Acoustic Source Location
There are a number of different ways by which the location of a noise source may be found. The best starting point is a binaural system based on that used within the human hearing system.
- The human hearing system uses a number of different cues to locate a noise source:
- Interaural Time Differences these become more difficult to calculate at higher frequencies due to the level differences between ears and the discrimination being so good using just intensity differences. Interaural time delays typically occur in the range -1 to 1ms. Therefore, the cross-correlations can be windowed to ignore values outside of these.
- Interaural Intensity Differences this was shown by Rayleigh to only work above approximately 1000Hz. This relates to a wavelength of about 0.34m giving a half wavelength that is approximately the width of the head.
- Monaural Spectral Cues these rely on the outer ear filtering the spectrum. The effect of the filtering due to location is only known if the spectral content of the noise source is known, if it is not there is no reference. The outer ear has most effect at higher frequencies, therefore, the ability to locate the noise source depends on conditions such that the higher frequencies are not attenuated severely.
- Head Movements moving the head allows the level difference between ears to be minimised and hence allow the direction to be sort.
- Grouping Cues if the sound is reflected from a wall the arrival of the reflected signal may be confusing when using interaural level and time differences. In this situation grouping cues allow different parts of the spectrum to be put together.
- "the cocktail party effect" is mentioned in a number of papers and is described as the ability of the human hearing system to effectively reject spurious noises and allow the hearing to "home in" on the sound that is arriving from a particular direction.
- A further addition to the human noise source location system is visual cues. The observer associates visual and sound cues, hence enhancing the location. This is of little interest in sound location systems as such, but other signals may be used to improve the description of the sound field. E.g. engine speed, road speed or even wheel hub vibration.
See also: Hearing, Percentage Articulation Loss of Consonants.
Subjects: Architectural Acoustics Noise & Vibration