Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
I am very excited to announce that AP is now an authorized reseller of microphones and acoustic test fixtures from G.R.A.S. Sound & Vibration. This means that we can now put together complete test solutions for our loudspeaker and electroacoustics customers on a single invoice. G.R.A.S. enjoys a very strong reputation for reliability and performance, and I see a lot of similarities with Audio Precision in their commitment to quality and customer service.
Meanwhile, all of AP is very focused on delivering APx500 v3.0 in April with its support for PDM and PESQ.
Thank you to all those who were able to join Tom Kite, Dan Foley, and me at our recent seminar in Boston, and to those that attended the AES LA chapter meeting where I spoke earlier this month. Both events were well attended and most enjoyable.
Output: Testing PDM Microphones and Inputs with APx
In January, we published an article highlighting the features of our upcoming PDM module. Since then, we’ve added even more features, including a choice of 4th or 5th order modulator; sample rate multipliers of 32x, 64x, 128x, and 256x; and an FFT view of the PDM bitstream before decimation. The article below is an excerpt from our new Technote 117, Testing PDM Microphones and Inputs with APx. We are currently taking orders for the PDM option module as an upgrade or as part of a new APx instrument, and plan to start shipping it at the end of April.
PDM audio measurements are generally similar to those for other audio devices. However, there are some details particular to PDM testing.
The six basic audio measurements (level, frequency response, THD+N, phase, crosstalk, and signal to noise ratio) form a good framework for discussing comprehensive PDM device testing (see Technote 104: Introduction to the Six Basic Audio Measurements, available at ap.com). Most other audio measurements are variations on these. In the following, we’ll discuss each one as they relate to PDM microphone testing (PDM Input testing is not included here, but is in the full version of Technote 117).
Measuring PDM Microphones
APx PDM module with microphone (DUT) connected.
Overload and maximum acoustic input level can be determined using the Stepped Level Sweep measurement, after following the dBSPL calibration procedure (described in Technote 117). To make sure that the source speaker can produce the required sound level for the test, first run the test using a calibrated measurement microphone. Measurement microphones can usually tolerate 145–160 dBSPL (check the datasheet for your capsule), which is well over the level that most PDM microphones can accept.
The APx500 Stepped Level Sweep measurement includes results for level, linearity, and THD+N—all of which can be used to determine the overload and maximum levels. The Maximum Output measurement may also be used to automatically determine the 3% overload level.
Level measurement will also be discussed in the context of other measurements below.
Acoustic Response measurement, Level result, with smoothing applied.
The 94 dBSPL output level has already been determined during the acoustic calibration procedure (included in the full Technote 117), and the maximum output has been determined with the Stepped Level Sweep. The only one that remains to be measured is the self-noise.
In order to make this measurement, you will need to isolate the microphone from acoustic noise by placing it in a very quiet room, and furthermore usually inside an isolation box within the room. The ventilation system should be turned off to prevent noise and rumble, and noise sources like the analyzer and computer should not be located close by. MEMS mics typically have a self-noise level of around 30–35 dBSPL. A very quiet room is typically in the range of 30-35 dBSPL, so usually the box will be a necessity to provide additional isolation.
You should first make the noise measurement using a measurement microphone to verify if the noise level in the room is low enough to accurately characterize the PDM microphone. Check the specifications for your measurement microphone to make sure it is suitable. Most general purpose 1/2" free-field capsules have self-noise between 15–20 dBA SPL.
Once the microphone has been isolated, go to the APx500 Noise (RMS) measurement and set the graph to read in dBSPL. The 20 kHz low-pass and 20 Hz high-pass filters should be selected. Set the weighting filter to A-wt if your specifications require A weighting. Now read the measurement on the graph.
The PSR of PDM mics is determined by superimposing an AC wave onto the DC power supply, and then measuring the level of the AC wave that is present in the audio. The PSR measurement has a selection of waveforms, including sine wave, 217 Hz square wave, pulsed 217 Hz square wave (duty cycles from 1/8 to 7/8), and TPDF noise. PSR is commonly specified on datasheets using a 217 Hz square wave at 100 mVpp. It’s used because GSM phones typically have a 217 Hz switching frequency that is the main source of noise on the power supply.
APx500 Scope monitor showing the 217 Hz square wave leaking into the audio signal.
Bandpass filtering is applied to the audio before measurement. In the case of the sine wave, the filter is at the stimulus fundamental; for the square wave, it is at the fundamental and odd harmonics (to H101); and for the pulsed square wave, it is at the fundamental and all harmonics (to H101). The filtering prevents random noise outside the fundamental or harmonic frequencies from affecting the results. It also gives the measurement some immunity to room noise being picked up by the mic, but for the most accurate results, the microphone should be isolated from noise using a chamber or box. No bandpass filtering is applied when using the TPDF noise stimulus, so it is especially important in this case to isolate the microphone.
APx500 Signal Analyzer, FFT Spectrum view of PDM Bitstream.
Downloads / Resources
Sound Advice: G.R.A.S. Measurement Microphones and Simulators
Audio Precision is pleased to become an official reseller for G.R.A.S. Sound and Vibration of Denmark. AP now offers a complete solution to customers looking to test devices with acoustic interfaces, matching the world’s best audio analyzers with the highest quality measurement mics, power supplies, calibrators, and accessories. G.R.A.S. products may be ordered along with the purchase of an analyzer, or at a later time by existing AP customers.
Both R&D and production test users now have a complete solution available from a single source for end-to-end testing utilizing acoustic, analog, AES digital, serial digital, HDMI, PDM (MEMS mics), and Bluetooth interfaces.
Founded in 1994 by Gunnar Rasmussen, G.R.A.S. Sound & Vibration (http://www.gras.dk) produces acoustic measurement products such as microphones, preamplifiers, and HATS (head and torso simulators). With over 50 years in the audio industry, Mr. Rasmussen is well known for his numerous contributions to the development of noise and vibration measurement instrumentation.
45BA Complete KEMAR Head & Torso with couplers
For smartphone and tablet customers, the KEMAR head and torso simulator, handset positioner, and ear and mouth simulators are essential tools for making measurements to international standards.
G.R.A.S. Kemar 45BM Head and torso simulator with mouth simulator.
Measurement Microphones and Accessories
46AE 1/2" Condenser microphone set, free-field, constant current power
Three high precision microphone sets for type 1 audio measurements are being offered that cover the vast majority of acoustic measurement situations that don’t utilize the specialized couplers above. The sets are comprised of a prepolarized capsule mated to a preamp body, sealed together with a label, and calibrated as a unit. This simplifies microphone selection and ordering. The capsules and preamps are identical to the regular modular offerings, so those who wish to interchange capsules in the traditional fashion may do so by simply removing the label.
G.R.A.S. 46AE microphone set (type 40AE capsule and type 26CA preamplifier sold as a set).
The free-field capsules have a frequency response that compensates for the air disturbance caused by the presence of the microphone itself, and are ideal for standard speaker measurements made in a room or anechoic chamber.
Pressure capsules lack this compensation, and have flat frequency response when used in close-coupled situations that are typical when measuring headphones and earphones.
The 1/2" microphones are by far the most popular in the industry, and the best choice in most cases—they offer excellent frequency response, low noise, and high overload level.
The 1/4" microphone, besides being more compact, offers extended frequency response to 100 kHz and has an extremely high overload level of 166 dBSPL. The trade-off is that self-noise is 36 dBA SPL, about the level of a quiet bedroom. This makes it unsuitable for measuring very low noise levels, but is of no consequence when making tone measurements at moderate to high levels.
The 40PH is a cost effective microphone suitable for type 2 audio measurements in production test. It is a one-piece integrated capsule/preamp design, with a 1/4" prepolarized free-field capsule. Although its self-noise, overload, and response accuracy do not match that of the high precision microphone sets described above, in many production line applications it is more than adequate.
AP’s new MMK-4 Measurement Microphone kit contains the G.R.A.S. 46AE microphone set and the G.R.A.S. 12AL CCP power supply. It replaces the MMK-2 kit, which will be discontinued on May 29th. The MMK-4 specifications are equal to or better than the MMK-2, and the capsule is exactly equivalent. If you currently have the MMK-2 and wish to purchase additional kits, you can confidently switch to the MMK-4.
The complete list of G.R.A.S. products that we offer may be seen at /products/accessories/gras.
Test Results: AP News & Events
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