Using Plot Spectrum for Audio Diagnostics

What You’ll Learn

You’ll learn to use Audacity’s Plot Spectrum tool to generate detailed frequency analysis graphs that show the exact amplitude levels of every frequency in your audio selection. This diagnostic capability allows you to make precise, data-driven decisions about EQ, compression, and noise reduction by identifying problem frequencies with scientific accuracy rather than guessing.

Key Concepts

Plot Spectrum is accessed via Analyze menu and generates a frequency distribution graph for any selected audio region, displaying frequency on the horizontal axis and amplitude in decibels on the vertical axis. Unlike the spectrogram which shows frequency changes over time, Plot Spectrum provides a single snapshot of the overall frequency content, making it perfect for analyzing sustained tones, identifying resonances, and comparing frequency profiles before and after processing. The graph includes precise numerical values and visual peaks that reveal the dominant frequencies in your audio, essential information for targeted EQ work.

• Selecting and Analyzing: Select a representative section of audio (at least 1-2 seconds) where you want to analyze frequency content, then navigate to Analyze > Plot Spectrum to open a window showing the frequency distribution as a detailed waveform graph with frequency markers.
• Reading Peak Frequencies: Peaks in the Plot Spectrum graph show dominant frequencies; use your mouse to hover over peaks to identify their exact frequency in Hz and amplitude in dB, allowing you to spot problematic resonances like a boomy 100 Hz peak or harsh 4 kHz presence peak.
• Logarithmic vs. Linear Frequency Scale: Toggle the “Logarithmic” checkbox in Plot Spectrum to switch between linear frequency spacing (which emphasizes low frequencies) and logarithmic spacing (which matches human hearing and makes mid and high frequencies more visible for analysis).
• Comparing Before and After: Make note of the frequency graph before applying EQ, then re-select the same audio after EQ processing and run Plot Spectrum again to verify that you’ve successfully reduced the problematic peaks and not created new resonances elsewhere in the frequency spectrum.

Practical Application

Select a 2-second portion of your audio containing a clear problem (excessive bass, harsh sibilance, or background hum), open Plot Spectrum, and identify the peak frequency causing the issue. Write down the frequency and amplitude value, then apply a narrow EQ cut at that frequency, re-run Plot Spectrum to confirm the peak has been reduced, and repeat this cycle until your frequency response matches your target.