Issues on anechoic chambers manufactured by other manufacturers -2

An anechoic chamber was delivered by one certain another company to the university B in Indonesia, and Sonora Technology conducted the measurement of inverse square law.
As a result, its performance was not within the allowable deviation required by the university B, and we confirmed that the disturbances especially in the high frequency range were found.

Sound-absorbing wedges in this anechoic chamber were made of perforated metal sheets, which was shaped as the wedge and may likely be filled with the sound absorber made in China.

The perforated metal sheet is literally a steel sheet with punched holes. It is generally believed that the sound can be absorbed based on the theory of Helmholtz resonators and the sound-absorption theory of the perforated plate structure, by placing a sound absorber behind the perforated metal sheet.

We do not deny this effect.

Even a sound absorbing wedge made of perforated metal sheet can show high sound-absorption properties when measuring the normal incidence sound absorption coefficient.

However, that’s only within a specific frequency range.

When measuring the sound absorption coefficient of a single sound-absorbing wedge, it is measured by the normal incidence sound absorption coefficient measurement method. This is not the reverberation chamber method, often to be misunderstood though. Measuring sound-absorbing wedges with the reverberation chamber method can result in excessively good sound absorption coefficient data.
* Some companies misuse it on purpose.

The frequency range measured by the normal incidence sound absorption coefficient method is generally centered around 500Hz or lower. This is because the sound absorption coefficient tends to exceed one (1) or higher at higher frequencies than 500Hz.

However, in reality, the high-frequency band measurement is also conducted in an anechoic chamber.

No matter how good the sound absorption coefficient of a sound-absorbing wedge made of perforated metal sheet on its own, it may not be practical to use it.

As mentioned above, the sound-absorption theory of perforated metal sheets is effective in terms of sound absorption design, while the metal itself does not absorb sound.

Some of you may say, “no, it absorbs sound up to around 4000Hz, right?”

• Q: Well, what measurement method was used to measure that 4000Hz?
• Q: How much sound absorption can be made in the higher frequency band than that?

Could you answer these questions?

In recent years, there has been a high demand for measurement in a lower frequency band than ultrasonic.
For example, it may be at 16000Hz. Will the sound in this frequency band be able to be made by perforated-metal wedges? Well, it would be difficult.

As mentioned at the beginning of this article, the perforated metal sheet is originally designed to absorb especially the low to mid-range frequencies, while tends to be weak in sound absorption in a high-frequency band. This is because the sound hitting the metal part can be reflected.

Above all, there is no advantage to using extremely expensive and heavy perforated metal sheets for a sound-absorbing wedge. It may work as a protective material, but it can be substituted with other materials. It is a well-known fact that the steel sheet is expensive

Then why are the perforated metal-made sound-absorbing wedges mounted in the anechoic chamber at the university B?

I guess the followings may be considered:

• 1. The manufacturer has no technology to create a shape of sound-absorbing wedge only using the sound absorber.
• 2. There is no sound absorber being able to maintain a certain degree of strength.

Sonora’s sound-absorbing wedges are made solely from sound absorbers. However, these materials should have both the “high strength feature” and the “excellent sound absorption property.”

As already described in the following articles, there are many materials that consist of anechoic chambers causing problems outside of Japan.

• → link 1: Issues on anechoic chambers manufactured by other manufacturers -1
• → link 2: Glass wool in Asia (China, Taiwan and Korea)

If you have difficulty in procuring materials suitable for use in sound-absorbing wedges, it would not be possible for you to manufacture them solely using sound absorbers like we do, unfortunately.

Regarding the sound-absorbing wedges at the university B, Chinese-made sound absorbers are filled inside perforated metal sheets. Based on my experience, it is not possible to manufacture sound-absorbing wedges using made-in-China sound absorbers. Its shape can only be created though, it would also require increasing the amount of binder and/or making them extremely dense. This would compromise their sound absorption properties. That’s why perforated metal sheets are probably used.

Therefore, Sonora Technology does not recommend to use sound-absorbing wedges made of perforated metal sheets, which have many disadvantages such as issues of sound absorption performance, being extremely expensive, too heavy…since these outweigh any potential advantages.

There are excellent anechoic-chamber manufacturers in the world, while not-so-good ones as well. So, we’d like to compete with excellent nominees.

Excellent proven anechoic-chamber manufacturers

• Company F in Krumbach, GERMANY
• Company I in Hampshire, UK
• Company E in Massachusetts, USA

AND

• Sonora Technology Co., Ltd. in Tokyo, JAPAN