Sound Level Meter Celebrates 50th Anniversary
2010 is the year we celebrate the 50th anniversary of Type 2203, the mother of all sound level meters. It was the world’s first precision hand-held and transitorised sound level meter. Produced for 21 years, it sold a record number of 17300 units. With its upgrades Type 2204 (Impulse time constant) and 2209 (peak detector), it accounted for a total of 32700 units. This the story of the world’s first hand-held sound level meter as recorded in the archives and as told by those who were there.
News from the Factory
Half a century ago, product introductions were modest announcements in the 'News from the Factory' section at the end of Brüel & Kjær’s Technical Review. Issue 3 in 1960 included the above bulletin.
Mr Sound Level Meter
Our expert sound level meter designer for decades, Peter Hedegaard, gives this account of how the hand-held meter evolved:
“Type 2203 was the first transportable Brüel & Kjær sound level meter. Who fathered this special design is not for me to say, but both P.V. Brüel and Gunnar Rasmussen are likely to have had a finger in the pie, while Kay Møller Petersen came up with the actual design. The special thing about the design was the cone-shaped front, which made it possible to place the microphone on the cabinet without significantly affecting the acoustics of the microphone. In other words, you could measure, hold and operate the meter without needing an extra hand.'
In a reprint of “Mobilia” No. 183, October 1970, the following appeared (translated from Danish): “Brüel & Kjær was about to develop a small handy sound level meter to be used in factories, in traffic and other locations where you have to shoot against moving targets. They faced the problem that the cabinet of the instrument reflected the noise towards the microphone, disturbing the accuracy of the measurement. Therefore, the cabinet could not be the usual box-shape but had to be conical. However, this reference to physical laws and technical necessities is only half the explanation.
Getting rid of the Assistant
The microphone used to be at one end of a cable, the other end plugged into a nice square box held by something or someone. Then the user would hold the instrument in one hand, operate it with the other hand and hold the microphone with a third. However, it is a universal experience that nothing is worse than an assistant with one hand unoccupied. Therefore, Brüel & Kjær chose to eliminate the most serious source of error – the assistant – by mounting the microphone directly on the instrument and give the instrument an exterior shape that would prevent the reflection of noise. The resulting creature appears to be very successful – we say appears, because not being an acoustician how would you know how she is to get along with.
Viewed today, where everyday life is filled with smart miniature technology (mobile phones, mp3 players, calculators the size of credit cards, etc.) Type 2203 doesn’t seem particularly small or light, but considering the technology available and the limited number produced, it was somewhat of a trick, and the concept with the cone shaped cabinet was copied by all our competitors. Let’s look at some of the technical opportunities and limitations.
Inside Type 2203
In 1962, Type 2203 and Type 2211 (Noise Limit Indicator) were the only devices using transistors. All the other instruments had valves. The available transistors were germanium types that generally did not have the required high gain, low noise and low leakage current. Thus it was necessary to sort the transistors into classes and use the best ones in the most critical places (the same was done with resistors, which were only available with a 20% tolerance and were then sorted into 2% classes).
The preamplifier for the microphone was also something special. Here transistors were totally out of the question and a valve had to be used. A local oscillator supplied the directly heated cathode in the small valve through a transformer, and the valve was then usable as cathode follower. The valve was suspended in a spring device to avoid excessive microphonic effects.
Making acoustic measurements on the microphone and the cabinet was not easy. Brüel & Kjær had no anechoic chamber and neither did the DTH (later known as DTU, the Technical University of Denmark). The measurements were made using a half-size model of the instrument, placed in a sound absorbing asymmetrical case about two metres long.
This setup was not particularly accurate and later measurements did indeed show that the acoustic performance were not as expected. The microphone was mounted right at the top of the cabinet cone and the cabinet influence was around 2 dB against the expected 1 dB. This was later corrected by inserting a “gooseneck” between the microphone and the cabinet. The 1965 international standard for Precision Sound Level Meters was based on Type 2203, which (with improvements along the way) stayed in production for 21 years.
In with Royalty
From its humble introduction in 1960, Type 2203 moved up to mix with high society. In this picture from 1970, the flamboyant Dr. Brüel demonstrates Type 2203 to King Frederik IX of Denmark (2nd from right) and Queen Ingrid of Denmark (4th from right). Next to the Type 2203 you see the innards of a measurement amplifier and in the background there are racks of generators, analyzers and a CRT screen.
With a Little Help from your Friends
Just like today, Brüel & Kjær’s mantra in the sixties was “we provide complete systems”. This is illustrated below, in a setup donated by a Danish consultancy. The A-weighted AC output signal from Type 2203 (centre) is taken to a Noise Dose Meter Type 4423 (top left), giving you a dose count you can convert to LAeq using the special slide rule provided. For intermediate results, the camera (right), triggered by a custom-built timer (bottom right) takes hourly photos of the dose meter counters for later reading, conversion and documentation.
Likewise, Type 2305 Level Recorder continuously traces the A-weighted sound pressure level on a strip chart, easy for you to annotate for documentation. For statistics, the level recorder pen position is detected by an adaptor, then via a cable activating the corresponding counter in the Statistical Distribution Analyzer 4420 (top right). A total counter is also provided, allowing the full statistics to be calculated. Thus the system will provide high-resolution Logging, Statistics, Overall Levels and even frequency analysis using the Octave Filter Set Type 1613, below Type 2203.
In the Hen-house
Working in the field with Type 2203 and its friends took stamina, determination and dexterity. Sitting for days in a hen-house, you had to check the clicking counters of the statistical analyzer (right), watch over the pen trace of the level recorder (centre), mark any acoustic events on its paper as it rolled by, and change the paper roll every other hour (operator, left). And with a fur coat, warm shoes and a lunchbox, life wasn’t all that bad. And on the bright side, Type 2203 was entirely free of software bugs.
On the Road
In this roadside scenery, Type 2203 is at the centre of attention. Tripod-mounted at the standard microphone height of 1.2 m and equipped with a windscreen for its 1-inch capsule, it will show the sound level of any passing vehicle. The operator, calm and confident, will check the level against legal limits and dispatch his motorbike crew in pursuit of the offender. Still a few questions remain:
When and where was this photo taken, and why the sidecar? Any clue or thoughts you may have will be thankfully received. (The motorbikes are BMW R60/5 or R75/5 produced between 1969 – 1973, if that’s any help).