Introduction to the Six Basic Audio Measurements - Part 1

We will measure our DUT at its power amplifier outputs (speaker connections), using the left and right channels. These are available as spring clips for connecting speaker cable. For our test we will ignore the surround and subwoofer channels. We will connect the left and right outputs to a pair of 8-Ω power resistors as a terminating load, and will also connect parallel lines from the resistors to our instrument analyzer balanced analog inputs.

Setting the DUT controls

Setting up the analyzer control software
Turn on the analyzer, and launch the AP2700 or ATS control software.

The Big Six Measurements
This section provides step-by-step instructions to make the Big Six measurements using an Audio Precision analyzer and our home theater receiver DUT.

The instructions are written assuming the use of a SYS-2722 audio analyzer. Where instructions for the ATS-2 differ, notes are added in [brackets].

Level

About Level measurements
Each DUT may have a number of level measurements that are of interest. You must choose which level you are seeking. Target levels include

• an input level that produces a given output level, such as 1 volt, or 1 watt, or unity gain (see below for a discussion of DUT gain);
• an input level that produces a certain output distortion, such as 1% THD+N;
• a level that provides good noise performance with comfortable headroom, often called the operating level;
• an input or output level specified in a testing document.

Any of these levels may be used as a reference level on which we can base further measurements. Frequency response measurements, for example, are expressed relative to the level of a mid-band frequency; THD+N measurements are made at specified levels, which should be reported in the results.

Gain considerations for level measurements
The ratio of a DUT's output voltage level to its input voltage level is the voltage gain of the DUT. For example, in a DUT with a gain of 2, an applied input of 2 volts will produce an output of 4 volts. A gain of 1, where the output voltage equals the input voltage, is called unity gain.

Some DUTs offer no gain adjustments, and are said to have fixed gain. The gain may be fixed at unity, or at some other value.

Measuring level in variable gain DUTs
A DUT with a volume control or other setting that affects gain is a variable gain device. When setting and measuring level, it is essential to consider whether or not the DUT gain is variable (not only volume controls, but tone controls and other settings can change gain), and, if it is, how to set the DUT controls for the desired test results.

Making Level measurements
We will make level measurements using three different methods that produce results that are commonly used in audio specifications.

With a 1 Vrms applied signal, we will set the volume control to the position that produces 1 Vrms at the speaker outputs (actually, across the 8-Ω terminating load resistors). Then we will readjust the volume control to produce 1 W in the load, and then we will drive the amplifier into distortion to find the level in watts at 1% THD+N.

Hint: making very small level adjustments
Our DUT has a stepped volume control, with steps of a few decibels. Very fine adjustment is not possible. To find the precise level where the DUT clips, we use a Bar Graph control in the AP software to make small adjustments in Generator level.

Here's how: first find the highest distortion less than 1%, using the DUT volume control. Then set up a Bar Graph to control the generator level, using very small steps.

Create a bar graph by right-clicking in the Amplitude setting field. Then set the Increment to 1 mV in the Bar Graph Setup dialog. Slide the bar control with the mouse, or click in the bar to engage the keyboard arrow keys. Click the arrow keys to adjust the level in these small steps until you find the highest THD+N reading you can get that is less than 1%.