prop versus engine noise.

And of course when we are discussing dB levels associated with sound and the human ear it should be referenced to dBA which further complicates the readings. Phil you allude to this phenomenon with "perceived and actual" levels. The following is an explanation from "WWW.the engineering toolbox" on how this affects the readings and our discussion. There are other complications associated with temperature, humidity and environment but lets keep it simple.

The human ear is more sensitive to sound in the 1 to 4 kHz frequency range than to sound at very low or very high frequencies. Regarding noise - higher sound pressures are therefore more acceptable at lower and higher frequencies than in the mid range.

Knowledge about the human ear is important in acoustic design and sound measurement. To compensate for the human hearing sound meters are normally fitted with filters that adapts the measured sound response to the human sense of sound. Common filters are

  • dB(A)
  • dB(B)
  • dB(C)
Sound - Decibel A, B and C Criteria




Regards.......Chook.
 
Chook is right to point out that if you are measuring noise relative to the impact upon the human ear then, it goes without saying (but it was remiss of me to not say it), that you must work with the « A » filter so that your sound pressure values are in dB(A) . This is very important for what Chook is trying to do and a bit less for Jetlag because he’s only trying to decide which is the louder of the two.

While I’m here I’ll add, from experience, that when you are taking measurements you should do it in the middle of the airfield (preferably on a hard runway/taxiway) so that you are measuring in a “free field over a reflective plane”. Don’t do it near a hanger because you will get a lot of reverberation that dopes your readings, also don’t stand behind the sound meter that also increases the values you measure.

What I’ve noticed with props generally is that they seem louder when they’re on the ground at full throttle at a given distance but they seem quieter when flying over head at the same distance. This is almost certainly because when it’s on the ground the sound power (SWL) is propagated as sound pressure (SPL) over the surface of a hemisphere and when it flies overhead the same sound power is propagated over a sphere of the same diameter giving only half the sound pressure compared to when on the ground.

Notice from Chook’s graph that if the 3 bladed prop of my example is at about 100 hz the “A” attenuation is about 19 dB but a 6 bladed prop would be at about 200 hz and the attenuation is only about 11 dB. On the other hand the 6 bladed higher frequency is more attenuated as it travels through the air than the lower 3 bladed frequency. I’m not convinced either way.

I wasn’t sure how much of my previous post people would understand, judging from the lack of comments/questions it was either crystal clear or totally opaque. So I was reluctant to go too deep into noise measuring because if you think vibration is “black magic” and complicated noise measurement is at a different level of complication and difficulty (been there done that).
Mike G
 
Chook is right to point out that if you are measuring noise relative to the impact upon the human ear then, it goes without saying (but it was remiss of me to not say it), that you must work with the « A » filter so that your sound pressure values are in dB(A) . This is very important for what Chook is trying to do and a bit less for Jetlag because he’s only trying to decide which is the louder of the two.

While I’m here I’ll add, from experience, that when you are taking measurements you should do it in the middle of the airfield (preferably on a hard runway/taxiway) so that you are measuring in a “free field over a reflective plane”. Don’t do it near a hanger because you will get a lot of reverberation that dopes your readings, also don’t stand behind the sound meter that also increases the values you measure.

What I’ve noticed with props generally is that they seem louder when they’re on the ground at full throttle at a given distance but they seem quieter when flying over head at the same distance. This is almost certainly because when it’s on the ground the sound power (SWL) is propagated as sound pressure (SPL) over the surface of a hemisphere and when it flies overhead the same sound power is propagated over a sphere of the same diameter giving only half the sound pressure compared to when on the ground.

Notice from Chook’s graph that if the 3 bladed prop of my example is at about 100 hz the “A” attenuation is about 19 dB but a 6 bladed prop would be at about 200 hz and the attenuation is only about 11 dB. On the other hand the 6 bladed higher frequency is more attenuated as it travels through the air than the lower 3 bladed frequency. I’m not convinced either way.

I wasn’t sure how much of my previous post people would understand, judging from the lack of comments/questions it was either crystal clear or totally opaque. So I was reluctant to go too deep into noise measuring because if you think vibration is “black magic” and complicated noise measurement is at a different level of complication and difficulty (been there done that).
Mike G
Thank you, Mike, for the excellent info. Am interested in purchasing a PB5 and will contact you later about it. Don't want to derail this thread. Cheers.

Brian
 
Hi @Mike G for my part, I have managed to follow all your comments, read twice and three times but got there. For my purpose it was because I had comments that my setup was much louder than the 914's. I know, one or two of the commenters pray at the later of Rotax so suspect possibly a bias, but, on another private forum, two others have the BMW K1200RS have also received comments re noise, so, my study is also the impact on the human ear and perceptions of noise perceived by those on the ground. Do you think that when if flight there is less loading on the propeller/engine as they both have forward momentum as apposed to static. Using your calculations my prop/engine frequencies are 92/183htz so I will in the next few days try and gather some data on that. Ooops, forgot to say thank you for your input @Mike G

phil
 
I feel the most useful information for your needs will come from a sound meter on the ground Phil.

Sound is very directional and it sounds different when you are on the ground and the people you would rather not annoy are likely on the ground.

Anything you can do to deflect the sound away from the ground will likely have value.

A twin is harder to muffle than a four and requires more volume in the muffler for the same level of silencing without hurting power output.

Good luck on your acoustic adventure.
 
I feel the most useful information for your needs will come from a sound meter on the ground Phil.
Another useful tool might be a recording made on the ground, then load the file in the free Audacity audio editing software. It can show you a spectral analysis, or at which audio frequencies the most noise energy is concentrated. Knowing that could lead to specific remedies.

I'd add that one other factor in perceived noise, aside from the response curves of the human ear, is the mix of frequencies coming from aviation powerplants with gearboxes. My personal experience with the Rotax 582 has convinced me that a 3.00-to-1 gearbox makes a less irritating sound than a 2.62 or 3.47, and it's likely because the reduction ratio is an integer, putting engine and prop noise on harmonically-related frequencies.

I don't know, but I'd guess the BMW conversion is using prime-numbered gear-tooth ratios to get a non-integer drive ratio. It's a common precaution against rapid gearbox wear, especially when thorough vibration studies have not been possible.
 
Phil this is turning out to be a very interesting thread- thanks for starting it.
And Mike thanks for bringing up the environmental measurement location (in the middle of the airfield). I suspect that we all sometimes assume things of our audience.
Which brings me to the next observation. A long time ago as a telecommunications tech we used to "tune" our telephone networks to work with speech frequencies between 300 and 3000 Hz. This gives an economical transfer of most of the intelligence of human speech (but not necessarily the clarity which is in the higher frequencies) and I would see no advantage in the current batch of mobile devices going much above that range. Yes I concede that the electret/MEMS microphone inserts used in these devices will give a much better frequency response but it is not used in the telco network. So using a mobile phone for noise readings may not be as accurate across a spectrum than say a dedicated compensated noise meter. Using your specific meter to take a series of relative measurements will be accurate and that is what I have done with my unfinished table.
And in the end it may be the particular note that your exhaust system is emitting that is the louder perception that you are trying to identify. Think the difference between a Harley and a Ducati - God I miss my Duke sometimes.

In the meantime keep the posts coming folks.

Regards........Chook.
 
@PW_Plack morning .. you are correct my gearbox is a Hirth 3:1. Interesting about the reductions and harmonics.

phil
 
@Chook not that I understand the technicalities of the telephone system, but I get the gist of your comments re the equipment used to capture data and its relevance to data captured on a dedicated machine.

Has anyone got a proper dB meter that they could make a comparison between that and, for example, an app free to all NIOSH SLM?

Each phone using its own internal microphone will have a variation with other phones and also with a proper dB meter, the NOISH SLM is recording in dB(A) and has a correction facility. How if at all this could be used, at a distance, to calibrate our phones to a known source I do not know.

I do not expect I can change much the noise I create but I strive to understand that noise and its perception by others on the ground when I overfly and at/near the airfield when I whip the donkey, not forgetting the very important work done by @Chook to better understand the risks to our hearing.

phil
 
I am arguing with the company that installed a new, and very noisy, heat pump in my house in Brittany. To demonstrate what the problem was I found this free app very useful.
very useful.
It gives you the spectrum in 1/3 octave bands and allows you to select the A filter.

For Jetlag who's basic question is "which makes most noise, the prop or the engine and would adding a second/bigger silencer reduce the overall noise?". If you set this app to "no filter" you should be able to see the dB level for the prop and the engine. This will tell you straight away which is making the most noise and from post20 you can decide if you need a new silencer.
Mike G
 
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