Full title: Intelligent low-frequeNcy acousTic Equalization of Rooms using Active ConTrol Subwoofers
The INTERACTS CTI-funded project is a technology transfer collaboration between EPFL, HEPIA (Geneva) as academic partners, and Goldmund SA (Geneva) and PSI Audio (Yverdon), two major companies in the professional audio industry.
The occurence of low-frequency resonances in rooms devoted to sound diffusion degradates the audio rendering quality of the low frequency bound of the audible spectrum. This has several prejudical impacts:
- on the uneven spatial distribution of energy,
- on the degradation of musical timbres (spectral degradations)
- on the sustain of low-frequency tones (modal extinction time)
spatial distribution of sound pressure levels in the reverberant chamber for mode (1,1,0) at 34,9 Hz
|frequency response of the reverberant chamber measured at one corner with a broadband noise source located at the same corner||time response of the room to a time-windowed pure sine at 34,9 Hz, measured at one corner (up: linear waveform; bottom: corresponding time-series in sound pressure levels)|
Examples: here are two versions of a musical excerpt:
- extracted from the original recording
- recorded in the reverberant chamber (better rendered with a subwoofer or good quality headphones)
The aim of this project is to develop low-frequency sound absorbers, effective on a relatively broad frequency range (ideally 20 Hz - 200 Hz), allowing an efficient equalization of the room acoustics down to the low-frequency bound of the audible spectrum, for application to recording studios and home-audio/-cinema spaces.
In this project, EPFL-LTS2 develops a dedicated broadband low-frequency sound absorber based on the principle of the electroacoustic resonator concept developped in the lab (EPFL thesis n°5331). The design methodology is the following:
- identify the frequency range of interest according to the room modal behavior and set the central frequency of the electroacoustic resonator
- identify a target acoustic impedance function to assign to the active electroacoustic resonator
- select loudspeakers according to a matching of resonance frequency and other passive properties (total losses, force factor)
- identify the electronic control law according to the target acoustic impedance function and the loudspeaker passive properties
- implement the control law on a digital signal processing platfrom (FPGA, DSP) combined with a voltage-controlled current amplifier (transconductance amplifier)
- test the performance in a standing wave duct (standard acoustic impedance measurement according to ISO-10534-2)
- test the overall performances in actual rooms
Outcomes of the project
- numerical model for the prediction of room modes damping
- identification of acoustic impedance targets and number/surface of active absorption as a functuin of the room properties in order to optimally set the electroacoustic absorbers
- set of active electroacoustic absorbers prototypes/demonstrators
- objective assessement in different rooms situations (reverberant chamber, listening rooms)
- perceptual evaluation of the overall performances with listening tests
Status: ongoing (started on December 1, 2012 - end on November 30th, 2015)
- Karkar, S. et al., "Electroacoustic absorbers for the low-frequency modal equalization of a room: what is the optimal target impedance for maximum modal damping, depending on the total area of absorbers?", presented at Forum Acusticum 2014, Krakow, Poland, September 7-12, 2014.
- Lissek, H., "Absorbeurs électroacoustiques actifs: application à l’égalisation des salles aux basses fréquences", presented at the CIDB-GIAC-SFA colloquium "Vibrations et bruits basses fréquences dans les bâtiments", Paris, France, June 8-9, 2015.
- Lissek, H. et al., "Experimental assessment of active electroacoustic absorbers for broadband room modes damping", in the Proceedings of INTERNOISE 2015, San Francisco, USA, August 9-12 2015.