Other methods

Microflown p-u probe

2.2.1

Further advances have been reported with a new approach based on hot wire transducers, which are sensitive to temperature changes. These are known as particle velocity transducers (Fahy, 1995), which are related to the fluctuations of value of particle velocity at that given coordinate. The p-u intensity probe was invented in 1996 by Hans

Elias de Bree in (de Bree, 1997) and later commercialised in (de Bree, 2003). The

company that manufactures this probe is called Microflown Technologies and is based in The Netherlands. This solution drastically minimizes the distance between the transducers, and is therefore, likely the smaller coincident array, showing the full bandwidth across frequency with a single probe. Owing to its small size, it is also possible to use the p-u intensity probe in smaller places than the p-p intensity probe (Druyvesteyn and de Bree, 1998).

In 2002, an American company named Meyer Sound3 _{created the first measurement}

system for measuring 3-Dimensional impulse responses of rooms using a custom p-u microphone array. It consisted of one pressure microphone and three orthogonal particle

velocities probes from Microflown (model PU regular one-dimensional) and custom

hardware and software. The acoustical parameters such as RT, EDT, etc., were calculated

using the Euclidean norm value of the instantaneous sound intensity ( i_inst ) instead of the

squared acoustic pressure ( _p2

). This process delivers lower floor noise than using only

acoustic pressure, since the acoustic pressure ( _p) and particle velocity (u) are

uncorrelated signals and each transducer type senses them separately. When both are multiplied, the net effect is to obtain a cleaner signal. It has passed through a kind of filter, which removes unwanted noise by making it smaller after multiplying two small noise signals. Identification of the source of reflections is possible by analysing the polarity of each of the Cartesian axis of each early reflection by plotting them as a

traditional Reflectogram, where the squared of the pressure signal ( _p2_{) is converted to a}

dBA level (2002).

According to Raangs, the three-dimensional Microflown p-u intensity probe model USP

(Ultimate Sound Probe) was introduced to the market (Raangs, 2005), with its signal conditioner model MFSC-4. It is reported that it has an accuracy of 7º in the estimation

of the particle velocity (u) when using steady state noise sources (Yntema et al., 2006).

Nevertheless, the accuracy for detection of early reflections has yet to be fully reported. The aim of this project is to select a measurement-grade instrument that can surpass the

minimum audible angle resolution of the human hearing system. Yntema, in his Ph.D.

thesis, (Yntema, 2008) still finds inherent problems on the self-noise of a p-u Microflown

intensity probe and on the angular accuracy achieved, but stresses the advantage of using only particle velocity sensors in order to overcome the discrepancies on signal-to-noise- ratio ( _SNR) found using pressure and particle velocities together. There was a low- frequency accuracy problem in the probe given that there is thermal noise in the hot wires

addressed by J. W. van Honschoten in(van Honschoten, 2004). A new Microflown probe

3 _{http://www.microflown.com/files/media/library/Applicationnotes/meyersound_3dp.pdf}

has been proposed with four pairs of wires instead of two. It exhibits a lower inner noise level because of an increased sensitivity than the one found in a previous two-wire design

(Yntema and van Honschoten, 2010). Nevertheless, the application of the Microflown

probe for measuring early-reflected energy is challenging because the requirements for analysis of transient noise sources are more demanding than the requirements of steady state noise sources.

Author Year Measurement System Angular

resolution Calculation method

Elko (Elko, 1984). 1984 2-D p-p intensity probe. 2º @ 1 kHz Finite differences intensity method. Sekiguchi (Sekiguchi et al., 1992). 1992 4 omni-directional microphones tetrahedral array.

3º Deconvolution technique with tone burst stimuli.

Abdou (Abdou, 1994). 1994 1-D p-p intensity probe

rotated. 3º@ 250 Hz

Instantaneous intensity (_

i_inst) in time domain. Van Lancker (van

Lancker, 2000). 2000 8 omni-directional microphones in the corners of a cube. 1º or 2º TDE method. Gover (Gover, 2001). 2001 32 omni-directional microphones spherical array. 22º Beamforming. Günel (Günel et al.,

2005, Günel et al., 2007).

2007 Soundfield microphone. 7º

Wavelet packet Decomposition.

Yntema (Yntema et al., 2006, Basten et al., 2009). 2006 2009 3-D Microflown p-u probe. a) 7º b) 10º with no correction & 3.5º corrected

a) particle velocity (u) only. b) instantaneous intensity (_ i_inst) method . Rechenberger (Rechenberger, 2009). 2009 5 omni-directional microphones tetrahedral array. 2º

TOA algorithm with up- sampling and minimum phase

IR.

Miah (Miah, 2009) (Miah and Hixon, 2010).

2009 7 omni-directional _{microphone array.} 4º ± 3º Finite Differences Intensity _method. Tervo (Tervo, 2010,

Tervo, 2012). 2012

3-D G.R.A.S. p-p intensity probe and TKK 3-D probe.

1º SRP method combining TOA and TDOA algorithm. Romero-Perez

(Romero-Perez, 2011, Romero-Pérez, 2011).

2013

1-D p-p intensity probe rotated with custom cradle.

2.971º ±0.226º

STFT method using

instantaneous intensity (I_act ) and Circular statistics

Table 2.1: Comparison of early reflections measurement systems, which reported quantifiable data.

2.3 Summary

After the survey on measurement of early reflections, it was found that a formal investigation on the accuracy of estimation of reflections was needed. In order to improve the B-Format angle estimation a new approach was taken. Therefore, the

motivation of using active instantaneous intensity (i_a), complex instantaneous intensity (



i_complex), envelope of intensity ( _i ) and short-time Fourier transform ( _STFT ) approach was implemented in the present work.

Chapter 3 : Applied Theory

The implementation of a measurement system is based in intensimetry. The concepts covered are the acoustic energetic quantities used for the diffuseness estimate, envelope and analytic function used to obtain the peak detector in the post-processing analysis, and several topics covering sound intensity such as the Euler equation to approximate the particle velocity (u_inst), instantaneous intensity (i_inst) and active instantaneous intensity (



i_a).

The second part covers the post-processing equations used, such as the Exponential sine sweep, the deconvolution equation to obtain the impulse response of an acoustic measurement, and the application of the single vector/matrix that represents the four- channel orthogonal impulse responses.