Electronics Guide

Architecture and Operation

A typical audio analyzer includes a precision signal generator capable of producing sine waves, multitone signals, and various noise types across the audio band and beyond. The analysis section accepts analog or digital signals and applies sophisticated processing to extract parameters including level, frequency response, distortion, noise, and crosstalk. Digital signal processing enables measurements impossible with traditional analog techniques.

The generator section requires exceptional spectral purity since any distortion or noise in the test signal corrupts measurements. High-end analyzers achieve residual distortion below -120 dB (0.0001%) and noise floors approaching theoretical limits. The analyzer section must handle signals spanning huge dynamic ranges, from microvolts of noise to tens of volts of signal, while maintaining accuracy and avoiding overload.

Key Measurements

Audio analyzers excel at distortion measurements. Total harmonic distortion (THD) quantifies harmonics generated when a pure sine wave passes through the device under test. The analyzer applies a notch filter to remove the fundamental and measures remaining harmonic content. THD+N (total harmonic distortion plus noise) includes both harmonics and broadband noise in a single figure. These measurements characterize amplifier linearity, converter performance, and overall signal path quality.

Frequency response measurements use swept or stepped sine waves to characterize gain versus frequency. Modern analyzers can complete full audio band sweeps in seconds while maintaining high accuracy. Simultaneous measurement of amplitude and phase reveals complete transfer function information. Automatic level control maintains constant input to the device under test regardless of its gain characteristics.

Intermodulation distortion measurements use two-tone test signals to reveal nonlinear interaction between frequencies. SMPTE IMD uses 60 Hz and 7 kHz; CCIF (or DIN) uses two closely spaced high frequencies. These measurements often reveal distortion mechanisms that THD measurements miss, particularly in amplifiers and loudspeakers where low and high frequency signals interact.

Digital Audio Analysis

Modern analyzers handle digital audio interfaces including AES/EBU, S/PDIF, ADAT, and various embedded formats. Digital domain analysis enables measurements impossible in analog, such as bit-perfect verification and interface jitter characterization. The analyzer can measure converters by providing digital stimuli and analyzing analog outputs, or vice versa, enabling complete characterization of analog-to-digital and digital-to-analog conversion systems.

Interface measurements verify signal integrity, jitter performance, and protocol compliance. Analyzers can inject controlled jitter to characterize device immunity. Eye pattern displays visualize signal quality. Embedded channel status and user data bits can be read and verified against expected values.

Leading Manufacturers and Models

Audio Precision dominates the professional audio analyzer market with instruments ranging from the portable APx515 to the flagship APx555. These analyzers set industry standards for measurement capability and are specified for production testing by most audio equipment manufacturers. Rohde and Schwarz offers the UPV and UPP series with particular strength in broadcast applications. Stanford Research Systems provides cost-effective alternatives with the SR1 series. Prism Sound dScope Series III combines analysis capability with focus on converter measurement.

Operating Principles

Traditional distortion analyzers use a tunable notch filter to remove the fundamental frequency from a sine wave test signal. The remaining signal contains harmonics and noise, which the instrument measures and displays as a percentage or decibel ratio relative to the fundamental. The sharpness and depth of the notch filter determine measurement floor; high-quality instruments achieve notch depths exceeding 100 dB.

Modern FFT-based analyzers take a different approach, digitizing the signal and computing its spectrum via fast Fourier transform. Individual harmonics can be measured and displayed separately. This approach eliminates the need for precision tunable filters and enables rapid measurements across multiple frequencies. However, FFT resolution and dynamic range must be adequate for the measurement requirements.

Measurement Considerations

Meaningful distortion measurements require appropriate test conditions. Signal level significantly affects distortion in most devices; specifications must state the measurement level. Frequency also matters since distortion often varies across the audio band. Load impedance affects amplifier measurements. Bandwidth limiting excludes out-of-band noise that would otherwise inflate readings.

The measurement system's own residual distortion and noise set the measurement floor. When measuring extremely low distortion devices, the instrument's residuals may dominate the reading. Specifications should state whether readings represent device under test performance or measurement system limitations.

Types of Generators

Sine wave generators produce the fundamental test signal for most audio measurements. Quality generators achieve very low distortion (below 0.001%) and flat frequency response across the audio band. Variable frequency and amplitude enable testing devices at various operating points. Some generators include sweep capability for automated frequency response measurements.

Function generators add square, triangle, sawtooth, and arbitrary waveforms to sine wave capability. While not as spectrally pure as dedicated sine generators, function generators provide versatile stimulus for various test applications. Modern arbitrary waveform generators can reproduce any waveform loaded from files or created through software.

Noise generators produce pink noise (equal energy per octave), white noise (equal energy per hertz), or other spectral distributions. Pink noise is standard for acoustic measurements since it better matches human hearing sensitivity distribution. Noise signals enable rapid frequency response measurement and room acoustic analysis.

Multitone generators produce multiple simultaneous frequencies for rapid testing. A carefully designed multitone can characterize frequency response, distortion, and crosstalk in a single measurement. Audio Precision's FastTest and similar proprietary signals enable production testing in a fraction of the time required for traditional sequential measurements.

Output Specifications

Generator output specifications include maximum level, output impedance, frequency range, and frequency accuracy. Professional generators offer balanced outputs for driving balanced audio equipment. Level accuracy and stability affect measurement repeatability. For distortion measurements, the generator's residual distortion must be substantially lower than the expected device under test distortion.

Measurement Methods

Traditional impedance bridges compare unknown impedance against precision standards using null-balance techniques. While accurate, bridge measurements are slow and require skilled operators. Modern impedance analyzers automate measurements using current-voltage techniques or reflection methods.

Current-voltage analyzers apply known voltage and measure resulting current (or vice versa) to calculate impedance magnitude and phase. Frequency sweeps reveal impedance variation across the audio band. Four-wire (Kelvin) connections eliminate lead resistance errors for low impedance measurements.

Audio analyzers with impedance measurement capability use their precision generators and analyzers to characterize impedance. This approach provides adequate accuracy for most audio applications while leveraging existing equipment.

Loudspeaker Measurements

Loudspeaker impedance curves reveal driver resonance frequencies and behavior. The impedance peak at resonance identifies the fundamental resonance (fs), a key Thiele-Small parameter. Measurements with the driver in free air versus mounted in an enclosure enable calculation of box parameters. Voice coil temperature affects resistance, enabling impedance-based thermal monitoring during power testing.

Instrumentation Options

Dedicated LCR meters and impedance analyzers from manufacturers including Keysight, Hioki, and Wayne Kerr provide comprehensive impedance measurement capability. These instruments offer accuracy specifications and frequency ranges suited to component testing. For audio-specific applications, the impedance measurement functions in audio analyzers typically provide sufficient capability at lower cost than dedicated instruments.

Classes and Standards

IEC 61672 defines Class 1 and Class 2 sound level meters. Class 1 instruments meet tighter accuracy tolerances and are required for precision measurements and legal compliance testing. Class 2 instruments are adequate for general field surveys and non-critical applications. Both classes specify frequency weighting accuracy, directional response, and performance under various environmental conditions.

Frequency weightings modify response to correlate with human hearing perception. A-weighting approximates the ear's sensitivity at moderate levels and is standard for noise measurements. C-weighting provides flatter response for measuring peak levels and low-frequency content. Z-weighting (zero weighting) provides flat response for scientific measurements. Some meters offer additional weightings for specialized applications.

Time Weighting and Integration

Time weighting determines how quickly the display responds to changing levels. Fast weighting (125 ms time constant) tracks moderately rapid fluctuations. Slow weighting (1 second) provides averaged readings for fluctuating sounds. Impulse weighting captures rapid transients. Modern integrating meters calculate equivalent continuous level (Leq), exposure dose, and statistical distributions over measurement periods.

Measurement Applications

Sound level meters verify loudspeaker sensitivity specifications, typically stated as SPL at 1 watt input measured at 1 meter distance. Noise surveys assess environmental and occupational noise exposure. System commissioning verifies adequate coverage and level distribution. Building acoustics measurements characterize transmission loss and background noise. Quality control verifies production consistency.

Leading Products

NTi Audio, Larson Davis, Bruel and Kjaer, and GRAS offer professional sound level meters meeting Class 1 standards. These instruments provide calibrated accuracy, data logging, and analysis software. Smartphone applications provide approximate measurements useful for informal surveys but lack the calibration and accuracy for professional work. Calibration with pistonphones or acoustic calibrators ensures measurement accuracy and traceability.

Display Formats

Traditional RTA displays show level in octave or fractional-octave bands, typically 1/3-octave resolution. This format matches how humans perceive frequency and corresponds to standard equalizer band spacing. Pink noise stimulus produces flat RTA response through systems with flat frequency response. The intuitive display enables rapid system tuning without detailed analysis.

FFT-based displays offer higher frequency resolution at the cost of increased visual complexity. Spectrogram displays add time history, showing how spectrum evolves. Combined displays overlay multiple formats for comprehensive visualization.

Transfer Function Mode

Dual-channel RTAs compare input and output signals to display system transfer function, rejecting ambient noise and non-test signals. Coherence indication shows measurement reliability. This capability enables system measurements in noisy environments where single-channel measurements would be unreliable.

Applications

RTAs are essential tools for live sound system optimization. Engineers use RTAs to verify system response, set equalizer curves, and identify feedback frequencies. Recording studio setup benefits from RTA visualization of room modes and monitoring system response. Automotive audio tuning relies heavily on RTA feedback. Broadcast facilities use RTAs for signal monitoring and compliance verification.

Product Options

Dedicated RTA hardware includes products from NTi Audio, Ivie Technologies, and Gold Line. Software RTAs running on computers or mobile devices provide cost-effective alternatives with excellent display capabilities. Smaart by Rational Acoustics has become the industry standard for live sound measurement. Room EQ Wizard (REW) provides comprehensive free measurement software for studio and home audio applications.

Audio-Relevant Features

For audio work, oscilloscopes need adequate vertical resolution to reveal small signal details and low-frequency capability extending below 20 Hz. AC coupling high-pass filters in some oscilloscopes attenuate low frequencies, potentially affecting audio measurements. Triggering must handle complex audio waveforms reliably.

XY display mode enables Lissajous pattern visualization for phase comparison between channels. This traditional technique shows phase relationships as patterns that trained operators can interpret instantly. Stereo audio balance and phase alignment are quickly verified using XY mode.

Digital Oscilloscope Advantages

Digital storage oscilloscopes capture and store waveforms for detailed analysis. Zoom and cursor measurements enable precise timing and amplitude characterization. FFT functions display frequency spectrum, providing basic spectrum analyzer capability. Some instruments include protocol decoding for digital audio interfaces.

High-resolution oscilloscopes with 12-bit or higher vertical resolution better suit audio work than standard 8-bit instruments. The additional resolution reveals small signal details and enables more accurate amplitude measurements.

Practical Applications

Oscilloscopes verify signal presence and waveform shape during troubleshooting. Clipping visualization shows when amplifiers or other devices are overdriven. Noise and interference identification benefits from waveform viewing. Power supply ripple measurement uses oscilloscopes to verify adequate filtering. Digital interface signal integrity analysis uses oscilloscope eye diagrams.

Measurement Types

Correlation meters display the statistical relationship between two channels, ranging from +1 (identical signals) through 0 (uncorrelated) to -1 (inverted polarity). This measurement indicates mono compatibility and reveals phase problems. PPM-style correlation meters show instantaneous and peak-hold values.

Goniometers (vectorscopes) display stereo signals as Lissajous patterns on XY displays. A mono signal produces a vertical line; stereo produces various patterns depending on channel differences. Out-of-phase content appears as horizontal deflection. Experienced operators can evaluate stereo width, balance, and phase issues at a glance.

Phase angle meters display the specific phase relationship in degrees between two signals at a single frequency. This measurement is useful for setting delays in distributed sound systems and verifying crossover alignment in multi-way loudspeakers.

Applications

Broadcast facilities require phase monitoring to ensure mono compatibility, as many listeners hear broadcasts in mono. Recording and mastering engineers verify phase coherence throughout the signal chain. Live sound engineers use phase meters when combining microphone signals and aligning subwoofers. Loudspeaker designers verify crossover phase behavior.

Measurement Principles

A precision 3150 Hz or 3000 Hz tone recorded on tape or disc provides the test signal. Speed variations frequency-modulate this tone. The wow and flutter meter demodulates the FM signal and measures the resulting low-frequency modulation. Filtering separates wow (slow variations below 6 Hz) from flutter (faster variations).

Different weighting standards exist for wow and flutter measurement. DIN weighting emphasizes frequencies where human hearing is most sensitive to speed variations. IEC standards define measurement procedures and weighting characteristics. Specifications must state which standard was used since results vary significantly.

Typical Values

Professional tape machines achieve wow and flutter below 0.05%. Consumer cassette decks typically measure 0.1% to 0.2%. Turntables vary widely depending on quality, from under 0.1% for high-end designs to over 0.5% for inexpensive units. These values indicate the pitch stability listeners experience.

Modern Relevance

Archival transfers from analog media benefit from wow and flutter measurement to characterize source quality. Restoration software can partially correct measured speed variations. Vinyl enthusiasts use wow and flutter measurement to evaluate turntable performance. Maintenance of professional tape machines requires regular wow and flutter verification.

Basic Continuity Testing

Simple cable testers verify conductor continuity and check for shorts between conductors. LED indicators or audible signals indicate test results. These testers identify open circuits from broken conductors and shorts from damaged insulation or connector faults. Battery operation enables field use.

Advanced Cable Analysis

More sophisticated testers measure cable resistance, capacitance, and in some cases characteristic impedance. Time-domain reflectometry (TDR) locates faults by measuring reflection timing. Length measurement uses capacitance or TDR techniques. These features are valuable for troubleshooting installed cable runs where fault location matters.

Multipin and Network Cable Testing

Professional audio systems use multipin connectors and network cables requiring specialized testers. Multipin testers verify all conductors systematically and display wiring maps showing connections between pins. Network cable testers verify Ethernet cables used for audio over IP systems, checking wire map, length, and signal quality.

Popular Products

Behringer CT100 provides basic testing at minimal cost. Pyle-Pro PCT40 offers phantom power testing. Galaxy Audio JIB series includes inline testing capability. Fluke and other professional brands offer advanced features for complex installations.

Operating Principles

Polarity checkers consist of a transmitter that generates a distinctive asymmetric test signal and a receiver that analyzes the signal and indicates whether polarity is preserved or inverted. The transmitter connects at the signal chain input; the receiver at the output. Acoustic versions use a loudspeaker output and microphone pickup.

Applications

System installation verification ensures consistent polarity throughout complex signal paths. Loudspeaker wiring verification confirms driver polarity matches across systems. Multi-microphone setups benefit from polarity checking to ensure proper combination. Troubleshooting phase cancellation problems starts with polarity verification.

Available Products

Galaxy Audio Cricket offers simple polarity testing. Behringer Ultra-G provides polarity checking with DI box functionality. NTi Audio MR-PRO integrates polarity testing with comprehensive signal analysis. Many audio analyzers include polarity testing in their feature sets.

System Architecture

Automated test systems typically include audio analyzers for signal generation and measurement, switching systems for connecting to different test points, computers running test software, and fixtures for connecting units under test. Communication interfaces enable instrument control and data transfer. Database systems store results for quality tracking and traceability.

Test Sequence Design

Effective test sequences balance thoroughness against test time. Statistical analysis identifies which tests best predict overall quality. Multitone signals enable multiple measurements in single acquisitions. Sequential versus parallel testing architecture affects throughput. Limit setting requires understanding manufacturing variation and specification requirements.

Software Platforms

Audio Precision APx500 software provides comprehensive test automation with the company's analyzers. National Instruments LabVIEW enables custom system development with various hardware. MATLAB and Python scripts automate measurements with programmable instruments. Most audio analyzer manufacturers provide software development kits for custom integration.

Quality and Compliance

Production test systems must be validated to ensure reliable results. Calibration procedures verify measurement accuracy. Statistical process control monitors for manufacturing trends. Data export enables quality analysis and regulatory compliance documentation. Audit trails track test history for individual units.

Application Requirements

Equipment selection starts with understanding measurement requirements. Laboratory development work demands the highest accuracy and most comprehensive feature sets. Production testing prioritizes speed and automation capability. Field service requires portable, battery-operated instruments. Budget constraints influence whether to invest in dedicated instruments or multi-function alternatives.

Performance Specifications

Key specifications include measurement floor (residual THD+N, noise floor), frequency range, dynamic range, and accuracy. The instrument's performance must exceed device under test performance by a comfortable margin, typically 10 dB or more. Digital interface support must match the formats used in the application. Environmental specifications matter for field and production floor use.

Practical Considerations

Software quality significantly affects instrument usability. Intuitive operation reduces training time and errors. Remote control capability enables automated testing. Data export formats must be compatible with analysis and documentation tools. Calibration requirements and costs affect total ownership expense. Manufacturer support availability varies by region and product line.

Calibration Requirements

Measurement accuracy depends on proper calibration. Manufacturers specify calibration intervals, typically one to two years for precision instruments. Calibration should be traceable to national standards through accredited laboratories. Calibration certificates document measured performance against specifications.

Verification Procedures

Between calibrations, periodic verification checks confirm instruments remain within specifications. Reference sources and loads enable quick checks of key parameters. Verification records demonstrate ongoing measurement reliability. Discrepancies trigger investigation and potentially early recalibration.

Care and Handling

Proper handling extends instrument life and maintains accuracy. Input protection prevents damage from excessive signals. Clean, temperature-stable environments optimize performance. Proper storage during non-use prevents corrosion and mechanical damage. Following manufacturer guidelines for maintenance ensures reliability.