Before W.G. Sabine around 1900 defined the reverberation time T₆₀ acoustic quality was a great mystery.
Nobody could understand why some rooms had a good acoustic and others, similar looking, had an inferior acoustic.
A few things were observed: Bare hard walls in rectangular rooms were nearly always bad, but not always.
Rooms with irregular surfaces, filled with sculptures, chubby- cheeked cherubs, wooden furniture's and some tapesty were often good, but not always.
So building a hall for both plays and concerts was an adventure, where some luck was needed.

After the reverberation time was introduced the worse mistakes were rare, but still some halls with equal reverberation time could have very different acoustic quality.
Another problem was that T₆₀ often was very different.
B&K became famous for in 1945 in Sweden to have developed a unique level recorder which was used for accurate T₆₀measurements.
B&K had in fact a world monopoly for level recorders for more than 40 years.
No competitors could match the principle, quality, and price. B&K made more than 25.000 level recorders during the years.

Now T₆₀ is measured with a PC for a fraction of the former cost.
With the level recorder it was possible to find out that T₆₀ depended on where in the room it was measured.
According to the simple Sabine theory T₆₀, should be the same anywhere in the room.

The reason for this apparent anormality, we now know, is that the reverberation field is built up of many different resonances, eigentones wandering around in the room with different damping.
Especially the one-dimension eigentones like flutterechos and two-dimensional eigentones have a less damping than the three- dimensional eigentones causing a reverberation curve not correct exponential.

Another more direct indicator for acoustic quality is Speech Transmission Index, where the percentage of correctly understood syllables is indicated.
Dr. Steeneken in Holland have developed in 1973 a more simple method called Rapid Speech Transmission Index -RASTI.
This method is now approved by IEC as Rec.268-16.

As RASTI is a new and excellent method for evaluating acoustics in rooms B&K became interested and with assistance from dr. Steeneken constructed RASTI-meter 3361.
This instrument was the first and is still the only commercial accessible RASTI in the world.
Unfortunately too few instruments were sold with the result that B&K did not follow up on the construction.
To-day the technique in the B&K design is more than 25 years old and accordingly very expensive to produce.

Brüel Acoustics could see the need for a good inexpensive STI-instrument and devlopped some instruments to support the sale of AcoustiCone.

Brüel Acoustics devlopped 44BA wich have some unique and new features, and is ideal for evaluation of room acoustic.

FOR GOOD ACOUSTIC: USE STI and ACOUSTICONE.

Misure acustiche al teatro dell'Opera di Roma (69 Kb)
Rasti 44BA (38 Kb)

Knowing the 25 years old B&K system 3361, Brüel Acoustics decided to develop a modern STI system and include some new and original thinking, which are as follows:

1) The instrument is formed as a human head and torso, which produce a correct directivity pattern.
(B&K with 2 KHz 10 dB).
2) The head is a sphere and a monopole can be made using very little material. (B&K: Rectangular heavy box).
3) Both power and battery operated. (B&K only battery).
4) Three levels: Standard, +10, and +20 dB. (B&K only +10 dB).
5) Both manual, computer and radio controlled. (B&K manual).
6) Receiver is an interface to PC which can be used to make highlevel programmes. (B&K special unit excellent, but heavy and expensive).
7) Can use B&K microphone, and other brands.
8) The whole instrument packed in a lightweight soft lined transport.
9) The instrument is patent protected. (B&K has no protection).

The general conclusion is that 44BA could be sold in large quantities by Brüel Acoustics BA.
There is a heavy pressure from customers to have a good and economical STI-system.

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SI AUTORIZZA LA COPIA E LA DIFFUSIONE CON CORTESE RICHIESTA DI INDICARE LA FONTE ( www.bruel-ac.com ) e l'E-Mail: bruel@mclink.it
Prof. Mario Mattia.

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