Patent Application
20090316944
This disclosure relates to headphone acoustics design, and in particular, relates to porting structures that control resonance in in-the-ear headphones.
Headphones are commonly used with a variety of electronic devices to provide mobile and/or personal access to audio content. For example, headphones can be used with music players, such as MP3, CD, and cassette players. Headphones can also be used with cellular phones, personal digital assistants, computers, and most other types of electronic devices that produce audio signals.
There are many types of headphones in existence. Some headphones are supra-aural, meaning that they sit on top of the ear. These headphones are particularly susceptible to external noise because they do not enclose the ear. Other headphones are circum-aural. These headphones surround the ear to create a sound-isolated cavity that blocks out external noise. Circum-aural headphones can perform very well, but are bulky and inconvenient for portable applications. Still another type of headphone is worn inside the ear. These headphones, also called earphones or earbuds, can sit outside the ear canal or be inserted into the ear canal. The latter type, often called canalphones or in-the-ear earbuds, can have better acoustic performance than the former types because the earbuds form an air tight seal in the ear canal to block out external noise.
Like loudspeakers, headphones convert electrical signals into audible sound via one or more transducers. One basic type of transducer comprises a coil of wire, called a voice coil, attached to the apex of a cone or dome shaped diaphragm. The voice coil is positioned in a permanent magnetic field, created, for example, by a pair of permanent magnets. Electrical current is passed through the voice coil, turning it into an electromagnet. The force generated by the fields of the electromagnet and the permanent magnet moves the voice coil back and forth, which in turn moves the diaphragm. The movement of the diaphragm creates longitudinal pressure waves in the air, which are perceived by our ears and brain as sound. In this manner, information carrying electrical current can be converted to information carrying acoustic waves.
The sound quality produced by a headphone is highly dependent on the design of its transducer(s). However, there are other parameters in the design of headphones, and in particular, in in-the-ear earbuds, that affect sound quality as well. In general, sound waves originating from a transducer must propagate through a volume of air before reaching the listener's eardrums. During this time, the sound waves can be corrupted by a variety of factors, such as ambient noise and energy loss. One particularly important factor that degrades sound quality in in-the-ear earbuds is resonance. Because in-the-ear earbuds are inserted into the ear canal, they form an air tight cavity between the earbud and the ear canal. This cavity can act like a resonator that preferentially energizes and amplifies sound waves of certain frequencies (the resonant frequencies). When a wave at one of these resonant frequencies propagates down the earbud and ear canal, it is reflected back in such a way that the amplitudes of the incident and reflected waves are in phase and additive. This creates a standing wave and distorts the original sound wave that was produced by the transducer. The result is undesirable distortion of the audio content being played by the headphones. Thus, there is a need in the art for in-the-ear earbuds that combats the detrimental effects of resonance to sound quality.
Accordingly, systems and methods are provided for controlling resonance in in-the-ear earbuds.
Nominally, an in-the-ear earbud includes at least one or more transducers and an air chamber (sometimes called a port tube) that couples the transducers to the wearer's ear canal. Sound waves generated by the transducers propagate through the port tube and into the ear canal. The port tube is generally a hollow cylindrical chamber surrounded by a flexible bulb tip that is configured to form an air tight seal against the walls of the ear canal. Thus, the port tube and ear canal act in concert to form a sound isolating and air tight cavity within the ear.
When the port tube is hollow, sound waves propagate down the cavity with little to no sound diffusion. The result is a situation that is prone to the generation of strong resonance effects. However, by adding acoustically resistive structures to the port tube, sound diffusion is advantageously produced and resonance effects are limited.
In one embodiment, the resistive structure that is added to the port tube is a spring coil. The spring coil disrupts steady and laminar flow of air in the port tube to cause sound diffusion. The spring coil can have several adjustable parameters to enable a fine-tuning of the frequency response of the cavity. These adjustable parameters can include, for example, the tension of the spring, the number of coils, the length of the coil, and the cross-sectional shape, as well as others.
In another embodiment, the resistive structure can include a pattern of grooves carved into the inner surface of the port tube. Like the spring coil, the grooves act to disrupt air flow and cause sound diffusion. The groove pattern can be, for example, a spiral screw thread. The grooves can have a variety of shapes. For example, the grooves can be semi-circular, triangular, and trapezoidal. By changing the groove shape, groove pattern, and the depth of the indentations, the frequency response of the cavity can be controlled.
The frequency response of a cavity with strong resonance effects generally has peaks at the resonant frequencies. The effect of resistive structures on the cavity's frequency response is, in general, to flatten the peaks. A substantially flat cavity frequency response indicates the absence of frequency-dependent distortion. The embodiments described above can be particularly advantageous because, although they flatten the resonant peaks of the cavity's frequency response, they do not unnecessarily dampen the frequency response at non-resonant frequencies. The result is a flatter frequency response that minimizes the amount of unnecessary energy loss.
In some embodiments, the port tube further includes one or more porous filters erected across the cross section of the tube. These filters also act to decrease the effects of resonance in the cavity. In particular, the filters act to decrease the acoustic energy across all frequencies. In addition to their acoustic properties, the filters can also provide the additional benefit of acting as dust caps to prevent foreign objects from entering the port tube.
Persons of ordinary skill in the art will appreciate that at least some of the various embodiments described herein can be combined together, or they can be combined with other embodiments without departing from the spirit of the present invention.
The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:
To provide an overall understanding of the invention, certain illustrative embodiments will now be described. However, it will be understood by one of ordinary skill in the art that the systems, methods and apparatus described herein may be adapted and modified as is appropriate for the application being addressed and that they may be employed in other suitable applications, and that such other additions and modifications will not depart from the scope hereof.
Electronic device 109 can output audio content by conducting an electrical signal encoding the audio content through wire bundle 105 to earbuds 103. The audio content may be encoded in the frequency, phase, and amplitude of the electrical signals. Wire bundle 105 can comprise a group of wires that are bundled together and wrapped with insulation. Different wires in wire bundle 105 can lead to each earbud 103. The signals carried to the left and right earbuds 103 can also be different, for example to create stereophonic sound. If earbud 103 is an in-the-ear earbud, earbud 103 can be inserted into the ear canal of the wearer. The functionality of earbud 103 is elaborated below in conjunction with
Electrical signals originating from an electronic device, such as device 109 of
Receiver 250 generally includes an outer casing 259 containing an armature 251 that drives a diaphragm 252 back and forth. The movement of diaphragm 252 compresses and decompresses the air inside encasement 259, creating sound waves. The sound waves propagate out of receiver 250 through outlet 253. If receiver 250 corresponds to receiver 207 or 209 in
Sound waves generated by receivers 207 and 209 are combined in outlet 211 and then introduced together into port tube 213. Port tube 213 can be designed to have a variety of shapes and sizes, and can contain acoustically resistive structures 215. Together, port tube 213 and resistive structures 215 function to improve the sound quality of earbud 200, as will be explained further below. A dust cap 217 may be located at the end of port tube 213. The primary purpose of dust cap 217 is to prevent dust, earwax, and other foreign objects from entering port tube 213. In addition, dust cap 217 can also act as an acoustically resistive structure, like resistive structures 215. The effect of dust cap 217 on sound quality is also explained further below.
Despite the presence of dust cap 217, foreign objects may still enter port tube 213. Thus, it may be beneficial in some embodiments to make port tube 213 removable. For example, port tube 213 may be kept in place in earbud 200 by screw threads. In this case, port tube 213 can be unscrewed out of earbud 200. After its removal, port tube 213 can then be serviced or cleaned. Port tube 213 can also be made to be disposable and replaceable.
Ear canal 225 is nominally approximately cylindrically shaped with a diameter of around 7 mm and length of around 25 mm, although this will vary from person to person. At the end of ear canal 225 is eardrum 229. Eardrum 229 is a thin membrane that separates ear canal 225 from the middle ear. Sound waves present in ear canal 225 are transferred to the rest of the auditory system through eardrum 229.
The section of earbud 200 that is inserted into ear canal 225 is enclosed in bulb tip 219. Bulb tip 219 can be composed of a flexible or moldable material, such as silicone or foam, that allows earbud 200 to fit snuggly and comfortably into ear canal 225. Bulb tip 219 can also form an air tight and sound isolating seal 221 against the sides of ear canal 225. Accordingly, receivers 207 and 209, eardrum 229, and the sides of ear canal 225 can form an enclosed cavity 227 that encompasses the volume in port tube 213 and ear canal 225. Sound waves produced by receivers 207 and 209 are contained within cavity 227 while sound waves from external sources are, for the most part, advantageously blocked out. Thus, sound isolating seal 221 and the resulting cavity 227 substantially eliminate acoustic corruption from external sources.
Like many systems that include volumes of air to operate (e.g., woodwind instruments, a partially-filled glass of water), cavity 227 can act like an acoustic resonator. Acoustic resonators, such as a Helmholtz resonator, selectively amplify sound waves with certain frequencies (e.g., the system's resonant frequencies). Resonance occurs in cavity 227 partly because sound waves introduced by receivers 207 and 209 into cavity 227 may reflect off the boundaries of cavity 227. Waves oscillating at resonant frequencies tend to reflect in such a way that the incident and reflected waves are in phase, and their amplitudes additive when the waves are superimposed. Conversely, waves oscillating at some non-resonant frequencies tend to reflect so that the incident and reflected waves partially, or even completely, cancel. Thus, when receivers 207 and 209 generate an audio signal comprising a variety of frequencies (e.g., a piece of music), cavity 227 selectively amplifies some frequencies and dampens others, thereby distorting the original signal.
Although wave reflection can occur at any part of cavity 227's boundaries and in any direction, the main path of sound wave propagation and reflection is along the length of cavity 227. Therefore, cavity 227's resonant frequencies are determined largely by the distance between receivers 207 and 209 and eardrum 229. In particular, since an incident wave must be reflected back in phase in order for full amplification to occur, the round trip distance between receivers 207 and 209 and eardrum 229 is equal to an integer multiple of each of the corresponding wavelengths of the system's resonant frequencies. In other words, cavity 227 can have a lowest (“fundamental”) resonant frequency who's corresponding wavelength is equal to cavity 227's round trip length, and a series of higher resonant frequencies (“overtones”) that are integral multiples of the fundamental frequency.
It can be seen from the discussion above that the properties of cavity 227 play an important role in determining the sound quality of earbud 200. Since the properties of ear canal 225 are uncontrollable, the sound quality of earbud 200 can be improved by appropriately designing port tube 213. For example, changing the length of port tube 213 changes the overall distance between receivers 207 and 209 and eardrum 229, thereby changing the resonant frequencies of cavity 227. Port tube 213 can also have a variety of shapes. Many port tubes 213 are cylindrical, as is show in
Port tube 213 can also be composed of different materials with different acoustic properties. Some materials reflect sound well while others absorb some of the energy carried by the waves. In one embodiment, port tube 213 is made from aluminum. Aluminum is particularly suitable because it is durable and easy to manufacture into small and thin parts. In another embodiment, port tube 213 is composed of plastic.
Although most port tubes in existence are hollow, port tube 213 can advantageously contain acoustically resistive structures 215. Acoustically resistive structures 215 can be designed to decrease the effects of resonance in earbud 200. In one embodiment, resistive structures 215 are self-contained parts that are inserted into port tube 213. In another embodiment, resistive structures 215 comprise textured alterations made to the inner surface of port tube 213. For example,
Accordingly, a cavity including port tube 300, such as cavity 227 of
The peaks in the frequency response of a port tube/ear canal cavity can be advantageously flattened by, for example, inserting acoustically resistive structures into the port tube in accordance with embodiments of the invention.
For example, one advantageously flattened frequency response 413 of a cavity which includes port tube 400 is shown in
In some applications of port tube 400, it may be beneficial to provide a large amount of attenuation at resonant frequencies and a small amount of attenuation across all frequencies. In this case, port tube 400 may include spring coil 401 to flatten the resonant peaks and a simple mesh filter to provide damping across all frequencies. An example of a mesh filter is dust cap 217 of
As mentioned previously, frequency response 413 associated with port tube 400 can be finely tuned by appropriately determining the parameters of spring coil 401. For example, the spring tension, spring length, number of coils, cross-sectional shape of spring coil 401, and cross-sectional shape of the coil wire are all parameters that can be adjusted during the manufacturing of the host earbud to “tune” the port tube to the desired resonant frequencies and optimal peaking magnitude. The cross-sectional shape of spring coil 401 may, for example, be rectangular or elliptical instead of circular. The tension of spring coil 401 can control frequency-dependent sound absorption. The length of spring coil 401 need not be the same length as port tube 400. Spring coil 401 can also have a diameter that is substantially smaller than the diameter of port tube 401. In this case, spring coil 401 can be secured in place by support fixtures connected to the interior surface of port tube 400.
The insertion of a spring into a port tube is not the only way to achieve the advantageous air flow disruptions, and subsequent reduction in resonance effects, described above. The same effects can be obtained, in accordance with one embodiment of the present invention, by altering the interior walls of the port tube. In
Like spring coil 401 of
One advantage of the design of port tube 500 is that additional components (e.g., spring coils) are not needed. Not only does this reduce the manufacturing complexity and parts cost of port tube 500, it decreases the probability of device failure, which generally increases with the number of additional and/or movable parts in a device. Additionally, like port tube 400 of
Continuing on to
Located at one end of the cavity are one or more transducers. The transducers may be part of a receiver unit and there may be an intermediate piece of hardware located between the transducers and the cavity, such as outlet 211 of
If the port tube does not contain any acoustically resistive structures, the air flow in the cavity can be steady and laminar (step 604). This can lead to particularly strong resonance effects due to wave reflection and superposition. The resonance effects can be frequency-dependent amplification and attenuation of sound (step 605). A quantitative way to characterize this phenomenon is to plot the cavity's frequency response, which in this case has peaks at the resonant modes, such as that shown in
Alternatively, fitting a cavity with acoustically resistive structures would disrupt the steady and laminar flow of air, leading to sound diffusion and a reduction in resonance effects (step 608). Two methods described above in accordance with embodiments of the present invention to add acoustic resistance to a hollow port tube include inserting a spring coil into the tube and/or carving screw threads into the inner surface of the tube. In those methods, the resistive structures can undergo minor alterations, allowing for fine tuning of the frequency response of the cavity. The general effect of resistive structures is to flatten the frequency response to approach uniform gain (step 609). Thus, the port tube designs described herein improve sound quality in in-the-ear earbuds (step 610).
Thus it is seen that systems, apparatus and methods for producing higher quality audio output signals using in-the-ear headphones are provided. Embodiments of the present invention produce improved audio output signals by disrupting the flow of the generated sound waves in order to reduce the effects such as laminar flow that result from smooth, unobstructed surfaces in traditional in-the-ear headphone devices. It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention, and the present invention is limited only by the claims that follow.
