Cabin noise (where does it come from)

arkvet

arkvet

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so I downloaded an app on the iPhone that is a decibel meter. I thought it would be cool to stick the phone in all sorts of places and see where the sound seems to be coming from. I'm not real sure how scientific this was/is?

Anyway. Seems like the loudest place in the place was right at the pilot / copilot head position. At a 70% HP power setting at cruise it was around 100 decibels. It seemed to be around that 100 decibel range all around the front of the plane except floor region where it was actually low 90's. I could also reach back behind me to the pax area and the sound dropped into the low 90's as well. Right up against the panel the sound level seemed to drop a little too. I'll attribute that to the panel blocking some sound.

I'm flying a pa32 so there is a forward baggage compartment that puts some space between me and the engine.

I figure this overall regional difference is pretty common within the plane. I'm just wondering if I can use this information to potentially identify where most of the sound comes from in an effort to quieten the plane in the future.

Door seals, old wind screen, insulation... where is most of the sound getting through or is it likely a combo of all of the above.

Would be nice to be able to let pax go headset free on occasion but sitting behind a 300 hp fan up front is going to be noisy.

And yes I understand I can dial back to lower power settings and decrease the sound level. That's obvious, but I'm looking for relative sound reduction.

Not a big issue for me just something I was looking at during my flight yesterday.
 
Wind noise, prop noise, and engine noise are the culprits. You can go to thicker windows, new (not squashed flat) seals, and more sound insulation. With those changes, you might achieve a modest reduction, but some of those come with a weight penalty.
 
I wonder how well acoustic panels would help. They absorb sound, not block it. I was looking into these years ago for making a room sound better to listen to music in. A place I worked at once had them on the walls to deaden sound. You don't have to cover the entire wall. Just a little here and a little there works. Putting some little pieces of it around in the cockpit could help, maybe. Google "sound absorbing."
 
Glider. Just saying, if ya wanna minimize noise. :D
 
arkvet said:
Seems like the loudest place in the place was right at the pilot / copilot head position.
Click to expand...

Tell them to stop shouting at each other!

I knew someone who used high tech (80's) dB equipment on his Queenair, then install the very best sound-deadening stuff in every conceivable place.
The net effect? Thousand spent, no change in cabin dB!
Stick to the ENC headsets, use slower prop speeds is the moral.
 
I'd say the primary noise source is engine noise, since on an industrial noise meter the noise at warm up is within a few dB of the noise at full power ops. The increase in noise from prop and wind is very small compared to the engine noise component when sitting in the tie down.
 
The engine and prop vibrate the entire airframe. Difficult to stop that. Even with an electric motor you'd still have to deal with the prop pounding the windshield and the vibration from assymetric blade AoA in the climb.
 
My new CFI from Maui informs me that if you buy an AOA which is now 10% off by the way, you can use it to drop the decibels sufficient enough to not need sound dampening headphones. Pretty cool
 
Wind noise is a small component. It's amazing how quiet the thing is with the engine off or even at idle. I believe most of the noise comes from the prop (and the prop blast) with the engine itself a good second.

Better dampning on the firewall and thicker windows helps a lot.
 
arkvet said:
so I downloaded an app on the iPhone that is a decibel meter. I thought it would be cool to stick the phone in all sorts of places and see where the sound seems to be coming from. I'm not real sure how scientific this was/is?

Anyway. Seems like the loudest place in the place was right at the pilot / copilot head position. At a 70% HP power setting at cruise it was around 100 decibels. It seemed to be around that 100 decibel range all around the front of the plane except floor region where it was actually low 90's. I could also reach back behind me to the pax area and the sound dropped into the low 90's as well. Right up against the panel the sound level seemed to drop a little too. I'll attribute that to the panel blocking some sound.

I'm flying a pa32 so there is a forward baggage compartment that puts some space between me and the engine.

I figure this overall regional difference is pretty common within the plane. I'm just wondering if I can use this information to potentially identify where most of the sound comes from in an effort to quieten the plane in the future.

Door seals, old wind screen, insulation... where is most of the sound getting through or is it likely a combo of all of the above.

Would be nice to be able to let pax go headset free on occasion but sitting behind a 300 hp fan up front is going to be noisy.

And yes I understand I can dial back to lower power settings and decrease the sound level. That's obvious, but I'm looking for relative sound reduction.

Not a big issue for me just something I was looking at during my flight yesterday.
Click to expand...
Headset free really isn't a problem in a Piper Malibu which has that 300 hp fan in front of you. I suspect the more substantial structure (windows included) and sealing from having a pressurized cabin does the job.

You can easily carry a conversation with normal voices in all parts of the cabin without a headset. I don't bother to give passengers in the back one at all. Mostly because I have no interest in hearing about the latest developments on the bachelor tv show while shooting an approach.
 
I found my notes from doing noise tests in my own airplane using a real noise meter.
PA-12, 160hp w/8242 prop. 1200 rpm warm-up 85db. 2450 rpm cruise 98db. Full throttle 2700 rpm cruise 102db. My Cessna 180 w/ 0-520 and 86" prop was 3-4dB quieter across the range. That surprised me at the time since I was comparing an 82" prop at 2700 rpm to an 86" prop at 2800+.
 
I used to tell the flight attendants that it was the speed of heat. Then to prove it to them I'd have them touch the windscreen, which was heated but they didn't need to know that. :D
 
I did not measure before and after, but my new 3 blade Hartzel scimitar seemed quieter than the old 2 blade it replaced. I called Hartzel tech support and asked if there was a correlation to the new prop.

Hartzel said yes, the fact that 3 smaller impulses are striking the windshield rather than 2 larger ones will affect sound level.
 
jesse said:
You can easily carry a conversation with normal voices in all parts of the cabin without a headset. I don't bother to give passengers in the back one at all. Mostly because I have no interest in hearing about the latest developments on the bachelor tv show while shooting an approach.
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That's why I like flying planes with the crew isolate switch on the intercom. :D
 
mscard88 said:
I used to tell the flight attendants that it was the speed of heat. Then to prove it to them I'd have them touch the windscreen, which was heated but they didn't need to know that. :D
Click to expand...

And they probably told you the warm chocolate chip cookies for the pilots was a special recipe so you wouldn't smear any on your uniforms. :)
 
arkvet said:
so I downloaded an app on the iPhone that is a decibel meter. I thought it would be cool to stick the phone in all sorts of places and see where the sound seems to be coming from. I'm not real sure how scientific this was/is?

Anyway. Seems like the loudest place in the place was right at the pilot / copilot head position. At a 70% HP power setting at cruise it was around 100 decibels. It seemed to be around that 100 decibel range all around the front of the plane except floor region where it was actually low 90's. I could also reach back behind me to the pax area and the sound dropped into the low 90's as well. Right up against the panel the sound level seemed to drop a little too. I'll attribute that to the panel blocking some sound.

I'm flying a pa32 so there is a forward baggage compartment that puts some space between me and the engine.

I figure this overall regional difference is pretty common within the plane. I'm just wondering if I can use this information to potentially identify where most of the sound comes from in an effort to quieten the plane in the future.

Door seals, old wind screen, insulation... where is most of the sound getting through or is it likely a combo of all of the above.

Would be nice to be able to let pax go headset free on occasion but sitting behind a 300 hp fan up front is going to be noisy.

And yes I understand I can dial back to lower power settings and decrease the sound level. That's obvious, but I'm looking for relative sound reduction.

Not a big issue for me just something I was looking at during my flight yesterday.
Click to expand...
What app did you use? My current plane is the quietest single engine plane I've ever flown in (or maybe I've just lost more hearing, IDK which), and I'd like to measure to compare.
 
GRG55 said:
And they probably told you the warm chocolate chip cookies for the pilots was a special recipe so you wouldn't smear any on your uniforms. :)
Click to expand...

Cookies? Warm chocolate chip ones? At the airline I flew at? :rofl::rofl::rofl:
 
Harold Rutila said:
That's why I like flying planes with the crew isolate switch on the intercom. :D
Click to expand...
It has that feature. However the wimmen don't like it if they know I muted them. However if I just don't give them headsets to begin with they don't connect the dots.
 
tawood said:
What app did you use? My current plane is the quietest single engine plane I've ever flown in (or maybe I've just lost more hearing, IDK which), and I'd like to measure to compare.
Click to expand...
The app is called Decibel 10th. Seems to work well. IIRC decibel values are not linear so the difference between 90 and 100 decibels seems pretty significant.

My plane has original windows that I'm sure don't block out much noise at all. I think I'm going to put new thicker windscreen on my to do list.
 
arkvet said:
The app is called Decibel 10th. Seems to work well. IIRC decibel values are not linear so the difference between 90 and 100 decibels seems pretty significant.

My plane has original windows that I'm sure don't block out much noise at all. I think I'm going to put new thicker windscreen on my to do list.
Click to expand...
So I measured mine last night...

Mine shows exactly what yours does...low 90's at floor and passenger area, right about 100 (+- 2) at pilot/copilot head high. So either we have planes at the same noise level, or that app is not that sensitive...
 
The dB apps show total noise trends but next to a real noise meter not very accurately. Not surprising when you look at the configuration of a cell phone microphone compared to a real noise meter.
 
arkvet said:
so I downloaded an app on the iPhone that is a decibel meter. I thought it would be cool to stick the phone in all sorts of places and see where the sound seems to be coming from. I'm not real sure how scientific this was/is?

Anyway. Seems like the loudest place in the place was right at the pilot / copilot head position. At a 70% HP power setting at cruise it was around 100 decibels. It seemed to be around that 100 decibel range all around the front of the plane except floor region where it was actually low 90's. I could also reach back behind me to the pax area and the sound dropped into the low 90's as well. Right up against the panel the sound level seemed to drop a little too. I'll attribute that to the panel blocking some sound.

I'm flying a pa32 so there is a forward baggage compartment that puts some space between me and the engine.

I figure this overall regional difference is pretty common within the plane. I'm just wondering if I can use this information to potentially identify where most of the sound comes from in an effort to quieten the plane in the future.

Door seals, old wind screen, insulation... where is most of the sound getting through or is it likely a combo of all of the above.

Would be nice to be able to let pax go headset free on occasion but sitting behind a 300 hp fan up front is going to be noisy.

And yes I understand I can dial back to lower power settings and decrease the sound level. That's obvious, but I'm looking for relative sound reduction.

Not a big issue for me just something I was looking at during my flight yesterday.
Click to expand...

The prop is the majority of the noise. I would say wind is the second. The engine comes in third.

Try this experiment. I do this in my Mooney quite often.

In cruise, pull the prop back to 2000-2200 rpm. You will find a DRAMATIC reduction in noise and only a slight decease in airspeed (due to the fact you are running lower % power). I pull my EDM data every leg I do this and there is not even a hint of detonation.

I know it doesn't seem quite right because we all know the wives tale of running over Square. But it is definitely a wives tale just like lean of peak. If you have an engine monitor, you can prove it to yourself.

It will be a bit unnerving the first time you do it because it is so quiet.


Sent from my iPad using Tapatalk
 
Go run a generator or any other internal combustion machine at a know speed at note the noise. Increase the engine speed and the noise gets louder. No propellor necessary for that simple test. Engine noise is relative to engine speed. How much noise you hear is relative to the airplane. That's why my Cub is noisier than my Skywagon, and everyone knows what a long prop on a Skywagon sounds like. Unless I have wheel skis installed in the retracted position the supersonic takeoff prop noise isn't a factor in the cockpit. According to a real noise meter, that is.
 
mscard88 said:
Cookies? Warm chocolate chip ones? At the airline I flew at? :rofl::rofl::rofl:
Click to expand...

The old Midwest Express/Midwest Airlines had great cookies, they baked them right on the plane. Then Republic bought them out and now no more cookies.
 
labbadabba said:
The old Midwest Express/Midwest Airlines had great cookies, they baked them right on the plane. Then Republic bought them out and now no more cookies.
Click to expand...

Never flew on Midwest but did hear about their cookies, and great service. Used to see them when they flew into ATL many years ago. Weren't they owned by Kimberly Clark, at least originally?
 
arkvet said:
The app is called Decibel 10th. Seems to work well. IIRC decibel values are not linear so the difference between 90 and 100 decibels seems pretty significant.
Click to expand...

That is correct. Decibels are measured on a logarithmic scale. A 3dB increase corresponds to a doubling of energy. Acoustics is a fascinating and very complex field. I took a few classes on it in college, but unfortunately that was a few years ago and I don't remember much of it.
 
I spent some more time thinking about this last night. For those of you that are so inclined, take a short video/audio recording of your aircraft in cruise (10 seconds would be plenty). Strip the audio from the recording and pass it through and FFT. This will give you the frequency spectrum of the cabin noise. From here you can try to figure out the source of the highest peaks. I would guess that you would likely see a peak a n*RPM of the propeller where n is the number of blades. You may also be able to detect the frequency of the pistons firing in your engine. For what it's worth, I've been told be acousticians at one aircraft OEM that similar techniques are used during crash investigations and frequently enable the investigators to pinpoint the exact source of a mechanical failure.

Back to cabin noise, if any peaks show up significantly above the other peaks, you can try to reduce those specific frequencies. Most noise reduction solutions are targeted to specific frequencies, so the most effective noise reduction may come from targeting any peaks that are significantly above the others. These will obviously change with power settings and propeller RPM, so if you want to use the data to attempt noise reduction, pick your most common cruise altitude/power settings and use a recording in those flight conditions.

I assume there's an app that will do an FFT on a sound signal, but if not, it can be done fairly quickly and easily in GNU Octave (which is an open source program that is similar to MATLAB). If you're stuck on this part and truly curious, send me a PM, I may be able to do this for you. It's been a while since I've opened those files.
 
What's the frequency response of the crappy mic in your phone.
 
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