Audio Testing Equipment Types

An audio analyzer is a test and measurement instrument used to objectively quantify the audio performance of electronic and electro-acoustical devices. Audio quality metrics cover a wide variety of parameters, including level, gain, noise, harmonic and intermodulation distortion, frequency response, relative phase of signals, interchannel crosstalk, and more.

As test and measurement equipment, audio analyzers are required to provide performance well beyond that of the typical devices under test (DUTs). High-quality audio analyzers must demonstrate vanishingly low levels of noise, distortion and interference in order to be deemed worthwhile and must do so consistently and reliably to be trusted by engineers and designers. Audio analyzers are used in both development and production of products. A design engineer will find it very useful when understanding and refining product performance, while a production engineer will wish to perform tests to rapidly confirm that units meet specifications.

Audio analysis requires that the device under test receive a stimulus signal of known characteristics, with which the output signal (response) may be compared by the analyzer in order to determine differences expressed in the specific measurements. Results of these measurements are processed by the analyzer into readable data using a variety of standard units and formats, such as volts, dB, dBu, SPL, ohms, relative percentage, etc., depending upon the specific measurement being reported.

Prior to the introduction of integrated audio analyzers, audio generators and audio frequency analyzers were separate pieces of equipment. One of the earliest reliable sources used for audio test was the first product made by Hewlett-Packard in 1939, the HP200A audio oscillator. The clever and inexpensive design of the HP200A allowed testers to generate very-high-quality, low-distortion sine waves that could be used for testing. These early analyzers could only determine total harmonic distortion and noise combined and worked by employing a steep notch filter to remove the fundamental frequency of the stimulus signal from the output of the DUT.

Subsequent products from HP, Wandel & Goltermann, Radford, Marconi, Sound Technology, and Amber continued to refine measurement capabilities from the 1950s through the 1970s, but the model of usage remained relatively constant; signal generators and analyzers were separate pieces of equipment, and testing involved careful tuning of each one by a person with high technical skills. This changed in 1980 with the introduction of the Tektronix AA501 Distortion Analyzer, which automated the processes of setting levels, frequency tuning and nulling. By the mid-eighties, Tektronix ceased production of audio test equipment, and in 1984 members of the team that had developed the AA501 started Audio Precision. The first Audio Precision product was the System One, which combined an integrated generator and analyzer with a connected PC to fully automate test procedures and provide a much higher degree of computational power than the simple microprocessors used in other products at the time. The combination of PC technology with audio analyzers was adopted by others, including Prism Sound (dScope), Rohde and Schwarz (UPL), and Stanford Research (SR1).

In addition to analog, audio analyzers today are frequently capable of generating and measuring audio signals over several different types of digital input and output. The signal analyzer can provide control to both the audio generator and the audio input stages, assuring that test conditions are met. In an open-loop test, the signal analyzer has no control over the audio source driving the DUT, and thus the user must take care to ensure that the source is providing a signal of appropriate characteristics.

Electro-acoustic devices such as loudspeakers and microphones present special problems for analysis, as they must receive or transmit signals through the air. In these cases, the DUT in the model shown above must be replaced with the complete electro-mechanical system, e.g., a power amplifier to drive a loudspeaker, a loudspeaker, a measurement microphone and microphone pre-amplifier. The actual device under test can be measured only when the other devices in this system are fully characterized, so that the contributions from these devices may be subtracted from the response. Many modern audio analyzers contain measurement sequences that automate this procedure, and the focus of recent developments has been on quasi-anechoic measurements. These techniques allow loudspeakers to be characterised in a non-ideal (noisy) environment, without the need for an anechoic chamber, which makes them ideally suited for use in high volume production line manufacturing. Most quasi-anechoic measurements are based around an impulse response created from a sine wave whose frequency is swept on a logarithmic scale, with a window function applied to remove any acoustic reflections. Additionally, the generator will allow for the definition of a precise frequency range and amplitude of the stimulus presented to the DUT.

Once equipment such as variable power supplies, multimeters, oscilloscopes have been acquired, the question is often, "what's next?" Well, if you're interested in audio, you'd probably want to look at the harmonic distortion of your amplifiers. For that you need a precision signal source -- one with very low THD. And you need some sort of distortion analyzer or spectrum analyzer. It doesn't have to be fancy. Something as simple as an HP 3581A Wave Analyzer can be used for distortion measurements. You would have to tune to each harmonic manually, measure their amplitudes, and calculate the THD manually. The 3581A is a frequency selective voltmeter. Its input stage is actually really good with a nice, low noise floor. It's on par with the HP 8903 and 3562A described below. With an X-Y plotter, it can be used as an inexpensive spectrum analyzer as its source can be swept automatically across frequency. With an A/D converter connected on its X and Y axis outputs, it would even be possible to import the resulting output into a computer for further data analysis. Another use for this instrument is to hunt down mains hum. The 50/60 Hz hum along with it's harmonics can be tough to trace.

The next significant step up is the HP 8903 Audio Analyzer. The 8903 comes in different 'flavours' (A, B, and E) with a couple of options. The 8903 (A or B) provides a precision signal source and a distortion analyzer in one package. So now that you know the amplitude response and total THD of your amplifier, the next question -- at least if you're building with vacuum tubes -- is usually, "well, is it mostly even-order or odd-order harmonics?" For this you need a spectrum analyzer (or the aforementioned HP 3581A wave analyzer if you're on a budget).

The HP 3562A does a lot of stuff. It's made for use on mechanical systems and audio systems, hence, offers a measurement range of 64 µHz to 100 kHz. It has a DDS-based signal source with very low THD. Its two inputs can be plotted measured and plotted separately, or one can be uses for a reference input while the other is measured for frequency response measurements. Its −144 dBV/√Hz noise floor also makes it a very handy instrument for measuring the noise floor of an amplifier. Furthermore, with a couple of simple test fixtures, it can be used for measuring the line regulation, load regulation, and output impedance of power supplies. Note that the HP3563A is the lesser known cousin of the HP3562A. The HP3563A is essentially an HP3562A with a digital output and a few other improvements. Also note that the HP3561A is essentially one analyzer channel of an HP3562A.

For amplitude response and THD+N measurements, I prefer using the HP 8903A along with some scripts to automate the frequency sweep. This method is faster and easier to set up.

Yes, the manuals can be expensive. Yes, they're hard to understand and in most cases rather boring to read. But the HP manuals of the 1970'ies and 1980'ies are really quite well written. I recommend getting both the Operating Manual(s) and the Service Manual(s). The operating manual will tell you how to use the gear. The service manual will tell you (in detail) how the gear works. Free of charge... Use the search function on the Agilent site to search the Test and Measurement Section for the manuals.

For good quality scans with readable schematics, I suggest contacting ARTEK Manuals. I used to recommend Manuals Plus, but sadly they have now closed. And, of course, there's always eBay. Just beware that some (less than honest) sellers sell the manuals that you can download for free from Agilent. Don't fall into that trap. Personally, I only use eBay for the printed manuals. Expect to pay $40~$100 per volume for a printed manual in good quality. About $5 for a scanned version.

Few people realize it, but an external USB or FireWire sound card is actually quite useful for measurements. However, it does have one significant limitation; its input voltage range is usually fixed at a pretty high value (> 1 Vrms). Personally, I prefer stand-alone test equipment. But those are my preferences. Feel free to disagree.

How to Use Audio Testing Equipment

Key Parameters Measured by Audio Analyzers

Audio analyzers are capable of measuring many types of parameters:

Level and gain: Level describes the magnitude of a signal and may be expressed in absolute or relative terms. Common absolute units may be volts, watts, dBV and dBu, while relative measurements are expressed most commonly in dB. Level may also be conditioned as a peak measurement or an RMS measurement.
Frequency response: Measures the output level of a DUT as a function of frequency.
Total Harmonic Distortion plus Noise (THD+N): Harmonic distortion products are multiples of stimulus frequencies, while noise is energy that is mathematically unrelated to the input signal.
Intermodulation Distortion (IMD): Distortion that is the result of non-linear mixing of two or more signals, typically two sine-waves at different frequencies or the sum of a sine-wave and square-wave.
Phase: The relationship in time between two signals of identical frequency, expressed as a fraction of the period of the signal.
Crosstalk: The unwanted presence of a signal from one audio channel as it appears in other audio channels of a DUT.

Examples of Audio Testing Equipment

Below are some examples of audio testing equipment available on the market:

DirectOut Technologies ANNA-LISA Battery Powered Mobile MADI Analyzer & Signal Generator
Gold Line ZM-1P Impedance Meter Plus Protection Relay
NTI XL2-M4262 XL2 Audio and Acoustic Analyzer with M4262 Measurement Microphone
NTI Audio M2215 High SPL Omnidirectional Measurement Microphone for XL2 - Class 1 Frequency Response
Gold Line ZM1 Portable Battery Operated Impedence Meter for 25/50/70/100 Volt Line Systems
NTI MR2 Minirator Audio Generator
NTI 600 000 018 Calibration Certificate for XL2/Measurement Mics/MR-PRO/MR2/TalkBox
Audix TM1 PLUS Measurement Microphone
Galaxy Audio Cricket CPTS XLR Audio Polarity Continuity Test Set
Gold Line DSP-30 Portable Digital Audio Analyzer with MK8A Instrument Microphone
Gold Line PROKIT 30 Portable Audio Analyzer Microphone Kit with Case
NTI 600 000 600 XL2 Limit Light for Live Sound Monitoring
NTI 600-000-339 XL2 Extended Acoustic Pack - EAP Option for the XL2 Analyzer
NTI Audio STI-PA Speech Intelligibility Option for XL2
NTI Audio XL2 Audio and Acoustic Analyzer - Microphone Not Included