Patent Application
20140294223
The present invention relates in general to headset porting and more particularly concerns headsets with linearized ports characterized by an acoustic impedance with a very low resistive component.
For background reference is made to U.S. Pat. Nos. 4,644,581, 5,181,252, and 6,831,984, incorporated herein by reference, including their file histories.
According to the invention the headset cup has a straight smooth port free of projections which introduce perturbations that could cause turbulence preferably made of metal, such as stainless steel, characterized by a linear acoustic impedance with low resistive component at high sound levels, such as those encountered in military applications that are above 120 dB SPL at between 60 and 100 Hz. By increasing the cross section of the port compared to one of small internal diameter , the resistive component is decreased. To keep the overall reactive+resistive impedance the same, the port is lengthened. An exemplary length is 37 mm for a cross section of 9.1 mm2. This construction also extends the range of sound levels over which the port acoustic impedance is effectively linear and maintains the same acoustic performance to 200 Hz. Linearizing the port in this manner allows noise reduction at higher sound levels. The headset cup preferably includes the high compliance driver disclosed in the aforesaid U.S. Pat. No. 5,181,252 in the active noise reducing system thus disclosed.
Other features, objects and advantages will become apparent from the following description when read in connection with the accompanying drawing in which:
With reference now to the drawing and more particularly
Both ports present an impedance to air flow that has a resistive and a reactive component. The resistive port 14 is of negligible length, so that the impedance of the port is dominated by the resistance of the port screen. The mass port 16 is significantly longer than it is wide, such that its impedance is dominated by its reactance, which depends on the acoustic mass of the volume of air inside the tube. The impedance of the mass port 16 varies with the frequency of the sound pressure in the rear cavity 13 that is causing air flow through them. In particular, as frequencies decrease, the contribution to total impedance from the reactive component of the mass port decreases, allowing the impedance to be dominated by the resistive component of the mass port's impedance at lower frequencies, which is relatively constant with frequency. The resistive component, however, varies with the sound pressure level inside the cavity, and this variable impedance results in the response being non-linear with pressure at frequencies where the resistive component dominates.
Non-linearity, i.e., impedance increasing with sound pressure levels, in the response of the acoustic system limits the output levels at which an ANR circuit can be operated—higher impedance requires more force to move the air, which requires more current through the motor of the transducer, potentially exceeding the capacity of the transducer or amplifier.
To address this problem, according to the present invention, the mass port is modified, relative to prior designs, to decrease the resistive component of its impedance, extending the frequency range in which the reactive portion dominates and in which the total impedance as a function of frequency is essentially linear. The resistance is decreased by increasing the diameter of the mass port 16. Increasing the diameter alone decreases the effective acoustic mass of the port, so to maintain the original reactance, the length of the mass port is also increased. Increasing the length has more effect on the acoustic mass than it does on the resistance, so this does not undermine the benefits of increasing the diameter. In one example, the cross-sectional area of the port tube is increased from 2.25 mm2 in conventional headsets to 9.1 mm2. To maintain the reactance, the length is increased from 10 mm to 37 mm (end-effects result in the effective length being slightly longer, an effect which increases with diameter). That is, a 4× increase in area is matched by a 4× increase in length.
The resistive port 14 in parallel to the mass port 16 also provides a resistive impedance, and it is desirable that the two impedances, resistive and reactive, remain parallel, rather than in series. The purely resistive port improves performance at some frequencies (where a back cavity with only a purely reactive port would have port resonance, significantly cutting output power), while compromising performance at others. Providing this resistance in a controlled, purely resistive port while the reactive port has as little resistance as possible allows that compromise to be managed and its benefits realized to the best advantage of the total system.
Thus, the performance of a headset for use in high-noise environments is improved by extending the operating frequency range at which the acoustic impedance of a mass port from the back cavity to ambient as a function of frequency is purely reactive, such that the total back cavity response remains effectively linear with respect to sound pressure levels. This is accomplished by increasing both the diameter and length of the port, but actually manufacturing such a port presents additional difficulty. As noted, the port in the example is 37 mm long, and has a cross-sectional area of 9.1 mm2, or a diameter of 3.4 mm, for a roughly 10× aspect ratio of length to diameter. Another way to consider the size of the mass port is that the volume of air inside the tube is 337 mm3, while the volume of the rear cavity (not including the volume occupied by the tube itself) is 11,100 mm3, giving a ratio of rear cavity volume to mass port volume of about 33:1. A conventional mass port would have a significantly smaller volume, and thus a significantly larger ratio of rear cavity volume to mass port volume. For example, for the conventional mass port described above with an area of 2.25 mm2 and a length of 10 mm, the volume is 22.5 mm3, and the ratio, in the same size rear cavity, is 493:1. Applying a ten percent tolerance to port volume and cavity volume, the ratio of the present design may vary from around 27:1 to 40:1, while the ratio using the prior port size may vary from around 400:1 to 600:1. The applicant has also found that it is preferable for the port to be of uniform cross-section, to provide consistency in response from unit to unit. It is also preferable for the port to be smooth inside, to avoid causing turbulence, which could reintroduce a resistive component to the response. Providing a long, skinny tube of uniform cross-section and free of internal projections can be prohibitively difficult in the ABS plastic conventionally used for forming the shells 12A and 13A of the headset. Molding a tube with such a long draw could not be done with uniform cross section, and assembling a port from multiple pieces would introduce rough edges, as well as potential assembly variation.
To resolve this, in the embodiment shown in
The exploded view of
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The headset cup of
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Power amplifier 32 amplifies the signal from compensator 31A and energizes headphone driver 17 to provide an acoustical signal in cavity 12 that is combined with an outside noise signal that enters cavity 12 from a region represented as acoustical input terminal 25 to produce a combined acoustic pressure signal in cavity 12 represented as a circle 36 to provide a combined acoustic pressure signal applied to and transduced by microphone 18. Microphone amplifier 35 amplifies the transduced signal and delivers it to signal combiner 30.
There has been described a ported headset characterized by a port having a linear acoustic impedance at high sound levels to allow improved noise reduction in a very noisy environment where the sound level may be greater than 120 dB SPL between 60 and 100 Hz. It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirited scope of the appended claims.
