Patent Description:
Internal combustion engines, such as reciprocating engines, are equipped with an air intake system for receiving, processing, and guiding fresh intake air into combustion chambers of the engine. As internal combustion engines usually comprise a plurality of cylinders, each of which accommodates one combustion chamber, known air intake systems typically divide intake air into a corresponding number of separate or partial intake air streams which are guided into one of the combustion chambers, respectively. For doing so, the air intake system is equipped with an intake manifold configured for dividing an air intake stream into the plurality of separate air intake streams before being directed into the cylinder.

For properly mounting the intake manifold to an engine block of the engine accommodating the plurality of cylinders, the use of spacers is known which are interposed between the intake manifold and the engine block and provide an airtight connection between these components.

However, during operation of the engine, e.g. upon actuation of an air intake valve, vibrations and pressure waves may be generated in the air intake system which may propagate from the air intake valve in an upstream direction through the air intake system, i.e. an intake duct thereof. As a result, e.g. by causing resonances in the air intake passage, the flow of intake air through the intake duct and thus the supply of intake air into the combustion chamber may be affected, thereby causing, for example, pollutant emissions of the engine, such as nitrogen oxides or particulate matter.

<CIT> discloses a multiple carburetor intake manifold with an air exchange passageway and method.

<CIT> discloses an intake air control apparatus.

<CIT> discloses an intake/exhaust air pressure balancer and method of reducing intake/exhaust air pressure resistance.

<CIT> discloses a structure of air control system.

<CIT> discloses an intake device for an engine.

Starting from the prior art, it is an objective to provide a solution for preventing an air intake system of an internal combustion engine from being subjected to a disturbed flow of intake air through its intake passage.

This objective is solved by means of an internal combustion engine according to the independent claim. Preferred embodiments are set forth in the present specification, the Figures as well as the dependent claims.

Accordingly, an internal combustion engine is provided which has an air intake system being equipped with a spacer. The spacer is configured for delimiting an intake duct between an intake manifold and a cylinder head of the engine and is provided with at least one flow-through passage and at least one air-accumulation cavity which are fluid-communicatively connected and constitute the intake duct.

Since the internal combustion engine is equipped with the above-described spacer, technical features which are described in connection with the spacer in the present disclosure may also relate and be applied to the proposed internal combustion engine, and vice versa.

In the following, the invention will be explained in more detail with reference to the accompanying Figures. In the Figures, like elements are denoted by identical reference numerals and repeated description thereof may be omitted in order to avoid redundancies.

<FIG> schematically shows a sectional view of an internal combustion engine <NUM>, also referred to as "the engine" in the following, which is provided in the form of a reciprocating engine, such as a diesel engine. The engine <NUM> may be installed in a vehicle, e.g. a vessel, as a main or auxiliary engine. Alternatively, the engine <NUM> may be used in power plants.

In the shown configuration, the engine <NUM> comprises at least one cylinder <NUM>, preferably three cylinders <NUM>, but may also comprise more or less than three cylinders <NUM>. Each cylinder <NUM> is provided with a combustion chamber <NUM> which is delimited by an inner wall of the cylinder <NUM> and a piston <NUM> accommodated in the cylinder <NUM>. The piston <NUM> is configured for reciprocatingly moving within the cylinder <NUM> and is connected to a crankshaft (not shown) of the engine <NUM> via a connecting rod <NUM>.

During operation of the engine <NUM>, each one of the combustion chambers <NUM> is supplied with a fuel mixture which is to be ignited therein so as to produce high-temperature and high-pressure gases which apply forces to and thus axially move the associated pistons <NUM>, thereby rotating the crankshaft of the engine <NUM>. In this way, chemical energy is transformed into mechanical energy.

The fuel mixture to be supplied to and ignited in the combustion chambers <NUM> is formed by mixing a fuel medium, i.e. diesel fuel, with intake air, i.e. comprising fresh air or ambient air from outside the vehicle.

For supplying intake air into each one of the combustion chambers <NUM>, the engine <NUM> comprises an air intake system <NUM> which is configured to collect air from outside the vehicle, to process the thus received air, e.g. by loading or pressurizing it using a turbocharger and/or by mixing it with exhaust gas from the engine, and to guide it into the combustion chambers <NUM>. In the context of the present disclosure, the term "intake air" refers to a gas or gas stream which is provided by the air intake system <NUM> to be supplied into the combustion chamber <NUM> so as to form the fuel mixture. Typically, intake air comprises fresh air from outside the vehicle. Additionally, intake air may comprise exhaust gas which may be recirculated from an exhaust passage of the engine <NUM> into the air intake system <NUM>.

More specifically, the air intake system <NUM> comprises an intake passage <NUM> which forms an intake duct <NUM> through which intake air flows prior to being feed into the combustion chambers <NUM>. In the context of the present disclosure, the term "intake duct" refers to a flow section or flow passage through which intake air is passed and guided prior to being discharged into the combustion chamber <NUM>.

For supplying intake air into the different combustion chambers <NUM>, the intake passage <NUM> comprises an intake manifold <NUM> which is configured to divide a common intake air stream <NUM> flowing through a common flow passage <NUM> of the intake passage <NUM> into separate and partial intake air streams <NUM>, each of which is guided to an associated one of the combustion chambers <NUM> via separate flow passages <NUM> of the intake manifold <NUM>. In other words, each one of the separate flow passages <NUM> of the intake manifold <NUM> is associated to one of the plurality of combustion chambers <NUM> and configured to direct one separate intake air stream <NUM> thereinto.

Each one of the cylinders <NUM> is provided with an intake air valve <NUM> which is configured to selectively direct and thus to variedly adjust the supply of intake air into the combustion chamber <NUM>. Each intake air valve <NUM> is provided in a cylinder head <NUM> and arranged at a downstream end of the intake duct <NUM>, i.e. of the corresponding separate flow passage <NUM>.

In the context of the present disclosure, the terms "downstream" and "upstream" refer to a flow direction of gases within the engine <NUM>, i.e. a flow direction of intake air flowing through the intake passage <NUM>.

To that end, for supplying the fuel medium into the combustion chamber <NUM>, each cylinder <NUM> is provided with a fuel injection valve or pump <NUM> which is arranged in the cylinder head <NUM> and provided for variedly injecting the fuel medium into the combustion chamber <NUM>. In an alternative configuration of the engine, the fuel medium may be injected into the intake duct, i.e. the separate flow passages <NUM>, before being supplied to the combustion chamber <NUM>.

The combustion chamber <NUM> of each cylinder <NUM> is further connected to an exhaust passage <NUM> for expelling combustion gases from the combustion chamber <NUM>, i.e. after combustion of the fuel mixture took place. For controlling the expelling of combustion gases, each cylinder <NUM> is provided with an exhaust gas valve <NUM> which is configured for variedly and selectively expelling exhaust gases from the associated combustion chamber <NUM> into the exhaust passage <NUM>. Exhaust gases are separately expelled from the combustion chambers <NUM> and are merged to a common exhaust gas stream <NUM> flowing through the exhaust passage <NUM> by means of an outtake manifold <NUM> which is arranged downstream of the combustion chamber <NUM>.

The basic structure and function of such an internal combustion engine <NUM> and its components are well known to a person skilled in the art and are thus not further specified. Rather, characteristics of the engine's air intake system <NUM> which are interlinked with the present invention are addressed in the following. The skilled person will understand that, although not further specified in the present disclosure, the internal combustion engine <NUM> may be equipped with further components, such as a turbocharger, an air intake filter, an exhaust gas recirculation system, etc..

For properly connecting the intake manifold <NUM> to an engine block of the engine <NUM>, in particular the cylinder head <NUM>, the air intake system <NUM> is equipped with a spacer <NUM> which, in a mounted state of the engine <NUM>, is disposed between the intake manifold <NUM> and the cylinder head <NUM>. In this way, intake air flowing through the intake duct <NUM> is successively guided through the intake manifold <NUM>, the spacer <NUM> and the cylinder head <NUM> before being discharged into the combustion chamber <NUM> via the intake air valve <NUM>. More specifically, upon flowing through the intake passage <NUM>, the common intake air stream <NUM> is, at first, guided through the common flow passage <NUM> and thereby is directed into the intake manifold <NUM>. Upon flowing through the intake manifold <NUM>, the common intake air stream <NUM> is then divided into the separate intake air streams <NUM>, each of which is directed into the associated one of the plurality of cylinders <NUM> via the associated separate flow passage <NUM>. By doing so, i.e. upon flowing through the separate flow passage <NUM>, the separate intake air stream <NUM> successively passes through the intake manifold <NUM>, the spacer <NUM> and the cylinder head <NUM>. In other words, the intake manifold <NUM>, the spacer <NUM> and the cylinder head <NUM> form and delimit successively arranged flow sections of the intake duct <NUM>, in particular the separate flow passage <NUM>.

Accordingly, the provided spacer <NUM> is designed and configured to delimit the intake duct <NUM>, in particular a flow section of the intake duct <NUM>, between the intake manifold <NUM> and the cylinder head <NUM>. Thus, as can be gathered from <FIG>, the spacer <NUM> fluid-communicatively connects a flow section of the intake duct <NUM> provided in the intake manifold <NUM> with another flow section of the intake duct <NUM> provided in the cylinder head <NUM>. In other words, the intake duct <NUM> forms a flow passage of intake air which extends through the intake passage <NUM>, the spacer <NUM> and the cylinder head <NUM>. Accordingly, the intake duct <NUM> is formed by the intake passage <NUM>, the spacer <NUM> and the cylinder head <NUM>.

For doing so, the spacer <NUM> is provided with a plurality of flow-through passages <NUM> each of which is associated to one of the separate flow passages <NUM>, respectively. In other words, each one of the flow passages <NUM> forms a part of the intake duct <NUM>, in particular a part of the associated separate flow passage <NUM>. Accordingly, the spacer <NUM> comprises a number of flow-through passages <NUM> which corresponds to the number of separate flow passages <NUM> of the intake manifold <NUM>.

In the shown configuration, the spacer <NUM> is provided as a multi-piece part which comprises or consists of a spacer plate <NUM> and a gasket <NUM>. In other words, the spacer <NUM> is provided in the form of an assembly unit. In an alternative configuration, the spacer may be provided as a one-piece or integral part.

As can be gathered from <FIG>, the spacer plate <NUM> and the gasket <NUM> are arranged one after the other, in particular directly one after the other. Accordingly, upon flowing through the spacer <NUM>, i.e. its flow-through passage <NUM>, intake air is subsequently guided through the spacer plate <NUM> and the gasket <NUM>. Thus, each one of the flow-through passages <NUM> is formed by both the spacer plate <NUM> and the gasket <NUM>.

In the shown configuration, the gasket <NUM> is interposed between the spacer plate <NUM> and the engine head <NUM>. Alternatively, the spacer <NUM> may be provided such that intake air, upon flowing through the flow-through passage <NUM> is, at first, guided through the gasket <NUM> and then through the spacer plate <NUM>. Accordingly, the gasket <NUM> may be interposed between the spacer plate <NUM> and the intake manifold <NUM>. Alternatively or additionally, the spacer <NUM> may be provided with a further gasket such that the spacer plate <NUM> is interposed between the gasket <NUM> and the further gasket. According to this configuration, each one of the flow-through passages may be formed or delimited by the spacer plate <NUM>, the gasket <NUM> and the further gasket.

<FIG> shows a perspective view of the spacer plate <NUM> in a disassembled state in which the spacer plate <NUM> is disassembled from the spacer <NUM>. Specifically, <FIG> provides a perspective view onto a front surface <NUM> of the spacer plate <NUM> which is configured to be mounted to and to get in contact with a correspondingly designed front surface of the gasket <NUM>. In other words, in an assembled state of the spacer <NUM> in which the spacer plate <NUM> and the gasket <NUM> are mounted to one another, the front surface <NUM> of the spacer plate <NUM> is connected to and fit tightly against the correspondingly designed front surface of the gasket <NUM>.

As can be gathered from <FIG>, the spacer plate <NUM> and thus the spacer <NUM> are provided with three flow-through passages <NUM> each of which is associated to one separate flow passage <NUM> and thus to one cylinder <NUM> of the engine <NUM>. The flow-through passages <NUM> are provided with a cross-sectional shape which is designed correspondingly to a cross-sectional shape of the intake duct <NUM> provided in the intake manifold <NUM>, i.e. the separate flow passages <NUM>, and in the cylinder head <NUM> so as to ensure a smooth transition of the intake duct among the spacer <NUM>.

Furthermore, the spacer <NUM> is provided with at least one air-accumulation cavity <NUM> which is fluid-communicatively connected to at least one of the flow-through passages <NUM>. Specifically, in the shown configuration, the spacer <NUM> comprises two air-accumulation cavities <NUM>, each of which is fluid-communicatively connected to two of the plurality of flow-through passages <NUM>.

In the context of the present disclosure, the term "air-accumulation cavity" refers to a cavity provided by or in the spacer <NUM> which is designed and configured such that, in the mounted state of the spacer <NUM> and during operation of the engine <NUM>, it accumulates intake air, while ensuring that a mass flow of intake air maintains constant or substantially constant upon flowing through the spacer <NUM>. In the following, the air-accommodation cavity <NUM> is also referred to as "the cavity".

In the provided spacer <NUM>, the flow-through passages <NUM> and the cavities <NUM> constitute the intake duct <NUM>, in particular a part of the intake duct <NUM> delimited by the spacer <NUM>. Specifically, in the shown configuration, each one of the cavities <NUM> is designed and configured to prevent gases or intake air present in the cavity <NUM> from being discharged from the intake duct <NUM>. To that end, each one of the cavities <NUM> is designed and configured to prevent gases from outside the intake duct <NUM> and the spacer <NUM> from being supplied into the cavity <NUM>. In other words, each one of the cavities <NUM> constitutes a section of the intake duct <NUM> which is hermetically sealed from an outside of the intake duct <NUM> and the spacer <NUM>.

By such a configuration, the spacer <NUM> is configured to, in the mounted state and during operation of the engine <NUM>, prevent a mass flow of a gas stream, in particular the air intake stream, flowing through the intake duct <NUM> from being increased or decreased upon flowing through the spacer <NUM>. Specifically, the spacer <NUM> is configured such that in each operational state of the engine <NUM> it prevents that the mass flow of the gas stream flowing through the intake duct <NUM> is increased or decreased.

During operation of the engine <NUM>, the intake air valves <NUM> are actuated in accordance with the reciprocating movement of the piston <NUM> in the associated cylinder <NUM>. In this way, i.e. upon actuation or operation, the intake air valves <NUM> may generate vibrations or waves, e.g. shockwaves, which propagate from the cylinder head <NUM> through the intake duct <NUM> in an upstream direction.

Each one of the cavities <NUM> is designed such that it is tuned to a frequency of vibrations generated in or waves propagating through the intake duct <NUM> during operation of the engine <NUM>, e.g. upon actuation of the intake air valves <NUM>. In this way, the cavities <NUM> may be designed to counteract, attenuate or affect the vibrations or waves propagating through the intake duct <NUM> during operation. In particular, the cavities <NUM> may be designed to counteract or attenuate resonances in the air intake system <NUM> which may be caused by the vibrations or waves propagating through the intake duct <NUM>. In this way, each one of the cavities <NUM> form a Helmholtz resonator. For doing so, the cavities <NUM> are configured and intended for increasing a volume of the intake duct <NUM>. In this way, vibration characteristics, e.g. a natural frequency, of the intake duct <NUM> and the gas stream flowing therethrough may be affected.

As set forth above, the flow-through passages <NUM> are fluid-communicatively connected to at least one of the air-accumulation cavities <NUM>. In the configuration depicted in <FIG>, for doing so, communication conduits <NUM> are provided which fluid-communicatively connect the cavities <NUM> to the flow-through passages <NUM>, respectively. Specifically, each one of the communication conduits <NUM> is provided such that it opens into the flow-through passage <NUM> in a direction that is substantially perpendicular to a flow direction of intake air to be fed through the flow-through passage <NUM> in the mounted state. Each one of the communication conduits <NUM> is provided in a side wall <NUM> of the spacer plate <NUM> which is arranged between a flow-through passage <NUM> and a cavity <NUM> and in particular delimits a flow-through passage <NUM> from a cavity <NUM>.

As can be gathered from <FIG>, each one of the two cavities <NUM> of the spacer <NUM> is arranged between and connected to two different flow-through passages <NUM>. Specifically, each one of the two cavities <NUM> is fluid-communicatively connected to a first flow-through passage <NUM> by means of a first communication conduit <NUM> and to a second flow-through passage <NUM> by means of a second communication conduit <NUM>, wherein the first and the second communication conduit <NUM> are arranged on opposed sides of the cavity <NUM>. In other words, the first and the second communication conduit <NUM> are provided in opposed sidewalls <NUM> of the spacer plate <NUM>.

Alternatively, the spacer <NUM> may comprise more or less than three flow-through passages <NUM> and more all less than two cavities <NUM>. For example, the spacer <NUM> may comprise a number of n flow-through passages <NUM> and a number of n-<NUM> cavities <NUM>, wherein n is a natural number greater than or equal to two. In such a configuration, each one of the plurality of cavities <NUM> may be arranged between two different flow-through passages <NUM>.

As set forth above, the spacer <NUM> in the shown configuration comprises or constitutes the spacer plate <NUM> and the gasket <NUM>. Specifically, in the shown configuration, the cavity <NUM> and the communication conduits <NUM> are delimited by both, the spacer plate <NUM> and the gasket <NUM>. For doing so, the spacer plate <NUM> is provided with a plurality of recesses which extend from the front surface <NUM> of the spacer plate <NUM> in a thickness direction of the spacer plate <NUM> and which form the cavities <NUM> and the communication conduits <NUM>.

The spacer may be configured and intended for delimiting an intake duct between an intake manifold and a cylinder head of the engine, wherein the spacer is provided with at least one flow-through passage and at least on air-accumulation cavity which are fluid-communicatively connected and constitute the intake duct.

By being provided with the air-accumulation cavity, flow and vibration characteristics of the intake duct may be affected so as to counteract or suppress unfavorable vibration or wave propagation phenomena occurring during operation of the engine.

It has been found that, during operation of the engine, an intake air valve configured for selectively injecting fresh air flowing through the intake duct into an engine's combustion chamber may induce vibrations or waves, e.g. shockwaves, which propagate therefrom in a flow upstream direction through the intake duct. These vibrations or waves, however, may induce resonances or other unfavorable vibration or wave phenomena which may affect flow of intake air through the intake duct and thus may disturb proper supply of intake air into the combustion chamber. Accordingly, by being provided with the at least one air-accumulation cavity, the suggested spacer may purposefully change the volume of the intake duct so as to change flow and/or vibration characteristics thereof, thereby affecting, in particular suppressing or counteracting, propagation of vibrations or waves through the intake duct.

The proposed spacer may be used and employed in an air intake system of any suitable internal combustion engine, in particular reciprocating engine, such as a diesel engine, a gas engine or dual fuel engine. For example, such an internal combustion engine may be utilized or be installed in vehicles, e.g. as main or auxiliary engines, or in power plants.

According to one configuration, the spacer may be provided as a separate part, i.e. which is provided separately from the intake passage, i.e. the intake manifold, and the cylinder head. Accordingly, the spacer may be detachably attached to the intake manifold and the cylinder head. However, the present invention is not limited to this configuration. Rather, the spacer, at least partly, may be provided in, in particular integrally provided, i.e. may at least partly form an integral part together with, the intake manifold and/or the cylinder head. Accordingly, in a further development, the at least one air-accumulation cavity may be provided or machined into an upstream end of the cylinder head which is closed off with a gasket provided between the cylinder head and the intake manifold. In this configuration, the gasket together with the upstream end section of the cylinder head constitute the spacer in the sense of the present disclosure. Alternatively or additionally, the air-accumulation cavity provided or machined into the upstream end of the cylinder head may be sealed or closed off with a correspondingly designed front surface of the intake manifold. In a further development, the front surface of the intake manifold may be provided with a recess which constitutes a part of the air-accumulation cavity. In this configuration, the downstream end of the intake manifold and the upstream end of the cylinder head constitute the spacer in the sense of the present disclosure. Alternatively or additionally, the at least one air-accumulation cavity may be provided or machined into the downstream end of the cylinder head which is closed off with a gasket provided between the cylinder head and the intake manifold. In this configuration, the gasket together with the upstream end section of the cylinder head constitute the spacer in the sense of the present disclosure.

As set forth above, the spacer may be used to delimit an intake duct, in particular a part of the intake duct, between the intake manifold and the cylinder head of the engine. In this way, the spacer may be employed to fluid-communicatively connect the intake manifold to the cylinder head. Further, the spacer may be configured and intended for properly mounting the intake manifold to an engine block, in particular the cylinder head of the engine. In the engine, at least one spacer may be provided. In general, the number of spacers used in the engine may depend on or correspond to the number of intake manifolds or number of cylinders of the engine.

As set forth above, the spacer is provided with at least one air-accumulation cavity, also referred to as "the cavity" in the following. The cavity may be designed and configured to prevent intake air present in the cavity from being discharged from the intake duct. Alternatively or additionally, the cavity may be designed and configured to prevent the cavity from being supplied with a fluid from outside the intake duct. Alternatively or additionally, the cavity may constitute a section of the intake duct which hermetically seals the intake duct from an outside of the intake duct.

Further, the spacer may be provided such that, in a mounted state of the spacer in which the spacer is installed in the engine and during operation of the engine, the spacer is configured to prevent a mass flow of a gas stream flowing through the intake duct from being increased or decreased upon flowing through the spacer.

In a further development, the cavity may be designed such that it is tuned to a frequency of vibrations generated in the intake duct or waves propagating through the intake duct upon operation of the engine. Specifically, the cavity form a Helmholtz resonator.

For fluid-communicatively connecting the cavity to the flow-through passage, the spacer may be provided with at least one communication conduit. In other words, the cavity is fluid-communicatively connected to the flow-through passage by means of the at least one communication conduit. Optionally, the fluid conduit may be provided such that it opens into the flow-through passage and/or the cavity in a direction that is perpendicular or substantially perpendicular to a flow direction of intake air to be fed through the flow-through passage in a mounted state.

In a further development, the spacer may be provided with a cavity which is fluid-communicatively connected to or which opens into at least two different and separately provided flow-through passages. Specifically, the cavity may be connected to a first flow-through passage by means of a first communication conduit and to a second flow-through passage by means of a second communication conduit. Optionally, the first and the second communication conduits may be arranged on opposed sides of the cavity. In this way, a compact design of the spacer may be provided.

The spacer may comprise at least two cavities, for example three or more cavities. Alternatively or additionally, the spacer may comprise at least two flow-through passages. The number of flow-through passages may correspond to or depend on a number of cavities provided in the spacer. By providing a plurality of cavities, in particular the number of which depends on the number of provided flow-through passages in the spacer, the vibration characteristics of the intake duct may be changed in a more effective and more precise way. Further, the number of flow-through passages may correspond to a number of cylinders the engine.

For example, the spacer may comprise a number of n flow-through passages and a number of n-<NUM> cavities, wherein n is a natural number greater than or equal to two. In such a configuration, each one of the cavities may be arranged between two different and separately provided flow-through passages. In this way, a compact design of the spacer may be provided.

In a further development, the spacer may be provided as an integral part. Alternatively, the spacer may be provided in the form of an assembly which is built up from more than one part. Specifically, the spacer may comprise or consist of at least two parts. In this way, the spacer may contribute to an improved or simplified manufacturing and/or maintenance process.

Specifically, the spacer may comprise or consist of a spacer plate and at least one gasket which, in an assembled state of the spacer in which the spacer plate and the gasket are mounted to one another, are connected to one another or fit tightly against each other at correspondingly designed front surfaces. In this configuration, the at least one cavity may be delimited by both the spacer plate and the gasket.

In a further development, the spacer plate may be provided with at least one recess which extends from a front surface of the spacer plate in a thickness direction thereof. The at least one recess may form the at least one cavity and/or the at least one communication conduit. By such a configuration, manufacturing of the spacer may be improved and/or simplified.

Claim 1:
Internal combustion engine (<NUM>) having an air intake system (<NUM>) which is equipped with an intake manifold (<NUM>) and a spacer (<NUM>) configured for delimiting an intake duct (<NUM>) between the intake manifold (<NUM>) and a cylinder head (<NUM>) of the engine (<NUM>), wherein
the intake manifold (<NUM>) is configured to divide a common intake air stream (<NUM>) flowing through a common flow passage (<NUM>) of an intake passage into separate intake air streams (<NUM>), each of which is guided to an associated one of a plurality of cylinders (<NUM>) of the engine,
wherein the spacer (<NUM>) is provided with a plurality of flow-through passages (<NUM>) and at least one air-accumulation cavity (<NUM>) which are fluid-communicatively connected and constitute the intake duct (<NUM>), wherein each of the plurality of flow-through passages (<NUM>) is associated to one of the separate flow passages (<NUM>), and wherein the cavity (<NUM>) constitutes a Helmholtz resonator.