Looks like the original flightglobal article got deleted, the link no longer works... Too late, the internet has it now, original text;
Airbus has modified its A320neo to eliminate the characteristic ‘barking dog’ noise generated by the A320’s power transfer system during ground manoeuvring.
The noise has been a familiar feature of the A320 family and originates in the power-transfer unit, a bi-directional device enabling the green and yellow hydraulic systems to pressurise one another.
It typically activates when the differential pressure between the two systems exceeds 500psi.
This activation is usually heard in the cabin as a distinctive repetitive sound from beneath the cabin floor, reminiscent of a high-speed drill, a buzz-saw or even a dog’s barking.
But Airbus A320 programme manager Klaus Roewe says that the airframer has developed a modification which “eliminates” this noise.
The noise is generated by vibrations within the pumps and the modification involves installing two additional dampers – one near the engines and the other near the wing root.
Airbus says the amendment means the issue has been “solved”.
But it stresses that, while the noise is a well-known aspect of the A320, it has never been a source of concern to customers.
Unfortunately the article is incorrect, how embarassing for Airbus. I assume the journalist made a gross misunderstanding, otherwise I am fearful of the single aisle ATA29 team's competency!
What is described, are probably the addition of attenuators / helmholtz resonators(which they are calling 'dampers'), which do help to reduce fluid borne pulsation and might help to reduce the in flight 'whine', but will do diddly squat to silence the PTU.
To stop the dog barking at the gate is very simple, simply turn the green system EMP off before shutting down the engines. Alternatively, press the PTU button on the hydraulics panel and it closes the PTU shut-off valve, if taxiing on the LH engine this would stop it operating un-necessarily during single engine taxi as well.
Personally I have seen passengers truly alarmed by it, and it annoys me because the purpose of the PTU is purely to help retract landing gear following a single engine failure, and can in fact be uni-directional - but because A320 is FBW, this drives you to a bi-directional one - and for some silly reason, the EMP is in the yellow system instead of the green where it's needed.
A bi-directional PTU with the EMP in the wrong system=dog bark.
This is fine except because the thirtiest landing gear consumers you're trying to support with the EMP are in the wrong system you have big efficiency losses trying to power them - the EMP is approx 76% efficient taking into account AC motor losses, volumetric and mechanical efficiency losses, but you're then turning two pumps connected by a shaft, losing a further 20% - in all, only half the delivered power becomes useable pressure. The rest becomes heat, and noise.
Heat in hydraulics is very bad. I personally spent a week in a lab watching a PTU slowly destroy itself from overheating (not A320, but easily could have been), as the elevated temperature breaks down the skydrol into corrosive microgel which then attacks the internals of the system, but this only occurs with low-demand prolonged 'chugging' that you all know as the barking dog. A single 'bark' by the way is the sudden spool-up of the PTU to 1500rpm or so before it crashes to a stop again. This creates a surge of flow until pressure balances, and is a saw-tooth stop-start profile.
As a result, I later sketched up a new design for a PTU that would greatly improve the redundancy and efficiency of a 3-system FBW aircraft by placing a back-driveable motor-generator between the two pumps driven through a differential gearbox and patented it, but at the time I was frustratingly not allowed to build it.
Rumour is, my patent is now being used on the Cessna longitude, and I'd love to see it implemented on A320, precisely because you would never hear it bark.
Resident TechOps Troll