Hi there "i625". Please read the forum rules for posting (click here). You seem to be missing a couple of things! :)
What units are you using? Give an example of what you calculated. You must use the correct units for the equation. If not, you wont get correct answers.and the resonant frequency seemed.. well.. incredibly high for a Helmholtz resonator.
Then again, maybe it's just that you aren't understanding what the equation says. :) Why did you want to mix metric and imperial in your calculations? Nobody said anything about measuring the speed of sound as 343 for the specific equation you are using... You cannot do that. Either use all metric or all imperial, and make sure you are using the right scale for all of your values. Eg. All inches, or all meters, or all feet, or all centimeters, etc. Inserting some numbers as imperial and some as metric will always give you the wrong answer in any equation, and even more so in one that includes a constant. You MUST use the same units as the constant, obviously, but you are not doing that. Better double-check your assumptions, and your units, and your calculations.Now, I may be missing something, or this may just be a completely fudged explanation on the author's part, but there is no possible way 343/(2*pi) can be 2160.
Yep... Very true. And notice the units... in that case the constant will not be 2160, and the dimensions could not be imperial either...but the Helmholtz resonator frequency is cited to be: f_res = c/(2*pi)*sqrt(A/(V*L)) where c is the speed of sound in m/s
... and you'd be wrong there too! :) The equation is correct as written, regardless of what you think about it, and it produces the correct results when used correctly, with the correct units. If you do that, then it will ALWAYS give an answer that is correct, but it requires a deeper knowledge of acoustics to determine if such a device can actually be built in practice, and whether or not it would provide useful absorption at the tuned frequency. You could, indeed build a slot-wall that is tuned to 35 Hz if you wanted to, but the dimensions for one that worked efficiently and effectively would be impractical for a real-world studio. To start with, the 1/5th wavelength "rule" would get in the way, as would the 1/10th volume "rule". Equations work when you use them correctly. And they don't work when you try to mix and match units that are unrelated to each other... They will still produce numbers, but the numbers will not bear any relation to reality. It requires understanding of what you are actually doing to determine if you areu sing the correct numbers.And I think it makes sense that the coefficient out front is supposed to be low, while all the other mass/spring-related terms stay within the square-root.
... and you'd be wrong there too! :) There's a lot of stuff that can be calculated very exactly in acoustics, and matches reality exactly as well. The equations do, in fact,work. I use them all the time when designing studios. No "jiggery pokery magic" required. That is only needed for people who don't really understand the underlying principles, and are just plugging numbers into equations blindly. There's still plenty of room for research in acoustics, for sure, but what is known so far is very usefully exact, and the equations that have been found so far are also quite reliable. They can still be refined, certainly, but they are plenty good and plenty accurate for the requirements of a home studio builder.but acoustics has too much jiggery pokery magic for anyone to try to be really exact about anything.
Really? They are? Silly people! They would be much better off just using the existing formula that are known to be correct, instead of "reinventing the wheel" so to speak, by trying to derive them all over again. Hopefully, they will arrive at the same equations that acousticians have already be using for years, quite successfully. 8) - Stuart -People are still trying to make empirical formulas for porous absorbers...!