resonant freq.

[Doug]

Does anyone know the formula for finding the resonant frequency of an enclosed space? I don't mean the resonant frequency of an enclosure with a speaker in it. The enclosed space could mean a speaker enclosure or a room.

Thanks; Doug

[Milzie]

Its not that simple as to apply a formula. It involves the various materials used, the geometry, and a whole lot of factors. If your interested in actually calculating this, get a good machine dynamics book.

[SY]

If you make simplifying assumptions, like perfectly relective, rigid boundaries and loss-free interior, the equations are quite straightforward once you've set the boundary conditions. What shape space do you have in mind?

[Kanga]

Doug

If you have a rectangular space, then there are a series of resonances across each of the parallel walls, starting when the half the wavelenth is equal to the distance between the walls, then 1x, 1.5x 2x etc the wavelength. You can calculate the wavelength from the formula :
Wavelength (m) = speed of sound in air (345 m/s) / frequency (Hz)

So a frequency of 100 Hz has a wavelength of 3.45m, and the first resonance will occur across walls 1.725m apart.

There are also tangential and oblique modes which occur between non-parallel walls.

Have a read of the Harman white papers by Floyld E Toole, which explains all about room acoustics and should answer the question much better than I can here. See http://www.harman.com/wp/index.jsp?articleId=65

Mick

[Milzie]

SY,
A simplidied equation is only that, it wont give the actaul natural frequency, it may be close enough for his purposes, but the purposes were not specified.

Kanga,
He wanted the natural frequency of the room or enclosure, not the modes of a given frequency within that space.

Im not trying to be rude or make it seem like its impossible, its just that the theory of vibrations is no where near as simple as a lot of people think.

The simplest way to calculate the natural frequency is with a signal generator and spl meter. Sweep the signal at constant amplitude, the frequency at which the spl meter is highest will be the spaces natural frequency.

Jeff

[Kanga]

Maybe Doug could clarify what he means by resonant frequency, so that we can get on the same wavelength...

[Milzie]

No pun intended right?????

[MJK]

To tell the truth, I don't understand the question. But I do have a comment on the following statement :

The simplest way to calculate the natural frequency is with a signal generator and spl meter. Sweep the signal at constant amplitude, the frequency at which the spl meter is highest will be the spaces natural frequency.

Depending on where you place the signal generator and the mike you may not pick up all of the space's modes. In my opinion, the simplest way to calculate all of the natural frequencies and mode shapes of a given space is with a finite element acoustic program and then correlate the results with some testing.

But I still do not understand what the original post was asking.