CAPÍTULO I. INTRODUCCIÓN GENERAL
1.4. ESTABILIDAD DEL DESARROLLO Y CALIDAD
Tervo et al. (Tervo et al., 2009, Tervo, 2012) recommends the use of a highly directional loudspeaker for the detection of the early reflections, since this method approaches the ray tracing method and analyses only a discrete number of reflections depending on the orientation of the source. The downside of this approach is that it is extremely time- consuming, because it requires performing several measurements rotating the highly directional speaker over the horizontal plane to cover steps of at least of 10º. The same case occurs in the vertical plane, however the highly directional speaker is more difficult to rotate in that direction.
In contrast, the same measurement can be done with one omni-directional or partially directional (source studio monitor) in only one measurement if a broadband monitor is used. That approach was partially fulfilled by this study by using a near field monitor
Genelec 8030A12. It has a reasonably wide directivity in the horizontal plane (x-y) and a
very tight directivity in the vertical plane (x-z)13. It has a waveguide in the tweeter to
limit the spreading in the vertical plane to some extent. Therefore, it may limit the SPL
level in the vertical plane, as it tends to show a more uniform SPL in the horizontal plane
regarding the angle sustained in the x-z plane. The configuration used for the
measurements was planed to cover a limit angle range constrained to the internal workable dimensions of the semi-anechoic chamber.
However, the two-driver monitor is contributing to the difference path from the woofer and the tweeter, which is frequency dependent. The active crossover divides the signal, and because the drivers are not concentric, there is a certain delay on the time of flight (
TOF), which additionally corresponds to a difference of direction of arrival ( DOA) for
certain frequencies covered by a different driver. Moreover, the transient response of both drivers is different because of the mass of each driver and the efficiency of the magnet and the power required to operate. Therefore, the woofer tends to add another delay to the frequencies that it covers. The result is that the measured impulse response of a room will present a phase difference, which is a function of the crossover frequency (
fcrossover). One way to overcome this problem is to use the acoustic centre of the monitor
for the estimation of the sound source real angle of arrival, but this will not be accurate
for all frequencies (see Figure 5.1). On the other hand, to improve localization of
reflections, Rechenberger (Rechenberger, 2009) recommends using a minimum phase
transformation applied to the impulse response ( IR). The non-flat frequency response of
the speaker similarly can be filtered with an inverse counterpart and then multiplied to the
spectrum of the impulse response (fft (IR)). This process may be applied to acquire an
ideal flat response from the speaker. Conversely, these processes introduce delays to the signal, which may affect the time domain analysis to some extent. For that reason, it was eluded in the post-processing of the analysis of reflections in this thesis.
13 http://www.princeton.edu/3D3A/Publications//Speaker_Directivity_Data.pdf [Online accessed on April
Figure 5.1: Acoustic centre of the near field monitor Genelec 8030A used in the measurements located by the laser pointer located at tangent to the woofer at a height of
19 cm from the Iso-Pod™ table stand of the monitor and valid from a distance larger than
0.7 m (2007).
A typical speaker has a frequency dependant directivity, which varies between omni- directional and directional. At low frequencies, it tends to show an omni-directional directivity, nevertheless the directivity increases with mid and becomes narrower at high frequencies. Ideally, the best source directivity for measuring all the reflections in a room should be omni-directional. One way to obtain that is to attach a cone to a speaker as shown in (Li, 1995, Miura et al., 2010). Using it in this way can correct the deviations at low and high frequencies. The downside is that it needs to have a single cone for each driver to cover different frequency ranges, which means that the measurement of an
impulse response ( IR) needs to be carefully done in several stages if a single point source
is needed. An alternative way to manufacture a ‘point source’ is by using a wide band compressor driver coupled with a tube termination of a certain length. Its frequency response is not flat because of the tube resonances impinged to the frequency response. However, the advantage is to be able to easily determine the acoustic centre of the source
as it is located with the two laser cross system as can be seen in Figure 5.2.
the driver and its inefficient acoustic radiation, because of the resonances of the coupled tube and the generated wave reflections by the sharp ending of the tube. This idea was partially tested with a custom simple prototype, composed of a compressor driver and a pipe, which was not used for results in this thesis, although it is considered for future
work (please refer to Figure 5.2 on the left side). For professional results, the
measurements should be done with the Brüel and Kjaer OmniSource Sound Source Type
429514 to avoid the coloration of the sound by the resonances of the tube and to manage
enough acoustic power to obtain good a signal-to-noise-ratio ( SNR).
Figure 5.2: Examples of two omni-directional sound sources. The left one was a custom- made at the University of Salford, and the one on the right is the Brüel and Kjaer
OmniSource Sound Source Type 4295, which compensates the resonances of the tube by
varying the diameter of the tube.