The performance measurement methods of anechoic chambers in China ~ widespread lies ~

Improper measurement method -vol. 1

One of our Chinese customers requested that they wanted to modify their existing anechoic chamber, and I (author) accompanied them to visit the company K, one of the most famous anechoic-chamber manufacturers in China.

As I was planning to measure

• 1) the background noise level inside the anechoic chamber and,
• 2) the lower frequency limit

At the site, I asked the company K what kind of equipment they would use and how they would conduct the measurements before visiting the client. However, the company K wanted us to leave it to them, so I did not intervene with them.

Then the on-site measurements were started. It should have taken at least several hours to actually conduct these measurements, but no way, those were completed in a few seconds.

While looking at the screen of a Chinese-made handheld acoustic measuring instrument in the anechoic chamber, the person in charge of the measurement from Company K said, ” The background noise level in this anechoic chamber is 30 dB(A), and the lower frequency limit is 63 Hz.”

According to that person in charge, two reasons for this are as follows:

• (1) The value indicated by the sound level meter was below OA 30dB(A).
• (2) The value at 63 Hz was below 30 dB.

Oh my goodness…!

Regarding 1), I tried to check with the company K since it was obviously not an accurate value in terms of specifications of the sound level meter and microphone. However, they just replied to me “we have no idea.”

Regarding 2), I asked Company K how they can conduct such a fraudulent measurement, however, they replied “this is the measurement method recommended by the manufacturer of the sound level meter. Any of the anechoic-chamber manufacturers use the same measurement method.”

In other words, the manufacturer of sound level meters has been spreading false measurement methods in order to hard sell their sound level meters whose specifications were not even publicly announced, while buyers have been deceived. That is considered to be the correct method in the Chinese industry, and no one is questioning it there.

What is the most concerning in this issue is that both buyers and users are too ignorant. It is totally unbelievable. When I pointed out this issue, the person from the company K clammed up, moved on to another topic and ended up leaving unanswered.

Improper measurement method -vol. 2

After constructing an anechoic chamber, its performance should be verified and confirmed by measurements. In general, the following three measurements are conducted in most cases.

• Measurement of the background noise inside the anechoic chamber
• Measurement of the sound insulation level in the anechoic chamber
• Measurement of the inverse square law

In China, the measurement of the inverse square law is not conducted by anechoic-chamber manufacturers, but by a certain public institution, since they are not trusted in China.

After constructing an anechoic chamber, the anechoic-chamber manufacturer pays a fee to a certain public institution, which then sends several staff members to conduct actual measurements and submit a report.

However, as mentioned above, it is impossible for an anechoic-chamber manufacturer without having general knowledge to design a full-fledged anechoic chamber, so there is no way to obtain a proper measurement value of inverse square law.

However,

According to the past measurement report from a certain public institution that we have received from the company K, the extremely excellent performance values ​​of the anechoic chamber were found, at a level I had never seen before.

I realized that the values were physically impossible and clearly been adjusted by manipulation.

Therefore, I asked the company K about this matter as well.

• 1) What kind of measurements were being made?
• 2) What kind of measuring instrument was used?
• 3) How long did the measurements take?
• 4) How was the anechoic chamber designed in the first place?
• 5) Obviously, the anechoic chamber of the company K should not have performed to this extent issued by a public institution?

Regarding these questions, the company K replied to me as follows

• (1) unclear
• (2) unclear
• (3) around 1 hour
• (4) designed by designers. It’s nothing to do with me.
• (5) thanks to the bribery.

Not worth discussing at all.

The report does not specify the specifications of the measuring instrument nor the method of measurement. And it is bloody impossible for around one hour or so to conduct the measurement of inverse square law, which may take several days depending on its scale.

It may be quite an unbelievable story; however, this presents the current situation of the anechoic-chamber industry in China. Many of those fraudulent anechoic chambers have been already delivered to major local Chinese companies, maybe not all, but some at least though.

If you are planning to procure from a Chinese manufacturer of anechoic-chamber, please be aware of the above-mentioned unbelievable issues anyway.

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