The eCathedral is an interdisciplinary Virtual Reality project and was born in 2012. This open access project aims at simulating audiovisual reality in the best possible/most immersive way. The famous Cathedral of Lausanne serves here as a beautiful example. Being located just a few kilometres away from EPFL, the Cathedral of Lausanne yields a great opportunity to assess real-world data whenever required.
The project focuses on two major disciplines:
High Quality Measurements of Room Impulse Responses A decent set of real-world data is mandatory for serious sound field analyses and simulation comparisons. So far, over 600 RIRs and nearly 90 Binaural RIRs (BRIR), each with a Signal-to-Noise Ratio (SNR) of over 70dB (broadband), were acquired resulting in a dense three-dimensional grid of high quality impulse response measurements over the entire seating area. The complete set of (B)RIRs is freely available and detailed information on the measurement procedure is given below.
Physically-based Auralization/3D Audio Rendering Analogous to visualization, the term auralization describes the simulation of sound propagation inside enclosures making audible the aural attributes of space. As a first outcome of eCathedral, enjoy a beautiful 360-degree audiovisual virtual tour in- and outside the Cathedral of Lausanne.
© Photo/VR Tour by Dirk Schröder and Loïc Baboulaz
Note: large file size, please allow demo to completely load.
The famous Cathedral of Lausanne (Cathédrale Notre-Dame) is considered as one of the most beautiful Gothic cathedrals in Europe. It is located approx. 500 ft. above Lake Geneva (Lac Léman) in the hillside city of Lausanne, a city in the french-speaking part of Switzerland (Romandie) and capital of the canton of Vaud. The construction of the cathedral began already in 1175 and it was consecrated in 1275 by Pope Gregory X in the presence of King Rudolf of Habsburg.
The cathedral is topped with towers and spires, and the portals and facade of the cathedral are richly ornamented with carved sculptures and reliefs. The interior is Gothic-style at its most elegant and beautiful, but most decorations were removed in 1536 by the combined forces of the Reformation and Bernese army including altars, statues and paintings. Fortunately, there are some notable exceptions of medieval art that survived until today. One example is the glorious Rose Window located at the south side of the cathedral - a giant Gothic window of stained glass from the 13th century. The rose was a popular medieval representation of the universe and Lausanne's huge rose contains images representing the four seasons, four elements, four winds, four rivers of paradise, and the twelve labors of the months and signs of the zodiac.
Inaugurated in 2003, the new organ is the only one of its kind in the world: manufactured by the famous American organ builder Fisk and designed by the Italian star designer Giugiaro, the organ's musical characteristics (French classical and symphonic styles, German baroque and romantic) and its dimensions (nearly 7,000 pipes, 40 tons, 150,000 hours of work to complete) are truly outstanding. Concerts are held on a regular basis, where detailed information on the current program can be found here.
© Photo by Dirk Schröder
In a joint project, the Institute for Audiovisual Communications (LCAV) at EPFL, Switzerland, and the Institute of Technical Acoustics (ITA) at RWTH Aachen University, Germany, carried out a huge measurement series of Room Impulse Responses (RIR) in the Cathedral of Lausanne using state-of-the-art measurement technology.
|Measurement Team (LCAV):||Dr. Dirk Schröder, Dr. Andreas Walther|
|Measurement Team (ITA):||Dr. Gottfried Behler, Martin Guski, Ingo Witew|
Note! The complete set of measurement data together with a short tutorial on how to process the data can be downloaded here (about 1.5 GB of data). The compiled measurement protocol can be viewed here summarizing all relevant data such as positions (speakers and microphones), room humidity, room temperature and measurement equipment.
Coordinate System In order to describe all relevant positions in an accurate way, the coordinate system is defined as follows: the x axis goes from the altar to the main entrance and the z axis from bottom to top. The y axis is defined consequently to build a right-handed coordinate system. The origin of the x coordinate has been chosen to be at the first step to the altar and z = 0 marks the floor of the audience area. The zero position of the y coordinate left most end of the seating area as seen from the main entrance. The two measurement loudspeakers (identical in construction) were positioned at the altar (L1, x=-1.15, y=4.25 m, z = 1.23 m) and at the main organ above the main entrance (L2, x=44.54 m, y=3.78 m, z=10.53 m).
General Equipment The measurements were conducted using a PC and the ITA-Toolbox. The ITA-Toolbox is an open source toolbox for MATLAB and contains numerous functions for acoustic data processing. The RME Hammerfall DSP Digiface was used as sound card. Three RME Octamic II were connected via ADAT and used as microphone front-ends. The Octamic II is an 8-channel microphone preamp with 24 bit AD converters. The frequency response of both measurement loudspeakers were equalized using the digital loudspeaker management system HD2 from Four Audio. The filters are based on the power averaged directivity measurement conducted in an anechoic chamber. The resulting linear phase filters were adjusted using a (broadband) delay to avoid destructive interferences at the crossing frequencies. The (inverted) transfer functions are combined with the frequency crossover filters and transferred to the HD2. Three stereo amplifiers of type Pro A 2000 from Neumann were used to amplify the 6 output signals of the HD2 and feed it to the two loudspeaker systems.
Measurement Loudspeakers The two dodecahedron loudspeakers used for the measurements were designed and manufactured by the ITA. This type of loudspeaker was developed to obtain an omnidirectional directivity and a flat frequency response over a wide frequency range to measure impulse responses suitable for auralization purposes. Since it is not possible to achieve an omnidirectional radiation over the full audible frequency range with one cabinet, the loudspeaker is realized using a three band design. The core of the subwoofer is a 12'' driver. The cabinet has a radiation opening of only 10 cm to ensure an omnidirectional radiation up to 300 Hz. The tuning of the housing results in a flat frequency response from 40 Hz to 300 Hz (-6 dB). The medium frequency range is covered by a spherical cabinet with a diameter of 30 cm. It consists of 12 loudspeakers each 12 cm in diameter and is placed 25 cm above the opening of the woofer (usable frequency range up to 6.3 kHz, directivity +/- 3 dB up to 2000 Hz). The high frequency unit is sphere made of Aluminum with a diameter of 9.5 cm. It is equipped with twelve tweeters of 0.8'' diameter. The directivity can be considered as omnidirectional up to a frequency of about 5 kHz.
A detailed brochure containing all technical information and conceptual details is given here: ITA Dodecahedron (50 MB, 10 pages).
Microphones Two different kinds of input devices were used: Omnidirectional microphones and dummy heads. For default room acoustic parameter evaluation (parameters such as Early Decay Time (EDT), Reverberation Time T30 or Clarity C80) omnidirectional microphones of type KE4 from Sennheiser were used. The 18 microphones were connected using custom made impedance converters. The dummy heads were used for parameter evaluation (IACC) and auralization. Two ITA dummy heads (in-house developments) and one Neumann KU 100 were used simultaneously.
Six microphone stands were used each equipped with three omnidirectional microphones. The heights of the microphones from the ground were z=1.20 m (seating height), z=1.60 m (standing height) and z=2.00 m (over standing height). Two additional tripods were used to mount the two ITA dummy head at an ear-height of z=1.60 m. The positions of all eight microphone stands were changed between every single measurement to cover the whole audience area. In total twelve rows and six positions each rows were measured (every second seat and every second row). The Neumann dummy head was positioned at the altar with an ear-height of z=2.42 m. The dummy head was placed on a turntable which was rotated by 5 degrees between every measurement. The orientation of 0 degree is defined by the artificial head oriented towards the organ over the main entrance. The detailed positions of all microphones (x, y and z in meters) are included in metadata of the measurements (channelCoordinates).
Excitation signal An exponential sweep was used as excitation signal. Compared to other excitation signals, such as linear sweep or maximum length sequences (MLS) it brings some important advantages. Sweeps are more robust against time variances for example caused by air movement (problematic in large rooms) or temperature drifts (problematic during long measurement sessions) [Mueller2001]. Exponential sweeps also show advantages for systems that have non-linear components. The harmonic components appear at defined positions in the impulse response mostly separated from the impulse response of the fundamentals. This allows on the one side a direct estimation of the magnitude of the nonlinearities and on the other side an elimination of most non-linear components using time windowing. The power density of an exponential sweep decreases inversely proportional to the frequency (pink spectrum). For typical ambient background noise (also proportional to 1/f) using exponential sweep results in signal-to-noise-ration that is constant over frequency. The exponential sweep used in this measurement has a frequency range from 20 Hz to 18 kHz and a sampling rate of 44.1 kHz. The total length of the sweep was 92 seconds followed by a silent part (called stop margin) of three seconds. The stop margin ensures that the last played frequencies of the sweep are able to decay below the ambient noise before the recording stops. Due to the fact that the latest part of the sweep only contains the highest frequencies, this stop margin can be chosen much shorter than the reverberation time of the room for the lower frequencies. Above 10 kHz the reverberation in any room typically is below 1 second. The deconvolution of the recorded signal has been performed in the frequency domain. For the inversion of the spectrum of the excitation signal regularization after Farina [Farina2000] has been used to reduce the influence of noise outside the considered frequency range.
Dodecahedron loudspeaker designed by ITA
Dummy head designed by ITA
Setting up the Measurement Instruments
|[Mueller2001]||S. Müller and P. Massarani, Transfer-Function Measurement with Sweeps, J. Audio Eng. Soc Vol. 49|
|[Farina2000]||A. Farina, Simultaneous measurement of impulse response and distortion with a swept-sine technique, 108th AES Convention, Paris 2000|