Audio Precision: How to write and read audio specifications

At Audio Precision, the analyzers we make are recognized around the world as the standard in audio test and measurement. Specifications, then, are very important to us. Engineers use our equipment to compose their own specifications for the audio devices they design. They use our equipment to verify that their manufacturing processes are correct, and that their products meet the goals they have set. They use our equipment to characterize their products’ competitive advantages, and they publish the specifications they derive as advertising bullet points.

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So we are very careful in how we spec our own analyzers, and we have developed some strong opinions about how speccing ought to be done. Specifications are necessary for every manufacturer, for every product. Internal engineering specs define the product and provide benchmarks for manufacturing quality assurance. Published specifications, usually less complete and rigorous than the engineering specs, inform the user of the device’s capabilities and limitations with the goal of safe and proper operation. Of course, a published spec is also a powerful sales tool for an audio device. What we read in an advertisement or even a data sheet might be, well, shaded a bit, in an attempt to place a device in an attractive position without actually lying. This tension between truth and marketing is especially evident in audio products targeted toward home entertainment use, as ordinary consumers are typically less informed than audio engineers, and less adept at interpreting arcane lists of numbers. In the professional and commercial audio fields, the tendency to use dubious power output ratings is certainly much lower than in the consumer goods sector; but a cursory sampling of pro audio, industrial audio and communications audio product specifications reveals incompleteness, inconsistency and just plain sloppiness, both in which specs are chosen and how they are expressed. 


This document discusses how to write audio specifications, and how to read them

• First, a look at our recommendations for writing specifications.

• What should good specifications look like?

• What are typical values?

• Appendix: audio standards.

• Appendix: THD and THD+N.

• Glossary: explanations of some of the measurements, standards and units found in audio specifications


We consider these guidelines as we write our own specifications, and think about them when we read specifications from other manufacturers. Each of these recommendations is discussed in more detail following.

1. Decide which characteristics are important.

2. Use the “best practice” method of measurement. If a standard applies, use the standard’s recommendation.

3. Spec conservatively.

4. State ranges and conditions; annotate when more detail is required.

5. Express in standard units. Display secondary units if applicable.

6. Express graphically (x-y, polar, waterfall, etc.) when appropriate.

7. Use uniform units, formatting, and numerical expressions throughout the product line.


The details of an audio device’s tested performance are the heart of a set of specifications. These are the numbers that are weighed for bragging rights, for market position, and ultimately, for a sale. In general, the results of four categories of measurements form the core of audio performance specifications for any audio device:

1. Level, as volts (V), watts (W), or sound pressure level (SPL).

2. Frequency Response, as a statement or a graph; or at least, Frequency Range.

3. Distortion, as total harmonic distortion (THD),total harmonic distortion plus noise (THD+N), and intermodulation distortion (IMD) of various flavors.

4. Noise, as an absolute voltage, as signal-to-noise ratio (SNR), or as dynamic range.


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