Compression driver theory

This section may be the length defined by the thickness of the magnetic return path backplate. This length is shown in FIG. This increase in the radius then limits the ability of the driver to produce a wide dispersion and broad radiation pattern as frequency is increased. There is a need, therefore, for a compression driver motor structure that overcomes these problems and provides a wider dispersion and broader radiation pattern as frequency is increased, for a given horn mouth configuration.

There has been summarized above, rather broadly, the prior art that is related to the present invention in order that the context of the present invention may be better understood and appreciated.

In this regard, it is instructive to also consider the objects and advantages of the present invention. It is an object of the present invention to overcome the above mentioned difficulties by providing a compression driver motor structure that overcomes these problems and provides a wider dispersion and broader radiation pattern as frequency is increased.

It is also an object of this invention to provide a compression driver motor structure that reliably generates sound in a wide dispersion in the horizontal plane over a larger bandwidth for a horn mouth of a selected rectangular configuration. The aforesaid object s are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined. Returning to the discussion on the horn's throat, applicant's inspection of FIG.

This is the point where the cross sectional area is the smallest and, as a result, the radius is minimal for any given design. There is no specific radius or associated area that will best optimize the performance. The applicant has discovered that an optimal area is a function of the plane immediately at the summation point of the phase plug.

The area at the summation point of the phase plug will be related to design features such as compression ratio and driver diaphragm area. In accordance with the present invention, a new horn throat configuration has a phase plug summation plane radius that is also the effective throat of the driver. At this plane the elements of the horn that provide directional information to the wave front are implemented.

Coupling the required radiation geometry to the driver i. From this example, it can be seen that there is substantial advantage in having a horn that imparts directional information to the wave fronts coupled to a driver using the smallest possible radius. This is accomplished by altering the geometry of the magnetic return circuit back plate, as compared to the prior art.

The geometry of the plate then immediately begins to form the desired horizontal and vertical or radial in the case of a circular or elliptical radiation pattern. As an example, the horizontal included angle beginning at the phase plug summation plane could be degrees and the vertical could also be degrees, or any other included angle that would be less than the limit imposed by the phase plug summation plane radius.

A typical practice would be to have a degree horizontal pattern and a 60 degree to 40 degree included angle in the vertical plane. Applicant's data describes the directional behavior of an un-flanged tube. The dispersion in actually increased i. The data shown compares a piston in an infinite plane baffle, a piston at the end of a long tube the data from tables 2 and 3 and a piston in free space no baffle. The addition of a horn to the exit aperture of a driver will alter the directional response and, for certain values of Ka will increase the dispersion angle, or beamwidth.

The horn will alter the dispersion characteristics much like the addition of a baffle in FIG. This effect is shown in the actual measured data. Diffraction is an effect that produces spreading of a wave form when that wave form encounters a gap, or slit. The smaller the slit relative to the wavelength, the wider the resultant spreading of the waveform relative to its original. This spreading will increase the dispersion pattern, or beamwidth of the radiated wave. Diffraction slots are an effective way to broaden a wavefront that has become narrow due to the exit radius of the driver being large relative to the radiated wavelengths.

The use of diffraction slots can present two basic problems. The first problem is that diffraction slots represent a change in cross sectional area. This area change, or discontinuity, will produce a reflected wave in the horn. The second difficulty with diffraction slots, if they are located between the driver exit and the horn mouth, is that they can introduce path length differences associated with the physical geometry required to transition from the driver exit geometry to the narrow slot required to produce the necessary diffraction to achieve a requires dispersion, or beamwidth.

These path length differences can result in uneven acoustical summing of the waveforms due to the phase differences associated with the different path lengths. It should be noted that when a horn is designed to produce a specific radiation or dispersion pattern, discontinuities are typical. The designer's goal is to minimize the number and magnitude of those discontinuities.

The Equivalent Throat driver of the present invention has an exit radius that is identical to and coincident with the phase plug summation plane. Based on applicant's data see below, in Tables 1 and 2 , a smaller exit radius will produce a wider dispersion pattern i. It is typical for a horn with a rectangular radiation pattern i.

It is also possible to have identical angles if the desired radiation pattern is square or a single angle if the desired pattern is oval in nature. A prototype embodiment of the Equivalent Throat ET driver of the present invention was designed to develop a rectangular radiation pattern.

The geometry within the section of the magnetic return path's steel back plate is shaped to form a portion of the actual throat geometry. The salient feature of the design is that the desired radiation geometry begins at the phase plug summation plane where the exit radius can be made a minimum for any given driver design. The conventional design's exit radius is displaced from the phase plug summation plane by the thickness of the magnetic return path steel back plate.

The conventional design exit radius is larger than the radius at the phase plug summation plane. An equivalent throat driver with a matching equivalent throat horn differs from a conventional driver with conventional horn in that the entrance geometry of the horn must match the exit geometry of the driver or be adapted to the radius of the phase plug summation plane. In the exemplary embodiment the horn's widest included angle in the case of a rectangular pattern implementation matches that of the widest angle that the radius associated with the phase plug summation plane.

In accordance with the present invention, the entrance geometry of the horn matches the geometry of the driver's exit as defined in the exemplary embodiment by the interior surfaces of the backplate's central opening when mated to the horn's throat to provide a virtually seamless horn interior sidewall from phase plug to well within the horn's throat.

The ET driver's exit aperture in the backplate has sidewalls that taper away from one another, and so are not parallel, as in conventional drivers. Other horn geometries, all with a rated beamwidth less than that supported by the phase plug summation plane radius may certainly be used with an equivalent throat driver.

Acoustical measurements of the horn and driver combination of the present invention indicate good agreement with the theory of the present invention.

The radius of the phase plug summation plane for an equivalent throat driver prototype is 0. The radius at the exit of the conventional driver is 0. The predicted difference in the high frequency dispersion limit is Hz. The measured difference is approximately Hz, representing excellent agreement. The measured frequency is approximately 11, Hz.

In both cases, the difference between the data calculated and the measured results are thought to be associated with acoustic end correction and boundary conditions. It has been discovered that the equivalent throat driver must incorporate the widest included angle to achieve the necessary dispersion, and other horn geometries may then be designed with narrower dispersion angles.

Other narrower dispersion pattern horns will function in a traditional manner with the ET driver of the present invention, as long as the horn entrance radius matches the phase plug summation plane radius on the ET driver. Conventional or prior art compression drivers have an exit radius that is larger than the phase plug summation radius and is separated some distance from the plane where the phase plug summation radius is located.

Because the exit radius on conventional devices is larger than the summation plane radius, the high frequency dispersion performance of the driver is limited by the exit radius. By utilizing the summation plane radius the high frequency dispersion limit is increased. The widest dispersion horn has a rear geometry that matches the exit geometry of the equivalent throat driver.

This type of driver consists of two motors and two annular diaphragms connected through similar phasing plugs to the common acoustical load. Summation of acoustical signals on common acoustical load provides extended frequency range compared to the design with identical diaphragms.

Theoretically maximum overall SPL sensitivity is achieved by the in-phase radiation of the diaphragms. These old horns can still be seen in a very few old cinemas and museums. Constant directivity. In recent times, computing power has enabled horn designers to be very innovative, bending the laws of physics for shortening truncating horns, and increasing horizontal dispersion constant directivity , with limited success The professional audio market is driven by trends.

When constant directivity horns were first marketed, many senior engineers were jokingly heard saying "They are not called bum horns for nothing". The principles behind constant directivity and wave guides are excellent in theory. The wave guide attempts to convert the circular throat opening of the driver to a vertical slot line-source at the throat of the horn. A line source is similar to the vertical slot of a water hose nozzle. Paint spray guns and many pressure pack cans use the same principle.

The vertical slot causes the water or paint to be spread horizontally. In practise, a line source wave guide results in loss of efficiency as the frequency increases which has to be compensated for.

Air radiation resistance to the diaphragm becomes nonlinear and the diaphragm is physically stressed with excessive movement at high power and can easily fracture. This also has to be compensated for by controlling power peak limiting to the driver.

Overall the compromises and questionable improvement of horizontal dispersion of constant directivity horns do not match the original circular exponential horn acoustical principles. The Lens. For circular and near circular horns a lens was the most effective means to increase horizontal dispersion without introducing lobe distortion and minimal loss of efficiency. An acoustical lens is the equivalent of the optical lens. A labyrinth of concave plates is put in front of the horn. Sound from the centre of the horn passes through unhindered.

Sound to the sides of centre in the horn, pass through the lens labyrinth, increasing the distance traveled and delayed in time. The sound waves are bent forming a wide horizontal dispersion. The lens improves horizontal dispersion as the frequency increases.

The lens is no longer used. It is large, fragile and expensive. Economic rationalism, not technical performance was and still is its downfall. Also the lens function was not well understood. Its physical appearance does not give an intuitive understanding of its function. Many old 60s - 70s roadie sound engineers believed the lens directed sound downward to the audience sitting in the front row.