Expectation vs. Reality in design methods for acoustic anechoic chambers based on ISO3745:2012

The design methods for anechoic chambers are specified in the International Standard, ISO 3745:2003 -Annex K3 (as recommended). However, Annex K has been deleted in the ISO 3745:2012 edition.

How will it go in terms of the design methods and specifications for anechoic chambers based on ISO 3745:2012? Why has it been revised as such? What are the problems for that?

Please read through this article to the end.

Comparison of the ISO 3745:2003 and ISO 3745:2012

Firstly, let’s look up the difference between pre-revision and post-revision.

According to the ISO 3745:2012, as long as the criteria of inverse square law is satisfied, it is recognized as an acoustic anechoic chamber even if the sound absorption coefficient of sound absorbers isn’t enough somewhat.

The definition of the acoustic anechoic chamber and semi-anechoic chamber

According to the Japanese Industrial Standard “JIS Z 8732:2000 (ISO/DIS 3745:2000) ~ sound power measurement methods – precision measurement methods in anechoic or semi-anechoic rooms”, the anechoic chamber and semi-anechoic chamber are defined as follows (excerpt):

• Anechoic room → a test room in which a free sound field is obtained.
• Hemi-anechoic room → a test room in which a free sound field over a reflecting plane is obtained.
• Free sound field → a sound field in a homogeneous, isotropic medium free of boundaries. In practice, a free sound field is a field in which the influence of reflections at the boundaries is negligible over the frequency range of interest.
• Hemi-free sound field → a sound field in a homogeneous, isotropic medium in the half-space above an infinite, rigid plane surface.

*a free sound field is a space in which no sound reflections occur.

It is also stated that a space that satisfies the following inverse square law.
(a free sound field ≒ to meet the criteria of inverse square law)

It is also stated that a space that satisfies the following inverse square law.
(a free sound field ≒ to meet the criteria of inverse square law)

According to the JIS Z 8732:2000, as long as the criteria of inverse square law is satisfied, it is recognized as an acoustic anechoic chamber even if any kind of sound absorbers are used.

How the anechoic chamber should be

As mentioned in the previous section 1 and 2, it can be said that anechoic chambers only need to satisfy the criteria of inverse square law in accordance with the ISO 3745:2012 and JIS Z 8732.

Anechoic chambers have generally been designed with long-type sound-absorbing wedges to accord with the target frequency, however, each of anechoic-chamber manufacturers will design their own anechoic chambers in line with the revision of relevant standards from now on. (e.g. with or without sound-absorbing wedges as such.)

Also, regarding the expression of the target frequency has so far been mainly used the term “cut-off frequency” for sound-absorbing wedges, however, it should be preferable mainly to use the term “lower frequency limit” targeted for anechoic chambers.

Issues due to the revision of standards

The revision of the standard have greatly increased the degree of freedom in the design methods of anechoic chambers, so to speak. The fact that anechoic chambers need only satisfy the criteria of inverse square law says that the measurement methods themselves are also free. (some methods are recommended though)

However, freedom can be also very scary. Because it may be possible for some manufacturers to take advantage of this and implement misleading expression (as-it-were, abuse its expression) when setting the specifications.

For example, the guaranteed value range can be set from the lower frequency limit to a specific frequency, while frequencies above that are not supposed to be covered by the warranties. Or high-frequency noise can be ignored since it is designed less than λ/4 of the sound absorption layer ~ manufacturers may be able to make their own calibration to the measured values of inverse square law (although the criteria of inverse square law is not actually satisfied.) Manufacturers own measurement method can be used to meet the criteria of inverse square law and issue proves of their own inverse square law…as such.

We’d like to introduce one tricky matter here. A high-performance anechoic chamber requires a lower frequency limit, as-it-were the high sound absorption in a low frequency range. However, to be honest, it is not so difficult “to increase ‘only’ the sound absorption coefficient of a specific frequency absorption” of sound absorber.

We cannot specify it in detail here, but it’s about to add certain materials to the sound absorber, and to work on the density and shape of the sound absorber as such.

*for you reference: an another article “the thickness of sound absorber = the performance of sound absorber!?”

In this method, if only the sound absorption coefficient of a specific frequency is increased, the sound absorption coefficient of the other frequency bands is decreased.
For example, a sound absorber, which only increases the sound absorption coefficient around 60 Hz, was made. Then this sound absorber has a poor sound absorption coefficient in the high frequency range.

However, when manufacturers say “thanks to our sound absorber to be used in the anechoic chamber, the lower frequency limit will be 60 Hz or above. The high frequencies with poor sound absorption can be ignored”, that specification will be accepted as an anechoic chamber, as long as there’s no doubt on why the high frequency range can be ignored.
This is so-called “tricky”

In the conventional design methods, these kinds of cover-up couldn’t be implemented due to the provisions regarding sound absorbers, however, unfortunately it may occur in the reality. More detailed explanations are as followings.

Why has the ISO standard been revised?

First of all, why the ISO standard has been reviewed and revised, even though the possibility of the above-mentioned cover-up may be occurred? We believe there are mainly two reasons for this as followings.

Due to the development of related technologies regarding the construction of anechoic chambers, only the conventional design methods are not sufficient to correspond with

Sound absorbers, such as sound absorbing wedges, used in anechoic chambers are getting thinner and thinner by years, not only domestically but also internationally.

In addition to the sound absorption properties of sound absorbers, many application technologies of those absorbers have been also developing. So it can be outdated idea that it should be enough simply to mount sound absorbing wedges of a length that fit with the target frequency.

Due to the convenience for anechoic-chamber manufacturers

The relationship between consensus member of the ISO standards and certain anechoic-chamber manufacturers may surely have an impact on that.
(Regarding its details, please get in touch with us, since we cannot speak it out loudly here.)

A true story of problem after revision

Regarding the above-mentioned negative aspects after revision, unfortunately we have encountered one of these problems.

According to the requirement by one of our users, Sonora has measured the value of inverse square law in the anechoic chamber constructed by a certain overseas anechoic chamber manufacturer.

This is because the user wondered whether the chamber was really performing as an anechoic chamber, since a general anechoic chamber has a space accompanied by a floating sensation that causes ears ringing. However, they didn’t feel any such sensation at all inside that so that they requested us to measure it.

As a result of our measurement, we found that it didn’t meet the criteria of inverse square law. In particular, the sound in the high-frequency band was not in scope of the allowable deviation at all and its measurement data was very unstable.

The product data measured in such an anechoic chamber has no meaning for users. As one of the competitors, we feel really regret this.

Expectations for the future anechoic-chamber industry

Why such a disappointing problem has occurred?

That’s because the user accepted a proposal by the anechoic-chamber manufacturer without any doubt.

On the other hand, the user’s knowledge and skill of judgement were not enough to procure an anechoic chamber.

We have no intention to criticize them, however, anechoic chambers are expensive equipment and it may occur only once in lifetime for persons in charge of procurement in user companies. Therefore, it is unlikely that many are familiar with technology of anechoic chambers.

Furthermore, the inverse square law to meet the criteria of free sound field in anechoic chambers is being measured by most of manufacturers. According to the ISO3745:2012, they can issue users with data to meet a criteria of free sound field by making their own measurement methods or calibrations. However, it won’t be proved in measurement by third-party experts.

There are many anechoic-chamber manufacturers all over the world. Although such problems would be constantly happening, we expect that users eventually notice it, and voluntarily ask measurement experts to provide the accurate measurement data of inverse square law after completion of anechoic chambers.

And later on, we believe that this chaotic situation caused by the revision of the ISO 3745 will be calmed down as getting to start up measurement-specialized companies trusted by users. And current regulations will be hopefully reviewed and appropriate ones will be newly created.

Summary

In this article, we have discussed on the revision of the ISO 3745.

As time progresses, we believe that the regulations on anechoic chambers will be also evolved to keep up with the times, however, the inverse square law, which is the basis for defining anechoic chambers, currently seems to be an absolute truth not to be affected by the flow of time.

Truth is truly a truth. And it cannot be distorted due to situations created by human beings.
We’re taking it seriously, and manufacturing anechoic chambers based on our consistent core value.