WO2000037962A1

WO2000037962A1 - Directional detection of acoustic signals

Info

Publication number: WO2000037962A1
Application number: PCT/DK1999/000707
Authority: WO
Inventors: Jacob Kortbek
Original assignee: Jacob Kortbek
Priority date: 1998-12-20
Filing date: 1999-12-17
Publication date: 2000-06-29
Also published as: AU1650300A

Abstract

Acoustic/electronic sound reception system consisting of a number of microphones located in a definite configuration (7), preamplifiers (8), multiplexer (9), band-pass filters (10, 14), time delay filter (11), comparators (12, 15) and microprocessor (17) designed to receive and directional detect acoustic signals. Via a multiplexer (9) controlled by a microprocessor (17) the system is able to join together a number of sound reception systems pointing in different directions. The microprocessor (17) measures the output signal from each of the directional orientated sound reception system where this output signal is a result of a measurement of the time delay between two signals and where this time delay is a function of the direction of the incidence of the acoustic signal. The smallest output signal among the sound reception systems decides the direction of the incidence of the acoustic signal. The resolution of the directional detection can be adjusted partly by means of the total number of sound reception systems and partly by a mathematical interpolation between the two smallest signals.

Description

Directional detection of acoustic s ignals The Field of Application for this Invention

The invention is an acoustic/electronic navigation aid, consisting of a number of microphones located in a definite configuration, preamplifiers, multiplexer, band-pass filters, time delay filters, comparators and microprocessors designed to receive and directionally detect acoustic signals from for instance:

Typhoons from another vessel than your own Light tower (main sailing routes) - Buoy (main sailing routes)

Persons in distress (hand carried signal transmitters)

and to reproduce visually the direction of this signal's incidence.

This system provides the rescue party and the shipmaster with an instrument to directionally detect a warning signal in an emergency situation as well as an extra security precaution when it is foggy and when visibility is reduced.

With the used technique, the system is constructed in such a way that the shipmaster among others receives correct information about the traffic in the sea routes in due time to enable him to take the necessary precautions.

This invention has been made in order to consider international requirements concerning registration of among others fog horn signals in order to increase safety for people and ships.

The Stage of the Technique

An existing method for directional detection of external sound sources with a guiding and non- detailed directional detection of the incidence of the received acoustic signal, with special microphones located physically far away from each other (which makes installation difficult) applies the following technique: The microphones first registrating an acoustic signal are the microphones nearest to the sound source. Another method with a guiding and non-detailed directional detection of the incidence of the received acoustic signal with specially designed physically big and expensive parabolas uses the following technique: The microphone with the biggest amplitude is the microphone with the best orientation towards the sound source.

A third method known from the patent magazine no. WO 95/27908 or GO I S 3/802, GOIV 1/00 with a big number of microphones and complicated signal processing technique uses the known "beamforming" technique.

The above existing methods for directional detection of acoustic signals are either of complicated and extensive technological nature or do not fulfil the requirement concerning a detailed directional detection of the incidence of the acoustic signal nor do they fulfil the requirement conccr ing a location of the microphones close to each other.

The Specific Advantage of this Invention Compared with the Stage of the Technique

With this invention consisting of a small number of microphones (f. inst. four) and simple signal processing technique without the use of parabolas and special microphones and with a location of the microphones close to each other, a detailed directional detection of the incidence of the acoustic signal is achieved.

The New Technical Means

The invention aims at a method for directional detection of a sound source and the method can be described as a number of directional detecting sound reception systems located in such a way that they point in different directions and the system transmitting the smallest output signal out of the total number of systems, determines the direction of the incidence of the acoustic signal.

The characteristic of this invention is that the output signal from each of the directional orientated sound reception systems is a result of a measurement of the time delay between two signals and this time delay is a function of the direction of incidence of the acoustic signal. This time delay, as being a function of the direction of the incidence of the acoustic signal, illustrated in a polar co-ordinate system has a characteristic cardioid form. The characteristic of this invention is that the output signal for the sound reception system reaches a minimum at one and only one angle of the incidence of the acoustic signal. Furthermore, this invention provides an accurate directional detection of the sound source as the sound reception system has a big relative change of the output signal when the angle of the incidence of the acoustic signal is changed at the angle in which the characteristic has its minimum.

The invention takes advantage of the fact that the smallest output signal among the sound reception systems pointing in different directions determines the angular correlation of the incidence of the acoustic signal. A more detailed angular definition can be obtained by a mathematical interpolation between the two smallest signals.

The discrimination of the directional detection can be adjusted partly by the total number of sound reception systems and partly by the mathematical interpolation between the two smallest signals.

The Technical Consequence

The invention consists of a number of microphones located in a definite configuration, preamplifiers, multiplexers, band-pass filters, time delay filters, comparators and microprocessors and depends on the fact that the time delay between the two signals on the output of the comparators determines the direction of the incidence of the acoustic signal on the microphone configuration.

For collection of the acoustic signals, a number of microphones are used (for instance elektretmicrophones) with an omnidirectional characteristic of direction, located in a special configuration consisting of microphones located in pairs. The microphones in these pairs of microphones are located in a well-defined distance. These pairs of microphones located so as pointing in different direction has the centre of the pairs of microphones in the same point.

The distance between two microphones of a pair of microphones is co-ordinated in such a way that the distribution time of the sound waves Td_A (acoustic time delay) between the two microphones has the same value as the time delay Tde_L caused by the time delay filter - that is when the sound wave is distributed in the same direction as the axis between the two microphones. This has the effect that the total time delay between the signals on the output of the comparators varies from the value zero to the value of 2xTdeL-

It is advantageous to determine the distance between the microphones to ! wavelength of the signal with the highest frequency within the frequency range within which the directional detection is requested.

Each microphone is provided with a preamplifier separating the microphone's voltage supply from the microphone signal and voltage amplifying the microphone signals with regard to an improved signal/noise relation.

The purpose of the multiplexer is to select by turns two signals coming from two microphones in a pair of microphones and by turns to re-transmit these two signals to one signal processing circuit including band-pass filter, time delay filter and comparator and to the other signal processing circuit including band-pass filter and comparator in such a way that both combinations of the two microphone signals of a pair of microphones and the two signal processing circuits are entered, and in such a way that all pairs of microphones can be selected. For this purpose a Nx2-multiplexer is used, a multiplexer which is able to multiplexe analogue signals and which can be considered as a stereo switch with N positions, N being the number of microphones in the configuration of microphones. (If there are four microphones in configuration of microphones, DG509A from Siliconic can be used). The different positions of the multiplexer are controlled by a binary code from the microprocessor.

To avoid undesirable noise signals, the two selected signals from the multiplexer are each lead through their own band-pass filter. The band-pass of the filter must be designed according to the frequency range within which the directional detection of the sound source has to be made. The band-pass filter ensures that undesirable frequencies are filtered off thereby increasing the system's capacity to measure the correct time delay between the two signals on the outputs of the two comparators.

One signal from the signals selected by the multiplexer is lead through a time delay filter, which exposes the selected signal to a constant time delay. The filter can be implemented in an active Bessel low-pass filter and has to be designed in such a way that it gives a constant time delay to the total frequency range within which the directional detection of the sound source has to be made.

The two signals from the signals selected by the multiplexer are each converted to square wave pulses by means of their own comparator. The comparators are designed to compare at the point where the signal voltage cuts the zero line. This means that in spite of a possible difference between the amplitudes of the two signals before the comparator, the two signals will be identical after the comparator apart from the time delay.

The microprocessor carries out more functions. The primary function is to measure the time delay between the two square wave signals from the comparators. The multiplexer is controlled by the microprocessor in such a way that the different combinations of microphone signals and signal processing circuits are selected concurrently with the microprocessor's measurement of the belonging time delays. These time delays are compared and the position of the multiplexer, which has caused the smallest time delay, determines the direction of the incidence of the acoustic signal on the microphones. When the direction of the acoustic signal's incidence is determined, the microprocessor reads out the direction in a display.

During the measurement of the time delay between the two square wave signals from the comparators the microprocessor can carry out a number of functions ensuring a more accurate measurement of the time delay and thus ensuring a more accurate readout of the direction of the incidence of the acoustic signal.

Description of Figures

Figure 1 Acoustic/electronic sound reception systems consisting of a number of microphones located in a definite configuration (7), preamplifiers (8), multiplexer (9), band-pass filters (10), (14), time delay filters (1 1), comparators (12), (15), and microprocessor (17).

Band-pass filter (10), time delay filter (11) and comparator (12) are named signal processing circuit 1 (13). Band-pass filter (14) and comparator (15) are named signal processing circuit 2 (16). The output signal from the signal processing circuit 1 (13) is named (signal 13), and the output signal from the signal processing circuit 2 (16) is named (signal 16).

Figure 2a to 2d The different positions a, b, c and d of the multiplexer (9) results in sound reception systems pointing in different directions and these combinations of microphone signals and signal processing circuits (13), (16) are illustrated in figure 2a to 2d. For each of the four locations of the multiplexer (9), the output signal of the sound reception system indicated by the value of the time delay between the output signals from the comparators (12), (15) is illustrated as a function of the direction of the sound source.

Figure 3 The way in which the microprocessor measures the time delay between the output signals (signal 13), (signal 16) from the comparators (12), (15).

Examples of Constructions

Please see position numbers on figure 1.

A given number of microphones (7) are located in a special configuration. The microphone configuration consists of a given number of microphones connected in pairs in such a way that the microphone configuration (7) can be described as a number of microphone pairs consisting of two microphones located in a well defined distance and in such a way that the microphone pairs point in different directions with a well defined angle between the given number of microphone pairs. On figure 1, a microphone configuration is shown consisting of two pairs of microphones (5), (6). One pair of microphone (5) consists of microphone 1 (1) and microphone 2 (2) and the other pair of microphone (6) consists of microphone 3 (3) and microphone 4 (4).

The distance between two microphones of a pair of microphones is co-ordinated in such a way that the time for the distribution of the sound wave Td.AK. (acoustic time delay) between the two microphones has same value as the time delay Tdg caused by the time delay filter (11), if of course the sound wave propagates in the same direction as the axis between the two microphones. All microphone signals are amplitude amplified in the preamplifiers (8) after which a multiplexer (9) selects two microphone signals from one of the given number of microphone pairs. From the multiplexer (9) one signal is lead through the signal processing circuit 1 (13) consisting of band- pass filter (10), time delay filter (11) and comparator (12). The other signal is lead through signal processing circuit 2 (16) consisting of band-pass filter (14) and comparator (15). The only difference between the two signal processing circuits (13), (16) is the presence of the time delay filter (11) in signal processing circuit 1 (13).

The multiplexer (9) is able to change in such a way that all pairs of microphones are selected by turns and in such a way that both combinations of the two microphone signals of a microphone pair (5), (6) and the two signal processing circuits (1 1), (14) can be carried out. When four microphones (two pairs of microphones) are used, the multiplexer (9) changes between four positions named a, b, c and d. From figure 2a to 2d the change of the multiplexer appears.

When the multiplexer (9) is in position a, the signal coming from microphone 1 (1) is lead through the signal processing circuit 1 (13), and the signal coming from microphone 2 (2) is lead through the signal processing circuit 2 (16). When the multiplexer (9) is in position b, the signal coming from microphone 3 (3) is lead through the signal processing circuit 1 (13) and the signal coming from microphone 4 (4) is lead through the signal processing circuit 2 (16). When the multiplexer (9) is in position c, the signal coming from microphone 4 (4) is lead through the signal processing circuit 1 (13) and the signal coming from microphone 3 (3) is lead through the signal processing circuit 2 (16). When the multiplexer (9) is in position d, the signal coming from microphone 2 (2) is lead through signal processing circuit 1 (13) and the signal coming from microphone 1 (1) is lead through the signal processing circuit 2 (16).

The output signals (signal 13), (signal 16) from the signal processing circuit 1 (13) and the signal processing circuit 2 (16) are square wave signals. The only difference between the output signal (signal 13) from the signal processing circuit 1 (13) and the output signal (signal 16) from signal processing circuit 2 (16) is that (signal 13) is delayed compared to (signal 16).

The size of the time delay Td between (signal 13) and (signal 16) consists partly of the contribution from the time delay filter (11) and partly from the acoustic time delay Td_A between two microphone signals of a given pair of microphones as a result of the final time for the distribution of the sound wave. The size of the time delay Td between (signal 13) and (signal 16) depends on the angle of the incidence of the acoustic signal on the pair of microphones selected by the multiplexer (9) and it varies between zero and the value 2 x Tde_L-

The time delay Td between (signal 13) and (signal 16) as a function of the angle for the incidence of the acoustic signal, shown in a polar co-ordinate system, is shaped as a cardioid. On figure 2a to 2d the time delay Td between (signal 13) and (signal 16) as a function of the angle of incidence is shown for each of the multiplexer's (9) four positions.

The time delay Td between (signal 13) and (signal 16) is measured by the microprocessor (17). The microprocessor (17) starts a counter on a rising flank from (signal 16) and stops the counter on a rising flank from (signal 13). The value of the counter is now a direct expression of the time delay Td. The way in which the microprocessor ( 17) measures the time delay between (signal 13) and (signal 16) is shown in figure 3.

When the microprocessor (17) has measured the time delay Td between (signal 13) and (signal 16) for all the positions of the multiplexer (9), the measured values are compared and the position of the multiplexer (9) in which the smallest value of Td was measured, decides the direction of the sound source. A more accurate indication of the sound source's direction can be achieved by an interpolation between the two smallest values of Td. Furthermore, the microprocessor can carry out a number of functions ensuring a more accurate measurement of the time delay, resulting in a more accurate read-out of the direction of the incidence of the acoustic signal.

Claims

1. Acoustic/electronic sound reception system consisting of a number of microphones located in a definite configuration (7), preamplifiers (8), multiplexer (9), band-pass filters (10), (14), time delay filter (11), comparators (12), (15) and microprocessor (17) designed to receive and directionally detect acoustic signals characterised in, that a method for directional detection of acoustic signals consists of a number of directional oriented sound reception systems located in such a way that they point in different directions and the system giving the smallest output signal among the total number of systems decides the direction of the incidence of the acoustic signal in the system.

2 Acoustic/electronic system according to requirement 1 characterised in, that the configuration of microphones (7) consisting of a given number of pairs of microphones consisting of two microphones located in a well defined distance and the pairs of microphones located in such a way that they point in different directions.

3. Acoustic/electronic system according to requirement 1 characterised in, that by means of the multiplexer (9) a number of sound reception systems pointing in different directions can be joined together, consisting of a pair of microphones (5), (6), preamplifiers (8), signal processing circuits (13), (16) and microprocessor (17) by selecting in turn pairs of microphones and by the fact that both combinations of the two microphone signals of one pair of microphones (5), (6) and the two signal processing circuits (13), (16) can be carried out.

4. Acoustic/electronic system according to requirement 1 characterised in, that the output signal from each of the directional orientated sound reception systems being a result of a measurement of the time delay between two signals, where this time delay is a function of the direction of the incidence of the acoustic signal and where the time delay as a function of the incidence of the acoustic signal is shaped as a cardioid or where the time delay in another way has a significant minimum at a definite angle for the incidence of the acoustic signal.