Helmholtz Resonators ![](0logo.gif) [rexresearch.com](../index.htm) --- Herman von HELMHOLTZ Resonator --- --- ![](cav5.gif) ![](cav.gif) ![](cav3.gif) ![](cav4.gif) --- Helmholtz Resonance A brass, spherical Helmholtz resonator based on Helmholtz's original design, from around 1890-1900. ![](hlmhltzrsnatr.jpg) Helmholtz resonance is the phenomenon of air resonance in a cavity. The name comes from a device created in the 1850s by Hermann von Helmholtz to show the height of the various tones. An example of Helmholtz resonance is the sound created when one blows across the top of an empty bottle. Qualitative Explanation When air is forced into a cavity, the pressure inside increases. Once the external force that forces the air into the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the neck, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time. This effect is akin to that of a bungee-jumper bouncing on the end of a bungee rope, or a mass attached to a spring. Air trapped in the chamber acts as a spring. Changes in the dimensions of the chamber adjust the properties of the spring: a larger chamber would make for a weaker spring, and vice-versa. The air in the port (the neck of the chamber) is the mass. Since it is in motion, it possesses some momentum. A longer port would make for a larger mass, and vice-versa. The diameter of the port is related to the mass of air and the volume of the chamber. A port that is too small in area for the chamber volume will "choke" the flow while one that is too large in area for the chamber volume tends to reduce the momentum of the air in the port. Quantitative explanation It can be shown[1] that the resonant angular frequency is given by: \omega\_{H} = \sqrt{\gamma\frac{A^2}{m} \frac{P\_0}{V\_0}} (rad/s) , ![](af98f42993f8cf38cd0767446726a892.png) where: \* ? (gamma) is the adiabatic index or ratio of specific heats. This value is usually 1.4 for air and diatomic gases. \* A is the cross-sectional area of the neck \* m is the mass in the neck \* P0 is the static pressure in the cavity \* V0 is the static volume of the cavity For cylindrical or rectangular necks, we have A = \frac{V\_n}{L} , ![](b3c05f0f6c2ecab620c818a8531e9eb6.png) where: \* L is the length of the neck \* Vn is the volume of air in the neck thus: \omega\_{H} = \sqrt{\gamma\frac{A}{m} \frac{V\_n}{L} \frac{P\_0}{V\_0}} ![](0bf765775301f908ec3416759d552512.png) By the definition of density: \frac{V\_n}{m} = \frac{1}{\rho} , thus: \omega\_{H} = \sqrt{\gamma\frac{P\_0}{\rho} \frac{A}{V\_0 L}}, and f\_H = \frac{\omega\_H}{2\pi} , ![](4f1661aca56daf6c2aa600788ee2ce00.png) where: \* fH is the resonant frequency (Hz) The speed of sound in a gas is given by: v = \sqrt{\gamma\frac{P\_0}{\rho}} , thus, the frequency of the resonance is: f\_{H} = \frac{v}{2\pi}\sqrt{\frac{A}{V\_0L}} ![](828b88b795f98db422137630067ccbff.png) The length of the neck appears in the denominator because the inertia of the air in the neck is proportional to the length. The volume of the cavity appears in the denominator because the spring constant of the air in the cavity is inversely proportional to its volume. The area of the neck matters for two reasons. Increasing the area of the neck increases the inertia of the air proportionately, but also decreases the velocity at which the air rushes in and out. Depending on the exact shape of the hole, the relative thickness of the sheet with respect to the size of the hole and the size of the cavity, this formula can have limitations. More sophisticated formula can still be derived analytically, with similar physical explanations (although some differences matter). See for example the book of F.Mechels [2]. Furthermore, if the mean flow over the resonator is high (Mach number above 0.3 typically), some corrections must be accounted for. Applications Helmholtz resonance finds application in internal combustion engines (see airbox), subwoofers and acoustics. In stringed instruments, such as the guitar and violin, the resonance curve of the instrument has the Helmholtz resonance as one of its peaks, along with other peaks coming from resonances of the vibration of the wood. An ocarina is essentially a Helmholtz resonator where the area of the neck can be easily varied to produce different tones. The West African djembe has a relatively small neck area, giving it a deep bass tone. The djembe may have been used in West African drumming as long as 3,000 years ago, making it much older than our knowledge of the physics involved. Helmholtz resonators are used in architectural acoustics to reduce undesirable low frequency sounds (standing waves etc.) by building a resonator tuned to the problem frequency thereby eliminating it. Helmholtz resonators are also used to build acoustic liners, which aim at reducing the noise of aircraft engines for example. These acoustic liners are made of two components: \* a simple sheet of metal (or another material), perforated with little holes that can be regularly spaced or irregularly spaced, called resistive sheet, \* a series of so-called honeycomb cavities (holes with a honeycomb shape, but in fact only their volume matters). Such acoustic liners are used in most today's aircraft engines. The perforated sheet is usually visible from inside or outside the airplane; the honeycomb is just under it. The thickness of the perforated sheet is of importance, as shown above. Sometimes, there are two layers of liners; they are then called "2-DOF liners", by opposition to the "single DOF liners" (DOF meaning Degree Of Freedom). This effect could also be used to reduce drag on aircraft by 40%.[3] --- --- Patents Novel Helmholtz Resonator CN101435356 Abstract -- The invention relates to a novel Helmholtz resonator which is arranged on an automobile exhaust pipe. The resonator comprises a resonance box and a tuning pipe, wherein the resonance box is spanned on the exhaust pipe which passes through the resonance box, and the tuning pipe is arranged on the exhaust pipe in the box and is positioned wholly inside the resonance box. Compared with the prior art, the resonator has the advantages of low resonance frequency and great noise reduction. The resonator is suitable for the low frequency noise abatement for an automobile exhaust system. --- SILENCER OF INTERNAL COMBUSTION ENGINE JP2009036185 Abstract -- PROBLEM TO BE SOLVED: To provide a silencer of an internal combustion engine capable of suppressing the intake and exhaust noise of a plurality of frequencies by one resonator. ; SOLUTION: In the silencer of the internal combustion engine including a hollow part for facilitating a sound attenuation function by resonance wherein a space communicating with the inside of an intake and exhaust pipe of the internal combustion engine is formed, a volume variable means for enabling a volume in the hollow part to be variable is provided. The volume variable means varies the volume by a throttle opening or engine negative pressure. A resonator switch means for switching a side branch type resonator and a Helmholtz type resonator is provided. ; COPYRIGHT: --- RESONATOR JP2008309050 Abstract -- PROBLEM TO BE SOLVED: To reduce the cost of a resonator by making a main-volume chamber and a sub-volume chamber communicate to an intake air duct via a neck part. ; SOLUTION: The resonator 1 includes a resonator body part 11 provided with the main-volume chamber 12 communicating to the intake air duct 4 via the neck part 10, and a long and narrow sub-volume chamber 13 directly connected to the main-volume chamber 12, and has a function of a Helmholtz type resonator of which resonance frequency is determined according to volume of the resonator body part 11 which is sum of volume of the main-volume chamber 12 and volume of the sub-volume chamber 13, and a function of a side branch type resonator of which resonance frequency is determined according to length along a longitudinal direction of the sub-volume chamber 13 together. Consequently, the resonator 1 can get two resonance frequencies without complicating a structure of the resonator body part 11, and can reduce manufacturing cost in general. --- Infrasonic Helmholtz resonator US6665413 Abstract -- An infrasonic Helmholtz resonator capable of producing clean sinusoidal frequencies in the range of 6-14 Hz is described. --- High intensity infrasonic tunable resonant acoustic test cell US2002085723 Abstract -- An apparatus and method are provided for high intensity acoustic test cells that employ the Helmholtz resonator principle, incorporating at least one moderately-sized volume or test cell that is tunable to a given infrasonic to low-sonic frequency and generates sound of high intensity with very pure sinusoidal waveforms in this test cell or volume, by varying the geometry of a port which is connected to the test volume and open to either atmosphere or a second or input volume and by introducing to the test volume or the input volume a driving acoustic signal at the given tuned frequency. The apparatus and method are used for testing or performing experiments on materials, structures, devices, products, biological entities or humans at high acoustic intensities and frequencies in the low-sonic to infrasonic ranges. --- ACOUSTIC MATERIAL JP2006337886 Abstract of **JP 2006337886 (A)** PROBLEM TO BE SOLVED: To provide an acoustic material which has excellent sound absorbing performance, can be made thinner than before, and is suitably used for, specially, an air-conditioning duct of an automobile. ; SOLUTION: The acoustic material is made of a fiber body or foamed body laminated on a surface of a base material and has a Helmholtz resonator structure comprising a void space 23 recessed in the acoustic material from a laminated surface 21 of the base material and a through hole 24 formed to reach the void space from the opposite surface from the laminated surface. When the volume V1 of the void space portion, the volume V2 of the through hole portion, and the volume V3 of the remaining portion of the acoustic material are so related that V1/(V1+V2+V3)=0.2 to 0.6, the acoustic material has excellent sound absorbing performance and can be made thinner than before. --- BARRIER SYSTEM WO2009108071 Process for the preparation of nanocrystalline hydrotalcite compounds GB2457771 Vibration damper for damping hydraulic vibration of hydraulic oil in pressure line DE102009004705 Pressure sensing device adapted to corrosive or explosive atmospheres and with means to filter disturbance frequencies GB2456865 Resonator device for e.g. non supercharged internal combustion engine FR2926608 INLINE HELMHOLTZ TYPE RESONATOR KR20090047083 Handheld electronic device having hidden sound openings offset from an audio source EP2081400 Hydraulic system for actuation of clutch of motor vehicle DE102008062336 Hydraulic system for motor vehicle DE102008060584 COOLING DEVICE AND ELECTRONIC DEVICE COMPRISING SUCH A COOLING DEVICE US2009168343 LITHOGRAPHIC APPARATUS HAVING ACOUSTIC RESONATOR US2009161085 Microphone for audio source tracking CN101449593 Novel Helmholtz resonator CN101435356 EXHAUST DEVICE FOR STRADDLE-TYPE VEHICLE AND STRADDLE-TYPE VEHICLE US2009139796 MICROREACTOR DEVICE JP2009090265 Noise attenuation device for a centrifugal compressor discharge or suction nozzle EP2063130 MEASUREMENT OF SOUND SPEED OF DOWNHOLE FLUID BY HELMHOLTZ RESONATOR WO2009055197 Method and device for achieving better heat transfer when using pulse heaters US2009084036 SILENCER OF INTERNAL COMBUSTION ENGINE JP2009036185 Supply gas cooling circuit for overheating turbo-compressor of motor vehicle FR2921123 Acoustic liner for gas turbine engine manufactured by an additive fabrication process GB2452476 HANDHELD ELECTRONIC DEVICE HAVING OFFSET SOUND OPENINGS SG149882 MOTORCYCLE AIRBOX WITH HELMHOLTZ RESONATOR JP2009013980 HERMETIC COMPRESSOR US2009022605 RESONATOR JP2008309050 TURBINE ENGINE FUEL INJECTOR WITH HELMHOLTZ RESONATOR WO2008153736 Recess lower cross member for motor vehicle, has Helmholtz resonator FR2917044 Cross head for dashboard of motor vehicle, has acoustic resonator FR2917055 Apparatus for delivery of pressurised gas CN101291703 Novel Helmholtz resonator CN201125766 Earphone with acoustical filter CN201114712 Adjustable helmholtz resonator CN101230818 COMPRESSOR SOUND SUPPRESSION US2008260547 INTAKE NOISE MUFFLING DEVICE AND MUFFLING METHOD JP2008223745 RECORDING APPARATUS JP2008213310 PNEUMATIC TIRE JP2008213596 RESONATOR WITH INTERNAL SUPPLEMENTAL NOISE ATTENUATION DEVICE US2008236937 ACOUSTIC FLOW STRAIGHTENER FOR TURBOJET ENGINE FAN CASING US2008236137 HELMHOLTZ RESONATOR WO2008116870 Porous acoustic material for Helmholtz resonator FR2914479 Flowmeter using tube waves GB2447691 System for Damping Thermo-Acoustic Instability in a Combustor Device for a Gas Turbine US2008216481 PNEUMATIC TIRE EP1964691 PNEUMATIC TIRE WO2008102780 Device for reducing pulsation in a variable displacement compressor US2008199329 Combustion chamber for gas turbine engine EP1962018 THERMAL EXCITATION TYPE SOUND WAVE GENERATOR JP2008167252 Piezo-Hydraulic Linear Actuator For Vibration Control US2008191397 Silencer for an exhaust system EP1959106 PNEUMATIC TIRE JP2008155810 PNEUMATIC TIRE JP2008155798 SPEECH COMMUNICATION DEVICE JP2008135864 Assessing Porous Media Properties by Downhole MRI and Acoustic Logging GB2445085 INTAKE ASSEMBLY HAVING HELMHOLTZ RESONATORS US2008135010 VEHICULAR SOUND-PROOF STRUCTURE JP2008114697 Device for attenuating cavity noise in a tire and wheel assembly EP1932689 ELECTRONIC DEVICE AND SOUND PROOFING METHOD FOR THE SAME JP2008090931 Combustion Control for a Gas Turbine GB2443838 Helmholtz resonator type marine signal US7360499 Combustion chamber for a gas turbine engine US2008087019 Exhaust silencer for automotive vehicles EP1908930 NOISE SUPPRESSOR, ELECTRONIC APPARATUS AND CONTROL METHOD JP2008058389 Vacuum cleaner equipped with a noise-attenuation device EP1905334 Thermoacoustic Device US2008060364 Gas turbine helicopter engine with reduced sound emissions due to acoustic treatment of a jet pump EP1902948 METHOD FOR CLEANING OF SPENT GASES... RU2361094 Resonator device at junction of combustor and combustion chamber US2008041058 A SILENCER, A MARINE VESSEL AND A METHOD OF SILENCING A MARINE EXHAUST WO2008012501 Combustion chamber for gas turbine of power plant system DE102005062284 CENTRAL BODY OF TURBOJET NOZZLE JP2007292064 INSULATION DEVICE FOR STEERING JP2007290432 CENTRAL BODY FOR A TURBOJET EXHAUST CHANNEL, TURBOJET US2007272477 PNEUMATIC TIRE JP2007269144 Handheld electronic device having hidden sound openings offset from an audio source US2007252810 Noise reduction of rotary mowers using an acoustical helmholtz resonator array US2007234699 Sound absorbing device for FR2899537 Flushing system for a vacuum toilet bowl, especially in an aircraft, has a Helmholtz resonator DE102006014011 A GOLF CLUB DRIVER HEAD HAVING HELMHOLTZ RESONATOR KR20070078374 Combustion chamber DE102006007711 A GOLF CLUB DRIVER HEAD HAVING HELMHOLTZ RESONATOR WO2007086676 AIR SUPPLY REGISTER JP2007139223 Acoustic resonator e.g. Helmholtz resonator, for use in motor vehicle FR2895714 Low-noise cabinet type air conditioner CN2893501 Silencer EP1785602 Resonator performance by local reduction of component thickness US2007102235 Heating device e.g. oil-heated heating device, has helmholtz-resonator DE102005052881 Cannula handle and storage system US2007093755 SPEAKER SYSTEM JP2007060099 Resonating device for combustion chamber EP1775515 CANNULA HANDLE AND STORAGE SYSTEM WO2007038433 Motor vehicle engine with Exhaust Gas Recirculation ... Helmholtz resonator FR2891015 MUFFLER FOR ELECTRONIC EQUIPMENT... JP2007047560 Exhaust system with external Helmholtz resonator and associated method US2007051556 Fuel injection device for internal combustion engine of motor vehicle... FR2890415 FLUID VOLUME DETECTING DEVICE JP2007017320 INTAKE DEVICE JP2007016652 Handheld electronic device having offset sound openings US2007025582 Silencer for use in exhaust system of motor vehicle, has helmholtz chamber and resonator pipe... DE102005033421 Damping device for damping frequency-variable pulsations in a hydraulic pipe has branch resonator... DE102005032319 ACOUSTIC TELEMETER SYSTEM... RU2291961 MUFFLER REDUCING HIGH FREQUENCY NOISE JP2006283625 DEVICE FOR REDUCING NOISE OF A RECIPROCATING COMPRESSOR... KR20050018155 DISK DRIVE ADOPTING A RESONATOR KR20050012042 STRUCTURE INSTALLED IN CAVITY BETWEEN HEAD LINING AND ROOF PANEL... KR20050008931 LOUDSPEAKER SYSTEM COMPRISING A HELMHOLTZ RESONATOR... KR0158885 HELMHOLTZ RESONATOR AIR INLET SYSTEM FOR VEHICLE KR20060001216 NOISE REDUCTION APPARATUS... USING HELMHOLTZ RESONATOR KR20010037807 RESONATOR OF ENGINE INDUCTION SYSTEM KR20010028178 BAMBOO SOUND-PROOF WALL KR20010011073 COMBINED AIR CLEANING AND RESONATING APPARATUS KR20000001532 COMPRESSOR KR100273658 NOISE-DECREASING DEVICE FOR INTAKE MANIFOLD OF A CAR KR960008782 IMAGE FORMING APPARATUS JP2006251590 RIM WHEEL AND MANUFACTURING METHOD THEREOF JP2006231966 DEVICE FOR MEASURING CHANGING AMOUNT OF VOLUME KR20020058784 APPARATUS FOR REDUCING INTAKE NOISE OF VEHICLE USING HELMHOLTZ RESONATOR KR20020038217 MUFFLER FOR VEHICLE KR20020030352 Silencer containing one or more porous bodies US7104358 Loudspeaker which can be used with or without stand which acts as a helmholtz resonator GB2422066 PIPE AND ITS MANUFACTURING METHOD JP2006144557 Combustion chamber for a gas turbine engine US2006123791 Exhaust gas muffler DE102004056889 SOUND ELIMINATING STRUCTURE AND STIRLING COOLING BOX JP2006105514 HELMHOLTZ RESONATOR AND AN EXHAUST LINE PROVIDED THEREWITH WO2006048557 Noise insulating partition for an internal volume, of an automotive vehicle, in particular. EP1657115 Devices and transducers with cavity resonator to control 3-D characteristics/harmonic frequencies US2006090959 Fluid pumping apparatus with Helmholtz-resonator EP1647713 Loudspeaker system with extended output and field cancellation US2006050915 Wide band sound diffuser with self regulated low frequency absorption and methods of mounting US2006042875 Silencer device for exhaust-gas turbocharger, has Helmholtz resonator DE102004040317 ACOUSTICALLY STIFFENED GAS-TURBINE FUEL NOZZLE WO2006014300 Apparatus for reducing thermoacoustic oscillations in combustion chambers EP1624251 Apparatus for reducing thermoacoustic oscillations in combustion chambers EP1624250 Gas-fired heating apparatus US2006014114 Determining the depth of perforation tunnels in a wellbore using the Helmholtz resonance effect GB2416398 Electronically controlled dual chamber variable resonator US2005252716 Resonator device and circuits for 3-d detection/receiving sonic waves US2005270906 ELECTRIC CONNECTION BOX JP2005304162 COMBUSTION APPARATUS HAVING INSTABILITY REDUCTION MEANS GB1203065 VIBRATION DAMPING DEVICE FOR A COMBUSTION CHAMBER GB1274529 ELECTRONIC SURVEILLANCE SYSTEMS GB1292380 Exhaust-silencer assembly for motor vehicle exhaust system, has Helmholtz resonator DE202005010304 RIM WHEEL AND TIRE RIM WHEEL ASSEMBLY JP2005219739 SONIC EMITTER ARRANGEMENTS WO2005051037 Burner, method for operating a burner and gas turbine US2005106519 Gas turbine US2005144950 RIM WHEEL JP2004352245 Muffler with helmholtz resonator having multiple degrees of freedom EP1482137 Device for damping thermoacoustic oscillations in a combustion chamber EP1596130 EXHAUST PART OF THE VACUUM DEVICE WITH REDUCED NOISE LEVEL SI21778 Improvement of acoustic situation in passenger cabin of vehicle, comprising use of Helmholtz-resonators DE102004016689 Vortex blower having helmholtz resonators and a baffle assembly US2005207883 Diffuser and turbine EP1559874 Device and method for damping thermoacoustic oscillations in a combustion chamber EP1557609 SOUND ABSORBING STRUCTURE JP2005146650 RESONATOR STRUCTURE IN EXHAUST SYSTEM FOR INTERNAL COMBUSTION ENGINE JP2005120853 SEAT JP2005080756 Radial blower for a leaf and waste collection/removal apparatus with operating noise suppression means US2004071546 2004-04-15 SUPPORT RING, TIRE ASSEMBLY USING THE SUPPORT RING, AND VEHICLE JP2005007928 Sound damping device having a Helmholtz resonator for installations with pulsating gas flows EP1381025 Multi-chamber resonator system eg for vehicle i.c. engine air intake silencer GB2391044 COMPRESSOR JP2004360630 SOUND ABSORPTION FLOOR JP2004338504 Fresh gas supply system for a combustion engine US2003230273 SONIC DRIER AND SONIC DRYING DEVICE JP2004317066 Speaker device US2004028246 Borehole telemetry system GB2399921 HELMHOLTZ RESONATOR US2004173175 Combined active noise control and resonator US2003178248 Feedback resistant stringed musical instrument US2004134334 Active noise control system with a Helmholtz resonator EP1313090 Radial fan for leaf and waste vacuum devices with Helmholtz resonator DE20221007 Mud pump noise attenuation in a borehole telemetry system GB2392762 Passenger restraint system for vehicle including a gas generator with noise reduction EP1288086 Infrasonic Helmholtz resonator US6665413 Broadband noise-suppressing barrier US2003006090 Tunable active sound absorber GB2387522 RESONATOR JP2003293873 Fan shroud with built in noise reduction US2003183446 VEHICLE JP2003267214 SOUND ABSORBING FOAM WO02065447 Sound absorbing foam US2002153199 Multiple frequency helmholtz resonator US2003136119 Exhaust gas silencer with combined acoustic resonator chamber and spark arrester GB2383604 MEDIUM WEIGHT SENSOR USING ACOUSTIC RESONATOR JP2002214104 Exhaust gas system with helmholtz resonator US2002108810 SOUND ABSORBING FOAMED PLASTIC AND METHOD OF MAKING THE SAME JP2003140659 HYDRAULIC ELEVATOR JP2003118950 Venting device for internal combustion engine... DE10143686 SOUND INSULATOR JP2002220817 Noise attenuation arrangements for pressurised-gas conduits EP1176355 SOUND INSULATION STRUCTURE USING DOUBLE BOARD JP2002356934 Sound-lowing cover for compressor CN2475837 Natural gas exhaust silencer for engine CN2475836 Apparatus for damping acoustic vibrations in a combustor US2002000343 SOUND ABSORBING METHOD IN BUILDING JP2002348985 RIM WHEEL JP2002234305 2002-08-20 RIM WHEEL WITH CORE AND TIRE RIM WHEEL ASSEMBLY JP2002234304 SOUND ABSORPTION STRUCTURE FOR BUILDING JP2002227322 ACOUSTIC MATERIAL JP2002189474 RIM WHEEL JP2002079802 Tyre EP1106395 Air induction system for internal combustion engine US6382161 ACOUSTIC ELEMENT WO0133906 ACTIVE MUFFLING DEVICE JP2002132265 Gas detection system has a laser FR2815122 SILENCER WO0123714 Noise attenuation arrangements for pressurised-gas conduits GB2365066 Tuned active helmholtz resonator with forced response EP1085201 Tuned Helmholtz resonator using cavity forcing US6758304 CHANGE OF PUMP JET ROTOR HOUSING... JP2001138990 Contactless inner eye pressure measurement DE10039896 Protective helmet, especially for motorcyclists EP1066766 Hard disk drive cover that contains a helmholtz resonator which attenuates acoustic energy US6493180 PRESSURE WAVE GENERATOR JP2001330000 RECIPROCATING COMPRESSOR JP2001263232 MUFFLER JP2001248509 Sound absorbing structure US6435303 HEAVY LOAD PNEUMATIC TIRE JP2001191734 Sound absorber for sound waves US6290022 Combined resonator and coolant store for an IC engine GB2357141 Hydraulic noise reduction assembly with variable side branch US6234758 Dosage arrangement in common-rail fuel injector of diesel engine DE19956077 Noise attenuating sound resonator for automotive cooling module shroud US6309176 DAMPING DEVICE FOR DAMPING VIBRATION AMPLIFICATION OF SOUND WAVE FOR BURNER JP2000146182 Tunable active sound absorbers US6778673 IMAGE FORMING DEVICE JP2001117451 PNEUMATIC TIRE HAVING SOUND ABSORBING CHARACTERISTIC JP2000118207 Method and arrangement for sound-suppression in wheels US6309026 LOW-FREQUENCY SOUND DECREASING DEVICE JP2001075574 MICROMACHINED ACOUSTIC EJECTORS AND APPLICATIONS TO MICRO THRUSTERS... WO0007735 ENGINE INTAKE NOISE REDUCTION STRUCTURE JP2001020717 Vehicle heating and/or airconditioning housing with improved acoustic performances EP0968857 Method of making composite chambercore sandwich-type structure with inherent acoustic attenuation US6231710 METHOD FOR MONITORING WHIPPED STATE OF CREAMS JP2000321253 RECIPROCATING INTERNAL COMBUSTION ENGINE JP11351085 INTERCOOLER AND INTAKE DEVICE... JP2000310124 RECIPROCATING INTERNAL COMBUSTION ENGINE JP11336555 VOLUMETER JP2000241226 VOLUMETER JP2000241225 Mechanically coupled helmholtz resonators for broadband acoustic attenuation US6069840 Vehicle wheel EP0936083 RECIPROCATING COMPRESSOR JP2000179458 RECIPROCATING COMPRESSOR JP2000161220 RECIPROCATING COMPRESSOR JP2000161219 RECIPROCATING COMPRESSOR JP2000161218 RECIPROCATING COMPRESSOR JP2000161216 PULSATION DAMPING DEVICE JP11201367 VACUUM CLEANER AND SUCTION PORT BODY THEREFOR JP2000126098 Sound-absorbing plate DE19841975 Combustion engine with EGR device, especially diesel engine EP0911510 Air induction device for IC engine has Helmholtz resonator DE19833326 Device for directed noise attenuation in a turbomachine EP0974788 Variably tuned helmholtz resonator with linear response controller EP0894970 Noise absorption device for a centrifuge motor-fan unit in an automobile air conditioning system FR2780454 Integrated resonator and filter apparatus US6048386 PASSIVE RADIATOR JP11346395 Combined air cleaner-resonator EP0896148 Noise attenuator for an induction system or an exhaust system US6009705 INTEGRATED DUCT AND RESONATOR WO9849440 HORN LOADED MICROPHONE WITH HELMHOLTZ RESONATOR ATTENUATOR WO9851122 STRUCTURE OF SOUND ABSORBING MATERIAL JP11259076 INTAKE DEVICE FOR INTERNAL COMBUSTION ENGINE JP10227261 PROCESS FOR REFINING THE MICROSTRUCTURE OF METALS WO9830346 Vehicle interior noise level suppression DE19755751 Motor air intake silencer DE19754840 TELEPHONE HANDSET JP10229435 EXHAUST MUFFLER FOR VEHICLE JP11062547 RESONATOR JP11044266 Noise determination method for road or railway DE19726608 COGGED BELT DRIVE OF INTERNAL COMBUSTION ENGINE MECHANISMS RU2131524 DIFFERENTIAL MICROPHONE ASSEMBLY WITH PASSIVE SUPPRESSION OF RESONANCES CA2205722 REFRIGERATOR JP10306972 AIR INTAKE SYSTEM FOR AN INTERNAL COMBUSTION ENGINE WO9742408 Refining microstructure of metals continuously cast by Hot Top technique FR2761624 Wheel tyre and road noise reducer DE19710029 SOUND ABSORBING DEVICE AND SOUND ABSORBING STRUCTURE FOR ROOM JP10227084 Refining microstructure of metals continuously cast by Hot Top FR2758101 Tunable acoustic system US5930371 Valve for an air flow channel EP0800030 LINING JP9175436 Resonator EP0778399 Sound absorption structure US5783780 Sound absorbing wheel-housing cover EP0769422 Tyre/road noise reduction arrangement EP0769421 Device for damping thermo-acoustic vibrations in combustion chamber of gas turbine DE19640980 Sound absorber element for absorber plates DE19640087 SOUND REPRODUCTION DEVICE JP10075494 Acoustic alarm employing a resonant cavity arrangement GB2316784 Sound absorptive hollow core structural panel US6021612 STANDING WAVE SPACER FOR INTERNAL COMBUSTION ENGINE JP10008990 POLYGON SCANNER JP9311286 LINT HANDLING SYSTEM CA2174460 HELMHOLTZ RESONATOR LOUDSPEAKER WO9631106 SILENCER JP9212176 Adjustable silencer for air intake of vehicular combustion engine FR2743848 SOUND ABSORBER JP9198051 Arrangement for reducing tire/road noise US5839761 MAGNETIC TAG EXECUTING ACOUSTIC OR MAGNETIC INQUIRY JP8249431 System for absorption of air noise DE19533623 SOUND ABSORBING MATERIAL JP9062267 TELEVISION RECEIVER JP8322095 Assembly for reducing tyre/road surface noise. EP0680866 DAMPING DEVICE FOR THERMO-ACOUSTIC VIBRATION IN COMBUSTION CHAMBER JP7293885 Engine enclosure air inlet/discharge sound attenuator US5625172 Sound absorber for motor vehicles US5681072 HELMHOLTZ HYDRAULIC PRESSURE PULSATION FILTER OF GAS SEALED TYPE JP8240202 GAS-TURBINE ENGINE AFTERBURNER RU2117806 Apparatus for acoustic noise reduction US5508477 Sound absorption system for automobile DE4446080 Structure of rear window shelf portion to partition vehicular passenger compartment... US5498050 PRODUCTION OF SOUND ABSORBING MATERIAL JP8166787 Dynamics free low emissions gas turbine combustor US5644918 Vehicle with IC engine DE4435296 Method and composite resonator for tuning an engine air induction system US5572966 INTAKE STRUCTURE OF ENGINE JP8093583 ADAPTIVE MANIFOLD TUNING WO9511373 Rotary throat cutoff device and method for reducing centrifugal fan noise. EP0638728 RESONATOR TYPE SOUND-ABSORBING BODY AND SOUND-ABSORBING CEILING JP8013642 Vibration-insulating panel element US5473125 MULTIBLADE BLOWER JP7332284 Sound-proofed swimming pool FR2718479 SILENCER FOR ATTENUATING DISCHARGE NOISES... WO9419596 Quantitative determination of air present in refrigerant sample US5524477 RESONATOR JP7208287 VOLUMETER JP7190834 PISTON POWER MACHINE OR WORKING MACHINE JP6213072 HAND-HELD APPARATUS RADIATING SOUND JP6223215 GAS TURBINE COMBUSTION DEVICE JP7139738 SOUND FIELD ADJUSTING DEVICE JP7160268 COMBUSTION CHAMBER OF GAS TURBINE JP6221563 SPEAKER SYSTEM JP6205491 SILENCER DEVICE FOR COMBUSTION ENGINE JP6202673 VOLUMENOMETER JP7083730 VOLUMENOMETER JP7083729 VOLUMENOMETER JP7083728 VOLUMENOMETER JP7083727 Silencer arrangement for combustion engines. EP0589516 Exhaust gas system of an internal combustion engine DE4330129 Sound absorption method for motor vehicles. EP0586831 SECONDARY BURNER JP6094227 SPEAKER SYSTEM JP7015783 Air intake device for an internal combustion engine. EP0569714 A refrigeration compressor. EP0561383 Piston-type engine with valve cover DE4305281 Covering element. EP0584442 Sandwich construction panel for acoustic insulation FR2700179 Noise-attenuating internal combustion engine air intake system US5424494 Sound-absorbing housing cladding for machine DE4241515 Piezoelectric sound sources US5386479 Low frequency sound generation system for use in vehicular passenger compartments US5394478 SOUND VOLUME CONTROLLER FOR PIANO JP6149232 Device for determining the volume of objects using a chamber with two resonators US5385069 Earplug and hearing devices formed in-situ US5333622 SPEAKER EQUIPMENT JP6038290 Acoustic muffler for high volume fluid flow utilizing Heimholtz resonators US5276291 Musical instrument incorporating a Helmholtz resonator US5248846 Gas turbine combustor. EP0576717 RECIPROCATING TYPE ELECTRIC RAZOR JP6000261 Self-supporting wall with sound absorbing properties. EP0518305 Guard-rail with sound absorbing properties. EP0518304 CALIBRATION OF SEISMIC STREAMERS IN A HELMHOLZ RESONATOR US5210718 AIR SUCTION DEVICE FOR INTERNAL COMBUSTION ENGINES DE4215416 EXTENDED FREQUENCY RANGE HELMHOLTZ RESONATORS WO9321625 NOISE REDUCTION DEVICE JP5195746 Sound attenuator for low frequencies, in particular for air ducts in paper mills. EP0495763 SOUND ABSORBING BODY JP5158484 CVD FILM FORMING DEVICE JP5117860 Silencer arrangement for internal combustion engines. EP0481450 NOISE REDUCTION DEVICE JP5059927 No title available JP5294230 SILENCER FOR FAN OF HOVERCRAFT(R) JP5016795 ACTIVE MUFFLING DEVICE JP4350314 SPEAKER SYSTEM JP4336795 Loudspeaker system. EP0456416 SOUND ABSORBING PANEL CA2040076 Exhaust system for small planing boat US5234364 Method and means to adjust sound characteristics of club head upon impact with golf ball US5064197 ACTIVE TYPE PULSATION PRESSURE ABSORBER JP6026448 HELMHOLTZ RESONANCE TYPE WIDE BAND TRANSMITTER-RECEIVER JP4278798 ROTARY TYPE ENCLOSED COMPRESSOR JP4265490 Tubewave damper probe for seismic applications US5171943 SPEAKER UNIT JP4369996 METHOD AND DEVICE FOR REGENERATING SOOTY SMOKE FILTER IN DIESEL ENGINE JP3242413 ROTARY COMPRESSOR JP4159490 INTERNAL SIDEBRANCH RESONATOR US5162621 INTAKE PIPE WITH INTAKE RESONATOR JP4109071 METHOD AND APPARATUS FOR MONITORING AND CONTROLLING A HEAT GENERATOR CA2020832 QUASI-OPTICAL GYROTRON HAVING A YOKE STRUCTURE... US5134341 Quasi-optical gyrotron US5052003 LASER PRINTER DEVICE JP4000469 EXHAUST DEVICE FOR SMALL SPEEDBOAT JP3295791 PULSE COMBUSTION APPARATUS CA2013491 DIRECT RADIATION TYPE SPEAKER JP3201796 Fan silencer apparatus GB2237323 VARIABLE TENSION TRANSDUCER JP2123900 Acoustic apparatus. EP0361445 VOLUMETER JP3108614 Acoustic apparatus US4987601 ELECTRIC ACOUSTIC UNIT CONVERTER JP2075299 NOISE-SUPPRESSING DEVICE JP2161134 Keyboard instrument. EP0347775 IC engine exhaust system - incorporates Helmholtz resonator DE3916429 GAS LASER JP2285686 GAS LASER DEVICE JP2281676 GAS LASER DEVICE JP2270384 Arrangement for diminishing the noise level within a motor car. EP0342353 Acoustic apparatus US5004066 Acoustic apparatus. EP0336303 Acoustic Apparatus. EP0334238 Method and apparatus of expanding acoustic reproduction range. EP0334217 Acoustic apparatus US5012890 Passive structural and aerodynamic control of compressor surge US5199856 Damping device for air transmitted and rigid-body sound vibrations. EP0337077 INTAKE DEVICE FOR ENGINE JP2176113 METHOD AND APPARATUS FOR MONITORING AND CONTROLLING A HEAT GENERATOR WO8906771 Sound insulation panel for preset frequency bands IT1225514 Compressor noise attenuation using branch type resonator US4927342 ACOUSTIC DEVICE JP2113697 MEASURING APPARATUS OF INTERNAL VOLUME JP2064418 ELECTRIC/ELECTRONIC MUSICAL INSTRUMENT JP2041098 ELECTRIC/ELECTRONIC MUSICAL INSTRUMENT JP2041097 PORTABLE ELECTRIC MUSICAL INSTRUMENT JP2029795 PORTABLE MUSICAL INSTRUMENT JP2029794 AUTOMATIC WASHER JP2026598 ELECTRONIC MUSICAL INSTRUMENT JP1319792 ELECTRONIC MUSICAL INSTRUMENT JP1319791 ELECTRONIC MUSICAL INSTRUMENT JP1319790 ACOUSTIC EQUIPMENT JP1254098 ACOUSTIC EQUIPMENT JP1254097 ACOUSTIC EQUIPMENT JP1241297 ACOUSTIC EQUIPMENT JP1241296 MAGNETIC RESONANCE IMAGING DEVICE JP1201247 VOLUMETER JP1184417 ACOUSTIC DEVICE JP1302998 ACOUSTIC DEVICE JP1302997 BODY RESONANCE INSTRUMENT JP1144099 VOLUME MEASURING INSTRUMENT JP1118722 PRINTER JP63239076 A silencing ventilating device EP0265000 Method and apparatus for nonintrusively determining mach number US4829813 VOLUMETER JP63298013 Acoustic inspection method EP0242060 Sound attenuating box US4787473 VOLUMETER JP63113315 VOLUMETER JP63044127 Method of mounting a piezoelectric helmholtz transducer on a printed circuit board US4630342 ACOUSTIC DISPLACEMENT METER JP62070705 Measure for reducing the sound emitted from railway rails DE3532765 SILENCER DEVICE FOR INTERNAL COMBUSTION ENGINE JP62038815 FLUID DRIVING APPARATUS JP61265396 Heating boiler DE3517859 Exhaust system for outboard motors US4897060 SILENCER DEVICE OF ADAPTABLE TYPE JP61129414 Infra-sonic intensification of glow bed SE456524 Method and apparatus for acoustically measuring the volume of an object US4640130 Sound probe DE3439546 SOUND ABSORPTIVE STRUCTURAL BLOCK WITH SEQUENCED CAVITIES CA1214396 Exhaust system for outboard motors US4607723 INTERNAL COMBUSTION PISTON ENGINE USING AIR CHAMBER IN PISTON CA1242121 SPEAKER DEVICE JP61020490 Broadband matching network US4644505 Low noise hand-held hairdryer US4596921 Sound-absorbing building element DE3412432 Sound attenuating ventilating box NL8304487 Burner for the processes of the autogenous welding technique DE3329937 PULSER BURNER JP60038514 PULSATING COMBUSTION APPARATUS JP60033408 GASEOUS COMPONENT DETECTOR JP59212761 General purpose modular acoustic signal generator US4602245 PIEZOELECTRIC TYPE LOW FREQUENCY TRANSDUCER JP59190795 Method and apparatus for low frequency active attenuation US4527282 APPARATUS FOR DETECTING GAS TEMPERATURE JP59160730 Gas pressure regulator GB2136095 BAR FOR A POWER CHAIN SAW CA1173330 Passive acoustic absorber device also absorbing low frequencies FR2540912 Vehicle roof liner. EP0079253 INFRARED RAY DETECTOR JP59054950 Low frequency muffler US4501341 SOUND SUPPRESSOR LINER CA1160850 CYLINDER BLOCK OF INTERNAL-COMBUSTION ENGINE JP58107839 Directional microphone with high frequency selective acoustic lens US4401859 Perforate tube muffler US4371053 Electro-acoustic transducer US4379212 Piezo-electric ringing transducer. EP0025955 COMPACT EXHAUST SILENCER FOR DIESEL LOCOMOTIVES CA1138780 Two stroke cycle engine with sustained power stroke US4370959 Mufflers for percussive pneumatic machines US4294330 ELECTRONIC AUDIO SIGNALLING DEVICE FOR TELEPHONES CA1138137 SOUND-ATTENUATING MUFFLER FOR EXHAUST GASES CA1123749 --- ![](0logo.gif) **Your Support Maintains this Service --** **BUY** **The** **Rex Research Civilization Kit** **... 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