Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having acoustic wave modifying structure
Reexamination Certificate
1999-09-16
2001-11-20
Le, Huyen (Department: 2743)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having acoustic wave modifying structure
C381S430000, C381S340000, C381S398000
Reexamination Certificate
active
06320970
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to high frequency compression drivers. Compression driver is an electro-mechano-acoustical transducer of electrodynamic type that converts electrical audio signal into an acoustical signal.
Electro-dynamic transducers that turn an electrical signal into radiated acoustical sound waves are well known. Such devices are generally broken down into two categories: direct radiating electro-dynamic loudspeakers, which directly radiate the generated sound waves into open air, and indirect radiators (consisting of horns and horn drivers, that are also called compression drivers), which require additional elements such as compression chamber, a phasing plug and a horn.
In a direct-radiating loudspeaker, the diaphragm, which is driven by the voice coil, vibrates and excites the particles of the surrounding air to generate the sound waves related to the input electrical signal. Low efficiency of direct radiating loudspeakers as well as the lack of controlled directivity of radiated acoustic energy make them impractical for use in sound systems requiring high sound pressure levels and controlled directivity.
Generally, compression drivers can generate much higher sound pressure levels when compared with direct radiators and are used, predominantly in sound reinforcement and in public address systems, where the loud sound signals are of essence.
In horn loudspeakers, such as compression drivers, the diaphragm moves against a surface closely spaced thereto and generates high-pressure acoustical waves which are passed through a phasing plug to a horn. Phasing plug is essentially an acoustical adapter that connects the air volume in front of the diaphragm (called compression chamber) to the input (throat) of the horn. Phasing plug has one or several inlets with overall area smaller than that of the diaphragm. Smaller area of the inlets of the phasing plug provides air compression and the increase of the sound pressure in the compression chamber therefore increasing efficiency of the transformation of mechanical energy of the moving diaphragm into the acoustical energy of a sound signal. The phasing plug is also used to reduce the volume of air to be compressed by the vibrating diaphragm to decrease the parasitic compliance of the air in compression chamber to prevent attenuation of high frequency signals. The phasing plug is also used to cancel high frequency standing waves in air chamber through carefully positioned passageways or holes through the phase plug, and it also used to eliminate certain interfering cancellations in the generated sound waves.
The phasing plug conveys the sound signal into the horn and is essentially the beginning of the horn. The horn provides transformation of a high sound pressure level signal at the throat into a lower sound pressure signal at the mouth of the horn. The horn with a phasing plug at its beginning is essentially an acoustical transformer which matches high mechanical impedance of the vibrating diaphragm to the low impedance of open air.
A horn speaker introduces distortions at high output levels which are perceived by a listener as a lack of quality and clarity of sound. The distortions of a horn speaker are caused by several reasons. Distortion may occur due to the high and non-symmetrical mechanical stiffness of the suspension of the diaphragm. This distortion is dependent on the amplitude of the excursion of the diaphragm. Since the amplitude of the excursion increases at lower part of the frequency range of the driver, the level of this distortion also increases at low frequencies. Great deal of distortion is generated in the compression chamber because of the non-linear nature of the compression of air. Strictly speaking, there are two air chambers in a compression driver. The chamber in front of the diaphragm, namely compression chamber, is open into the horn through the orifices in the phasing plug. The chamber behind the diaphragm, called rear chamber or back chamber, is usually sealed. In spite of the similar basic nature of the air compression-related distortion in front and rear chambers, its behavior is different. The air trapped in the back chamber acts merely as a non-linear spring, somewhat similar to the non-symmetrical mechanical suspension of the diaphragm. The air in the front chamber is also non-linearly compressed during the operation of the driver, but since the front compression chamber is open into the horn, the process of compression is more complicated and so is the behavior of the corresponding distortion.
In order to understand the non-linear behavior of air enclosed in a chamber, one may consider that the diaphragm acts as a piston, reciprocating in a cylinder, which is either closed, which is typical for the rear chamber, or has an orifice of an area which is equal to the entrance of the phasing plug (this holds true for the front chamber). For adiabatic change of pressure which occurs in the cylinder, which is a compression chamber, the relationship between the total pressure and volume in the cylinder is expressed by the Boyle's law, (P
0
+P(t)) (V
0
−V(t))
&ggr;
=P
0
V
0
=const , where P
0
is atmospheric pressure, V
0
is the initial volume, P(t) is the instantaneous change of the pressure in the cylinder, V(t) is the change of the volume of the cylinder, and &ggr;=1.4 is the ratio of the specific heat of the air at constant pressure to the specific heat at constant volume. As the cylinder reciprocates with equal displacement on either side of the initial reference position, the minimum and maximum values of the displacement and correspondingly, the volume, cause non-equal changes of pressure around its initial value P
0
. The positive change of the pressure (this corresponds to decrease of the volume) has higher amplitude than the negative change of the pressure, which corresponds to the increase of the volume. For a sealed cylinder, the volume V is expressed as V(t)=X
d
(t)S
d
where X
d
is the displacement of the cylinder (diaphragm), S
d
is the area of the cylinder (diaphragm). For the partly open cylinder, the front chamber, the change of the volume is expressed as V(t)=X
d
(t)S
d
−X
t
(t)S
t
where X
t
is the displacement of the air particles at the orifice of the cylinder at the entrance of the phasing plug and S
t
is the area of the orifice. The air in the front chamber is partly compressed and partly displaced into the entrance of the phasing plug to propagate down the horn to be radiated from the mouth of the horn. Input acoustical impedance of the horn with the phasing plug being at the beginning of the horn is frequency-dependent. It is essentially zero at low frequencies, and then it grows with frequency and reaches the constant value
Z
=
ρ
c
S
t
,
where &rgr; is the air density, c is the speed of sound, and S
t
is the area of the entrances in the phasing plug. At low frequencies the compression chamber is practically open, there is no air compression and no air-related distortion occurs. At higher frequencies the impedance increases and the chamber gets “closed” (not completely though), and the pressure inside the chamber increases. As the compression of the air increases the distortion grows. Therefore, the distortion increases with frequency until the impedance of the horn reaches its maximum constant value. Obviously, the distortion also grows with the increase of pressure in the chamber. The smaller area of the entrances in the phasing plug causes higher pressure in the chamber, and correspondingly, higher level of air compression-related distortion. To decrease the level of air compression distortion in the rear chamber, its volume should be large as compared to the displacement volume of the diaphragm. Opening in the back chamber decreases the pressure in the back chamber and, correspondingly, decreases the level of distortion. Rear chamber can be opened into the cavity underneath the top plate, between the magnet and the pole piece.
The level of air compression d
Czerwinski Eugene J.
Voishvillo Alexander G.
Johansen W. Edward
Le Huyen
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