Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
We’re getting ready for AES Oct 9-12. I’m personally excited for two reasons. First, we’ll be kicking off our 25 year anniversary celebration with a little event thanking our customers for their support. If you’re at AES on Sunday at 5pm, do please join us at booth #442 for a slice of cake.
But AP isn’t really about looking back. We’re in the business of providing audio test gear for modern audio devices. So we’re also introducing a new bit of hardware that should turn some heads. I’m not going to spoil the announcement, but let’s just say you’re going to see things you’ve never seen before in an audio FFT.
That’s all for now and hope to see you at the show.
Output: Tech Tips and New Applications from AP
Measurement Microphone Selection
To complement the APx Series' acoustic response measurement capability, and our recently released MMK-2 Measurement Microphone Kit, we have decided to take a more in-depth look at measurement microphone selection in this issue of Audio.TST.
Measurement microphones are designed to have very flat frequency response and to be very stable over time. Selecting the proper measurement microphone from the many choices available is essential for getting valid test results.
The first decision to make is the powering method, as this affects the product category from which you choose all the other system components. Both externally polarized and pre-polarized microphone systems require an outboard power supply for the microphone electronics. Externally polarized microphones, however, need an additional 200 Volt line from the power supply to charge the condenser diaphragm. Because of this, they use an expensive multi-pin cable between the microphone and the power supply, instead of a simple inexpensive BNC cable. While in the past pre-polarized microphones were noisier and less stable than their externally polarized cousins, the latest models perform just as well and have taken over the market. A feature included in many recent preamplifiers is TEDS, or Transducer Electronic Data Sheet. A TEDS equipped preamplifer can store calibration information in memory.
Capsule size is the next criteria. For most applications, 1/2" is a good choice. This size provides flat response over the 20 Hz to 20 kHz audio bandwidth, self-noise levels far below a typical quiet room, and plenty of headroom to handle loud sources. The larger 1" capsules have extremely low noise, but often less headroom and more limited high frequency response. The small 1/4" and 1/8" capsules have extremely wide response and ultra high overload levels, at the expense of higher self-noise. Sometimes a manufacturer will offer more than one capsule in the same size and response type, each with a different set of trade-offs.
Once the capsule is selected, you'll need a mating microphone pre-amplifier/body. For capsule sizes other than the standard 1/2", some manufacturers make matching preamplifiers, while others sell adapters to insert between the capsule and the standard 1/2" preamplifier. It's even possible to mix capsules, preamplifiers, and power supplies from different manufacturers.
The last major decision is the response type of the interchangeable capsule. Free-field capsules optimize response to compensate for the air disturbance caused by the microphone itself in free air. Most loudspeaker measurement applications use this type. Pressure capsules are designed to respond uniformly to sound pressure at the face of the capsule. They are used with various enclosures and test jigs, such as artificial ears, where the air does not flow around the microphone body. A third capsule response type, random incidence, is intended for noise measurements and not normally used to measure audio devices.
AP has made a kit available utilizing the most common configuration for speaker testing—a pre-polarized, 1/2" system with a free-field capsule. The MMK-2 kit components, made by PCB Piezoelectronics (Larson Davis), include the 337B02 capsule, the 426E01 preamplifier, and the 482A21 power supply. The Larson Davis CAL200 calibrator is also available as the Audio Precision MMC-3.
Sound Advice: AP Knowledge Base
Measuring Amplifier Damping Factor with APx500
AP offers an expanding list of add-on utilities that extend the capabilities of the APx Series analyzers. The latest utility and accompanying knowledge base article, both written by Director of Technical Support Joe Begin, make it easy to measure amplifier damping factor.
The Damping Factor of an amplifier is defined as the ratio of its rated load impedance to its output (source) impedance. In loudspeaker systems, the damping factor is considered to be a measure of the amplifier’s ability to control undesirable movement of the speaker cone near the resonant frequency of the speaker system.
Damping factor can easily be calculated by measuring an amplifier’s output voltage with and without its rated load impedance attached (typically 4 Ω or 8 Ω). Consider the loaded and unloaded circuits below, where RO represents the amplifier’s output impedance, RL represents its rated load impedance, VNL is the voltage measured with no load, and VL is the voltage measured under load. Note that for the purposes of this measurement, RO is assumed to be totally resistive, and RL is a non-inductive load resistor.
The load resistance acts as a voltage divider, such that
The above equation can be rearranged to yield
But by definition, the damping factor (DF) is equal to RL / RO. So, we can determine the damping factor directly from the measured voltages as
The amplifier’s output impedance can vary with frequency, and therefore it may be desirable to measure damping factor as a function of frequency. This can be done by conducting sweeps of the loaded and unloaded amplifier circuit and applying the damping factor equation at each step in the sweep.
APx Damping Factor Measurement Utility
Audio Precision has developed a utility for conducting damping factor measurements with an APx500 Series audio analyzer. The utility was developed in LabVIEW 8.6, and is available as LabVIEW source code for users who have a LabVIEW development system, or as a compiled application for those who don’t. The source code and the compiled application are designed to work with APx500 version 2.4. To install the application, unzip the installer and run the setup program. The installer places a shortcut on the desktop as well as in the Start menu.
When the utility is started (Figure 1), it opens the APx500 software if it isn't already running. If a project file has been loaded, it displays the name. It also enumerates the APx500 project and gets lists of all the Signal Paths and Measurements in the project. A damping factor measurement can be based on any of the measurements used to conduct a level versus frequency measurement, (i.e. Frequency Response, Continuous Sweep, Stepped Frequency Sweep, or Multitone Analyzer) except for Acoustic Response. A button is included on the front panel to allow you to load an APx500 project file, if required.
To run a damping factor measurement, use the controls provided to select the Signal Path and Measurement to be used for the test. By default, these controls are set to the first Signal Path in the project and the first Frequency Response measurement in that path (if one exists). Then click the Run Damping Factor Test button.
The utility will first prompt you to disconnect the load from the amplifier (Figure 2). Once the OK button is clicked, it conducts the specified level versus frequency measurement on the unloaded amplifier.
Next, the utility prompts you to connect the amplifier load (Figure 3). As noted, at this point the units should not be changed, the graph should not be re-sized, and auto scaling should be disabled. Clicking OK will run the test again on the loaded amplifier.
Once Part 2 is complete, the utility calculates the damping factor using equation (3) and plots the results on the graph (Figure 4). It displays results for each of the analog input channels enabled in the APx software. As shown, it also retrieves the channel labels from the system (in this case, “Left” and “Right”).
For a hard copy of the damping factor results, click the Print Results button. This opens a Print Results dialog window (Figure 5), which includes various test details and a note field where you can add an annotation, if desired. Click the Print button to print this results window to the system default printer.
The Export Data button allows you to save the data in tab-delimited text format, which can be opened by typical spreadsheet programs like Microsoft Excel. Figure 6 shows the data after the text file has been opened in Excel.
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Test Results: AP News & Events
AES New York, Oct 9-12
©2009 Audio Precision Inc.