FIELD OF THE DISCLOSED TECHNOLOGY

The disclosed technology relates generally to a passive impedance matching device for sound radiation from a sound source such as a loudspeaker. More specifically, the disclosed technology is related to enhancing acoustic performance of a loudspeaker by achieving impedance matching of sound radiation mainly on the front side of the loudspeaker using acoustic metamaterial (AMM) methods.

BACKGROUND OF THE DISCLOSED TECHNOLOGY

A loudspeaker is an electro-mechanical transducer, which radiates sound to both front and back side. While the front surface of the cone of a loudspeaker is pushing forward to create a sound wave by increasing air pressure, the back surface of the cone is lowering the air pressure.

The enclosure on the back side of a loudspeaker separates sound waves created from front side the speaker from the ones generated on the back. Because these waves are out of phase, they will cancel each other out when they meet or are allowed to merge, which would happen for a loudspeaker without a baffle or enclosure.

All loudspeakers require some form of isolation sound energy that will radiate off of the speaker backside. Baffles or some sort of enclosure is needed to maintain and define low frequency output. Enclosures are a way to implement infinite baffle on a loudspeaker. Loudspeaker enclosure can be thought of as a baffle wrapped around it on the back side. Thus, (loudspeaker) enclosure contains all the back radiation, which would have otherwise radiated away, as well its own modal characteristics imposed on it. The enclosure will obviously influence loudspeaker's front radiation.

Since loudspeaker diaphragm radiated on both sides, front and back, it creates acoustic pressure on its back inside the enclosure. Below the resonance frequency of the enclosure, acoustic pressure changes from positive to strongly negative which causes sudden drop in performance of the loudspeaker. Build up of negative pressure in the enclosure causes negative axial forces on the diaphragm, which do not allow it to create sound waves efficiently on the front side. This phenomenon is due to the pressure build-up around resonance peak in the back enclosure and exists all the way down to the lowest frequency. Thus, performance of the loudspeaker below the enclosure resonance frequency is controlled by pressure build-up in the enclosure.

Apart from the electro-mechanical impedance, loudspeaker, on the front side, is subjected to radiation impedance of the ambient medium, i.e., air.

Most speakers are actually very inefficient; only about 1% of the electrical energy sent by an amplifier to a typical home loudspeaker is converted to acoustic energy. The remainder is converted to heat, mostly in the voice coil and magnet assembly. The main reason for inefficient operation is the difficulty of achieving proper impedance matching between the acoustic impedance of the drive unit (i.e, loudspeaker diaphragm) and that of the air into which it is radiating. The efficiency of loudspeaker drivers varies with frequency as well. It is well known that if the acoustic impedance of the two media are very different, most sound energy will be reflected (or absorbed), rather than transferred across the border.

When the radiated wave from high impedance of the loudspeaker diaphragm reaches the transition of low impedance in the radiation (e.g., ambient medium) field, most of the power in the incident wave is reflected, rather than transmitted. For higher transmission to be achieved a matching intermediate impedance between the two regions is needed.

Consumer electronic devices, such as cell phones, tablets, and the like with more features and capabilities are ubiquitous and are positioning to become entertainment centers. However, they also exhibit severe audio deficiencies and provide many challenges to maintain the acoustic performance as enclosed acoustic volume size, power and diaphragm size are reduced significantly. Due to the smaller size of the speaker used in such devices, the low frequency response is severely affected. For example, as the size of the cell phone decreases, the volume of air behind the diaphragm is reduced. This small amount of volume behind the speaker limits the range of motion of the diaphragm. The speaker does not produce enough force to compress the air beyond a certain point, hence causing the air to push back. This reduces the displacement of the speaker diaphragm, which in turn lowers the output. Thus, low frequencies are affected the most by this phenomenon, as the diaphragm moves with the largest amount of displacement at these frequencies. Consequently, the frequency response usually rolls off faster at low frequencies (<1000 Hz). Thus, two key phenomena, namely high resonance frequency of diaphragm and small amount of back volume, severely impact performance of micro-speakers. Moreover, the back enclosure of a micro-speaker used in smartphone is usually sealed and is not accessible. This means that no modifications to the back enclosure can be made.

The total mass Mtot, and compliance Ctot, of the loudspeaker constitute a simple mass-spring system and create a resonance at the so-called fundamental resonant frequency f0, determined b

where, Ctotis the total compliance of the inner and outer suspension system and the Mtotincludes mass Mcof the voice coil and voice-coil cylinder and Mdmass of the cone. This frequency is mostly chosen as low as possible since it determines the lower limit of the frequency response of the loudspeaker (in a sufficiently large baffle pressure and power responses decrease with 24 dB per octave with decreasing frequency below/p). The resonance frequency of micro-speakers used on consumer audio devices is around 1000 Hz. Below the diaphragm resonance frequency, the sound output of the micro-speaker falls at the rate of 12 dB per octave.

Previously, investigators have tried to modify the front radiation impedance of loudspeaker using active control. An active acoustic impedance system comprised of a loudspeaker in a closed cabinet connected to a feedback control loop based on a combination of pressure measured with a microphone and the velocity of the loudspeaker's membrane, acquired through an impedance bridge-motional feedback principle patented by Philips. Although this system covers a large bandwidth, it rapidly becomes instable as the gain of the counter reaction is increased. Furthermore, it is difficult to adjust the central frequency that the loudspeaker will absorb.

WIPO Publication No. WO 2014/053994 to Lissek et al (hereinafter “Lissek”) provides an electric impedance synthesis method to be used in an electroacoustic device having broadband noise-reduction capacitors, which includes a step of defining the acoustic specifications based on a complex specified acoustic impedance, which varies with the frequencies, and responds to requirements in term of noise reduction. However, such methods are applied to using loudspeaker as a noise reduction device rather than its original function as an electro-acoustic transducer. One limitation of this system is that it is intrinsically unsteady depending on the type of electric impedance that is connected to the loudspeaker.

WIPO Publication No. WO 2016/083971 to Roux et al (hereinafter “Roux”) provides an adaptive device that adjusts to absorb the predominant resonant frequencies of a closed area or a large room. It also provides a large active absorption area, significantly larger than the area of the transducers used. This device aims to transform an electroacoustic transducer into a versatile electroacoustic resonator for absorbing sound energy in a space or to contain this energy between two adjacent spaces without using sensors to reach the desired noise reduction.

Published article “Advanced control for modifying the acoustic impedance at the diaphragm of a loudspeaker,” Acoustics 2012, April 2012, Nantes, France. hal-00810907, authored by R. Boulandet et al. (hereinafter “Boulandet”) addresses active control of a loudspeaker via an active electrical source connected at the terminals. By sending back a control voltage proportional to the sensed acoustic variables, the diaphragm acoustic impedance can be matched to the characteristic impedance of air, i.e. Zc=ρc. It results in an optimal sound absorption, specifically in the low-frequency range where the usual passive soundproofing means are ineffective, or their embodiment would become almost impractical. The diaphragm of electroacoustic transducers is used as refracting surface that controls the reaction of some boundaries of the sound field. The general idea is to absorb the incident sound energy or to contain it, simply by altering the transducer dynamics in a controlled fashion. A direct-radiating electro-dynamic loudspeaker can thus be transformed into an active electroacoustic resonator. Through the use of sensors and controller, it is proposed how to achieve broadband sound absorption at the transducer diaphragm. They used loudspeaker diaphragm impedance as control error for the active control strategy.

Radiation resistance represents the energy lost by the piston and/or loudspeaker. Indeed, the radiation resistance determines the amount of power, which propagates into the medium. On the other hand, the imaginary part, the radiation reactance, represents the amount of energy stored in the medium in the vicinity of the piston to facilitate this propagation.

Metamaterials are broadly defined as artificial composite materials specifically engineered to produce desired unusual properties not readily available in nature.

Accordingly, there is a need for a passive impedance matching device for enhancing acoustic performance of a loudspeaker by achieving impedance matching of sound radiation mainly on the front side of the loudspeaker using acoustic metamaterial (AMM) methods.

SUMMARY OF DISCLOSED TECHNOLOGY

The present disclosed technology provides an acoustic metamaterial (AMM) passive impedance matching device and system, designed to provide optimum impedance on the front side of a loudspeaker to significantly improve its broadband acoustic performance and overcome and/or match the adverse complex impedance load presented by the radiation impedance of a loudspeaker. The AMM device includes a combination of resistive and inductive acoustic elements to match the resistive and reactive features of the radiation impedance load of a loudspeaker. A combination of resistive and a reactive impedance including inductive and capacitive elements in the transmission line model may also be used for enhancing loudspeaker performance over a given broad band frequency range. Passive management of acoustics of loudspeaker radiation can thus be achieved with various compatible configurations of the AMM impedance device.

In some embodiments, the acoustic metamaterial passive impedance matching device includes a baffle including an upper end and a lower end adapted to attach to the loudspeaker, the upper end opposite the lower end, a plurality of plates stacked concentrically with respect to one another from the upper end to the lower end, each of the plates including a central region including a plurality of perforations, a plurality of slots positioned between the plates, a slot of the plurality of slots positioned between two adjacently stacked plates, a side channel extending annularly around a circumference of at least one of the plurality of plates, the side channel defining a chamber including a height that is greater than the height of the plates and an interior volume enclosing a predetermined volume of air, and a spoke interconnecting the plurality of plates to one another, the side channel and the perforations generating an acoustic resistance that matches the acoustic resistance of the loudspeaker.

In embodiments, the side channel includes a plurality of side channels spaced periodically between the plates.

In certain embodiments, the plurality of side channels and the plurality of plates alternate in arrangement.

In other embodiments, the plurality of side channels each include open ends to provide an inductive reactance.

In some embodiments, each of the plurality of plates includes a different quantity of perforations with respect to each of the other of the plurality of plates.

In embodiments, the quantity of perforations per plate decreases from the upper end toward the lower end, such that the topmost plate includes more perforations while the lowermost plate includes the least perforations.

In certain embodiments, the quantity of side channels and the quantity of perforations on each individual plate are functions of the resistance of the loudspeaker, the quantity of the side channels and the pattern and the quantity of the perforations are dependent on the resistance reactive impedance of the loudspeaker.

In other embodiments, the dimension of the side channels is a function of the reactance of the loudspeaker, the dimensions of the perforated plates dependent on the resistance of the loudspeaker.

In some embodiments, the plates increase in diameter from the upper end to the lower end, such that the plate furthest from the loudspeaker includes the smallest diameter and the plate closest to the loudspeaker includes the largest diameter.

In embodiments, the plates are uniform in diameter from the upper end to the lower, such that the plates include substantially equal diameters.

In some embodiments, the plurality of side channels further comprises a second set of side channels, each side channel including closed ends to provide a capacitive reactance.

In certain embodiments, the lower end of the baffle includes a bracket for mounting the baffle to the loudspeaker.

In other embodiments, the bracket includes a plurality of mounting apertures protruding outwardly from baffle, each of the mounting apertures corresponding to mounting holes of the loudspeaker, the mounting apertures configured to receive a fastener there-through for securing the mounting apertures to the mounting holes.

“Metamaterial” refers to “any material engineered to have a property that is not found in naturally occurring materials, which may be made from assemblies of multiple elements fashioned from composite materials such as metals and plastics”. “Impedance” refers to “the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.” “Inductance” refers to “the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing.” “Resistance” refers to “the degree to which a substance or device opposes the passage of an electric current, causing energy dissipation.” “Capacitance” refers to “the ratio of the change in an electric charge in a system to the corresponding change in its electric potential.” “Radiation” refers to “the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles which cause ionization.” “Resonance” refers to “increased amplitude that occurs when the frequency of a periodically applied force is equal or close to a natural frequency of the system on which it acts.” “Resonance frequency,” also know as “resonant frequency,” refer to “the natural frequency where a medium vibrates at the highest amplitude.” “Resonator” consists of “an electronic device consisting of a combination of elements having mass and compliance whose acoustical reactances cancel at a given frequency.” “Acoustic transducer” refers to “a device that converts acoustic energy to electrical or mechanical energy.” “Bulk modulus” refers to “the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume of a substance.” “Anisotropic” refers to “having a physical property that has a different value when measured in different directions, or varying in magnitude according to the direction of measurement.” “Resistor” refers to “a device having a designed resistance to the passage of an electric current.”

Any device or step to a method described in this disclosure can comprise or consist of that which it is a part of, or the parts which make up the device or step. The term “and/or” is inclusive of the items which it joins linguistically and each item by itself. “Substantially” is defined as “at least 95% of the term being described” and any device or aspect of a device or method described herein can be read as “comprising” or “consisting” thereof.

