D. Guicking Patents on Active Control of Sound and Vibration { an Overview { Second revised and enlarged edition May 2009 Contact: Dr. Dieter Guicking Schl¨ozerweg6 37085 G¨ottingen Germany Tel.: +49-551-4 61 06 E-Mail: Dieter.Guicking(at)phys.uni-goettingen.de Homepage: http://www.guicking.de/dieter Emeritus member of: Drittes Physikalisches Institut Universit¨atG¨ottingen Friedrich-Hund-Platz 1 37077 G¨ottingen Germany ⃝c Dr. Dieter Guicking 2009 PREFACE Patent titles and patent abstracts often are not really informative. During my work on a patent bibliography it appeared to me therefore reasonable to write this text. Based on about 2060 patent families out of which about 1740 have been selected (removing redundancies and less important ones), this text gives an overview of old and new patents on Active Noise and Vibration Control (ANVC), including related fields such as algorithms, sound design, active flow control, transducers, etc. The patents are grouped in 17 sections which are subdivided into subsections if appropriate. Some emphasis is laid upon patents on algorithms (Section 2). The References in Section 18 are given in short form; concerning the citation style, see [1]. This text is the second revised and enlarged edition of the first version, published in January 2001 [2, 3]. Although I have done my best to deliver a reliable and useful document, I cannot take any re- sponsibility for the correctness and completeness of the data given. Gottingen,¨ April 2009 Dieter Guicking 3 CONTENTS 1. Introduction . 5 2. Algorithms. 5 3. ANC in Ducts and Mufflers . 14 3.1. Ducts . 14 3.2. Mufflers. 15 4. ANC in Interior Spaces . 17 4.1. Room Acoustics . 17 4.2. Local Control in Rooms . 18 4.3. Audio Reproduction in Rooms . 19 4.4. Propeller Aircraft and Helicopter Cabins . 20 4.5. Jet Aircraft Cabins . 21 4.6. In-Vehicle Sound Field Control . 21 4.7. Miscellaneous . 24 5. ANC in Communication (except Headsets) . 25 5.1. ANC in Radio Links . 25 5.2. ANC in Cable Links . 25 5.3. Acoustic Echo Cancellation in Communication Links . 26 6. ANC for Personal Noise Protection . 30 6.1. Hearing Protectors and Earplugs with ANC . 30 6.2. Communication Headsets with ANC . 31 6.3. Hearing Aids with ANC. 32 7. ANC for Domestic Appliances . 32 8. ANC for Exterior Noise . 33 8.1. ANC for Propeller Aircraft and Helicopter Noise . 33 8.2. ANC for Jet Aircraft, Gas Turbine and Fan Noise . 33 8.3. ANC for Road Traffic Noise . 34 8.4. Miscellaneous . 34 9. Sound Radiation and Transmission Control, ASAC . 35 10. Other ANC Applications . 36 11. Active Flow Control . 37 12. AVC for Beams, Plates and Structures . 38 13. AVC for Buildings . 40 14. AVC for Rotating Machinery . 40 15. Active Vibration Isolation . 42 16. Various AVC Applications. 44 17. Transducers . 47 18. References . 51 4 1. INTRODUCTION Unlike most other fields in physics, noise and vibration control is a subject where many re- searchers apply for patents before publishing their results in the open literature. This is even true for fundamental ideas. Early patents on basic principles of active noise control are briefly summa- rized in the following paragraph. A fundamental concept in both active noise control (ANC) and active vibration control (AVC) is feedback. As a seminal patent on negative feedback systems is considered [4]. The first patents on the idea of active noise control by interference with an anti phase sound wave have been granted to Henri Coanda [5, 6], although his arguments are acoustically incorrect. Widely known are Paul Lueg’s German and US patents [7] where the basic idea of active cancellation by superposition with a phase-inverted replica is correctly described for one- and three-dimensional sound fields; the pro- posed experimental set-ups were, however, far from being applicable to practical situations. First experimental demonstrations were given by H. F. Olson in the 50s, based on two patents [8, 9]: the first one describes a loudspeaker with a microphone just in front, with feedback control to serve as a local sound absorber, the second one is related to active vibration isolation. Various fields of possible applications are listed in both patents. Two further early patents are those granted to M. Brute de Remur on the electroacoustic cancellation of a sound wave passing through an orifice in a baffle [10], and to M. Jessel on the cancellation of sound waves radiated from a given source distribution by an array of Huygens sources (monopoles plus dipoles) distributed along a closed shell surrounding the primary sources [11], the now so-called JMC theory (after Jessel, Mangiante and Canevet).´ A more recent patent applying this method is [12]. A simple feedback system for (local) free-field cancella- tion of machine noise is disclosed in [13], employing a conventional loudspeaker or a “stentorphone” where the sound is produced by a modulated air flow. Three patents have been applied for by the German acoustician O. Bschorr in 1969: one describing the basic principle and listing a variety of unconventional sound sources [14] (see also Section 17), the next one applying to noise shielding by a grid of anti sources [15], and the third one to reduction of sound radiation from vibrating panels by distributed or point force input [16], a technology which has later been termed ASAC (an acronym for Active Structural Acoustic Control, see Section 9). All these patents have in common that they have never been exploited commercially. 2. ALGORITHMS Practical applications of active noise control (ANC) and also of many aspects of active vibra- tion control (AVC) became possible only with modern electronics, in particular adaptive digital sig- nal processing. The fact that algorithms are essentially mathematics, and mathematics cannot be patented, can be circumvented by describing the algorithms as block diagrams so that the patents are granted for an assembly of signal processing units. Algorithm patents in this sense will be summa- rized below in chronological order of first submission. Before 1970: An early application of active sound cancellation is the “Apparent sound translator” of Atal and Schroeder [17] which applies head-related crosstalk cancellation to a stereo set-up with two loudspeakers. Provided that the left loudspeaker is fed by a signal recorded with the left channel of a stereo microphone, the sound from this left loudspeaker should only reach the left ear of the listener. The component yet arriving at the right ear is cancelled by a compensation signal superimposed to the right loudspeaker, and vice versa. The compensation signals are obtained from the loudspeaker input signals by filtering with transfer functions derived from the transfer functions from the loud- speakers to the ears (head-related transfer functions, HRTF). More patents based on this method are discussed in Section 4.3. A method of howling prevention in public address systems is disclosed in 5 [18]: the microphone signal is frequency-shifted by a few Hertz so that positive acoustic feedback is prevented; furthermore, a suppressor is introduced eliminating weak spectral components which are supposed to result from multiply reflected and thereby frequency-shifted reverberant sound. This acoustic feedback stabilization by frequency shift has been re-invented repeatedly [19, 20, 21]. A method of speeding up the convergence of an adaptive line echo canceler is disclosed in [22]. 1970–1980: Automatic amplifier gain control is often necessary to obtain a good signal-to-noise ratio, for overload prevention, and to ensure stability. An early patent on this problem is [23], offering a digital feedback controller which provides dynamic level compression of time-varying signals es- sentially without nonlinear distortion. An important concept in many fields of ANC is adaptive noise canceling which became widely known since 1975 by B. Widrow et al.’s seminal Journal paper [24]: A “primary” sensor picks up a desired signal which is corrupted by additive noise, its output being sp, and one or more “reference” sensors are placed such that their output sr is correlated (in some unknown way) with the primary noise, but does essentially not contain the desired signal. Then, sr is adaptively filtered and subtracted from sp to obtain a signal estimate with improved signal-to-noise ratio (SNR). This concept, realized by a linear predictive filter employing the least mean squares (LMS) algorithm, has been patented in [25] and since then has proved to be a valuable tool for many fields. A nonlinear signal processing strategy of noise reduction from two essentially equal signals with uncoherent noisy distortion (such as in the cocktail-party effect) is outlined in [26]. The task of mea- suring transfer functions of time-varying systems can be solved by pseudo-random time series as test signals [27]. G. B. B. Chaplin (England) and his coworkers applied for many patents on ANC, starting with a description of non-adaptive and adaptive feedforward systems for active fan noise cancellation in ducts, including feedback cancellation, directional microphones and loudspeaker arrays, and ad- justable time-delay units in analog electronics [28]. While in this patent no specific adaptation algo- rithm is presented, another patent followed on the quasi-adaptive technology of waveform synthesis, applicable to essentially repetitive noise and vibration; a trigger signal is required to provide the fun- damental period, and the adaptation is performed by a trial-and-error method in the time domain, eventually minimizing an error signal [29]. Two more patents followed on feedforward control with fixed filters, including feedback cancellation [30, 31]. The application of an adaptive filter to another aspect of signal processing, the improvement of the signal-to-noise ratio for narrowband spectra in braodband noise by the LMS algorithm, is the subject of a patent on Widrow’s Adaptive Line Enhancer, which eliminates uncorrelated signal portions [32].