engine noise pitch change

[airball]

Can some one explain why the jet engine noise going over head changes so radically in pitch some times. The aircraft are invariably coming towards and the change can be up and down sequentially.I do not believe it to be throttle inputs,is it doppler effect if so why only on jet engines? Is the noise produced by certain A/C types or engine?
Thanks i a.

[speedbird_heavy]

I think what it is, is as the aircraft is coming towards you, you hear the high pitch whine of the turbine blades and as it moves away you hear the rumble of the cumbustion process.

[BRISTOLRE]

There was a very detailed thread on this recently. Maybe it was in this forum or rumours, can't remember.
It discussed the same topic but specificly in relation to the B777 and GE vs. RR engines.

[747FOCAL]

speedbird_heavy has it mostly right. A lot depends on the which engine your talking about.

Another cause of change in tone or the addition of a tone will happen when a High Pressure Bleed Valve opens during spool down on approach. Then you get the tuba effect. Pratt powered 757s are famous for this. Also why RR powered 747 and 767 have been in the news every now and then........ :E

[Milt]

Doppler

Doppler

Doppler effect.

[dudduddud]

Notice how the same thing occurs with passing cars when you are standing still? Maybe I am about to describe the doppler effect, who knows but as I understand the situation, here's what happens:

Wavelengths are measured in meters and centermeters (or w.h.y.) and take a period of time to complete a cycle. If the object is moving towards you, the movement during the cycle will compact it and increase its frequency. Vice versa when the object is moving away from you.

[Notso Fantastic]

Simple mentions of the Doppler Effect do not explain the noises. Where I am in Portsmouth, I get a lot of London bound jets descending and at about 10,000'+. As they pass overhead, there is a 'spool up' sound' then the noise appears to resume something similar to its previous sound. Doppler explanations do not cover the few seconds of this previous noise. It happens again and again, so I am inclined to think that if you were to draw a frequency 'map' of the sound below jets with this sort of power set, it would show some funny effects immediately below the aeroplane.

[747FOCAL]

Without going into Noise 101 the noises that you are hearing are bleeds opening or they are other turbo machinery noises that are always there you just can't hear them until power comes down to the point they are no longer buried in the broadband noise.

[airball]

Notso Fantastic
I am in the Goodwood area and obviously experience the same dramatic engine noise .Doppler is not the sole reason for the change.I like the turbine and bleed explanations though.

[LoGo]

Probably has got something to do with the speedbrake being raised or lowered.

[Notso Fantastic]

I think 747Focal is most probably right. I live in north Portsmouth and it repeatedly seems to happen in the same area of sky. I descend in this area myself and there is no reason for a particular event to take place in this location. But the sound change is large in frequency- sometimes it seems to be more than just bleeds. I think maybe we need to go into Noise 301 for the explanation- I'm inclined to think the noise footprint over the ground has a frequency spectrum along it that isn't explained by Doppler effects.

[Farty Flaps]

Who cares, i just fly em. All i can hear is the sound of my own voice,....ah sweet music.:}

[WHBM]

I guess many of us on here know of all sorts of aircraft features that change noise when they are operated.

Out here in East London, if you are in say Stratford 146s outbound from London City will retract flaps as they overfly at 3,000', a very distinctive noise when inside the aircraft but also audible from the ground.

And the Heathrow inbounds also change noise considerably as they spool up as they stop descendind, which corresponds to what you hear on board.

It's a good post to ask a point I've noticed from our office car park in Slough looking up at the Heathrow departures. What is it on the IAE powered A321s (eg BMI, Lufthansa) that makes them sound like a buzz-saw sometimes when on climb power?

[747FOCAL]

The buzzsaw noise is the blade tip interface with the fan ring and the turbulent air coming into the nacelle. The A320 now offers a hardwall forward acoustic panel that lowers buzzsaw noise. Things like probes etc that stick out into the nacelle will cause fan tones as well because of the swirl put off by the probe. Fan blades like to see uniform no turbulent air or lots of different sounds pop up. :)

[WHBM]

Just to follow on about the IAE buzzsaw noise, was on a BA A320 yesterday with these, seat row 5, on departure the noise was sufficiently pronounced that pax all started commenting on it, and it was loud enough to be irritating. It was throughout the climbout. You would have thought we were fitted with props !

Given that it's quite audible from the ground too, how does this get past the noise regs ?

[747FOCAL]

The buzzsaw noise is accounted for in aircraft noise certification. It just seems more pronounced as it resides in the "irritant" spectrum of noise. I.e. same as dragging finger nails on a chalkboard.