A new high-frequency impedance tube for measuring sound absorption coefficient and sound transmission loss

A high-frequency Impedance Tube (HF Tube) that can measure acoustic performance (sound absorption coefficient and sound transmission loss) up to 12.8 kHz was developed. This frequency range has become important due to trends in the automotive and cell phone industry. The trend toward more electric and hybrid vehicles has brought about new noise sources and challenges. One in particular is electrical invertor noise in the 10 kHz range, which is one of the dominant noise sources in these vehicles. In cell phone and smart phone applications, the trend is to improve telephone speech quality. High frequency audible noise has a major influence on perceived quality. Previously, conventional impedance tubes (for example the Bruel and Kjaer Type 4206) could not evaluate acoustic performance in the frequency range of 8~10 kHz. The usable frequency range of the standard tube was limited to 6.4 kHz based on the microphone spacing and tube diameter. Therefore, based on ASTM E1050/ISO 10534-2 standard (for sound absorption coefficient) and ASTM E2611 standard (for sound transmission loss), a newly developed HF Tube allows for measurements of acoustic performance up to 12.8 kHz. The tube was designed with a 15 mm inner diameter and 11.9 mm microphone spacing. It was designed to use Bruel & Kjaer's 1/4 inch microphones and preamplifiers, Type 4187/2670, from the conventional impedance tubes. In this paper, the HF Tube was verified by measurements an air cavity (empty tube) and some acoustic absorber materials with the results compared with the conventional impedance tubes.