Engine noise

[Matt Piatkowski]

Hello,
The manufacturer of the digital noise meter I am considering gives the measurement range of 30 to 130 dB. and the frequencies range of 31.5 to 8,000 Hz.
What are the approximate border frequencies of the noise produced by typical 40-60 size 2 stroke engine on the ground? Do they fall in the range given above?

Comment: left border frequency is the lowest frequency produced by the engine and the right border frequency is the highest.


Thanks,
Matt

[Eric Viglione]

No idea on the border freq quesiton... it's like the tree falling in the forest as far as I'm concerned. The only sounds that matter are the ones I can hear and can cause loss, so I use a device to measure the potentially dangerous ones.

The Tandy (radioshack) Digital sound meter is the best I have found for the price, and provides A-C weighting for different curves. A-weighting is supposed to best represent sound pressure that can damage human hearing with emphasis on higher freqs and filtering out some of the lower, and is what I use to measure our engines. C-weighting is more flat and includes more of the lower freqs.

https://en.wikipedia.org/wiki/A-weighting

You might get a 115db reading off of a TeeDee like I did with A weighting at the distance prescribed by AMA over grass (look it up, there are different suggested levels above ground and distance from plane for over grass vs/hard surfaces), but you will get something totally different with C weighting. Does that mean one is right or wrong? Nope... just measuring different parts of the sound.

Higher freq is more annoying and it is argued that it can do more damage, lower can be louder but much less painful, to a point, (witness 4-strokes).

Hope that helps,
EricV

[Chris Gilbert IRL-1638]

Frequency in Hz multiplied by 60 gives RPM. In this part of the world a fluorescent mains lamp will report 3000 rpm on a tacho (50 Hz mains here), so that frequency range covers all model engine RPMs that I've ever heard. I can't say anything about the intensity levels.

[Keith Renecle]

Hi Matt,

As Eric said, the Radio Shack sound level meter is very good value for money. It's not for professional use but I've had one for a long time and used it to check my own mufflers where you really need a relative reading like greater or lesser than what you had. The decibel is a logarithmic thing so 3 dB up or down doubles or halves the sound pressure level. You need around a 10 dB shift to hear the difference with your ears. With my "not-so-young" ears probably more!

The specs on my meter do not mention a frequency range but 8 KHz is high enough for engine work. I would personally doubt that upper end limit because they use electret microphones which can work into the ultrasonic range. Of course the manufacturer can limit the range electronically. I know that my meter reads even at 30 KHz. I used it to check a high frequency screamer to sort out a neighbours dogs!

Keith R

[Bill Johnson]

Matt, I have one on my android phone downloaded for free. It has proven to be very accurate when compared to the calibrated meter we use to measure noise levels in our hangars.