Unwanted frequencies in music files?

phantom4

Member
2016-01-17 1:02 pm
I decided to learn some signal analysis with a spectrum analyzer. I have been using baudline as analyzer software.

baudline is able to display full spectogram or average levels of all present frequencies. user manual's exact explanation for averaging function: the average window is a way of collecting and displaying accumulated spectral traces.

i am using averaging function in order to get a summary of freq vs. level for a whole track. i detected extremely low frequencies (below 25hz) at some tracks :confused:.

track 1: a standard house music track. encoded in 128Kbps VBR MP3.
track 2: a test track from Ultrasone Test CD. it is a realistic image of a shore with sounds of soft waves, some breeze and birds. encoded with 16/44 FLAC.
track 3: a test track from Focal JMLab Demo CD 4. a trash metal song. encoded with 16/44 FLAC.
track 4: another house music track. encoded in 320Kbps MP3.


(all graphs are averaged and zoomed starting 0hz, displayed wrt the above order);

[IMGDEAD]https://dl.dropboxusercontent.com/u/100840512/forum%20images/diyaudio/Unwanted%20frequencies%20in%20music%20files/1.png[/IMGDEAD][IMGDEAD]https://dl.dropboxusercontent.com/u/100840512/forum%20images/diyaudio/Unwanted%20frequencies%20in%20music%20files/2.png[/IMGDEAD][IMGDEAD]https://dl.dropboxusercontent.com/u/100840512/forum%20images/diyaudio/Unwanted%20frequencies%20in%20music%20files/6.png[/IMGDEAD][IMGDEAD]https://dl.dropboxusercontent.com/u/100840512/forum%20images/diyaudio/Unwanted%20frequencies%20in%20music%20files/8.png[/IMGDEAD]

what is happening in tracks 2&3 :boggled::boggled::boggled: ?

just look at the freq band 0-50hz. it seems the encoded signal contains frequency below 25hz.

i also checked the regular spectogram (not the averaged graph) for the whole timeline of the track. those <25hz frequencies do exist along both of the tracks.

am i misinterpreting the graphs :confused: ?
 
i detected extremely low frequencies (below 25hz) at some tracks :confused:.

what is happening in tracks 2&3 :boggled::boggled::boggled: ?

just look at the freq band 0-50hz. it seems the encoded signal contains frequency below 25hz.

i also checked the regular spectogram (not the averaged graph) for the whole timeline of the track. those <25hz frequencies do exist along both of the tracks.

am i misinterpreting the graphs :confused: ?
CD Audio spectrum is DC to 22.05 kHz in theory. Perhaps some recordings do not limit the low frequency content, note these are test CDs.
 
I used to live directly over the road (80m) from the beach in north part of Perth, WA.
I can well assure you that there are some ultra/very low frequencies/pulses present in what seems to be quiet gentle surf sounds, and at surprisingly/disarmingly high amplitudes.
These ULF events are relatively rare compared to the VLF/LF events, and at 80m distance from the shoreline caused the house/surrounds/grounds to shake even on what seemed to be a calm and still evening.

I now live further north and about 800m from the shoreline.
On still, no wind nights (silent neighbourhood after 7.00 pm) these ULF/VLF events are also clearly audible/felt/sensed.
The rising ULF characteristic (LF pink noise characteristic) present in the Ultrasone Test CD (which I have and I am familiar with) shore sound recording you mention accords with 1/f noise/events present in nature and is also generated in semiconductors/passives.
In the shore sound recording you quote, the reasons for the such steeply rising ULF measure are not clear.
Looking at the ULF slope of your graph, I feel that there is a LF roll out of the mic/recording gear combined with real ULF audio signal combined with maybe some DC servo action.
I should have another close listen to that recording and see what I can subjectively discern.
Try researching 1/f noise....this is a subject in it's own.

Dan.
 
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phantom4

Member
2016-01-17 1:02 pm
Perhaps some recordings do not limit the low frequency content, note these are test CDs.

i found another one, this is Dire Straits/So Far Away; 16/44 FLAC, original recording (not a re-master). this is not a test recording :confused:.

[IMGDEAD]https://dl.dropboxusercontent.com/u/100840512/forum%20images/diyaudio/Unwanted%20frequencies%20in%20music%20files/so%20far%20away.png[/IMGDEAD]

pls notice the low level, as it is known that Brothers In Arms (1985) has a low level (dBFS) in general. this is perfectly displayed on the average graph.
 

phantom4

Member
2016-01-17 1:02 pm
i have Audacity already installed. it has a Plot Spectrum feature. this is an excerpt from the documentation;

Plot Spectrum take the audio in blocks of 'Size' samples, does the FFT, and averages all the blocks together.

here is the analysis window shot from Audacity for Dire Straits/So Far Away with the same FFT size and window function selected.;

[IMGDEAD]https://dl.dropboxusercontent.com/u/100840512/forum%20images/diyaudio/Unwanted%20frequencies%20in%20music%20files/so%20far%20away_audacity.png[/IMGDEAD]