Acoustics project

Hello guys, I am working on a project. Hope you had a great thanksgiving.

The train measurements give a very low decibel SPL (dB). It turns out that it was the highest and most uncomfortable for my ears. According to reliable sources, it should give me a high number of decibels. Whar should i change or how should i analyze this sound?

How are you measuring that? dB SPL levels are positive. Do you have an SPL meter?

0dB SPL is approximately the quietest sound you can hear. Digital levels are dBFS (decibels full-scale) and 0dBFS is the “digital maximum”. Electrical dB levels are also usually negative.

There is a direct correlation between acoustic & digital levels. If the SPL drops by 3dB the digital level will also drop by 3dB (more negative).

But, there is no fixed calibration. i.e. If you are recording and turn-up the microphone volume, or if you simply have a more sensitive microphone, the digital levels will be higher even with the same acoustic loudness.

Yes, I used audacity to record the noise source. Here is a better explanation to understand. I believe the fast train should give me higher decibels numbers right? Because is a louder sound.

They are negative decibels. A bigger negative number is quieter and a lower negative number is louder. Audacity itself can go even higher into positive decibels, but 0dB should be considered the the maximum.

It looks right. The top waveform is hitting 0dB (=100% or +/-1). It actually appears to be clipped which means it would go louder but your analog-to-digital converter can’t go over the “digital maximum” of 0dB.

The bottom waveform is much quieter.

Okay, thanks!
Last concern. So would in my report would I say that the train has an average btw 20-60 dB. Whereas the water fountain has an average between 40-80 dB?

Remember, those are NEGATIVE digital levels, correlated-to but not calibrated-to acoustic SPL levels.

The -20 to -60dB range compared to the -40 to -80dB range, means the train is about 20 louder (which doesn’t “feel like” enough difference to me.)

I hope you are comfortable with negative numbers… Sometimes I imagine a ladder where positive height is above ground, and negative is a hole in the ground. A 100 ft ladder going down is lower than a 10 ft ladder going down.

-10dB is quieter than +10dB and -100dB is quieter than -10dB. Although we don’t know the SPL loudness, usually we don’t hear anything less than about -60dBFS (digital). But of course, if you crank-up the volume control high enough you will hear it.

An SPL Chart may be useful.

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