The MeldaProduction Analyzer is the best free VST Audio Spectrum Analyzer there is! It has a nice big and adjustable display with a clear graphical representation of the sound. You can enlarge sections for a better view and copy/ paste the image to a clipboard and compare the results in a different analyzer window. Voxengo SPAN is one of the most popular and widely used spectrum analysis plugins that you can find. It is a FREE plugin that manages to compete shoulder to shoulder with industry giants. SPAN has an extremely flexible system that allows users to set up the spectrum analyzer preferences.

If you use Wavosaur for mastering / premastering, or signal analysis in a more general way, you need tools for audio analysis : spectrum analyzers, phase analyzers, sonogram.
Wavosaur comes with analyzers, you’ll find them in the ‘tools’ menu, along with oscilloscopes & statistics about the sound file (maximum sample peak, RMS power, average value..).

But you can also use VST plugins (for some people it can be more convenient).
We have searched deep deep inside deep deep down inside into the web, to find the best free VST plugins for audio analysis, here’s the selection:

1) Spectrum Analysis

1.1) Voxengo SPAN

This spectrum analyzer uses FFT to display the signal frequency in real time.
You can freeze the display, and use the mouse to shows dB values / Frequency value
Frequency is displayed in Hz and in musical note.
The Voxengo SPAN analyzer also displays statistics : RMS power, and it features a clipping detection.

More info and download @ http://www.voxengo.com/product/span/
It is worth mentioning these plugins too, freeware!

1.2) Mdsp SmartElectronix Fre(a)koscope

Very nice, with interesting options for the spectrum display : linear/logarithmic/semitones ..
Adjustable size for the FFT buffer. Had sometimes display problem with Wavosaur.
More info and download @ http://mdsp.smartelectronix.com/freakoscope/

1.3) BlueCat FreqAnalyst

Like the previous VST effect, this one can show spectrum of left / right channel or both, has a “hold peak” feature that is interesting, when dealing with EQ level in your mix.
More info & download @ http://www.bluecataudio.com/Products/Product_FreqAnalyst/

1.4) Seven phase Spectrum Analyzer

This one is freely resizable inside the host application (works in Wavosaur)
More info and download @ http://sevenphases.wordpress.com/spectrum-analyzer/

1.5) MultiInspector free

Download @ http://www.vertexdsp.com/downloads.html#MultiInspectorFree

2) Sonogram

2.1) ag-works sonogram sg-1

This free plugin displays a scrolling sonogram for frequency analysis.
You have the choice for the sonogram colors, and many options to tweak the sonogram displays
This is the only sonogram VST we have found for free ! Fortunaly it’s very good.
More info & download @ http://ag-works.net/plugins.sg1.htm

3) Goniometer

3.1) Flux Stereo Tool

When you need to visualize your stereo position, and the repartition in the field, this plugin is very useful. You can check for phase errors in your mixes.
download @ https://www.fluxhome.com/download

3.2) Gonio3

download @ http://www.uk-music.de/index.php?page=downloads

3.3) HOFA 4U Goniometer & Korrelator

download @ http://hofa-plugins.de/pages/start_en/4u_en.php

If you think that we have forget some free VST analyzer plugin in this list, do not hesitate to comment !

Edit:

We just have found this pretty nice analyzer somewhere on our harddisk :

Dust Analyzer:

It provides three modes : spectrum / scope / phase , with “hold peak” option, freeze option, dB/Freq display with mouse, linear / logarithmic scale

more info & download @ KvR audio forum & http://www.signaldust.com/

What Is Spectrum Analyzer?

Spectrum analyzers are tools that provide a user with detailed information about the sounds by dividing the frequency spectrum to show the range of frequencies. The horizontal (x) axis corresponds to the frequency and the vertical (y) axis corresponds to the amplitude of the frequencies.

The frequencies displayed on a spectrum analyzer are normally from 20Hz to 20kHz which is the range that can be heard by a human ear.

What Is The Purpose Of The Spectrum analyzer?

Spectrum analyzers are very useful to check the overall frequency balance of the mix or when comparing certain parts of a mix. This makes it especially useful when mixing using lower-quality monitors or in a non-treated (acoustically) room which can lead to an accidentally unbalanced mix.

The 6 Best Ring Modulator VST Plugins in 2020 | KiloHearts, Melda
11 Best Granulizer Plugins 2020 For a Future Sound Design
10 Best Opto Compressor Plugins (VST,AU,AAX) For Transparent Sound
37 Best Free Vst Compressor Plugins For Mixing & Mastering
10 Best Noise Gate Plugins (VST,AU, AAX) of 2020

What is the difference between a Signal Analyzer And Spectrum analyzer?

Both signal and spectrum analyzers have the same function. The only difference is that signal analyzers have digital signal processing (DSP) in order for it to solve functions like fast Fourier Transform (FFT). Signal analyzers can also have very high bandwidths.

What Is The Difference Between an Oscilloscope and a Spectrum Analyzer?

Oscilloscopes are normally used to find the time delay between two signals. On the other hand, spectrum analyzers are used to display and edit the frequency properties of a signal.

The difference between an oscilloscope and a spectrum analyzer is that the graph displayed in an oscilloscope is a graph of the signal against time, whereas the displayed graph in a spectrum analyzer is of the signal against frequency.

(Image from https://electricalacademia.com/instrumentation-and-measurements/oscilloscope-definition-and-basic-operation/)

The figure above shows an example of an output waveform in an oscilloscope and it shows a square wave in a time domain.

(Image from https://iconcollective.edu/spectrum-analyzers/)

The figure above shows an example of an output waveform of a spectrum analyzer showing a graph of amplitude (in dB) against frequency (Hz).

What Is Reference Level In Spectrum analyzer?

The reference level is the maximum level of amplitude that can be displayed. For example, if we have the highest peak in volume of our signal at 20 dB and we set the reference level at 15 dB, then half of the waveform of that peak will be cutoff.

(Image from https://electronics.stackexchange.com/questions/467062/whats-the-reference-db-level-in-spectrum-analyser)

The 10 Best Spectrum Analyzer VST Plugins

1. Voxengo SPAN Review

More Info & Download

Voxengo SPAN is one of the most popular and widely used spectrum analysis plugins that you can find. It is a FREE plugin that manages to compete shoulder to shoulder with industry giants.

SPAN has an extremely flexible system that allows users to set up the spectrum analyzer preferences. Users can specify the Fourier block size in samples, set the percentage for the FFT window overlap, and specify the spectrum’s visual slope. In addition to that, Voxengo SPAN supports multi-channel display in both real-time and all-time maximums.

Key Features:

Voxengo SPAN has a real-time FFT spectrum analyzer
This plugin has a phase correlation meter that enables users to detect phase cancellations
Colorful interface with interchangeable colors
Users can listen to a specific frequency through a bandpass filter
K-level metering

Free Vst Spectrum Analizer