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Landtek ND9 Microphone Level Calibrator
For a SLM or spectrum analyzer to report decibel values accurately, it must be calibrated with the microphone used to a known reference. The commonly used calibration device is a small cylinder which fits tightly over the microphone's diaphragm and creates a cavity of fixed volume. The calibrator emits a tone of 1,000 Hz, at 94 or 114 dB while the system's sensitivity (in SpectraPLUS) is adjusted to give the correct 94 or 114 dB measurement. Standard practice is to check calibration periodically and reset if necessary. IEC942 is the main standard for these devices, which usually combine electronics and a tiny speaker at a fixed distance in a cavity over the mic. The precision of the microphone calibrator in both frequency and loudness is obviously important for SPL accuracy.
Brüel & Kjær offers something called a pistonphone, which in theory should be the most accurate microphone calibrator as it uses a physical piston to produce a 250 Hz tone. The physical dimensions of the piston cavity are the determining factor in the resulting pressure and it is supposed to be less affected by environmental conditions than the electrical circuitry in a mic calibrator. Its price is beyond our reach; even a refurbished current model (Type 4228) was listed recently for $2,400 by a test equipment vendor.
The ND9 Microphone Level Calibrator is a low cost alternative with excellent specifications:
- SPL Accuracy ± 0.3 dB (20°C, 760 MM Hg)
- Frequency of 1000 Hz ± 0.1%
It looks like a simple cylinder.
Insert 1" mic on this end; rubber grommet makes tight seal.
Turn 1,000 Hz tone on at 94 dB or 114 dB at this end.
The ND9 came with no individual test results. The SPL accuracy cannot be checked without some other reference, but SpectraPLUS confirms that the frequency is 1000 Hz, give or take 10 Hz, as claimed. The inexpensive $115 unit was purchased from an eBay store called Easy Life Products, which claims to be "a professional Internet seller based in Hong Kong" specializing "in electronic instruments, laboratory equipment and other gadgets." It's clear they are not the manufacturer, which appears to be Landtek of China.
With the help of Professor Murray Hodgson at the Acoustics and Noise Research Group of the University of British Columbia, the ND9 was compared to several other mic calibrators and a high precision digital SLM. In the mix was a Brüel & Kjær pistonphone that works at 250 Hz, 124 dB. Amazingly, all the calibrators in the UBC sound lab gave results with 0.1 dB of each other, and the ND9 calibrator fell within that range. It's certainly high enough accuracy for our purposes.
A NEW AUDIO PC
Last June, the Shuttle Zen which had been our audio PC broke for the second time in three years. Again, it appeared to be associated with the VRM; a bad capacitor problem is suspected. But rather than replace the board this time, a new PC was assembled for our audio test duties. A newer motherboard with more uptodate features and better capacitors might not be a bad thing.
The new audio PC is also a HDD test bed. Note easy access to SATA and power cable atop.
The system was built with the following components:
- Antec NSK-3480 micro-ATX case - chosen for good airflow potential, and separate thermal zone for the PSU.
- SilverStone ST30NF 300W - Still our favorite fanless PSU, it runs cool in the Antec NSK-3480 case.
- Abit NF-M2 nView micro-ATX AM2 motherboard - A discontinued but quite nice product based on the nVidia 6150/NF430 Chipset, it has many features, including passive cooling, Firewire, optical S/PDIF In/Out, 4 RAM sockets, and 4 SATA ports.
- AMD Athlon 64 X2 4850e - This 45W TDP dual-core CPU running at 2.5 GHz has plenty of processing power for our needs while ensuring low power consumption, especially at idle, which is the operating state 90% of the time.
- Sycthe Ninja (gen. 2) heatsink with a Scythe SlipStream 120mm SL fan at 500rpm was more than adequate to cool the CPU in this system.
- A pair of 1GB Corsair CM2X1024-6400 DDR2 RAM ensures plenty of memory for the system.
- A Western Digital single-platter 320GB Caviar SE16, one of the noisier samples, in a Smart Drive Enclosure sitting atop a bit of soft foam at the bottom of the case ensured that the hard drive noise would be absolutely minimal.
- Windows XP Pro operating software
There are no fans other than the one on the heatsink, and the only other noise source in this PC is the Smart Drive enclosed WD drive. I'm getting a bit ahead of myself here, but its measured SPL (with the side cover on) is under 15 dBA@1m in the new anechoic chamber.
One of the really handy aspects of this Antec case is that it pulls apart easily without tools if the top piece is just friction-fitted into place. (See the details in our review.) The top is actually left off most of the time, as there's no noise from the fanless Silverstone PSU. The cables up top make this system very useful for hooking up hard drives temporarily for testing, formatting, wiping, etc.
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