Engine Tone (pitch actually)

[deafsound]

I spent a lot of time flying over the past couple of weeks, and most of it was solo and over mountains, so I was really zoned in with the airplane.

I noticed something interesting. The pitch of the airplane engine would change depending on which way my head was facing. If I was looking forward it'd be one pitch, and if I looked left or right it'd be another pitch, about a half step or so lower, although it might have been less than a half step. I know the engine wasn't actually chaging pitch because I was watching the tach. It must've been some psycho-acoustic thing.
Anyone else ever noticed that? First few times it happened to me, it kinda freaked me out.

 

[Dave Krall CFII]

I spent a lot of time flying over the past couple of weeks, and most of it was solo and over mountains, so I was really zoned in with the airplane.

I noticed something interesting. The pitch of the airplane engine would change depending on which way my head was facing. If I was looking forward it'd be one pitch, and if I looked left or right it'd be another pitch, about a half step or so lower, although it might have been less than a half step. I know the engine wasn't actually chaging pitch because I was watching the tach. It must've been some psycho-acoustic thing.
Anyone else ever noticed that? First few times it happened to me, it kinda freaked me out.

That's just due to sterio acoustics of the two ears.

 

[Pilawt]

I know what you mean. It may be that when you are facing straight ahead, part of what you are hearing is reverberations from the side window just an inch or two from your head. If you turn your head 90 degrees to one side, the nearest sound-reflecting surface is much farther away, resulting in a different perceived tone.

Not long ago I was concerned that my engine sounded different than it had in the past, and it had nothing to do with what direction my head was turned.

The answer was right on top of my head. I was wearing a different baseball cap from my usual airborne attire, and it caused my noise-cancelling headset to fit less snugly over my ears, resulting in a different perception of the normal sound.

-- Pilawt

 

[gismo]

The spectrum of sounds in a typical GA cockpit is a combination of broadband (white or pink noise) and several specific frequency peaks. Many of those frequencies are related directly to the rotational speed of the engine and some are unrelated. When you turn your head or move it around the cockpit you are likely to find that different peaks are loudest due to your orientation to the surface conducting the sound in, and any resonances of those surfaces. In addition, IIRC there is an acoustic illusion where changes in the sound level can be perceived in pitch changes even though the spectra was unchanged.

 

[mmilano]

That's just due to sterio acoustics of the two ears.

you'de more than likely hear different sounds if you recorded a single microphone rotating 360 degrees, .. so it's probably not all stereo acoustics.

my guess would be that you can hear more of the high end spectrum when facing something directly. since higher sounds don't travel as far or penetrate through things like the larger lower sound waves.

i haven't payed attention myself, but with this theory, the sound would go lower in tone the more you turned your head away from the engine.

 

[deafsound]

you'de more than likely hear different sounds if you recorded a single microphone rotating 360 degrees, .. so it's probably not all stereo acoustics.

You're right, and that's due to phase shift, but I'm talking about my head being stationary looking out the left window, so it's not ( I don't think) that or doppler shift.

T In addition, IIRC there is an acoustic illusion where changes in the sound level can be perceived in pitch changes even though the spectra was unchanged.

Yes, bass players wearing headphones in recording studios have a hard time hearing the correct pitch of their instrument. Usually they wind up tuning a little sharp. Something about having a set of speakers a half inch from your ear makes it sound flat. This is a psycho acoutic thing.

Anyway, very disconcerting when it sounds like you've lost 50-100 rpm everytime you look out the window.

 

[judypilot]

Also, if you have slight differences in your hearing in each ear, this would make a difference. I have substantially less hearing acuity in one ear, and it's very uneven (I hear high and low frequencies better than midrange), and I get all kinds of weird aural effects because of that. Of course, I've been like this since I was an infant so most of the time I mentally adjust even before I notice it, but sometimes the effects are very noticeable.

Judy

 

[Don Jones]

Always happens when you fly over rough mountains or open water. The airplane always sounds different and goes into auto-rough mode. lol
Don

 

[Alan]

Had an instructor riding safety pilot a few years ago while I brushed up on approaches. We were in a Baron and it was dark. Somewhere along the way he lurched and yelled "What was that?" I hadn't heard anything and told him so. After 4-5 seconds he figured out that his headset had touched the side window and changed the tone. I quietly had a good chuckle.

 

[Everskyward]

I noticed something interesting. The pitch of the airplane engine would change depending on which way my head was facing.

I'm surprised you noticed anything at all, based on your user ID.

Sorry, I had to say it!

 

[deafsound]

I'm surprised you noticed anything at all, based on your user ID.

Sorry, I had to say it!

Heh, heh.
My user ID came about many years ago thanks to AOL not having room for enough characters to fit "deafsoundguy".
Kinda like slowhand or stumpy as a nickname.
My hearing is quite good. Last time I had it checked (about a year ago, and it was fine), I noticed the the machine that tests your hearing was not playing accurate tones. It was supposed to be a tone at 1khz, then 2khz, and the 2khz was a little flat.
After that, they asked me if I wanted to take part in a hearing perception study at MIT. They paid me enough for a couple hours of flying, thank you very much.
Anyway, I'm very sensitive to pitch to the point of aggravation, or in the case of flying over mountains and turning my head, fright.

 

[Dave Krall CFII]

Heh, heh.
My user ID came about many years ago thanks to AOL not having room for enough characters to fit "deafsoundguy".
Kinda like slowhand or stumpy as a nickname.
My hearing is quite good. Last time I had it checked (about a year ago, and it was fine), I noticed the the machine that tests your hearing was not playing accurate tones. It was supposed to be a tone at 1khz, then 2khz, and the 2khz was a little flat.
After that, they asked me if I wanted to take part in a hearing perception study at MIT. They paid me enough for a couple hours of flying, thank you very much.
Anyway, I'm very sensitive to pitch to the point of aggravation, or in the case of flying over mountains and turning my head, fright.

Don't let it give you a start too much in the mountains when you turn your head. Studies have shown the engine can't see you turn your head any better in MTNs than over flatlands or water...