WO2013064602A1

WO2013064602A1 - Adaptive helmholtz resonator

Info

Publication number: WO2013064602A1
Application number: PCT/EP2012/071665
Authority: WO
Inventors: Stanislaw PIETRZKO; Qibo MAO
Original assignee: Empa Eidgenössische Materialprüfungs- Und Forschungsanstalt
Priority date: 2011-11-04
Filing date: 2012-11-01
Publication date: 2013-05-10

Abstract

A semi-active adaptive Helmholtz resonator (1) with a resonator cavity (11), comprising an inner microphone (14) and a loudspeaker (13) positioned inside the cavity (11) and connected in an inner control loop (15) to a controller (18) with P- and I-gain parts, whereas the loudspeaker (13) driven by the controller (18) generates according to analyzed signals of the inner microphone (14) sound pressures within the resonator cavity (11) with variable adapted frequencies (fa) in a frequency band around the natural frequency (fn) of the Helmholtz resonator (1), adapted to reach maximum damping values and therewith maximum noise absorption of an acoustic disturbance (3) with a frequency of disturbance (fs), should be improved in order to reach a separate adjustment of adapted frequency (fa) and adapted damping (da) values. This will be achieved by using loudspeaker (13) with certain relation between resonance frequency (fL) of the loudspeaker and using an additional outer control loop (16) for estimation of difference frequency and damping values inside and outside the cavity (11) in real time.

Description

Adaptive Helmholtz resonator

TECHNICAL FIELD

The present invention describes a semi-active adaptive Helmholtz resonator with a resonator cavity, comprising an inner microphone and a loudspeaker positioned inside the cavity and connected in an inner control loop to a controller with P- and I-gain parts, whereas the loudspeaker driven by the controller generates according to analyzed signals of the inner microphone sound pressures within the resonator cavity with variable adapted frequencies in a frequency band around the natural frequency of the Helmholtz resonator, adapted to reach maximum damping values and therewith maximum noise absorption of an acoustic disturbance with a frequency of disturbance. The present invention also describes a method of absorbing a acoustic disturbance comprising a frequency of the disturbance with an adaptive Helmholtz resonator.

STATE OF THE ART

The well known Helmholtz Resonators noise control mechanism is based on adding the acoustical damping to the target acoustic mode at some disturbing frequency. The performance of a Helmholtz Resonator is closely related to its natural frequency established by the volume of the resonator body, the resonator cavity, and neck dimensions and orifice and damping ratio. Searching for and tuning the optimal values of the damping and natural frequency of the Helmholtz Resonator to the particular small band of disturbing noise is essential for successful application of a Helmholtz Resonator for noise reduction. These two parameters guarantee the coupling of a Helmholtz Resonator to the disturbing acoustical modes as well as the reduction of this disturbance.

Once a passive Helmholtz Resonator is manufactured, the natural frequency and damping ratio are fixed, and the noise control performance is only applicable to a single frequency or within a narrow band around it. A conventional fixed geometric Helmholtz Resonator is very sensitive to tuning errors and variations in the disturbing frequency and therefore unfavorable. If the natural frequency of the Helmholtz Resonator can be adaptively tuned to the changing disturbing frequency, the Helmholtz Resonator will work for a wide range of frequencies and thus remove this kind of disturbing acoustical mode as well . An adaptive Helmholtz Resonator is essentially a passive device whose parameters, natural frequency and damping ratio, can be tuned in real time by using an electronic control system. It should be noted that a control system is used only to tune the parameters of Helmholtz Resonator, not to control the acoustic noise directly. The control energy does not input directly into the system being controlled. Thus, it never increases the system's overall energy as opposed to active noise and vibration control technology.

A possible way to tune the resonance frequency of a Helmholtz Resonator is to adjust the resonator cavity volume. However, the maximum cavity volume of a Helmholtz Resonator is reserved for the lowest natural frequency. The tuning mechanism used to change the resonator volume must occupy additional cavity volume and undermine the resonator efficiency. Such types of adaptive Helmholtz Resonator are also quite bulky, as can be seen in M . Bedout et al ., "Adaptive-passive noise control with self-tuning resonators", J . Sound Vib. 202 (1997) 109-123. According to S. J. Esteve et al ., "Adaptive Helmholtz resonators and passive vibration absorbers for cylinder interior noise control", J. Sound Vib. 288 (2005) 1105-1130, another method of adaptive Helmholtz Resonator is to adjust the resonator neck length or opening. According to experimental results, the damping ratio of the Helmholtz Resonator varies significantly and non-linearly with increasing neck length. If the neck opening is varied with a wire mesh screen placed over the opening, the damping ratio can be maintained relatively constant around a 5% value.

EP0586831 discloses an adaptive Helmholtz resonator comprising a fixed resonator cavity and a neck with additional electroacoustic features. With an external microphone noise to be absorbed outside the Helmholtz resonator cavity is sensored . The signal of the microphone is fed to a transmission system which controls the output signal of at least one loudspeaker, which is integrated in the resonator cavity. With the controller a tunable sound signal can be generated leading to a tunable resonance frequency of the Helmholtz resonator, whereas different resonator cavities can be simulated. The transmission system shows either a PDT transmission behaviour or a transmission behaviour of an additive or multiplicative amplification in case of using a second passive Helmholtz resonator. According the EP0586831 different resonance frequencies are reachable, but it is not disclosed if a separate control of resonance frequency and damping values is possible.

Birdsong et al ., "An electronically tunable resonator for noise control", SEA paper No. 2001-01-1615, proposed a semi-active Helmholtz resonator to compensate for the acoustical disturbance in a pipe. This approach permits the simultaneous tuning of natural frequency of the resonator and the damping value. As proposed by Birdsong et al . the semi-active Helmholtz resonator is able to simultaneously change its natural frequency and damping value by means of a supporting feedback control loop.

In this case the loudspeakers used must be specially conditioned as a volume velocity source by a separate closed feedback control loop. Beside the more complex electronic setting with an additional compensation device compensating the dynamics of the speaker, the compensation is introducing additional noise into the speaker's output. According to the document of Birdsong et al . this system is not able to separate the tuning of the natural frequency from the damping value, thus considerably limiting the practical application fields.

DESCRIPTION OF THE INVENTION

The object of the present invention is to create an adaptive Helmholtz resonator which allows the separate tuning of the resonator natural frequency and the resonator damping value, whereat no special and expensive hardware is necessary and which is resulting in improved disturbance absorption.

The invention presented provides significant improvements to remove the weaknesses of the prior art. According to the invention adaptive Helmholtz resonators with comparable setup but improved absorption results and simplified operation are obtainable.

This invention provides an adaptive Helmholtz resonator with tunable natural frequency to the frequency of the disturbing noise, whereat also the damping value can be adjusted separately in response to the disturbing sound wave. It is an essential property of this device that the tuning of the natural resonator frequency and resonator damping value is established by proportional and integral gain parameters of the feedback controller, which is a part of this Helmholtz resonator in response to the signal of the inner microphone.

Beside the PI controller the electrical characteristics of the loudspeaker are adapted to a specified frequency of the disturbance.

Due to the concept of the presented Helmholtz resonator with only one adaptive system control loop the Helmholtz resonator is tuned to the frequency and damping of the disturbance and the same adaptive loop is simultaneously used for the performance evaluation of the tuned Helmholtz resonator and its coupling to the surrounding acoustical field . Therefore a second microphone has to be used which is sensing the disturbance field outside of the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention is described below in conjunction with the attached drawings.

Figure 1 shows a schematic view of one embodiment of an adaptive Helmholtz resonator. DESCRIPTION

The objective of this invention is a high-performance, low-cost semi- active adaptive Helmholtz resonator 1 for semi-active noise control, noise or generally noise modification of an acoustic disturbance 3. The acoustic disturbance 3 is characterized by a resonance frequency fs and a damping ds.

In Figure 1 the Helmholtz resonator 1, comprising a resonator body 10 with a resonator cavity 11 which ends in a neck 12 as known in the field of classical Helmholtz resonator 1 is shown. Possible forms of the resonator body 10 are cylindrical or conical shaped resonator bodies 10, but according to the later use of the Helmholtz resonator 1 also other forms are possible. The resonator body 10 respectively the resonator cavity 11 only has one opening in the neck 12. The volume of the resonator cavity 11 and the design of the neck 12 regarding length and/or cross section of the neck 12 are leading to a fixed natural frequency fn of the Helmholtz resonator 1. The Helmholtz resonator 1 is absorbing sound energy possessing the natural frequency fn of the Helmholtz resonator 1 in an optimum way.

The Helmholtz resonator 1 is further comprising electroacoustic features, namely at least one electroacoustic transducer or actuator 13, in the form of a loudspeaker 13, which is mounted inside the resonator body 10 respectively inside the resonator cavity 11 facing to the neck 12. Instead of positioning the loudspeaker 13 spaced from the end wall of the resonator body 10 as shown in the figures, it can also build the end wall of the resonator body 10 opposing the neck 12. The loudspeaker 13 used in this Helmholtz resonator 1 as proposed is selected from loudspeakers 13 comprising resonance frequencies fL which are much greater than the frequency fs of the disturbance. For optimum results the loudspeaker resonance frequency fL is four times greater than the disturbance frequency fs. The loudspeaker 13 shows a known sound characteristic and is driven by an analogue or digital controller 18. The controller 18 is connected to an at least one inner microphone 14 and the loudspeaker 13 inside the cavity building an inner control loop 15. Electrical audio signals from the controller 18 are converted in acoustic signals by the loudspeaker 13 and fed in the resonator cavity 11. The used favoured controller 18 has proportional and integral gain parts or is a PI- controller 18 normally accompanied by amplification electronics. The at least one inner microphone 14 is mounted inside the resonator cavity 11. The inner microphone 14 is disposed at a distance from the walls of the resonator body 10, spaced from the loudspeaker 13 and from the neck 12 in a free-standing position. There are other positions of the inner microphone 14 possible inside the cavity 11, where the inner microphone 14 can be glued or welded.

Via the inner control loop 15 sound signals from inside the cavity 11 sensored by the inner microphone 14 defined by an inner frequency fi and an inner damping di can be measured .

The Pi-controller 18 is part of the inner control loop 15 and is analyzing the input signal of the inner microphone 14, processing and feeding an output signal to the loudspeaker 13 with an adapted frequency fa and an adapted damping da.

In an outer control loop 16 an outer microphone 17 is connected via an adaptation unit 19 and the Pi-controller 18 with the actuator 13. Due to the positioning directly in the acoustic disturbance 3 the frequency fs and the damping ds of the acoustic disturbance 3 can be determined directly with the outer microphone 17 by the adaptation unit 19. By the outer control loop 16 the actual disturbance frequency fs and disturbance damping ds are fed in the Pi-controller 18. By using the electroacoustic features 13, 14, 18, 17 and 19 different Helmholtz resonators with different acoustic characteristics can be simulated and different natural frequencies fn can be reproduced by a signal of the loudspeaker 13 with a adapted frequency fa. The aim is the maximum absorption of the acoustic pressure of the acoustic disturbance 3. The natural frequency fn can be manipulated by the electroacoustic features in a frequency band resulting in different adapted frequencies fa. After estimation of the disturbance frequency fs and the inner frequency fi a difference frequency Af=fs-fi can be calculated which is used to tune the P-gain of the Pl-controller 18.

After estimation of the damping values of disturbance ds and the inner damping di, a difference damping Ad=ds-di is calculated leading to the tuning of I-gain of the Pl-controller 18.

The difference frequency Af and the resulting optimum P-gain as well as the difference damping Ad and the resulting optimum I-gain are calculated and estimated by an algorithm leading to P-gain and I-gain values which are tuned online in real time in order to reach maximal disturbance reduction of the acoustic disturbance 3.

The tuning of the Helmholtz resonator 1 natural frequency fn by generating the sound signal with the adapted frequency fa and a certain amplitude, which leads to a certain adapted damping da are fully decoupled and can be realised separately by changes of the proportional and integral values of the Pl-controller 18. The tuning of the resonator natural frequency fn by generation of the loudspeaker signal with adapted frequency fa and adapted damping da, can be accomplished over a wide range of frequencies both up and down the frequency scale around the natural frequency fn. The necessary condition for separate tuning of the natural frequency fn and damping value requires the above mentioned relation between the resonance frequency fL of the loudspeaker used and the natural frequency of the disturbance fs. The sensored values of the outer microphone 17 and the inner microphone 14 are used to optimize the P- and I-parameters of the controller 18, where this tuning is accomplished in real time. Due to the used loudspeaker 13 with a resonance frequency fL which is about four times greater than the frequency of the disturbance fs, it is possible to control the adapted frequency fa with the P-gain independently from the adapted damping da, which is controllable with the I-gain of the Pi-controller 18.

When the adaptive Helmholtz-resonator is in service, the difference frequency Af and the difference damping Ad are analyzed online in real time and appropriate P-gains and I-gains are identified.

For reaching more efficient controlling of the P- and I-gains by directly sensoring the actual inner frequency fi and inner damping di, an optional performance loop 20 between the inner microphone 14 and the adaptation unit 19 can be established. The performance of the adapted Helmholtz resonator 1 is therewith directly measureable and analyzeable. This kind of adaptive Helmholtz resonator 1 has a wide range of commercial applications whenever one has to reduce an acoustical disturbance 3 at one or many tonal frequencies or in a narrow frequency band. Removing of disturbing frequencies can also be a valuable contribution in the design of a sound field to guarantee some required quality. Sound quality has been found to be the second most important factor related to improvement of customer satisfaction and product acceptance. This process is called 'engineered sound quality' and provides specific design goals for the sound of a product in terms that product planners can specify and engineers meet. Engineered sound quality has been used in setting design goals for several appliance products. Possible applications are in ventilation ducts, heating systems, all kinds of mufflers as used in automobile exhaust systems or combined with a device through which the exhaust gases from an internal-combustion engine are passed to attenuate or reduce the airborne engine noise. Another possible application is to damp internal acoustical modes in cabins of cars or other kinds of vehicles. Wide range of applications is possible in architectural acoustics (room acoustics) to damp low frequency room modes and adapts a room to speech quality requirements.

Instead of using a loudspeaker 13 directly with an optimum resonance frequency fl_ it is also possible to use electronic means for active tuning of the resonance of a chosen loudspeaker 13. Using the electronic means loudspeaker 13 are adaptable to existing Helmholtz resonators 1. Also in cases when the frequency of disturbance fs is changing, one can react by simply tune the loudspeaker 13 resonance frequency fl_ flexible to the desired changed value.

The signals of the outer microphone 17 can be additionally used in the Pi-controller 18 to evaluate the performance of the semi-active adaptive Helmholtz resonator 1. Possible embodiments of the Pi-controller 18 is an analog circuitry or a digital signal processor (DSP) with analogue digital converter respectively digital analogue converter if necessary. It is preferred to use some amplification electronics to generate the adapted sound signals.

For estimation of sound characteristics by the microphones 14, 17 leading to frequency and damping values fs, fi and ds and di, methods known in the art as for example Fast Fourier Transformations can be used.

LIST OF REFERENCE NUMERALS

1 Helmholtz resonator

10 resonator body

11 resonator cavity

12 neck

13 actuator/loudspeaker

14 inner microphone

fn natural frequency

fi inner frequency

di inner damping

fL resonance frequency of loudspeaker

15 inner control loop

16 outer control loop

17 outer microphone

18 analogue or digital controller / Pl-controller fa adapted frequency

da adapted damping

19 adaptation unit

20 performance loop

3 acoustic disturbance

fs frequency of disturbance ds disturbance damping

Claims

PATENT CLAIMS

1. Semi-active adaptive Helmholtz resonator (1) with a resonator cavity (11), comprising an inner microphone (14) and a loudspeaker (13) positioned inside the cavity (11) and connected in an inner control loop (15) to a controller (18) with P- and I-gain parts,

whereas the loudspeaker (13) driven by the controller (18) generates according to analyzed signals of the inner microphone

(14) sound pressures within the resonator cavity (11) with variable adapted frequencies (fa) in a frequency band around the natural frequency (fn) of the Helmholtz resonator (1), adapted to reach maximum damping values and therewith maximum noise absorption of an acoustic disturbance (3) with a frequency of disturbance (fs),

characterized in that,

an outer control loop (16) is applied, comprising an outer microphone (17) leading sound characteristics of the acoustic disturbance (3) via an adaptation unit (19) in real time into the controller (18),

whereby the sound signal with adapted frequency (fa) and adapted damping (da) is generated by the loudspeaker (13), which resonance frequency (fl_) is at least approximately four times greater than the frequency of the disturbance (fs), leading to a fully decoupled adjustment of the proportional and integral gains of the Pi-controller (18).

2. Semi-active adaptive Helmholtz resonator (1) according to claim 1, whereas a performance loop (20) between the inner microphone (14) and the adaptation unit (19) is established.

3. Semi-active adaptive Helmholtz resonator (1) according to one of the preceding claims, whereas the relation between the resonance frequency (fl_) of the loudspeaker (13) and the frequency of disturbance (fs) is tuned by additional electronic means in order to reach a resonance frequency (fl_) which is at least approximately four times greater than the frequency of the disturbance (fs).

4. Method of absorbing a acoustic disturbance (3) comprising a frequency (fs) of the disturbance with an adaptive Helmholtz resonator (1), characterized by the steps:

- identifying the frequency (fs) of the disturbance and the damping (da) of the disturbance (3) outside of a cavity (11) of the Helmholtz resonator (1) with an outer microphone (17) and a adaptation unit (19)

- identifying the frequency (fi) and the damping (di) inside the cavity (11) with an inner microphone (14)

- leading the measured values to a controller (18) with at least one P-gain and one I-gain part

- estimation of the difference frequency (Af=fs-fi) and a difference damping (Ad = ds-di)

- using the difference frequency (Af ) for adjusting the P-gain of the controller (18)

- using the difference damping (Ad) for adjusting the I-gain of the controller (18),

- generating a sound signal with an adapted frequency (fa) and decoupled adapted damping (da) inside the cavity (11) with a loudspeaker (13) whose resonance frequency (fl_) is at least approximately four times greater than the frequency of disturbance (fs), by fully decoupled adjustment of the

proportional and integral gains of the controller (18).

5. Method of absorbing a acoustic disturbance (3) according to claim 4, whereas the performance of the Helmholtz resonator (1) is analyzed in real time by a performance loop (20), leading the current signals of the inner microphone (14) directly into the adaptation unit (19).

Method of absorbing a acoustic disturbance (3) according to one of the preceding claims, whereas

identifying the frequency (fs) of the disturbance and the damping (da) of the disturbance (3) outside of a cavity (11) of the Helmholtz resonator (1) with an outer microphone (17) and a adaptation unit (19)

and

identifying the frequency (fi) and the damping (di) inside the cavity (11) with an inner microphone (14) is done

simultaneously.