Read the paper from the OP now. Where does it come from? It doesn't appear to be written like any technical paper I've read - how many bother to tell you the colour they painted the box? It seems like a justification for a DIY project.
Not by the thread-starter is it?
Not by the thread-starter is it?
No, the document in question was written by Dr Marek Natkaniec.
The OP considers it very important because it was written by a university professor.
A couple of other details are relevant, though:
A) It was written while Dr Natkaniec was still a student.
B) It was published in a student magazine at a university of mining and metallurgy.
C) Neither Dr Natkaniec's field of study, qualifications, subsequent career or any of his other published work has anything to do with either acoustics in general or loudspeakers in particular.
While I respect Dr Natkaniec and have no doubt he is an expert in his field, I think it is worth noting that his field of expertise has nothing to do with acoustics.
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Your explanation, in either post, makes no sense to me. In my opinion, your reference to line arrays is not relevant to TL designs and is just another diversion. Clearly you do not understand the boundary conditions applied to open ended pipes when solving the acoustic wave equation, Dave provided a pretty good reference. You have also avoided the three questions that would assist people in assessing your method, so I ask again.
A driver with an fs of 20 Hz is optimal in a TL tuned to 30 Hz? None of the frequencies associated with 20 Hz will play a significant role for this TL length.
Is this for any driver with an fs of 20 Hz independent of the rest of the T/S parameters?
What is the cross-sectional area of the TL?
If you will not or can not provide some more specific information about your TL design I don't really see much point in continuing the discussion.
The standing waves pulses don't require reflection in this case to cancel one another. The net result of two pulses of opposite pressure confined in a resonant cavity is cavitation - based cancellation.
There are entire textbooks written on the phenomenon of cavitation. Before you mock or criticize without a factual basis, I suggest you read one thoroughly.
For yet one more analogy to instruct those who claim to have a firm grasp on the physics involved - ever witness a propeller undergoing cavitation? The propeller continues to rotate at high speed but it no longer continues to pump water. Ever consider how that's possible? Do you think this phenomenon is possible with a speaker exciting sound waves in a pipe or have you already conducted intense scientific studies and experiments on standing waves in pipes to disprove that assumption? If so, would you mind sharing your experimental data?
That's your assertion- would you mind sharing your experimental data?
Maybe I've missed it, but have you shown ANY experimental data?
Cavitation has nothing to do with the fundamental TL engineering design questions being covered in this thread, this is just another attempt to divert the discussion from the topic at hand. So far you have provided nothing beyond some simple wavelength calculations and a lot of hand waving and random discussion.
There are three states of matter, solid, liquid and gas. A plasma is sometimes referred to as a 4th state of matter. Cavitation involves a change of state form a liquid to a gas. In pumps cavitation occurs when the pressure, for some reason, drops below the vapor pressure of the liquid being pumped resulting in flashing of the fluid from the liquid phase to the vapor phase. Since there is no phase change involved with a speaker driving air in a duct there is no possibility of cavitation.
Here is proof: http://en.wikipedia.org/wiki/Bonneville_(crater)
I guess it is as relevant as cavitation. 🙂
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Re: Cavitation in air
The theoretical limit for a sinusoidal sound wave is a minimum pressure of zero (a total vacuum) and a maximum pressure of 2 atmospheres, which is a level of 191.1 dB SPL
Sort of shocking to think the "loud" car folks have gotten to within 10dB of that.
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The more you get to know me, the more evidence you will see that what I say and believe is true:
Practically everything I know, I've learned from others. In my frame of reference, the contributions of many are the key to advancing understanding. Without these collective contributions and a general tone of respect for the contributions of others, progress as we know it would grind to a halt.
I don't need to be the one providing textbooks and experimental data to be able to understand the concepts involved and to support conclusions by others that are rational and coherent. The world doesn't revolve around me. I read a great deal because I am genuinely fascinated by and appreciate the work of others. I don't see my understanding or lack thereof as a weakness or opportunity to compete. To me, it's all about the thrill of enlightenment and appreciation for our "world of wonders".
So in that spirit, I would like to redirect your attention to the published works from experts such as Meyer Sound and F. Ronald Young (Cavitation). I didn't make this stuff up. But in the course of reading, maybe you can decipher whether they have or if there really is something to what they claim.
You know the old saying, believe 1/2 of what you see and none of what you hear.
"More importantly, you keep talking about a reflected wave off the open end of a pipe as if both ends were closed off. The only way you can get a reflection of a sound pressure wave is with a substantial shift in density. Since no density shift exists to support that theory - I'm not sure we can have a meaningful, productive conversation."
The first statement is completely wrong, I agree with the conclusion.
The first statement is completely wrong, I agree with the conclusion.
It would appear that someone with the same misguided through process has also contaminated the Wikipedia page on Acoustic Transmission Lines , and just this morning!
Let me provide a simple explanation of why there is a reflection off an open ended duct. Consider a plane wave propagating down the length of the duct. The air in front of the wave is at rest. As the wave passes through the duct, it accelerates the air behind it to some velocity, u. The direction of u will depend on whether the wave is a compression or expansion wave. U will be toward the open end for a compression wave, and in the opposite direction if the wave is an expansion wave. When the wave reaches the end of the duct, it continues to propagate into the surrounding environment. Since it is no longer confined the the duct, if it is a pressure wave it will expand into the lower pressure environment. Since we are talking about a wave of low amplitude the velocity, u, is less then the speed of sound. Thus, a pressure discontinuity can not exist at the end of the duct once the wave passes. As a result, an expansion wave is initiated at the end of the duct when this propagates back up the duct toward the source of the original wave. It is a simple result based on conservation of mass, momentum and energy.
Okay, then, what's preventing you from taking a different driver and designing a different TL for it based on your chosen design principles, then modeling it and letting all of see predicted performance in graphical form? Is there some reason this isn't possible, or are you simply reluctant to present your design for criticism? [I was going to say "present your design for peer review", but I doubt you think many people here qualify to be your peers.]
Paul
Paul
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