The six basic audio measurements are level, frequency response, total harmonic distortion plus noise (THD+N), phase, crosstalk, and signal-to-noise ratio (SNR). A useful measurement starts by defining the exact signal path through the device under test (DUT), its connections and load, and the gain and processing settings. This introduction focuses on that setup and on measuring level; it also previews frequency response.
What are the six basic audio measurements?
David Mathew of Audio Precision organized the measurements as the “Big Six” in an article published by EE Times on 14 November 2007. Part 1 names all six, but its hands-on emphasis is measurement setup and level. Frequency response is introduced; the related Part 2 covers the remaining measurements in greater detail.
- Level: The magnitude of an audio signal, commonly expressed as voltage or power for a stated measurement point and condition.
- Frequency response: How a device’s output level changes as the input frequency changes.
- THD+N: Total harmonic distortion plus noise, assessed at a stated signal level and test condition.
- Phase: The phase relationship between signals at specified points or channels.
- Crosstalk: Unwanted signal coupling from one channel or path into another.
- SNR: Signal-to-noise ratio, comparing the wanted signal with the noise under stated conditions.
The names identify different properties; they are not interchangeable scores. A meaningful result depends on documenting what was connected, where it was measured, and the conditions used.
Define the DUT path before connecting instruments
Decide which input and output of the DUT you are testing. A receiver, for example, may be tested from its left and right CD analog inputs through to its left and right power-amplifier speaker outputs. That is a different setup from a power amplifier tested on its own. A playback-only DVD player has outputs but no audio inputs, so a test may instead require prerecorded test signals.
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Match connections to the device
Professional, industrial, and broadcast equipment commonly uses balanced analog connections; consumer analog equipment typically uses unbalanced connections. These are tendencies, not rules for every product. Mathew’s receiver example uses unbalanced RCA inputs and balanced amplifier outputs, so a single test path can involve both types. Select cables and instrument connections to match each endpoint rather than assuming one cable arrangement applies to all devices.
Use the required load and document settings
Some devices need a specified load impedance to operate as intended or to meet a particular test condition. In the receiver example, an 8-ohm power resistor takes the place of a loudspeaker, and the measurement instrument connects across the load. That resistor is specific to the example; use the load called for by the DUT’s documentation or the test you intend to perform.
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Input level, output level, gain, volume, equalization, tone controls, and digital signal processing can all affect a result. For a baseline test, the article disables processing and sets controls to neutral unless a different condition is required. Record the settings you actually use so another person can interpret or repeat the measurement.
Choose and report the level target
There is no single correct level for every measurement. Depending on the purpose, a test may target a specified output voltage or power, unity gain, a particular distortion threshold, a practical operating level with useful headroom and noise performance, or the level required by a named test specification. State the target and the DUT’s relevant gain or volume setting, especially when that level will serve as a reference for later measurements.
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Mathew defines voltage gain as “the ratio of a DUT’s output voltage level to its input voltage level.” For a variable-gain device, controls such as volume, tone, or processing can change that ratio. Set them to the intended test condition rather than treating the control position as incidental.
Receiver example: three illustrative level targets
The 2007 tutorial uses a 1 kHz sine wave and walks through three example targets. These are procedure examples, not universal audio-device specifications.
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- Unity gain: Apply 1 Vrms at the input and adjust the receiver to produce approximately 1 Vrms at the output.
- Power reference: Adjust the output to 1 W into the example’s 8-ohm load.
- Distortion threshold: Increase the output toward 1% THD+N to identify the level at that threshold.
In the EDN republication’s account of this particular receiver and setup, the output reached about 97 W, or about 28 Vrms into 8 ohms, at the below-1% THD+N maximum-output threshold. That is an example DUT result, not a general receiver rating or benchmark. The article also describes about 1 Vrms as a nominal operating level for much equipment, while noting that specialized devices can operate well below or above it.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How a basic frequency-response measurement works
A basic frequency-response measurement records output level at different known input frequencies. A quick check can compare two or three tones. To obtain a fuller curve, sweep a sine wave from low to high frequency and plot the output level against frequency. The appropriate range and setup depend on the DUT and the question being tested; the tutorial does not prescribe one sweep configuration for every device.
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Choose equipment for the signal path
A dedicated audio analyzer can generate a test signal and measure the DUT’s output, as in the historical tutorial. Suitability depends on the channels, bandwidth, signal domains, connection types, and load handling your measurements require. You may also need compatible balanced or unbalanced cables and, for a power-amplifier test like the example, an appropriate load resistor. The SYS-2722 and ATS-2 are equipment references in the 2007 article, not current product recommendations.
What to record with a measurement
A measurement is easier to interpret when its conditions travel with the result. Record the details that define the test, including:
- The DUT and the specific input-to-output path tested.
- Analog or digital signal path, and balanced or unbalanced connections at each end.
- Any required load impedance and the load actually connected.
- Input signal type, frequency, and level.
- Output target and how it is expressed, such as voltage, power, unity gain, or a distortion threshold.
- Gain, volume, tone, equalization, and processing settings.
These details matter because results taken at different levels, loads, or control settings may not be directly comparable.
Sources: David Mathew, Audio Precision, “Introduction to the Six Basic Audio Measurements – Part 1,” EE Times, 14 November 2007; EDN republication; eeNews Europe republication; Part 2.
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