Audacity Plot Spectrum: Linear vs Logarithmic
When analyzing audio in Audacity using the Plot Spectrum tool, choosing between a linear and a logarithmic frequency display fundamentally changes how frequency data is scaled and visually interpreted. While a linear display distributes frequencies in equal mathematical increments, making it ideal for technical troubleshooting and harmonic analysis, a logarithmic display arranges frequencies in octaves to mirror human hearing, making it the preferred choice for musical analysis and equalization. Understanding the operational differences between these two views allows you to accurately diagnose issues, read spectral peaks, and make informed audio editing decisions.
What is the Plot Spectrum Tool?
Audacity's Plot Spectrum feature uses a Fast Fourier Transform (FFT) algorithm to break down an audio signal into its individual frequency components. The resulting graph displays frequency on the horizontal axis (X-axis) and amplitude in decibels (dB) on the vertical axis (Y-axis). The setting that governs the X-axis can be toggled between "Linear frequency" and "Logarithmic frequency."
Linear Frequency Display
In a linear display, every Hertz (Hz) is given the exact same amount of horizontal space across the graph. The visual distance between 100 Hz and 200 Hz is identical to the distance between 10,000 Hz and 10,100 Hz.
- How It Behaves: Because the human audible range spans from 20 Hz to 20,000 Hz, a linear axis dedicates the vast majority of its visual space to high frequencies. The entire bass and lower-midrange spectrum (20 Hz to 1,000 Hz) is compressed into the first five percent of the graph.
- Best Use Cases: Linear display is best for identifying mathematically spaced phenomena, such as evenly spaced harmonics generated by distortion, carrier signals, or electrical hum (e.g., 60 Hz mains hum and its multiples at 120 Hz, 180 Hz, and 240 Hz). It is also useful when you need precise detail in the upper register, above 5 kHz.
Logarithmic Frequency Display
In a logarithmic display, the horizontal scale expands exponentially. Each octave—a doubling of frequency—occupies an equal amount of horizontal space. The distance from 100 Hz to 200 Hz is visually identical to the distance from 1,000 Hz to 2,000 Hz, or 10,000 Hz to 20,000 Hz.
- How It Behaves: Human hearing perceives pitch logarithmically rather than linearly; we perceive the musical interval between 110 Hz (A2) and 220 Hz (A3) as the same distance as the interval between 440 Hz (A4) and 880 Hz (A5). The logarithmic display reflects this psychoacoustic reality by spreading out the bass and midrange frequencies where most musical fundamentals and speech formants reside.
- Best Use Cases: Logarithmic display is the industry standard for musical mixing, mastering, and tonal balancing. It is best used for identifying bass resonances, vocal fundamentals, and determining exact equalization curves for acoustic cleanup.
Summary of Differences
- Visual Distribution: Linear spreads frequencies equally by value; logarithmic spreads frequencies equally by ratio/octave.
- Low-Frequency Detail: Linear compresses bass frequencies into a narrow, hard-to-read band; logarithmic expands bass and midrange frequencies for easy inspection.
- High-Frequency Detail: Linear gives extreme detail to high frequencies above 10 kHz; logarithmic compresses high frequencies toward the right edge.
- Primary Application: Use linear for technical diagnostics, harmonic detection, and high-frequency noise. Use logarithmic for musical analysis, speech editing, and general audio equalization.