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Spectrum Analyzer

Spectrum Analyzer

Overview

This tool analyzes and displays the frequency components of audio signals in real-time.
You can visually confirm the magnitude of each frequency band for sounds input via microphone or line-in.
In addition to general FFT (Fast Fourier Transformation) analysis, it also features advanced measurement functions such as PSD (Power Spectral Density) display useful for noise analysis, and weighting (A-weighting/C-weighting) that takes hearing sensitivity into account.

☕ Coffee Break: What is the Spectrum Analyzer actually doing?

Sound is simply "vibration of air." An oscilloscope graphs this vibration over "time," but when a complex sound plays, the waveform becomes a messy jumble.

The Spectrum Analyzer takes this complex waveform and passes it through a mathematical magic formula called FFT (Fast Fourier Transform), breaking it down to show "which pitches" are mixed in and "how strong" they are. This is exactly the same principle as passing sunlight through a prism to separate it into its rainbow-colored components (red, green, blue, etc.). The role of this widget is to separate the "colors" of the sound and show them to you.

Operation

Starting and Stopping Measurement

  • Start Analysis / Stop Analysis button: Toggles the measurement start and stop.

Reading the Graph

  • Horizontal Axis (Frequency): Represents frequency (the pitch of the sound). Moving to the right indicates higher pitch. It is displayed on a logarithmic (Log) scale.
  • Vertical Axis (Magnitude): Represents the size (strength) of the signal. Moving upwards indicates a stronger signal. The unit for the scale depends on the settings (Unit).
  • Cursor: When you hover the mouse cursor over the graph, the exact frequency and level at that point are displayed in "Cursor: ..." at the top of the screen.
  • Overall: Displays the total signal level (root mean square) across the entire frequency range.

Settings

Basic Settings (Analysis Settings)

  • Mode

    • Spectrum: The most common mode. Displays the peak levels for each frequency. Suitable for measuring signal levels such as sine waves.
    • PSD (Power Spectral Density): Displays the power spectral density. Use this when you want to evaluate the noise distribution uniformly. The unit is /√Hz/√Hz.
    • Cross Spectrum: Displays the correlation components between the L and R channels (advanced setting).

  • Channel

    • Left / Right: Displays only the specified channel.
    • Average: Displays the average of the left and right channels.
    • Dual: Displays both left and right channels simultaneously on the graph (Left=Green, Right=Red).

  • FFT Size (Frequency Resolution)

    • Specifies the number of samples used for analysis. Think of this like the megapixel count of a digital camera.
    • Higher numbers (e.g., 131072, 1M): High megapixel (high resolution) state. The frequency scale becomes finer, making it easier to distinguish dense peaks. However, just as taking a high-res photo requires a longer "exposure time," the graph's response speed becomes slower.
    • Lower numbers (e.g., 1024, 4096): Low megapixel (low resolution) state. The frequency scale becomes coarser, but because the shutter speed is fast, it briskly follows fast-moving sounds.
    • Usually, a value between 4096 and 16384 is recommended for a good balance between resolution and response speed.

  • Window (Window Function)

    • A process to suppress errors (spectral leakage) that occur during FFT analysis.
    • 💡 Why is it necessary?: FFT assumes that the "sound is repeating the exact same pattern infinitely." However, in reality, we cut out (sample) a certain length of sound and calculate that. The unnaturally cut edges of the waveform (as if cut by scissors) generate "non-existent noise (spectral leakage)" in the calculation results. The window function acts to smoothly fade out the edges of the cut waveform, preventing this noise.
    • hanning: The most versatile and common window function. Choose this if you are unsure.
    • rect (Rectangular): No window function is applied. Errors will be large for any signals other than transient signals or signals whose cycles match perfectly.
    • Multitaper: When the Multitaper feature (described below) is turned ON, a dedicated window function is automatically applied.

  • Weighting

    • Z: No weighting (flat). Use this when measuring physically accurate voltage or sound pressure.
    • A: A-weighting. A filter that matches the sensitivity characteristics of the human ear.
      • 💡 The mystery of the human ear: The human ear does not hear all pitches equally. We are most sensitive to 3kHz-4kHz (the pitch of a baby's cry or a siren), and insensitive to low and very high frequencies (this is said to be an evolutionary advantage for survival). A-weighting converts the physical strength of the sound picked up by the mic into this "loudness perceived by humans" (weighting). This is the standard setting for noise level measurements.
    • C: C-weighting. Closer to flat than A-weighting, but it cuts out very low and very high frequencies that human ears cannot hear. It is often used to evaluate the sound pressure you "feel" with your body at loud venues like live houses.

  • Unit

    • dBFS: A relative value with 0 dB as the digital full scale. It is the level relative to the input limit of the audio interface.
    • dBV: Voltage level with 1 V as 0 dB (requires calibration settings).
    • dB SPL: Sound pressure level (requires calibration settings such as microphone sensitivity correction).

Advanced Controls

  • Smoothing

    • Smooths out the jaggedness of the graph to make it easier to read.
    • 1/3 Octave, etc., are common display formats used in audio analysis.

  • Avg (Averaging)

    • Specifies the strength of the averaging process in the time direction.
    • Moving the slider to the right makes the graph move more slowly, suppressing fluctuations in noise components for easier viewing.

  • Multitaper

    • When turned ON, it uses multiple window functions to reduce the variance (scatter) of the spectrum estimation. This can result in smoother and more reliable results in noise analysis. When ON, the Window setting is disabled.

  • Peak Hold

    • Continues to hold the maximum level from the past with a red dotted line.
    • Convenient for checking loud sounds that occur only momentarily.
    • Clear Peak button: Resets the held peak display.

Usage Examples

Below are a few specific usage scenarios for the Spectrum Analyzer.

Basic Input Check

Basic usage to confirm that the microphone is picking up sound correctly.

  1. Press the Start Analysis button to begin measurement.
  2. Select Spectrum for Mode and Average or Left for Channel.
  3. Speak into the microphone or clap your hands.
  4. If the graph reacts and changes shape, the input is functioning normally.
  5. By turning ON Peak Hold and clapping, the frequency components of the momentary impact (the spectrum of the pulse sound) will remain as a red line, making it easier to observe.

Noise Floor Measurement

Checks the noise level (noise floor) when there is no sound. This is useful for improving the S/N ratio or finding power supply noise.

  1. Connect the input device (microphone or line input) and ensure no sound is being produced.
  2. Change Mode to PSD. PSD is suitable for viewing the distribution of noise.
  3. Increase the Avg (Averaging) slider to about 50% to 90%. The jumpiness of the waveform will subside, and the average noise line will become visible.
  4. If there is a sharp peak at a specific frequency (e.g., 50 Hz or 60 Hz), there may be power supply hum noise mixed in.
  5. Setting Smoothing to 1/3 Octave makes it easier to grasp the overall noise trend (whether it's closer to white noise or pink noise, etc.).

Speaker Frequency Response

By playing a test signal (such as pink noise) and picking up the speaker's output with a microphone, you can perform a simple check of the frequency response.

  1. Prepare and play a Pink Noise source separately.
  2. Place a measurement microphone in front of the speaker.
  3. Set Mode to Spectrum and FFT Size to around 16384.
  4. Set Avg to a high value (90% or more).
  5. Set Weighting to Z (flat).
  6. As the graph approaches flatness, the speaker's characteristic is flat. If the low or high frequencies are drooping, that is the limit of the speaker's reproduction range.
    • Note: This is not a rigorous measurement as it also picks up room reflections, but it is effective for knowing trends.