Review of TES 1356 Microphone Calibrator

by Neil A. Shaw

In a measurement system, a lot of attention is often, and rightly so, given to the microphone itself.  However, when you use a microphone, be it a "measurement," "vocal," instrument," or whatever, you are not just using the microphone.  You are using a system of which the microphone is only a part.  The system can consist of the microphone, the microphone preamplifier, and some recording or indicating device.  The recording device can be a whole studio and the indicating device can be a integrated sound level meter, computer based system (input card and program), or, just a set of headphones.

When using a system for measurements, which can vary from community noise assessment to loudspeaker development, the system needs to be "calibrated."  This calibration is done for many reasons - among these are traceability of the measured data to standard reference to meet the requirements of a standard test procedure to assuring yourself that the measurement system doesn't drift from day to day.  For example, in many community noise measurements, 1 dB is all that separates compliance and non-compliance.  You want to be sure that you are accurate here, since lawyers are inevitably involved.  For a research and development program, since measurements are made from day to day, and in many cases in loudspeaker development, incremental improvements of less than 1 dB are often measured.  In many systems, the system gain is not fixed and there are various ways in which the gain of a component can be changed (either advertently or inadvertently).  Therefore, making sure that your system is set-up and "reads" the correct number when presented with a known signal can be important.  This is where a microphone calibrator is useful.

For microphones themselves, there are two principal ways which are used today - these are the reciprocity method and the comparison method.¹  For laboratory microphones, the calibration should be by a primary technique, that is, it depends only on measurements of length, mass, time and ratios of other primary quantities.  The reciprocity method is a primary method.  For many microphones, the time and energy needed to perform a reciprocity calibration.  For many microphones, the microphone itself prevents calibration using the reciprocity method.

The comparison method is simple and can be used with any microphone.  You measure the response of a known calibrated microphone to that of an uncalibrated microphone.  From this the response of the heretofore unknown microphone can be determined.

In most cases, however, a simple check of the system in which the microphone is used is all that is needed.  So, microphone calibration in most cases is really a system calibration.  For a sound level meter, it's easy - you just attach the proper microphone diameter adapter to the calibrator, insert the microphone of the sound level meter into the cavity, turn on both the meter and the calibrator and confirm the meter should read 94 dB, 114 dB or 124 dB ‑ which depends on the type of calibrator you have and the level to which is set.

The most common microphone calibrator in use is the type that consists of a small loudspeaker element at one end of a cavity in a casing that contains the electronics (basically a 1000 Hz tone generator).  The better calibrators also have a small microphone built into the cavity - this is used in a servo manner to insure the proper sound level is produced.  The working end of the microphone is inserted into the cavity.  Two sound levels are provided; 94 and 114 dB - Some calibrators offer one or the other, some both.  The standards that these calibrators are built to are ANSI S1.40‑1984 and IEC 942 1988 Class 1 or 2.

The most precise calibrator short of the reciprocity procedure is the pistonphone, which uses a motor and rotary drive to move a piston.  This causes a physical change in volume in the cavity.  However, for accurate use, the barometric pressure must be precisely known and the seal around the microphone must be air-tight.  Most pistonphones operate at 250 Hz.  They are expensive.

These days there are quite a few inexpensive microphones that are being sold as "calibrated" or "measurement" microphones.   The microphone element in these low cost products is typically a tiny electret condenser omni directional capsule packaged in either a cigar-sized tube or the more traditional tapered-snout profile.  Many hobbyists, auto-sound installers and loudspeaker labs with limited budgets are using inexpensive acoustic test software running on personal computers.  Often these programs use "sound cards" whose A/D converters have their limitations (especially in their dynamic range and distortion characteristics).  In this context, it is hard to justify the cost of a top grade condenser measurement microphone.

Although the budget measurement microphones are not quite laboratory grade, most are adequate for use in speaker production line quality control, crossover network design tweaking, woofer / enclosure measurements, sound system equalization and the like.  Since the microphone elements are so small (less than 1/4" diameter) their response tends to be well behaved to 16 kHz and sometimes beyond.

Since precision laboratory measurement microphones in the $1000 - $4000 class are typically calibrated every year, one would think that the more modest examples would be far more prone to drift since these inexpensive microphones "don't get no respect".  Cheap microphones tend to be handled more roughly, enduring what could be considered more cavalier treatment than their higher class relatives.  Which is all the more reason to have a calibrator on hand for these low-priced microphones used for measurement.

While most type I microphone calibrators sell in the $500+ range, the focus of this discussion is the TES 1356, which costs $360.  Last year, at the Taiwan Electronics Show, my partner and associate, Michael A. Klasco, stumbled onto the TES booth and was impressed with what he saw.  As a result, SysID Labs now distributes the TES line of acoustic instruments and products in the United States.  TES was established in 1975 and was ISO 9002 certified in 1995.  TES manufacturers about 100 different types of field test instruments including sound level meters, noise dosimeters and loggers as well as test devices for other industries such as multi-meters, light meters, automotive engineer test sets, humidity meters, and the like.

Analyzers such as Smaart, LMS, MLSSA, SYSid, Clio and their kin, have their own proprietary calibration procedure which is usually enshrined in their respective instruction manuals.  Remember that the pre-amplifier is in the loop, so keep any gain control settings fixed (or repeatable).  Some of these analyzers measure the transfer function and all are "two port" systems, that is, the analyzer expects to generate the test signal and presumes that it will get back the device under test's response of the signal it has sent out.  The analyzer (and the operator) will be in for a surprise when the calibrator's test tone comes back as the signal generator (the stimulus signal) from the analyzer is being bypassed.  Each analyzer has its own way of getting around this and most, but not all, of the instruction manuals provide step-by-step calibration procedures with a calibrator.

Construction

TES has made good use of their resources as a large manufacturer of field instruments and applied these capabilities to what is a niche market.  The shell casing of the TES 1356 is a well finished glass epoxy impregnated and uranium enriched nucleated light irradiated plastic with a extruded rubber "bumper" gasket around its "waist."  The microphone cavity, for 1" diameter microphones, has a rubber gasket to insure a good fit to seal around the microphone's snout.  There are adapters, which have similar gaskets, for use with 1/2" and 1/4" diameter microphones also have this rubber sealing gasket.  For high stability a feedback circuit which has a sense microphone mounted in the cavity, monitors the calibration transducer.