Do enclosure designs even matter?

[Vasubandu]

I have a question about all of the subwoofer designs out there. I understand the idea of appropriate volume, and to a degree, I understand the idea of controlling air flow and pressure. Those affect the physical environment and the physical actions of the driver.

What I do not understand is the idea that the enclosure has any acoustic effect of significance. A 20 Hz sound wave is 56 feet wide. 40 Hz is 28 feet, and 50 Hz is 22.5. All of those will pass through walls, and the lower frequencies will pass through concrete or steel So what exactly in a 6 square foot piece of MDF is going to stop or even affect one of those sound waves? What is left of the physicist in me says nothing. What would happen to one of those subwoofers if all of the internal elements were removed? I have no idea, but it is hard to see how the sound waves actually produced would change.

This is meant as a legitimate question. I do not know the answer, but things do not add up to me. If anyone can explain, I would appreciate it.

[Bill Fitzmaurice]

If not for the enclosure those long wavelengths would not be produced at all, the front and rear waves from the cone would meet and cancel each other.

A 20 Hz sound wave is 56 feet wide. 40 Hz is 28 feet, and 50 Hz is 22.5. All of those will pass through walls, and the lower frequencies will pass through concrete or steel

Sound waves do not pass through walls. They cause walls, floors and ceilings to vibrate. If they vibrate sufficiently they will cause a sound wave to be produced in the air on the other side. To make a wall, floor or ceiling soundproof isn't difficult, it just requires a combination of mass and stiffness adequate to keep them from vibrating enough to create a sound wave on the other side. With the large areas of typical walls, ceilings and floors that can require very robust construction, like a couple of feet of solid concrete. With the small areas of speaker cabinet panels they can be made from sufficiently braced plywood and still be soundproof.

[Vasubandu]

Thanks again Bill. That makes sense. I cannot tell you how many times I have read about low sound waves passing through walls. Your ability to make sense of these things in a way that I understand is invaluable. My own background is in light, Out of curiosity, do you know if the frequency of sound changes when it is emitted out of the back side of the wall? When light enters a denser medium such as glass, it slows down and refracts, but the wavelength does not change. Early on, I made the mistake of equating light and sound waves, but they are fundamentally different. Here, I am just curious if the phenomenon you describe would provide a lower Hz signal. Thanks.

[Bill Fitzmaurice]

Frequency doesn't change, but the spectral content does. It takes relatively little to block very high frequencies. At 10kHz a piece of paper is effective. The mass and stiffness required to contain sound grows exponentially as frequency goes down, so what's soundproof at 1kHz can be totally ineffective at 100Hz. That's why low frequencies are so difficult to contain. Light and sound waves do behave very differently. Many of the incorrect assumptions made about how speakers work are the result of equating them with light bulbs. Aside from both being sources of wave forms they have almost nothing in common.