1/4 wavelength placement for velocity or pressure?

[johndykstra]

THis has been confusing me for a while now.

I've read that porous (velocity based) absorption is most effective when placed in the 1/4 wavelength region of the frequency you are trying to absorb. But isn't 1/4 wavelength the region of highest pressure, telling us that a pressure based absorber would be more effective at 1/4 wavelength? Go easy on me.

[avare]

Originally Posted by johndykstra

THis has been confusing me for a while now.

I've read that porous (velocity based) absorption is most effective when placed in the 1/4 wavelength region of the frequency you are trying to absorb. But isn't 1/4 wavelength the region of highest pressure, telling us that a pressure based absorber would be more effective at 1/4 wavelength? Go easy on me.

THIN porous absorbers are most effective at 1/4 wavelength from a reflective surface. If you think about sound as ping-pong balls, the are slowest when right against a wall, and fastest when not slowed down by the wall.

Andre

[johndykstra]

But not necessarily fastest at 1/4 wavelength right? I mean, I understand the spacing from the wall, but 1/4 WL seems arbitrary... velocity should be pretty consistent throughout the wavelength. Shouldn't it?

[avare]

Originally Posted by johndykstra

But not necessarily fastest at 1/4 wavelength right? I mean, I understand the spacing from the wall, but 1/4 WL seems arbitrary... velocity should be pretty consistent throughout the wavelength. Shouldn't it?

Okay, think of a playground swing. Frequency is speed at which the swing goes back and forward. At the swing, meaning at it highest, the swing is not moving. This is the highest pressure point. At the lowest point of teh swing, it is moving at its fastest. This is the maximum velocity.

Andre

[johndykstra]

I see. That makes sense. I guess I was confused by the explanation of the "hump" of a sine wave being "greater than normal atmospheric pressure".