outside, and that provides the power to keep the oscillation going." (University of South Wales: http://www.phys.unsw.edu.au/jw/Helmholtz.html)
However, this "lump" contained by the neck is just as compressible as the air inside. Similarly, it is just as springy. Therefore, how is it possible to separate this lump of air in the neck from the air inside?
This is supported by observing a reed pipe Helmholtz resonant system. The "neck" of the port is the same diameter as the body of the chamber. The lengths are adjusted, thus adjusting volume, while everything else remains constant. Intuitively, the pipes with larger internal volumes play lower tones.
It has then been established that by adjusting the cross sectional area of the neck and its length, you can adjust the resonant frequency of the system in a formulaic manner. This formula being greatly based upon the relationship between the mass of the air contained by the neck and mass of air contained inside the chamber. However, according to my assumption above, the total volume is simply being adjusted. This is where I run into a problem. If my assumption were true, the following would not be possible:
Given a fixed volume of air, we can create different resonant frequencies by adjusting the size and length of a port. Let's say that the port has a large diameter, but shallow depth. This would result in a relatively high resonant frequency. Now, let's make the port considerably smaller but keep the same length. The resonant frequency is now much lower. This is counter intuitive to previous assumption because the total volume has been lowered, but the resonant frequency has also been lowered.
My assumption is therefore wrong, and I am back to square one.
I then look towards Bernoulli's Principle. This principle intuitively relates the area of an opening with pressure. I just cannot see how it works with this system. This is greatly due to the inquires above. I am yet again back to my initial question. I am desperately trying to understand how this system works in an intuitive way. I am the type of person who needs to "see" what is happening to really "get it." I would appreciate any help. I have some more questions from which these questions have stemmed in regards to speaker enclosures. However, maybe if I can understand this, those questions will also be answered.
Thanks for your time,
Tony
An example of its used:
The port of a Helmholtz Resonator resonates at the system's resonant frequency.
The port itself is not that thing resonating correct? It is rather the air within the port.
Second, how does the Helmholtz resonator actually work. It has been defined as follows by multiple sources:
"The vibration here is due to the 'springiness' of air: when you compress it, its pressure increases and it tends to expand back to its original volume. Consider a 'lump' of air at the neck of the bottle. The air jet can force this lump of air a little way down the neck, thereby compressing the air inside. That pressure now drives the 'lump' of air out but, when it gets to its original position, its momentum takes it on outside the body a small distance. This rarifies the air inside the body, which then sucks the 'lump' of air back in. It can thus vibrate like a mass on a spring. The jet of air from your lips is capable of deflecting alternately into the bottle and outside, and that provides the power to keep the oscillation going." (University of South Wales: http://www.phys.unsw.edu.au/jw/Helmholtz.html)
However, this "lump" contained by the neck is just as compressible as the air inside. Similarly, it is just as springy. Therefore, how is it possible to separate this lump of air in the neck from the air inside?
This is supported by observing a reed pipe Helmholtz resonant system. The "neck" of the port is the same diameter as the body of the chamber. The lengths are adjusted, thus adjusting volume, while everything else remains constant. Intuitively, the pipes with larger internal volumes play lower tones.
It has then been established that by adjusting the cross sectional area of the neck and its length, you can adjust the resonant frequency of the system in a formulaic manner. This formula being greatly based upon the relationship between the mass of the air contained by the neck and mass of air contained inside the chamber. However, according to my assumption above, the total volume is simply being adjusted. This is where I run into a problem. If my assumption were true, the following would not be possible:
Given a fixed volume of air, we can create different resonant frequencies by adjusting the size and length of a port. Let's say that the port has a large diameter, but shallow depth. This would result in a relatively high resonant frequency. Now, let's make the port considerably smaller but keep the same length. The resonant frequency is now much lower. This is counter intuitive to previous assumption because the total volume has been lowered, but the resonant frequency has also been lowered.
My assumption is therefore wrong, and I am back to square one.
I then look towards Bernoulli's Principle. This principle intuitively relates the area of an opening with pressure. I just cannot see how it works with this system. This is greatly due to the inquires above. I am yet again back to my initial question. I am desperately trying to understand how this system works in an intuitive way. I am the type of person who needs to "see" what is happening to really "get it." I would appreciate any help. I have some more questions from which these questions have stemmed in regards to speaker enclosures. However, maybe if I can understand this, those questions will also be answered.
Thanks for your time,
Tony