System Echo-Free 

An ACAD-developed system for echo-free measuring rooms

System Echo-Free is a Swedish system for echo-free measuring rooms developed by ACAD-International AB in collaboration with Acophon AB and Nordisol Akustik AB.

All development and documentation of products from the viewpoint of sound require proper, repeatable measuring conditions. In order to measure sound propagation, to begin with it is necessary that measurement is not disturbed by uncontrolled reflections. Such conditions can sometimes be attained outdoors. Weather and other uncontrolled sources of disturbance comprise, however, a factor of uncertainty.

With System Echo-Free, the free sound propagation of outdoors can be reproduced indoors under completely controllable conditions year-round.

The absorbent system can be dimensioned according to ISO-3745 for measuring rooms with spherical or semi-spherical sound propagation.

Quality Requirements

Sound data will be increasingly in demand, and requirements on maximum values will be placed for ever more product types. This sound data must of course be measured under standardised conditions, suitably in accordance with the international ISO-3745 standard.

Measuring according to this standard lends great strength to all products that are exported, as the procedure has gained international currency and acceptance.

An acoustic measuring room equipped with System Echo-Free can be dimensioned according to ISO-3745.
Thus, a simple standardised measuring procedure is obtained, which gives measuring data that can be used both for internal product development and for production control and official product data. The measuring room is of course eminently suited to the mapping of sound radiation in different directions from a sound source.

Development

ACAD-International AB and Nordisol AB work on a continual basis to improve System Echo-Free. We conduct extensive test measurements of element constructions and materials. For this purpose, we have among other things built a seven metre long Kundt’s pipe, with which we can measure absorptivity to the order of 1/100th.

Acoustical Characteristics

The system is dimensioned based on requirements on measuring methodology, measuring precision and the design of the measuring room. The very high sound absorption, a=0.99 within a broad frequency range, is achieved using a customised absorbent arrangement.

A layer of absorbents, possibly in combination with an air gap, is fitted next to the hard reflection surface. Outside of this layer, oblique absorbent elements that form a wedge-shaped pattern are fitted. The absorbent elements are attached to a concealed metal construction to avoid reflections.

How does the system work?

The arrangement with the absorbents works as three different types of absorbents:

• Porous absorbent
• Cavity absorbent
• Membrane absorbent

Incident sound waves are absorbed in the absorbent elements or underlying absorbent layers. If a lesser part of the sound wave is reflected when it first reaches the absorbent elements, it is reflected deeper in to the arrangement of absorbents until it is completely absorbed. The flow resistance in the absorbent elements’ exposed edges is so low that the sound wave is not reflected. An appreciable degree of absorption is achieved by allowing the absorbent elements to oscillate (membrane effect).

In the very lowest frequency range, the system is also dimensioned with consideration taken of the vibrations of the material fibres. This way the effect is better than can be expected with traditional calculation. During dimensioning, the dimensions of the absorbents, air gaps and absorbent angles are determined. The absorbents’ inner characteristics can also be chosen to suit the given pre-conditions.

The total thickness of the arrangement of absorbents is calculated for the lower limit frequency (fO Hz), i.e. the lower frequency for which the established reflection requirement applies. For e.g. fO = 200 Hz, the total thickness would be approx. 0.5 metres.

Specifications

The system is as a rule dimensioned according to ISO-3745 for an absorption factor of a=0.99 between the lower and upper limit frequencies. This corresponds to a reflection factor=0.1. In this situation, according to the law of distance in free propagation, the sound reduces by 6 dB per doubling of distance in the case of a point-shaped sound source.

Quality Control

We ourselves perform extensive quality control of the rooms prior to delivery in order to assure ourselves that they keep what we promise.

Such control is done as a measuring in in accordance with standard and results in us assuring the room’s function in connection with completion. If the customer subsequently wishes to measure in the room himself for his own purposes, this is naturally perfectly acceptable.

Contact ACAD-International AB for references and more information regarding System Echo-Free.