## audacity-users

 [Audacity-users] Details about "Plot Spectrum" frequency analysis From: David A. Riggs - 2013-01-16 22:46:33 ```Hi all, I frequently use the Analyze -> Plot Spectrum function to view the frequency content in test recordings. Specifically, I use the default "Spectrum" algorithm, which plots frequency in Hz on the X axis and dB on the Y axis. I'd like to know precisely what I am looking at in this display. * Is the Y axis showing me the peak amplitude for that FFT bin, or is it some sort of average for that bin over the recording? * When I see a peak value of, say -18dB at my test frequency of 75kHz (my recordings are typically 250kHz, 300kHz, or 500kHz samplerate!), what exactly does that "-18dB" mean, what is it in relation to? 18dB less than maximum sample value? I ask because another piece of software, which I'm trying to replace with Audacity, displays what I believe to be the same information ("peak hold" plot as it plays back the recording) with the Y axis showing dB on a scale from 0dB up, where Audacity plots this scale from 0dB down. Thanks in advance for helping me to understand! I'm particularly happy to see that this frequency analysis functionality has seen some improvements in recent versions of Audacity, allowing me to use it for our ultrasonic recordings. - DR -- David A. Riggs ```
 Re: [Audacity-users] Details about "Plot Spectrum" frequency analysis From: Dave - 2013-01-18 02:36:44 ```Hi if it is any help dB is relative to something else so -18db below 0db , 20db is 10 times whatever 0db was. THANKYOU DAVE-211 ----- Original Message ----- From: "David A. Riggs" To: Sent: Thursday, January 17, 2013 9:46 AM Subject: [Audacity-users] Details about "Plot Spectrum" frequency analysis > Hi all, > > I frequently use the Analyze -> Plot Spectrum function to view the > frequency content in test recordings. Specifically, I use the default > "Spectrum" algorithm, which plots frequency in Hz on the X axis and dB > on the Y axis. > > I'd like to know precisely what I am looking at in this display. > > * Is the Y axis showing me the peak amplitude for that FFT bin, or is > it some sort of average for that bin over the recording? > > * When I see a peak value of, say -18dB at my test frequency of 75kHz > (my recordings are typically 250kHz, 300kHz, or 500kHz samplerate!), > what exactly does that "-18dB" mean, what is it in relation to? 18dB > less than maximum sample value? > > I ask because another piece of software, which I'm trying to replace > with Audacity, displays what I believe to be the same information > ("peak hold" plot as it plays back the recording) with the Y axis > showing dB on a scale from 0dB up, where Audacity plots this scale > from 0dB down. > > Thanks in advance for helping me to understand! I'm particularly happy > to see that this frequency analysis functionality has seen some > improvements in recent versions of Audacity, allowing me to use it for > our ultrasonic recordings. > > - DR > > -- > David A. Riggs > > ------------------------------------------------------------------------------ > Master Java SE, Java EE, Eclipse, Spring, Hibernate, JavaScript, jQuery > and much more. Keep your Java skills current with LearnJavaNow - > 200+ hours of step-by-step video tutorials by Java experts. > SALE \$49.99 this month only -- learn more at: > http://p.sf.net/sfu/learnmore_122612 > *********** ASKING FOR HELP ************* > > When asking for help on this list, please include the following > information, so we can > help you properly: > > * What operating system you are using (for example, Windows XP or Mac OS X > 10.5.1) > > * Exactly what three digit version number of Audacity you are using (Help > > About > Audacity, or Audacity > About Audacity on a Mac computer) > > * If this is a recording problem, what equipment you are recording with, > and how is it > connected to the computer? > > Mailing list: Audacity-users@... > To UNSUBSCRIBE, use the form at the bottom of this web page: > https://lists.sourceforge.net/lists/listinfo/audacity-users ```
 Re: [Audacity-users] Details about "Plot Spectrum" frequency analysis From: David A. Riggs - 2013-01-18 04:55:09 ```On Thu, Jan 17, 2013 at 9:36 PM, Dave wrote: > if it is any help dB is relative to something else so -18db below 0db , 20db > is 10 times whatever 0db was. Thanks for your reply, but I'm very familiar with the dB scale and logarithmic relationships. I'm trying to find out the "something else"... On the Menu -> Plot Spectrum frequency analysis window, when viewing the "spectrum", I assume that 0 dB on the Y axis refers to maximum amplitude... But what precisely is the plot showing me? Is it plotting the maximum amplitude at the given frequency FFT bin? It is actually plotting the cumulative amplitude of the bins for the entire selection? Is it some averaged value? I did RTFM, but it didn't make this clear. http://manual.audacityteam.org/o/man/analyze_menu.html I'm just trying to find a precise definition of what this value means with regard to a particular frequency in relation to the selection. Thanks! - DR -- David A. Riggs ```
 Re: [Audacity-users] Details about "Plot Spectrum" frequencyanalysis From: Dave - 2013-01-18 11:45:09 ```Hi the y axis is dB 0dB at top you can adjust this in "Meter/Waveform dB range" in the Interface Preferences, Plot Spectrum take the audio in blocks of 'Size' samples, does the FFT, and averages all the blocks together. There is an audio version of the Uncertainty Principle that says "Frequency_Resolution*Time_Resolution = 2." Size lets you choose the FFT size which is the number of consecutive audio samples to be analyzed. If you choose 1024 samples, you will get 512 frequency bins covering the frequency range up to half the sampling frequency. Most projects use 44.1kHz sampling frequency. Thus, in this example, Time_Resolution = 1024/44.1 = 23.2 milliseconds and the frequency resolution is 44100/2/512 = 43 Hz. the above is what is in the page ,put your values in the formulas and see if it makes sense. The function affects different parts of the waveform THANKYOU DAVE-211 ----- Original Message ----- From: "David A. Riggs" To: "Discussion list for Audacity users" Sent: Friday, January 18, 2013 3:55 PM Subject: Re: [Audacity-users] Details about "Plot Spectrum" frequencyanalysis > On Thu, Jan 17, 2013 at 9:36 PM, Dave wrote: >> if it is any help dB is relative to something else so -18db below 0db , >> 20db >> is 10 times whatever 0db was. > > Thanks for your reply, but I'm very familiar with the dB scale and > logarithmic relationships. I'm trying to find out the "something > else"... > > On the Menu -> Plot Spectrum frequency analysis window, when viewing > the "spectrum", I assume that 0 dB on the Y axis refers to maximum > amplitude... > > But what precisely is the plot showing me? Is it plotting the maximum > amplitude at the given frequency FFT bin? It is actually plotting the > cumulative amplitude of the bins for the entire selection? Is it some > averaged value? > > I did RTFM, but it didn't make this clear. > > http://manual.audacityteam.org/o/man/analyze_menu.html > > I'm just trying to find a precise definition of what this value means > with regard to a particular frequency in relation to the selection. > > Thanks! > > - DR > > > > -- > David A. Riggs > > ------------------------------------------------------------------------------ > Master HTML5, CSS3, ASP.NET, MVC, AJAX, Knockout.js, Web API and > much more. Get web development skills now with LearnDevNow - > 350+ hours of step-by-step video tutorials by Microsoft MVPs and experts. > SALE \$99.99 this month only -- learn more at: > http://p.sf.net/sfu/learnmore_122812 > *********** ASKING FOR HELP ************* > > When asking for help on this list, please include the following > information, so we can > help you properly: > > * What operating system you are using (for example, Windows XP or Mac OS X > 10.5.1) > > * Exactly what three digit version number of Audacity you are using (Help > > About > Audacity, or Audacity > About Audacity on a Mac computer) > > * If this is a recording problem, what equipment you are recording with, > and how is it > connected to the computer? > > Mailing list: Audacity-users@... > To UNSUBSCRIBE, use the form at the bottom of this web page: > https://lists.sourceforge.net/lists/listinfo/audacity-users ```
 Re: [Audacity-users] Details about "Plot Spectrum" frequencyanalysis From: Steve the Fiddle - 2013-01-18 13:27:19 ```The dB axis is normalized such that a 0 dB sine wave will produce 0 dB for that frequency bin. Steve On 18 January 2013 11:44, Dave wrote: > Hi > the y axis is dB 0dB at top you can adjust this in "Meter/Waveform dB range" > in the Interface Preferences, > Plot Spectrum take the audio in blocks of 'Size' samples, does the FFT, and > averages all the blocks together. There is an audio version of the > Uncertainty Principle that says "Frequency_Resolution*Time_Resolution = 2." > Size lets you choose the FFT size which is the number of consecutive audio > samples to be analyzed. If you choose 1024 samples, you will get 512 > frequency bins covering the frequency range up to half the sampling > frequency. Most projects use 44.1kHz sampling frequency. Thus, in this > example, Time_Resolution = 1024/44.1 = 23.2 milliseconds and the frequency > resolution is 44100/2/512 = 43 Hz. > > the above is what is in the page ,put your values in the formulas and see if > it makes sense. > > The function affects different parts of the waveform > > THANKYOU DAVE-211 > > > > ----- Original Message ----- > From: "David A. Riggs" > To: "Discussion list for Audacity users" > > Sent: Friday, January 18, 2013 3:55 PM > Subject: Re: [Audacity-users] Details about "Plot Spectrum" > frequencyanalysis > > >> On Thu, Jan 17, 2013 at 9:36 PM, Dave wrote: >>> if it is any help dB is relative to something else so -18db below 0db , >>> 20db >>> is 10 times whatever 0db was. >> >> Thanks for your reply, but I'm very familiar with the dB scale and >> logarithmic relationships. I'm trying to find out the "something >> else"... >> >> On the Menu -> Plot Spectrum frequency analysis window, when viewing >> the "spectrum", I assume that 0 dB on the Y axis refers to maximum >> amplitude... >> >> But what precisely is the plot showing me? Is it plotting the maximum >> amplitude at the given frequency FFT bin? It is actually plotting the >> cumulative amplitude of the bins for the entire selection? Is it some >> averaged value? >> >> I did RTFM, but it didn't make this clear. >> >> http://manual.audacityteam.org/o/man/analyze_menu.html >> >> I'm just trying to find a precise definition of what this value means >> with regard to a particular frequency in relation to the selection. >> >> Thanks! >> >> - DR >> >> >> >> -- >> David A. Riggs >> >> ------------------------------------------------------------------------------ ```
 Re: [Audacity-users] Details about "Plot Spectrum" frequencyanalysis From: - 2013-01-18 21:33:36 ```On Fri, 18 Jan 2013 13:27:10 +0000 Steve the Fiddle wrote: > The dB axis is normalized such that a 0 dB sine wave will produce 0 dB > for that frequency bin. > > Steve After 2.0.3, I suggest we add a little about this to the Manual, especially . as the normalization changed significantly from 1.3.8 onwards. Prior to that, a sine tone at 0 dB amplitude on the waveform would show at anything up to +15 dB in Plot Spectrum, but "real music" with a peak amplitude of 0 dB on the waveform usually looked like about 0 dB in Plot Spectrum rather than well below 0 dB as it appears now. I also notice that Philip van Baren who made this change in 2009 commented at the time: "If the sine tone doesn't exactly match one of the frequencies in the FFT, the energy will be split between two bins, and neither will be exactly 0dB in that case. However, there is a peak interpolation which attempts to report the actual peak amplitude and frequency. It looks like that interpolation could use a little touch-up too, because it tends to report the interpolated peak amplitude a little high." Gale > > ----- Original Message ----- > > From: "David A. Riggs" > > To: "Discussion list for Audacity users" > > > > Sent: Friday, January 18, 2013 3:55 PM > > Subject: Re: [Audacity-users] Details about "Plot Spectrum" > > frequencyanalysis > > > > > >> On Thu, Jan 17, 2013 at 9:36 PM, Dave wrote: > >>> if it is any help dB is relative to something else so -18db below 0db , > >>> 20db > >>> is 10 times whatever 0db was. > >> > >> Thanks for your reply, but I'm very familiar with the dB scale and > >> logarithmic relationships. I'm trying to find out the "something > >> else"... > >> > >> On the Menu -> Plot Spectrum frequency analysis window, when viewing > >> the "spectrum", I assume that 0 dB on the Y axis refers to maximum > >> amplitude... > >> > >> But what precisely is the plot showing me? Is it plotting the maximum > >> amplitude at the given frequency FFT bin? It is actually plotting the > >> cumulative amplitude of the bins for the entire selection? Is it some > >> averaged value? > >> > >> I did RTFM, but it didn't make this clear. > >> > >> http://manual.audacityteam.org/o/man/analyze_menu.html > >> > >> I'm just trying to find a precise definition of what this value means > >> with regard to a particular frequency in relation to the selection. > >> > >> Thanks! > >> > >> - DR > >> ```