MultiTone Generator: Create Custom Multitone Audio Signals Quickly

How to Use a MultiTone Generator for Audio Testing and CalibrationAccurate audio testing and calibration are fundamental for sound engineers, acousticians, audio equipment manufacturers, and serious hobbyists. A MultiTone generator — a tool that produces multiple simultaneous sinusoidal tones at different frequencies and amplitudes — speeds up and improves many measurement tasks compared with single-tone or swept-sine methods. This article explains what a MultiTone generator is, when to use it, how to set up and run tests, interpret results, and avoid common pitfalls.

What is a MultiTone Generator?

A MultiTone generator produces two or more sine waves simultaneously. Each tone can be individually configured (frequency, amplitude, phase) and combined into a single output signal. Common uses include:

• Measuring frequency response and linearity
• Detecting intermodulation distortion (IMD)
• Stress-testing amplifiers and converters
• Calibrating room correction systems and loudspeakers
• Characterizing noise and dynamic range

Key advantage: a MultiTone test can examine many frequencies at once, greatly reducing test time while revealing non-linear behaviors that single tones or sweeps might miss.

Types of MultiTone Signals

• Discrete spaced tones: evenly or selectively spaced frequencies (e.g., 31 tones across 20 Hz–20 kHz).
• Comb-like tones: harmonically related or evenly spaced to probe periodic responses.
• Random-phase vs. aligned-phase sets: phase settings affect crest factor (peak-to-average ratio) and thus stress levels on devices.
• Optimized low-crest multi-tone: uses algorithmic phase selection to minimize peaks, useful for reducing clipping during tests.

When to Use MultiTone Tests

• Quick frequency-response snapshots: measure many frequencies simultaneously to build a response curve.
• Intermodulation and distortion detection: multi-frequency interactions reveal device non-linearities.
• High-resolution FFT analysis: combine MultiTone output with a high-resolution FFT to separate and quantify harmonic and intermodulation products.
• System calibration: room correction filters or equalizers can be tuned using MultiTone-derived frequency-response data.

Equipment and Software Needed

• MultiTone generator software or hardware (standalone devices, plugins, or DAW tools).
• Audio interface with adequate sample rate and dynamic range.
• Measurement microphone (for acoustic tests) with a calibrated preamp and known mic response.
• Analyzer software that can perform FFT, spectral averaging, and display harmonic/intermodulation components.
• Cables, stands, and an acoustically appropriate test environment (anechoic chamber, or a treated room for consistency).

Preparing for a Test

1. Define test objectives

   • Are you measuring frequency response, distortion, or dynamic range?
   • What frequency range and resolution do you need?

2. Choose frequencies and spacing

   • Use logarithmic spacing for audio-range response curves (mimics musical perception).
   • Use linear spacing when probing narrowband behavior or specific harmonics.

3. Set amplitudes and headroom

   • Start with moderate levels to avoid clipping in the signal chain.
   • For distortion testing, select higher levels to stress the device but stay within safe power limits.

4. Select phase strategy

   • Random-phase to simulate realistic or “noisy” signals.
   • Optimized low-crest phases to maximize average power while avoiding large peaks (helps avoid clipping and heating issues).
   • Aligned-phase for worst-case peak levels when testing handling of transients.

5. Calibrate your measurement chain

   • For acoustic tests, perform a reference sweep or single-tone calibration with the known microphone response.
   • Verify the audio interface’s level and linearity using loopback or reference equipment.

Running the Test: Step-by-Step

1. Generate the MultiTone signal

   • Configure frequencies, amplitudes, and phases in the generator.
   • Ensure the sample rate and bit depth are appropriate (48 kHz or 96 kHz common; 24-bit recommended).

2. Route the signal through the device under test (DUT)

   • For electronics: feed into amplifier, DAC, or speaker driver.
   • For acoustics: play through the loudspeaker and measure with the microphone at the listening position.

3. Record the output

   • Use the analyzer or DAW to record the DUT output with the same sample rate/bit depth.
   • Include a synchronized reference channel if possible (direct generator output) for deconvolution or transfer function measurement.

4. Analyze with FFT and spectral tools

   • Identify the primary tone bins (where tones were placed).
   • Measure amplitude deviations across bins to derive frequency response.
   • Inspect harmonic bins (integer multiples of each tone) and intermodulation products (sum/difference frequencies) for distortion.

5. Repeat and average

   • Take several measurements and average them to reduce random noise.
   • Vary level or position (acoustic tests) to assess behavior across conditions.

Interpreting Results

• Frequency Response

  • Plot amplitude vs. frequency using the tones’ measured magnitudes.
  • Look for dips, peaks, and slope deviations indicating resonances, cancellations, or room modes.

• Harmonic Distortion

  • Harmonic components appear at multiples of each fundamental. Sum their power relative to fundamentals to calculate THD (total harmonic distortion).

• Intermodulation Distortion (IMD)

  • Look for non-harmonic products at sums and differences of the test tones (e.g., f1 + f2, |f1 − f2|). These indicate non-linear mixing.

• Noise Floor & Dynamic Range

  • Measure spectral noise between tone bins. Dynamic range ≈ difference between tone levels and noise floor.

• Crest Factor Effects

  • High crest factors reduce average test energy and might mask temperature-related or long-term nonlinearities. Low-crest signals increase average power and stress components more.

Practical Examples

• Loudspeaker frequency-response check:

  • Use ⁄₁₂-octave spaced tones from 40 Hz to 16 kHz, measure on-axis and off-axis, average multiple positions.

• DAC linearity test:

  • Use discrete tones spanning audible bands, record direct electrical output, compute THD+N and IMD.

• Amplifier stress-test:

  • Use an optimized low-crest MultiTone at elevated levels to observe thermal compression and clipping thresholds.

Common Pitfalls and How to Avoid Them

• Clipping in the chain: monitor peaks and use headroom; prefer low-crest multi-tone only when safe.
• Aliasing: ensure sample rates and anti-alias filters are appropriate for the highest tone.
• Poor phase alignment: if comparing reference vs. output, ensure timing sync or use time-alignment tools.
• Room reflections masking results: use gated measurement techniques or perform tests in a treated space.
• Misinterpreting IMD vs. harmonic distortion: map expected product frequencies to identify their origin.

Advanced Tips

• Use windowed FFTs with sufficient resolution to separate closely spaced tones.
• Apply notch filters to remove very strong tones if you need to inspect weak intermodulation products near them.
• Automate sweep-and-multi-measure sequences to compare before/after calibration quickly.
• Combine MultiTone tests with impulse-response (time-domain) measurements to get both steady-state and transient behavior.

Conclusion

A MultiTone generator is a powerful and efficient tool for audio testing and calibration, offering simultaneous multi-frequency excitation that shortens test time and exposes nonlinear behaviors. Proper setup — choosing frequencies, managing phase and crest factor, ensuring headroom, and using correct analysis techniques — is essential for reliable measurements. When used carefully, MultiTone testing yields clear insights into frequency response, distortion, noise, and system stability across real-world listening conditions.