消声室声学技术的革新

Innovative technology for anechoic chambers

半个多世纪以来,在室内建立自由声场,以尖劈为基本吸声构造,尖劈的长度基本定于所需的低频截止频率,建造尖劈占用大量体积的事实,似乎都是无需讨论或必须接受的。随着科学技术的发展,尖劈的长度要求(1/4波长),已经成为自由场室扩展低频截止频率的实际障碍。工业技术发展需要的自由场室,不仅有它特殊需要的低频下限,例如到50Hz,决定于它的环境条件,对于吸声构造又有相应的使用和维护方面的要求。本文从室内稳态声场的理论分析出发,提出扩展低频截止频率,以阻尼房间的简正振动为基本,界面的反射,应从在室内更接近球面声波的实际出发,建立自由声场的新思想。介绍了新开发的三种新型无纤维吸声构件和适应不同要求的两类自由场室。以一间已经建设成功的半自由场室为例,用复合板共振吸声器(CPA)和非对称构造吸声器(ASA)组合,吸声构件的厚度仅620cm,得到的自由场结果是:以1/3倍频程频带信号测试(这正是该室的使用测试信号),25Hz至80Hz,±2.5dB,100～16000Hz,±1.0dB,自由场范围9m至12m。以正弦信号测试,100Hz以上达到和频带噪声类似结果,100Hz以下至50Hz,自由场范围7m至9m。已建成50间以上类似的自由场室。

For more than 50 years acoustic engineers and scientists seem to have accepted that anechoic (freefield) chambers are equipped with porous or fibrous wedges or pyramids the depth of which being determined by the lower limiting measuring frequency. This conventional acoustic lining technology requires a large portion of occasionally more than 30% of the room's raw volume with no other practical benefit whatsoever. Apart from this, delicate fibrous structures have to be carefully protected and sealed against abrasion and damages. With an increasing need for technical sound sources to be tested at very low frequencies (even below 50 Hz) and the manufacturers' ambitions to maintain attractive and durable test-bed environments for their products and personnel called for a fundamental innovation in the acoustic performance and design of anechoic linings. This prompted a group of researchers at IBP to develop a new concept for the general layout of freefield or semi-freefield rooms. It enables a marked reduction in lining depth by employing Compound Panel Absorbers CPA for primarily damping the low-frequency room modes. These are individually tuned and spatially adapted to the geometry of the respective test environment. In combination with an additional Asymmetrically Structured Absorber ASA cladding with more than 99% absorption efficiency for normal as well as oblique sound incidence this guarantees precision measurements according to ISO 37 45 down to 50 Hz for narrow bands and down to 25 Hz for third-octave bands. More than 50 anechoic rooms have so far been equipped with the new absorber modules.