Brüel Acoustics

Anecho Rooms

Anecho Test Facilities: 62BA and 62BB

Brel Acoustics have developed two modular Anecho Test Rooms for industrial research, acoustical measurements and calibration.
The test chamber is a light construction named type 62BA (modul 40 cm) and the system for larger rooms type 62BB (modul 60 cm).
The modular system makes use of a 1imited number of standard elements which can be connected together and form more than 100 rooms with different lenght, weight and height.
On a given space all dimensions can be made by choosing the nearest number of moduls.
The price for the rooms are proportional to the element units.

Cremer Contra Wedges:

Brel Acoustics are using the Cremer principle which has proved very useful and economic.
The absorbing walls are built up with cubes which near the inside of the room are small and made of special glass fibres with a very low density.
The cubes increase in both size and density as you go into the wall. In this way you obtain an extremely good impedance matching with the heavy absorbing material in the internal part of the room.
This principle is analogous to the exponential horn in front of a loudspeaker driver unit.

Wedges are normally build of the same material both at the bottom and the top. In the loudspeaker analogy this correspond to a linear cone in front of the driver.
Consequently the Cremer room is much better at high frequencies than a Wedge room of the same size.
At mid frequencies the two types are equel, and at low frequencies the Wedge type is normally slightly better.
From an economical point of view it can be said that a Wedge room contains more absorbing material than a Cremer room, but a Cremer room requires more sophisticated work which means higher labour cost.
At B&K we used only Cremer type rooms because it is very important that the room is good at higher frequencies up to 15 kHz.
The lower frequencies below 300 Hz are not so important as the wavelength is large so that other methods can be used.
With frequencies over 3 kHz a Wedge room will always give some uncontrollable fase shifts because the Wedges have large plane surfaces whereas the Cremer rooms have an acoustical jungle.

Both Cremer types are built of tubes where no plane surfaces exist.
Consequently all reflections are minimized in all directions.
Most of the supporting structure is imbedded in the absorbing material.

Type 62BA have a modul length of 40 cm and also a wall thickness of 40 cm. Supporting tubes have a diameter of 10 mm.
This room works very accurately from 350 Hz to 25 kHz depending of the size.
Type 62BB has a modul length and a wall thickness of 60 cm and is intented for bigger rooms and working frequencies from 200 Hz to 20 kHz.
Both rooms should be erected on a plane smooth surface e.g. reinforced concrete or very hard wood.

A concrete slab are ideal and can conveniently be supported by steel springs for vibration isolation.

Both types can work with and without an absorbing floor and therefore satisfy all standards in force today.
The absorbing floor consists of easy removable units.
It takes only a few minutes to remove the floor.
It is therefore important that the base reflects the sound and is plane.

The door can consist of one or two units in width and two, three, or four units in height.
The door itself consists of units which like the floor fits in the normal wall units.
The door is rolling out perpendicular to the wall.
When it is out, it is rolled parallel sidewards.
This movement is done on rails with roller bearings.

How much space is needed?:
The rooms can be erected from the "inside" and therefore the space required is only 5 to 10 cm space between testroom and fixed building.
But remember the test rooms can only be delivered in units of 60 or 40 cm.
The space from the wall where the door is installed has to be the wall tickness (60 or 40 cm) + 10 cm.
This small space is minimum.
The erection is faster and easier if more space is available.

Air should not be blown into the test room.
Heavy draught will break some of the glass fibres and produce dust. But the air around the room can be airconditioned without problems.

Lisht and Electric Cables are easily installed in a Cremer room as there is a lot of space in the absorbing walls.
It is very easy to change the cables.

Technical Review 96-02

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- updated on 2003, April

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