Patent Description:
Slow driving electric vehicles produce too little noise to be noticed by pedestrians. This clearly causes a safety issue, for example in the neighbourhood of schools, at pedestrian crossings or traffic lights. Legislation has been adapted to address this matter by making mandatory the generation of an artificial sound. An Acoustic Vehicle Alerting System (AVAS) is designed to emit vehicle warning sounds and alert pedestrians to the presence of electric drive vehicles. These include hybrid (HEVs), plug-in hybrid (PHEVs), and full battery electric vehicles (BEVs) travelling at low speeds, especially in the lowest speed range beyond which the noise generated by rolling tires can be easily heard.

A horn intended for producing a warning signal is installed in every vehicle. Horn signals have a typical sound that is automatically recognized by people as a horn. Worldwide people have grown accustomed to the sound of vehicle horns, in spite of tonal differences that may be observed between horn sound signals. Most horns use a similar principle of operation, based on a hammer knocking on a metal disk or vice versa to resonate in a specific way, hence its tonal character. The hammer or the disc itself is moved by means of electromagnetism in a fixed manner since the frequency of the hammer or disc movements is defined by an electromechanical interruption process. Direct current from the vehicle battery, e.g. 12V battery, is fed into a coil. The hammer moves towards the disc or vice versa. By moving forward the contact with the battery is interrupted and after hitting the disc or the hammer the contact is restored.

The monotonous stimulation of a resonating object brings forward a recognizable alarming tone, albeit with a tonal character defined by the frequency of excitation and by the characteristics of the resonating object. Typically a single horn produces a spectrum in which a low frequency component is clearly present (between <NUM> and <NUM>) combined with a louder high frequency component (around <NUM>) amongst other less pronounced tones. From a legislative point of view the required amplitude levels for a horn signal are high and the bandwidth is narrow. The loudspeaker on the contrary has a relatively broad bandwidth, while the output amplitude levels are limited to avoid too loud output signals. Therefore the broadband AVAS loudspeakers cannot reach the sound pressure levels (SPLs) required for a horn unless they are overdesigned for the requirements imposed on an AVAS system.

In the art systems are known wherein the warning functionality of AVAS is combined with horn functionality. For example, <CIT> discloses a vehicular horn device that can be used as a dynamic speaker so as to generate a false engine sound. The shortage of a low-pitched sound in a parametric speaker device is complemented with a false engine sound which the vehicular horn device generates. As the vehicle approaches a pedestrian, a sound tone of the false engine sound which the pedestrian hears changes, enabling the pedestrian to easily notice the approach or presence of the vehicle.

<CIT> relates to a vehicle comprising a multi-purpose automotive sound device for alerting pedestrians. The sound device operates as a horn in a first mode in response to an external input (e.g. from the driver) and as a speaker or other sound generating device in a second mode in response to the vehicle moving in reverse or moving forward at a speed satisfying a given threshold.

<CIT> presents a system to generate external sound for use in electric motor vehicles. A hybrid transducer controlled by a common interface that assures distribution of power between a piezo-electric transducer and a magnetic transducer according to a required function, e.g. alarm function and sound warning function.

In <CIT> an acoustic vehicle warning system for a motor vehicle is disclosed. The system comprises at least one loudspeaker and a control unit designed to output a continuous acoustic signal by means of the loudspeaker during driving operation of the motor vehicle. Further, the control unit can actuate the loudspeaker after an actuation unit has been actuated, the loudspeaker being designed to output an acoustic warning signal after receiving the actuation signal.

<CIT> is concerned with acoustic sound design for an AVAS system of an electric vehicle. The proposed solution aims at minimizing annoyance from sound leaking into a vehicle compartment. Road and other functional vehicle made sounds (tread noise, fans, wind noise etc.) are used to create an alert sound that mimics these existing vehicle sounds and can be potentially masked, interiorly, by those same sounds. However, the creation of horn signals is not discussed in this document.

In <CIT> a sound generating system for an electric vehicle is disclosed. The system comprises a sound output unit which performs a warning sound function to warn a pedestrian about an approaching vehicle. Horn signals, however, are not discussed in this document.

Hence, there is a need for a system designed for use of a horn sound signal as well as for use of a warning signal to make road users aware of the presence or the approaching of the vehicle.

It is an object of embodiments of the present invention to provide for an audible vehicle warning system that offers both warning functionality and horn functionality, while meeting all relevant legal requirements.

The above objective is accomplished by the solution according to the present invention as defined by the appended claims.

In a first aspect the invention relates to an audible vehicle warning system arranged for performing AVAS functionality and for generating a horn signal and comprising.

and wherein a <NUM>/<NUM> octave frequency band averaged response of the loudspeaker has in a <NUM>/<NUM> octave frequency band corresponding to the reference frequency band of the respective sound signal an amplitude level which is at least <NUM> dB above the average amplitude level in a frequency range from <NUM> to <NUM>.

The proposed solution indeed allows for operating the system for generating artificial sound to make e.g. pedestrians aware of the presence of a vehicle and for generating a horn signal. It is made sure both types of signal meet the respective legal requirements. Avas sounds are typically synthesized in real time making use of speed, acceleration, etc. info from the car. Each of the horn sound signals in the sound library has one <NUM>/<NUM> octave frequency band that serves as reference and contains at least a predetermined part of the total power of the sound signal. The loudspeaker is then so designed that its frequency response, when split up in <NUM>/<NUM> octave frequency bands, comprises a <NUM>/<NUM> octave frequency band corresponding to said reference frequency band wherein the average amplitude level over that band is at least a certain amount of dB higher than the average amplitude over a selected frequency range. The selected frequency range comprises at least the frequencies from <NUM> to <NUM>. The selected frequency range contains the reference band. In preferred embodiments the selected frequency range includes also the part of the frequency spectrum where a horn signal typically displays a peak, for example the range from <NUM> to <NUM>. The resulting peak in that <NUM>/<NUM> octave frequency band ensures the requirement is met that the horn signal produce a louder high frequency component.

In preferred embodiments the amplitude level of the <NUM>/<NUM> octave frequency band averaged response of the loudspeaker is <NUM> dB higher than the average amplitude level in the rated frequency range. This yields the advantage that a prominent peak is obtained in the response that meets the legally required high sensitivity. In other embodiments a gap of for example <NUM> dB or <NUM> dB or <NUM> dB is provided.

In preferred embodiments the loudspeaker of the loudspeaker system is mounted in a cavity.

Preferably the loudspeaker system has at least one acoustic resonance falling within the <NUM>/<NUM> octave frequency band having an amplitude level at least a <NUM> dB above the average amplitude level in the frequency range from <NUM> to <NUM>.

In some embodiments the cavity has a first opening in front of the loudspeaker's diaphragm.

In one embodiment the audible vehicle warning system comprises a horn mouth in front of the first opening.

In another embodiment the cavity is provided with a second opening behind the loudspeaker diaphragm at the backside of the cavity.

In some embodiments the cavity comprises both the first and the second opening. In advantageous embodiments the second opening is in connection with at least one vent outlet oriented in an opposite direction of the first opening.

Advantageously, the loudspeaker system comprises a single loudspeaker. This leads to a compact and efficient implementation.

In preferred embodiments the selected frequency range comprises the range from <NUM> to <NUM>. In other preferred embodiments the selected frequency range goes from <NUM> to <NUM>.

In another aspect a method for creating a sound signal to be added to a sound library for an audible vehicle warning system, is discussed. Said sound library comprising one or more sound signals intended as a horn signal when output by a loudspeaker of said audible vehicle warning system. The method comprises :.

Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein, as soon as it does not depart from the scope of the appended claims.

However, it is understood that embodiments of the invention may be practiced without these specific details, without departing from the scope of the appended claims.

The present invention proposes an audible vehicle warning system that is arranged for performing AVAS functionality and for generating a horn sound. The horn sound signal is designed to be recognizable as such when output by a loudspeaker of the warning system. As already mentioned above, this requires that the horn signal meet the specifications imposed by legislation. The horn needs to be homologated both at car level and at device level. At device level the power sum of A-weighted <NUM>, <NUM> and <NUM> third octave frequency bands of the generated horn signal should be at least <NUM> dB sound pressure level (SPL), measured in free field conditions and with a microphone on-axis at <NUM> from the device under test. As commonly known in the art, the upper band edge frequency of a <NUM>/<NUM> octave frequency band equals the lower band edge frequency times <NUM><NUM>/<NUM>. At vehicle level the A-weighted sound pressure level should be at least <NUM> dB(A), measured in half space free field outside and on-axis at a distance of <NUM> from the vehicle, with the microphone at least <NUM> above the ground. Further, also the design of an audible vehicle warning system is subject to a set of legal requirements.

Loudspeakers in a free-field or an half-space free-field condition can be characterized acoustically by the transfer response between an electrical input voltage and the sound pressure output generated at a reference point at a stated distance on a reference axis (usually on-axis) in the far field. Loudspeaker sensitivity is defined as the sound pressure level in a given frequency band generated by a voltage corresponding to an input power of <NUM> W at rated impedance Zn for a distance of <NUM>.

Legal requirements define minimum sound pressure levels for both the horn signal and the loudspeaker of the vehicle warning system but do not specify how they should sound. Today, typical vehicle warning system loudspeakers have a sensitivity of <NUM> to 85dB/W/m over a frequency range of <NUM> to <NUM> to meet global AVAS requirements and a power handling capacity between 10W and 20W. This, however, is insufficient for implementing horn functionality, if one assumes the horn signal needs a SPL response with a minimum sensitivity of <NUM> to 105dB/W/m within the octave band of <NUM> (i.e. in the range of <NUM> - <NUM>) in order to meet the legal requirements.

In the approach according to the present invention the design of the system, in particular the loudspeaker system, and the design of the horn signal are combined. A loudspeaker in an audible vehicle warning system of this invention is a broadband loudspeaker, i.e. a loudspeaker which is arranged for outputting sound signals at least in a rated frequency range from <NUM> to <NUM>. The solution of this invention proposes designing the loudspeaker system so that a peak in its SPL response corresponds to a peak in the horn power spectrum, or vice versa, which allows for a loudspeaker with a low average sensitivity while capable of meeting the legislation concerning the horn sound signal.

This can be achieved for example by relying on the use of resonance effects. Resonances are inherently present in any broadband loudspeaker, but are certainly not designed towards the reproduction of a specific and fixed sound. Moreover, they are generally regarded as unwanted and, if present, one likes to smoothen a peaky SPL response by means of damping. In the present invention, however, peaks in the frequency response are created on purpose in order to ensure a faithful reproduction of the horn sound signal. More details are provided later in this description.

The audible vehicle warning system of the present invention comprises as main components a sound library of one or more sound signals intended to be used as a horn signal and a broadband loudspeaker system comprising a loudspeaker for outputting a sound signal selected from the library.

A horn has a typical frequency spectrum. An illustration is provided in <FIG>, showing a power spectrum of a horn signal. The spectrum displays a fundamental tone at a frequency corresponding to the rate at which the hammer of the vehicle horn beats against the tone disk. This frequency usually lies in the octave between <NUM> and <NUM>. Further the spectrum shows several overtones, some of which are higher in amplitude than the fundamental tone. In the example of <FIG> the strongest overtone is around <NUM>, which is in the most sensitive region of the human auditory system. This overtone also falls within the <NUM> octave band which is the window of the homologation procedure for vehicle horns, as already mentioned.

Any sound signal of the sound library comprised in the system of the present invention is characterized by a <NUM>/<NUM> octave power spectrum. A <NUM>/<NUM> octave frequency band of the horn sound signal containing at least a predetermined amount of the total power is taken as a reference frequency band for the signal in question. This <NUM>/<NUM> octave frequency band representative of the sound signal under consideration and taken as reference contains at least one third of the total power, for example half of the total power i.e. <NUM> % of the power, or <NUM>% or <NUM>% or of the power.

An illustration is provided in <FIG>, where the power spectrum of <FIG> is depicted split up in third octave frequency bands of the horn signal. The centre frequencies of the various bands can be chosen for example, but not necessarily, in line with the ANSI S1. <NUM> standard. The relative contribution of each <NUM>/<NUM> octave frequency band of the various bands to the power spectrum can then be determined. In this example it is found that the <NUM> third octave frequency band contains more than <NUM>% of the total power of the signal. In this case it is clear the <NUM> third octave frequency band is to be taken as reference frequency band for the horn sound signal depicted in <FIG>. This reference frequency band plays an important role when designing the loudspeaker frequency response, as will be shown later in this description.

In a more generic case, if the predetermined part of the power in the horn signal would be set higher than <NUM> %, there would obviously be at most only one third octave frequency band qualifying as reference frequency band. If said predetermined part would be set at <NUM>% or less, there may be more than one frequency band meeting this criterion. In that case any of those bands can be considered for use as reference frequency band.

This is illustrated by means of some examples in <FIG>. In <FIG> the predetermined portion of the power in the reference frequency band is taken to be half of the total power. Obviously there is only one <NUM>/<NUM> octave frequency band that meets the criterion and thus there is only one candidate to be used as reference. In the example shown in <FIG>, on the contrary, the predetermined part is set at one third of the total power and two <NUM>/<NUM> octave frequency bands are found that at least contain one third of the total power. As mentioned above, in principle any of the two can be selected to serve as reference frequency band. In practice, in this particular example, the higher frequency band around <NUM> is chosen in order to meet the legal requirements concerning the horn signal.

In some embodiments the reference frequency band is determined when creating the sound library. Information related to the <NUM>/<NUM> octave frequency band that serves as reference frequency band is stored in the sound library, along with the sound signal itself in the sound library. In other embodiments the audible vehicle warning system comprises processing means to determine the <NUM>/<NUM> octave frequency band that is to be used as reference frequency band on the fly. It may then be sufficient to store only the sound signal itself.

The reference frequency band of the horn sound signal is used to determine the frequency response of the loudspeaker system. The frequency response of the loudspeaker is so designed that, when representing the frequency response using <NUM>/<NUM> octave frequency bands and determining for each of those bands an average value of the response, there is a <NUM>/<NUM> octave frequency band frequency band corresponding to the reference frequency band that has an amplitude level which is at least a certain amount, e.g. <NUM> dB or <NUM> dB or <NUM> dB or <NUM> dB above the average amplitude level in the rated frequency range, i.e. in the frequency range from <NUM> to <NUM>.

The inventors have found that a <NUM> dB peak in the amplitude level yields satisfactory results in practice.

The <NUM>/<NUM> octave frequency band corresponding to the reference frequency band may in some embodiments have the same centre frequency as the reference frequency band. In other embodiments its centre frequency may have shifted with respect to that of the reference frequency band.

Apart from the high sensitivity in the frequency band corresponding to the reference frequency band, which is needed to meet the horn homologation requirements, the loudspeaker is preferably so designed that it also has a good sensitivity around the fundamental tone of the sound signal from the sound library, for example in the third octave frequency band comprising that fundamental frequency. In other frequency ranges (and thus in <NUM>/<NUM> octave frequency bands corresponding thereto) a lower sensitivity is acceptable, as only the requirements for acoustic vehicle alerting need to be met, which are less strict than for a horn. For example, in the frequency range from <NUM> to <NUM> outside the <NUM>/<NUM> octave frequency band corresponding to the reference frequency band and possibly the frequency band corresponding to the fundamental tone, or in the corresponding <NUM>/<NUM> octave frequency bands, a 85dB/W sensitivity may be sufficient for the loudspeaker frequency response.

A loudspeaker has a membrane, also often called diaphragm, having a front surface facing in a forward direction for producing sound to be radiated outwardly from the loudspeaker in the forward direction and a back surface facing in a backward direction. Both the forward and backward directions extend along the longitudinal axis of the loudspeaker. The diaphragm is suspended from a frame of the loudspeaker by a rim which extends continuously around the outer edge of the membrane.

It was already mentioned above that the envisaged frequency responses can be obtained for example by exploiting acoustic and/or mechanic resonance effects. The basic principle used is based on the well-known mass-spring interaction. Resonance occurs when the frequency of the applied oscillating force is equal or close to a natural frequency of the system. The system then oscillates at a higher amplitude than when the same force is applied at other, non-resonant frequencies. The natural frequency of a mass-spring system is defined by following relation : <MAT> wherein F<NUM> denotes the natural frequency [Hz], K the spring stiffness [N/m] and M the mass [kg]. K can be a mechanical spring or an enclosed volume of air. M can be a mechanical mass or a mass of air acting on an enclosed volume of air (Helmholtz resonator). <MAT> which leads to <MAT> where (see also <FIG>) V denotes an enclosed volume of air [m<NUM>], S the surface area of opening [m<NUM>], L the length of opening [m], ρ<NUM> the density of air [kg/m<NUM>]. Other acoustic principles can be used to increase the SPL at a particular frequency, e.g. making use of standing waves in tubes, impedance matching between loudspeaker and air by using a horn or waveguide,. Some possible ways to implement the loudspeaker system are now presented.

A first embodiment of a loudspeaker system is illustrated in <FIG>. The loudspeaker depicted in <FIG> comprises a front resonator implemented as a cavity with an opening in front of the diaphragm. The opening can act as a mass and the air volume between the cavity and the diaphragm as a spring. Together they can be tuned as needed to meet the SPL requirements in the appropriate frequency band for the horn sound signal being considered. The loudspeaker box volume acts as a spring for the moving mass of the loudspeaker, which is formed by the diaphragm, coil and a part of suspension. Here a high quality-factor (low losses) of the resonant system enables an increased SPL at resonance frequency.

In <FIG> the resulting frequency response is shown. The highest peak results from the tuned SPL peak of the front resonator. A less pronounced SPL peak is obtained from the tuned resonance frequency of the loudspeaker box. This SPL peak can be increased by lowering the damping (by increasing the quality factor Q).

In some embodiments a phase plug can be used to optimize the tuning of the peak frequencies. In further embodiments also other acoustic attributes (tube, horn, etc.. ) can be used to maximize the SPL in the frequency band of interest.

Another loudspeaker embodiment is illustrated in <FIG>. In this embodiment a pressure chamber is provided between the horn mouth and the loudspeaker diaphragm. In this way the impedance of the horn outlet can be matched with the impedance of the air and thus a better efficiency can be obtained in a selected frequency range, dependent on the size and shape of the horn design. A passive diaphragm is provided at the backside of the loudspeaker box. A possible resulting frequency response shape is also depicted in <FIG>.

Yet another illustration is provided in <FIG>. In this embodiment the loudspeaker comprises a bandpass enclosure with two vent outlets facing forward. The small volume and small mass of air on the front of the loudspeaker result in a higher tuning frequency as compared to the lower tuning frequency due to the larger volume of air and higher mass of air acting on it; this can be seen in the resulting SPL frequency response, which is also shown in <FIG>.

Also more complex implementations can be realised. <FIG> depicts a loudspeaker with a sixth order bandpass enclosure. A robust implementation is obtained in this embodiment thanks to the single outlet and the loudspeaker unit well embedded inside the loudspeaker box. With this setup a frequency response can be obtained with more peaks than in the previously discussed embodiments.

For the skilled in the art it will be apparent that any combination of these known techniques can be utilized to obtain a desired SPL level at a selected frequency.

A resulting <NUM>/<NUM> octave frequency band averaged response of the loudspeaker system is illustrated in <FIG>.

<FIG> provides an overview of a possible process to obtain a sound signal for the sound library. The procedure illustrated in <FIG> uses a recording of a time signal of an existing horn in operation measured at evaluation distance as prescribed. The recorded signal is taken as input signal and from this input signal is derived a signal one would like to use as horn signal and store in the sound library. The latter signal is illustrated in step A. This is the time domain signal one wants to be reproduced for the listener. In step B the signal is converted into a high resolution spectrum Fast Fourier Transform. It can be seen from the spectrum that a lot of harmonics are present in the recorded horn signal. Dominant spectral lines are extracted by finding local maxima in the spectrum. This results in step C in a table with dominant spectral lines and their respective amplitudes. From this table one or more spectral lines are extracted that lie in <NUM>/<NUM>rd octave frequency bands that are relevant for the device homologation, e.g. bands in the neighbourhood of <NUM>, <NUM> and <NUM>. The extracted spectral lines may already have a relatively high amplitude value compared to the average, but that is no requirement. In the example shown in <FIG> only one spectral line is selected, namely at <NUM>. The value(s) of the selected frequency or frequencies is/are increased in a synthesis table, shown as step D. The amount with which the value is increased may depend on various factors. The resulting peak has to be high enough to meet the requirement that the average amplitude level is exceeded by a certain number of dB. Further also the design of the loudspeaker has to be taken into account. It is an advantage to use only one or a few selected spectral lines in that so the tonal characteristics of the signal is not much affected. Other values in the synthesis table can be modified based on the capability and frequency response curve of the loudspeaker intended for the signal playback. For example, the signal can be band limited to exclude a part of the spectrum that does not result in significant loudspeaker output. From the synthesis table a synthesized stimulus signal is generated by using the frequencies and amplitudes of the synthesis table, whereby the phase for each individual spectral component is randomized. The resulting additive signal synthesis spectrum is illustrated in step E. Note that the lowest frequencies (below ± <NUM> in this example) are absent. The synthesized signal has at least one dominant signal line in the relevant frequency band for device homologation. For the rest the tonal characteristics of the input signal from which was started, are not substantially altered. The corresponding time domain signal is shown in part F of <FIG>. The synthesized signal (represented either in the time domain or in the frequency domain) resulting from the synthesis table is stored in a sound library. As illustrated in the last part of <FIG> the stored sound signal can be used for later loudspeaker playback. In part G of <FIG> a graph of the <NUM>/<NUM> octave frequency band spectrum of the output signal is shown. The output spectrum closely resembles the reference signal. The peak in the relevant frequency band makes it loud enough to pass the device homologation.

Alternative ways to create a sound signal for the sound library are available. For example, a manufacturer may artificially produce a sound signal wherein the power is spread over the various frequency bands in such a way that a peak is present at a convenient position in the spectrum.

In embodiments of the invention the sound library comprises one or more sound signals that can be used as horn signal. For each sound signal of the library a suitable loudspeaker frequency response. One sound signal may differ from another sound signal stored in the sound library for example because one starts from another reference signal, or because another selection of spectral lines is used to build a synthesis table as in <FIG>.

Obviously, the loudspeaker system is so designed (for example according to one of the embodiments illustrated in <FIG>) that it allows producing all sound signals stored in the sound library in such a way that they are recognizable as a horn signal.

Claim 1:
An audible vehicle warning system arranged for performing AVAS functionality and for generating a horn signal and comprising
- a loudspeaker system comprising a loudspeaker configured to output sound signals at least in a selected frequency range comprising at least frequencies from <NUM> to <NUM>,
characterized by further comprising :
- a sound library comprising one or more sound signals intended as a horn signal, when output by said loudspeaker, each of said sound signals having a <NUM>/<NUM> octave frequency band which is taken as a reference frequency band, said reference frequency band containing at least one third of the total power of said sound signal,
and wherein a <NUM>/<NUM> octave frequency band averaged response of said loudspeaker system has in a <NUM>/<NUM> octave frequency band corresponding to said reference frequency band of the respective sound signal an amplitude level which is at least <NUM> dB above the average amplitude level in a frequency range from <NUM> to <NUM>.