understanding waveform db and correlating to dBSPL... another thread

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amaterasu
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understanding waveform db and correlating to dBSPL... another thread

Post by amaterasu » Sat May 08, 2021 5:33 am

hi all, new to this forum and to the audacity software.

context: im recording airplane sounds that go over my home using a digital recorder to track frequency of airplanes that go over my home (i live 20+ miles away from the airport and i have no idea why i have planes going over my home every 5-10min for the majority of the hours from 7am-12am, well, i do but it makes no sense and it's still bonkers!). also using this dBSPL reader to get dB readings.

i'm trying to understand how to read the waveform dB in audacity, and I cannot for the life of me understand how to read this. i've tried scouring this forum and just googling, and wat i did get is that the waveform db in audacity is relative/a ratio and it does not provide dBSPL. beyond that, i do not understand. :(

my goal: i would like to understand the waveform DB. my sound decibel reader is very buggy and doesn't always record. but the recordings i do have, i wanted to see if i could correlate to the waveform dB measurements, and then calculate the dbSPL for all the other waveform DB points. so then i'm thinking i need to figure out:

1) truly understand the waveform DB that audacity generates.
2) is there a way to export the RMS and max peak point of every second in an audio file from the waveform that audacity generates? I understand that the waveform is essentially many pixels generated/averaged for any point in time, but I'm just looking for the peak pixel value and the RMS at every second. wondering if this also might be a good way to log plane activity without physically going thru the audio files and identifying planes by waveform patterns (need a more programmatic way of identifying planes). altho, i do have some loud ass birds in my backyard (prefer the birds over the planes, 100%) that may cause false positives if i went down the every-second route? granted the birds are much closer to the recorder than the actual plane. see birb chirp image below.
3) once i have that, could i correlate the dBSPL measurements i got from the sound meter to the audacity waveform data, and then calculate all the other times (reverse engineer) when the sound meter recording did not work. i've been placing the recorder and sound meter in the same position, whenever i record, so trying to reduce external confounding factors...

Here are two examples. First one i had a sound meter reading. Second one, i did not. The third is birb chirps and trills. These planes are freaking loud and clock in >65decibels at the loudest. i dont even live next to the airport!
example01.png
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example02.png
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example03 - loud biirrbb chiirrpps.png
example03 - loud biirrbb chiirrpps.png (504.46 KiB) Viewed 304 times
anyone willing to spend time and help me understand waveform DB and if i can possibly do this correlation? let me know if i need to clarify anything.

many thanks!

kozikowski
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by kozikowski » Sat May 08, 2021 10:16 am

At first pass, no Sound Pressure Level Meter I ever saw worked in stereo (two blue waves). Drop-Down Menu on the left > Split Stereo To Mono. [X]Delete one of the two waves.

Then we note that the dB level labels on the left aren't the most useful the way they are, so I pull Audacity and the window a lot taller.

Screen Shot 2021-05-08 at 2.10.57 AM.png
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Now you should see better correlation between the blue waves and the bouncing sound meter.

And then I pull the sound meters a lot bigger.

Screen Shot 2021-05-08 at 2.17.15 AM.png
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Making them even easier to see.

The Light Green meters and the light blue waves are roughly loudness. The darker green meters and darker blue waves are instantaneous peaks.

There is no direct correlation between dB Sound Pressure Level in air and dB sound in Audacity. It's a common problem/request. Or there might be if you jump through some hoops.

You need to come up with a steady tone in the air, measure it with your meter, record it, and then note the Audacity readings. They should track as long as you don't move anything or change any settings. Or your meter wasn't buggy.

~~~

Oh, that's not an SPL meter. That's just a voice recorder.

That's a dBSPL meter.

Image

It doesn't record anything. It just wiggles the meter to tell you how loud something is.

The reason your recorder seems buggy is it naturally does Sound Activated Recording. It says so in the Advertising. It only records when the sound is loud. You might be able to turn Sound Activated Recording off in the recorder settings and get much more stable recordings. I have a cousin to that recorder and I can turn mine on and off.

It's important that my SPL meter has that A/C switch on the left. There's two ways to measure loudness. "A" measures roughly the same way your ear does. It ignores very high and very low tones and concentrates on the tones you're most sensitive to. You know what these tones are. Fingernails on Blackboard and Baby Screaming on a Jet. Those two are very "A".

C measures more or less everything, annoying or not. Your voice recorder works in C.

~~~

About the airport. You may be able to find the landing patterns for your airport. The north pattern at LAX has jets coming in over Santa Monica on the coast, turning right over downtown Los Angeles, turn right again over Montebello, and then landing over Inglewood. Roughly 15 miles give or take. That's the Big Jets. Landing is the big deal because they have to lose altitude quickly for a safe landing.

Taking off, they just floor it, gain altitude quickly, and leave. Elevation has a lot to do with it, too. East Coast jets have to come in over the San Gabriel Mountains and they don't clear the peaks by all that much.
i live 20+ miles away from the airport
Are you sure? There's no shortage of tiny airports with commercial (tiny) jet service. Hawthorne Airport is a busy field that if you spit just right you can hit LAX. But it's a completely separate field with separate landing patterns and planes.

Also see: Santa Monica Airport and El Monte Airport. Burbank has a large commercial airport.

There was a book written about Kennedy Airport in NY called Cedarhurst Alley.

Screen Shot 2021-05-08 at 3.02.09 AM.png
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The upshot was Air Traffic Control brings big jets in one way until the neighborhood complaints get too loud and then they switch to another way and wait for the complaints.

Repeat.

Koz

kozikowski
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by kozikowski » Sat May 08, 2021 10:33 am

One other note about your voice recorder.

It's not unusual for voice recorders to try and adjust loudness to follow your voice. Their job is capture every word no matter how loud it is. This can mess up scientific measurements because as your voice gets louder, the recorder turns the volume down. Some adverts call that AGC or Automatic Gain Control. So there is no correlation between recorded sound and sound pressure level...ever.

I do have a Sound Recorder. The Zoom H1n is not a voice recorder. It's a sound recorder and defaults to recording anything I point it at with no tricks or automatic anything.

Koz

amaterasu
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by amaterasu » Sun May 09, 2021 1:13 am

koz, thanks for your response.
kozikowski wrote:
Sat May 08, 2021 10:16 am
At first pass, no Sound Pressure Level Meter I ever saw worked in stereo (two blue waves). Drop-Down Menu on the left > Split Stereo To Mono. [X]Delete one of the two waves.

The blue waveform images I attached are from a digital recorder, not a sound pressure level meter. however, i did also record the dBs using a sound pressure level meter as linked in my first post. Unless that's not a sound pressure level meter. The sound pressure level meter I purchased has a record function (data logging). To reiterate, it is not my recorder that's buggy, it's the data logging feature on the sound pressure level meter. it's doesnt always record properly, but the recorder always records, hence me trying to figure out if i can reverse engineer the dBSPL since I have samples.
kozikowski wrote:
Sat May 08, 2021 10:16 am
You need to come up with a steady tone in the air, measure it with your meter, record it, and then note the Audacity readings. They should track as long as you don't move anything or change any settings. Or your meter wasn't buggy.

that's basically wat i was doing, and how i was able to get that 73.6dB reading from the sound level meter records and match it to the audio recording from the digital recorder. but the sound level meter data logging is super buggy and doesn't always record, hence looking for correlation to reverse engineer. but looks like that's not possible. also, sound level meter was always set to dbA and not dbC. trying to point out how loud it is to human ears, so dbA makes most sense.
Are you sure? There's no shortage of tiny airports with commercial (tiny) jet service. Hawthorne Airport is a busy field that if you spit just right you can hit LAX. But it's a completely separate field with separate landing patterns and planes.

Yeah, 20+ miles. And these are big jets. So close (low altitude), I can literally see the logos. International, domestic, all of them. I don't think this should be allowed so I'm trying to record everything and build case with data/evidence. yeah it's freaking annoying, they need to have these planes fly over freeways over major industrial areas, not residential areas, unless you're next to the airport.
It's not unusual for voice recorders to try and adjust loudness to follow your voice. Their job is capture every word no matter how loud it is. This can mess up scientific measurements because as your voice gets louder, the recorder turns the volume down. Some adverts call that AGC or Automatic Gain Control. So there is no correlation between recorded sound and sound pressure level...ever.
looks like i need to purchase sound recorders, not voice recorders.

kozikowski
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by kozikowski » Sun May 09, 2021 1:53 am

Your forum posts have to wait for an elf to review them and make sure you're not trying to sell us screen doors or adult entertainment. Don't double post.

It still might be instructive to find the pattern for your airport. The LAX approach over Santa Monica—15 miles out—is nearly inaudible. I bet if you asked people on Santa Monica Blvd if they minded the jets, they would say, "What jets?" I think there's something odd about your situation. You can't see logos on jets over Los Angeles without field glasses, and that's only about eight miles out.

This may be a bad time to say that Audacity can't be used for law enforcement, surveillance, or conflict resolution. You might be able to use your works to convince a company or agency to come out and measure it for real with certified instruments, etc.

And yes. "A" is the one burned into laws and regulations. "Your Donna Summer disco rating shall not be louder than.....etc."

If you're in the US, do you remember early on when the dump truck backup tones violated the loudness ordinances? So you had to choose between being crushed or deaf.

Do you know that part? What are the noise regulations for your situation or environment? "Our town has a noise regulation after 10 PM of so and so many dBSPL."

"I don't like the noise" isn't going to get you very far. See: "Cedarhurst Alley."

Koz

amaterasu
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by amaterasu » Sun May 09, 2021 3:05 am

Your forum posts have to wait for an elf to review them and make sure you're not trying to sell us screen doors or adult entertainment. Don't double post.
literally thought my post didn't go thru, that's why i double posted. work, elf, work!
This may be a bad time to say that Audacity can't be used for law enforcement, surveillance, or conflict resolution. You might be able to use your works to convince a company or agency to come out and measure it for real with certified instruments, etc.

civilian data points are better than nothing, and may lead to better things (like having an agency come to actually confirm). gotta start somewhere.
I think there's something odd about your situation. You can't see logos on jets over Los Angeles without field glasses, and that's only about eight miles out.

i have the pictures and videos from my smartphone that show some of the jets flying so low that i can see the logo. the closeness, loudness, and frequency of these planes flying over borderline like noise abuse to me. whether that's legally true, i dont know. here's one foto. not zoomed in when i took the photo, just took it from where i was standing in my room next to the window. foto has been cropped tho (which makes it look zoomed in... sigh. but it's not!). this particular airplane was a korean airlines jet.
ka.JPG
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If you're in the US, do you remember early on when the dump truck backup tones violated the loudness ordinances? So you had to choose between being crushed or deaf.
nope. did they institute some regulation to protect the truck drivers?
Do you know that part? What are the noise regulations for your situation or environment? "Our town has a noise regulation after 10 PM of so and so many dBSPL."

hmm.. don't know, despite a quick search thru city site. but im pretty sure it's the generic "no loud noises after 10pm and before 6am".
"I don't like the noise" isn't going to get you very far.
duh, hence the data gathering in a systematic and reproducible manner. i believe there's more power with data, altho politics and money also play a huge roll, let's be real. but again, gotta start somewhere!

one last question, do you know if there are any plugins available that allow to export the RMS and max peak point of every second in an audio file from the waveform that audacity generates? I understand that the waveform is essentially many pixels generated/averaged for any point in time, but I'm just looking for the peak pixel value and the RMS at every second. i need a more programmatic way of identifying the timestamps of when airplanes fly over my area, and right now, i've been going thru each file using the waveform visuals, but it's very manual and inefficient. one possible issue: i have lots of bird chirps in my audio files (see image 3 in original post), so maybe i have to go down to milliseconds to get continuous RMS and peak data points from the planes versus the staccato chirp chirps from the birb birbs. thank you for any input/feedback!

kozikowski
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by kozikowski » Sun May 09, 2021 4:23 am

literally thought my post didn't go thru
Isn't there a message somewhere on the screen telling you about this? The other elves keep insisting there is, and yet people still double and triple post.
ka.jpg
That's pretty low. The patterns at LAX don't look anything like that until they final-final and dip into the runway.
did they institute some regulation to protect the truck drivers?
Pedestrians. Truck drivers can't see directly behind them when they back up, hence the beeping. And everybody was fine until they actually measured the beeping. Dueling regulations.
any plugins available that allow to export the RMS and max peak point of every second in an audio file
I think so, but I also think you just wandered out of my field of expertise. We need a heavy editor/developer elf.


Did you know palm trees were grasses?

Amaze your friends.

Koz

kozikowski
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by kozikowski » Sun May 09, 2021 4:43 am

You don't have to do it manually. There are some second-hand tools.

Select a chunk of the show. Effect > Amplify. Don't apply it. Read the top number. If the goal is 0dB (the bottom number), then the top number is the boost needed to get there, and the peak sound value (add the -).

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Analyze > Contrast > Measure Selection.

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That's the RMS value.

Koz

amaterasu
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Re: understanding waveform db and correlating to dBSPL... another thread

Post by amaterasu » Sun May 09, 2021 8:39 pm

That's pretty low. The patterns at LAX don't look anything like that until they final-final and dip into the runway.

right?? so you can imagine how loud and frustrating it would be to get these jets flying over your home once every 5-10min from the hours of 6am all the way into 12am. noise abuse.
Did you know palm trees were grasses?
TIL! tree imposters, all of them. SUS.
I think so, but I also think you just wandered out of my field of expertise. We need a heavy editor/developer elf.
i'll create a separate post and see if any elf bites. or i will have to dive into developer mode.
You don't have to do it manually. There are some second-hand tools.

i dont think this method will work, but i'll give it a shot. my recording is for many hours and so i would basically need the print out of the whole file since many planes are recorded with diff peaks and RMS values. thanks tho.

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