A320 noise change in descent at 1.8 Delta PSI

[applecrumble]

Hey guys,

I have noticed that in the descent there is a noise change at 1.8 delta PSI. (Cabin VS-300 and cabin alt 500 feet).
This sounds like a change to the air con air as if there is more air flow coming out of the vents. A slight rushing sound. I'm sure that it's not anything to do with the air cond but that's the only way I can describe it.
This happens every descent. Any ideas?

[Intruder]

High-pressure bleed valve opens when the engines are brought to idle, causing a temporary surge.

Outflow valves open to reduce the pressure differential, allowing greater air flow through the airplane.

[Check Airman]

The sound is evedint in climb and descent at a delta of 1.8-1.9 PSI, usually around 3500-4000ft.

I always thought it was caused by the seals at the windows closed up properly. The change in noise is quite noticeable below that altitude.

[LEVEL600]

Increased cabin air flow due to fuselage drain valves opening? Don't remeber pressure treshodl for ops. just now..

[Zaphod Beblebrox]

Isn't there some type of change in configuration of avionics cooling? The exhaust door closes and the whole system re-configures to the closed mode. I can't remember the actual conditions but it is both temperature and pressure differential dependent.

This would account for the perceived change in sound on the flight deck.

[lurkio]

You will find it happens on departure at that diff pressure as well. My guess has always been avionics cooling outlet flap opening/closing.

[dream747]

I've always noticed this sound too, especially on descent passing around 4,000ft. From my experiences, the bleed page shows that the HP bleed valve still closed.

[mcdhu]

Isn't it to do with the "keel valves" as discussed below:
http://www.pprune.org/tech-log/435469-a320-cockpit-air-noise-change-climb-descent.html?highlight=keel+valves

mcdhu

[Goldenrivett]

Isn't it to do with the "keel valves" as discussed below:

Yes.

Also discussed in 2007 by Aztec Kid http://www.pprune.org/tech-log/291248-a320-noise-again.html

& a Boeing Patented design 1983: Patent US4463774 - Fuselage-mounted valve for condensate drainage and cabin-air pressurization - Google Patents (http://www.google.co.uk/patents/US4463774)

"The invention pertains to condensate draining, air pressure sealing valves that are mounted on an aircraft fuselage at various positions along the belly to drain off condensate collected in the bilge-like area of the fuselage, and to close in response to cabin air pressurization to form an airtight seal."

On A320 they open / close around 1.8 delta psi.

[applecrumble]

Brilliant! Great info thank you for that.
What actually causes the increase in sound in the descent when they open?
Is this the cabin air escaping the fuselage via those holes?