With this set-up, the measured hologram’s size is 0.90 cm by 1.76m. This implies a frequency range from 377 Hz up to 2125 Hz that can be analyzed using near-field acoustic holography (NAH). NAH can still be used below 377 Hz for sound source localization but calculated sound power levels are then no longer accurate. However, resulting sound power values still give the user a relative idea of their importance. Above 2125 Hz and up to 4250 Hz, near-field focalization can be used on the same data set to localize the source. With one single data set, a frequency range from 377 Hz to 4250 Hz is covered with a minimal spatial resolution of 8 cm.
An A-weighted power spectrum shows the major sources. Figure 3 on the left shows the highest ranked source with detailed source localization at low frequency (220-270Hz). The picture on the right of figure 3 shows an analysis at frequency range 2410-2530Hz, which is above the limit for holography (2125Hz in this case). The near-field focalization technique doubles this frequency range, allowing source localization up to 4250Hz to be analyzed with 8cm spatial resolution.
Figure 3: exhaust at low frequency and near-field focalization at high frequency
More information on:
- Sound Source Localization technologies
- Acoustic beamforming technique and near-field focalization
- Acoutics holography technique
- Spherical beamforming technique for interior sound source localization
