# Development of a Panel Membrane Resonant Absorber
## 1 Introduction  
At the beginning of the 20 th century, [PERSON] proposed the famous reverberation time (RT) theory, which brought room acoustics into the scientific realm. However, many acousticians have proposed different methods to inspect the pros and cons of room acoustics. [PERSON] and [PERSON] [1] believe that a RT below 500 Hz used in the field of music should be higher than the middle frequency. [PERSON] [2] experimented with 250 Hz and found that when the masking sound is 20 dB, the 250 Hz threshold of the same frequency is increased by about 10 dB, the masking sound is 80 dB, and the 250 Hz test signal threshold is raised by about 50 dB. [PERSON] [3] proposed the bass ratio (BR) indicator and believed that in the acoustic design of a hall, the RT for low frequencies (125-250 Hz) should be increased by 20% compared to intermediate frequencies (500-1000 Hz), suggesting that a concert hall could be even up to 50%, which can make the sound warm and brilliant. Therefore, while low frequency is of considerable importance in a space, too much low-frequency energy will have the opposite effect. [PERSON] and [PERSON] [4] proposed that both language and music have non-negligible energy in the low frequency, which may generate standing waves in space, indirectly strengthen the low-frequency sound field energy, and shield the middle and high frequencies that are extremely important for clarity, thus affecting speech clarity (C\({}_{50}\)). Furthermore, in addition to the feedback on the physical level of the low-frequency sound, the psychological impact is also an important factor. [PERSON] and [PERSON] [5] found that low-frequency noise mainly occurs in indoor environments, while [PERSON] [6] considered the low-frequency noise band to be 10-200 Hz and pointed out that LAeq underestimated low-frequency noise most of the time. [PERSON] et al. [7] foundthat low-frequency noise caused users to feel annoyed, whereas [PERSON] and [PERSON] [8] proposed that the low-frequency noise of ventilation equipment was prone to higher levels of psychosocial symptoms, sleep disturbance, and headaches for people who are annoyed. [PERSON] [9] found that underestimating the interference caused by LAeq can affect sleep conditions, and [PERSON] et al. [10] found that low-frequency background noise causes reduced speech intelligibility and users feel stressed and fatigued. [PERSON] et al. [11] conducted a noise test with 35 young males aged 20 to 30 years old and found that noise between 65 dB and 75 dB obviously caused psychological fatigue, increased heart rate, and reduced working memory. Therefore, in order to reduce the likelihood of generated low-frequency sound, some researchers have proposed the sound absorption method. Common sound absorption systems on the market, such as Helmholtz resonance, perforated panel resonance (PPR), micro-perforated panel (MPP), and membranous vibration (MV), each have sound absorption characteristics at a different frequency. Helmholtz resonance is for mid-low frequency, but the frequency band is narrow. PPR is also for mid-low frequency but is wider than that of Helmholtz. The MPP has better sound absorption performance than PPR, but the manufacturing cost is higher. MV is the only one that can facilitate artistic creation with sound absorption ability at mid-low frequency. Since this research is focused on low frequency, we adopted the PPR and MV systems. As a result, in this paper, we took advantage of the sound absorption characteristics of PPR and MV to reach better performance at a low frequency. A bass trap is normally used to solve the problem of acoustics at a low frequency. Some people will create a bass trap by themselves since they are expensive and enormous, but such DIY products are without measurements to confirm the sound absorption performance. Therefore, this research designed an absorbent material for low frequency (125 and 250 Hz).