JPS6316957B2 - - Google Patents

Description

TECHNICAL FIELD The present invention relates generally to pressure wave transduction, and more particularly to electro-acoustic transducers, such as loudspeaker drives, in a medium through which pressure waves propagate, such as air. novel devices and techniques that combine
The present invention relates to a novel device that significantly improves the bass response of pressure wave conversion systems, such as loudspeaker systems.

BACKGROUND ART Olney's U.S. Pat. No. 2,031,500, which initiates a labyrinth speaker design that uses acoustic transmission lines, eliminates cavity resonance in cabinet-type speakers, extends low frequency response, and increases acoustic damping. There is. In this patent, the rear of the speaker cone is rigidly coupled to one end of a conduit lined with sound absorbing material and open at the other end.
This patent also discloses folding the conduit within the cabinet and placing the open end at the bottom of the cabinet. A more detailed discussion of transmission line loudspeaker systems can be found in the 1975 best paper A STUDY OF
TRANSMISSION LINE LOUDSPEAKER
SYSTEMS”.

OBJECTIVES An important objective of the present invention is to provide an improved acoustic transducer.

SUMMARY OF THE INVENTION According to the present invention, there is provided a converting means having a vibrating surface for converting one of a pressure wave form and an electrical form into the other, and at least one converting means for transmitting energy between a medium and a vibrating surface. low loss pressure wave transmission line means, having one end proximate the vibrating surface and the other end proximate the medium, the effective length of which is in communication between the medium and the vibrating surface. approximately corresponds to a quarter wavelength at the lowest frequency of the pressure wave energy, and the low loss pressure wave transmission line means consists of a hollow tube with a non-absorbing inner wall having a cross-sectional area less than the area of the vibrating surface; A pressure wave energy transfer device is provided that is capable of suppressing the magnitude of the response peak at the lowest frequency while maintaining the system response at a predetermined level at low frequencies.

According to further features of the invention, there is provided a device defining at least first and second spaced apart apertures, a vibrating device for generating pressure waves, one end of the vibrating device having a first aperture and the other end a second aperture. and a device for coupling to. The first and second apertures are provided at a predetermined distance so close that low frequency characteristics are not degraded and far enough apart that no deep cut occurs in the frequency response of the system at high frequencies. The preferred spacing is
1/8 to 1/8 of the pressure wave path length between the longer wave path length between the vibration device and the first and second openings and the vibration device
It is between 1. Preferably, the device for coupling the vibrating device to the at least one aperture includes a predetermined length of the device that alters the pressure wave impedance match between the vibrating device and a medium (typically air) proximate the first and second apertures. It is a pressure wave transmission line device. The pressure wave transmission line device preferably consists of a tube and the vibrating device consists of a diaphragm, the cross-sectional area of the tube being smaller than the cross-sectional area of the diaphragm. The tube length between the diaphragm and the first aperture is desirably less than the tube length between the diaphragm and the second aperture (e.g.
The cross-sectional area of the diaphragm is 1.5 to 2 of the cross-sectional area of the tube.
times). Also, the input end of each tube is preferably in close proximity to the diaphragm. The loudspeaker preferably has a diaphragm and is characterized by a pressure wave impedance and a Bl product that cooperates with the length of the tube, and is characterized by a relatively wide frequency range that extends to a relatively low bass range by the use of an equalizer. A loudspeaker with a nearly constant frequency response over a range.
Form a system. The tube has a rectangular cross section formed by alternating internal panels within the speaker cabinet.

(Explanation of Examples) The present invention will be explained in detail based on examples.

Referring to FIG. 1, a front view of an embodiment of the invention is shown. Speaker system 11 is typically rectangular and includes a top panel 12, a bottom panel 18,
It includes side panels 14 and 15 and a front panel 16. A vertical interior panel 21 descends from the top panel 12 and carries a speaker 22, which is typically a 4 1/2 inch (11.43 cm) speaker driver used in the commercially available BOSE802™ speaker system. A receiving opening is provided. The speaker 22 is installed between the vertical panel 21 and the second vertical panel 23, and the panel 23 is lowered from the panel 12 and horizontally arranged inside the panels 24, 2.
5, 26 and 27 to define a rear tube of rectangular cross-section extending between the front panel 16 and the rear panel 17, coupling the rear side of the speaker 22 to the upper opening 28, the cross section of which opening is surrounded by a rectangle. is the same as the cross-sectional area of the pipe. The bottom panel 24 cooperates with the vertical panel 21 to form a front tube that couples the front surface of the speaker 22 to the opening 31 in the front panel 16 .
Aperture 31 also has approximately the same cross-sectional area as the right-angled rectangular tube between speaker 22 and aperture 31 . Although the speaker 22 can be full range, the tweeter is placed on one side of the front panel,
A suitable crossover network connects the left and right stereo
It is desirable to direct the high frequencies from the channel to a tweeter to enable stereo sound reproduction in a compact cabinet.

The length of the longer tube between the back of the speaker 22 and the upper opening 28 is about three times the length of the shorter tube between the front of the speaker 22 and the lower opening 31. The straight distance between openings 28 and 31 is approximately half the length of the short tube between speaker 22 and opening 31. All internal panels are rigid or non-absorbing, creating a high Q pressure wave or acoustic transmission line between the loudspeaker 22 and each of the apertures 28 and 31, creating a high standing wave ratio within the tubes. Establish. The present invention effectively utilizes tubes to couple the pressure waves of the loudspeaker into the air outside the apertures 28 and 31 over a relatively wide frequency range extending down to very low sounds, thereby transferring the low frequency energy to relatively high sound pressures. At the level,
A relatively small displacement of the diaphragm of the speaker 22 is then coupled to the listening area to keep distortion very low. A tube can be thought of as a transmission line transformer with a transmission line medium characterized by an impedance and length that reduces the mismatch between a vibrating diaphragm at one end of the tube and the impedance of the medium at the other end.

Having described the physical layout of an embodiment of the invention, the principle of operation will now be described. Over the averaged effective bandwidth of the system, the present invention provides a loudspeaker system with higher sensitivity than the same loudspeaker in an infinitely full or perforated enclosure of the same volume, and efficiency comparable to that loudspeaker. . Traditional methods of using acoustic transmission lines use sound absorbing materials to minimize resonance phenomena within the tube, but according to the present invention,
The tubing is preferably rigid (non-absorbing) and without sound-absorbing materials, taking advantage of resonance phenomena in the acoustic transmission line to achieve improved impedance matching between the loudspeaker and the external surroundings of the cabinet. Improve power transmission.

Referring to FIG. 2, one end of a rigid tube 33 acts as an acoustic transmission line of length l, having the same cross-sectional area as the loudspeaker and having an open end 34 at the other end for radiating the waves sent out by the loudspeaker 32. A speaker 32 provided in is shown. In this simplified analysis, it is convenient to consider speaker 32 as a velocity source. Since the acoustic impedance provided at the open end 34 does not terminate the acoustic transmission line 33 with its characteristic acoustic impedance, the pressure waves emitted by the speaker 32 are reflected at the open end 34 and create standing waves within the tube 33. . The boundary conditions for the ideal state are that the particle velocity at the source end of the tube (X=o) must match the velocity of the loudspeaker drive source 32, and the incremental pressure at the open end of the tube (X=l) must be equal to zero. For a given driving frequency, the envelope of the standing wave inside the tube is sinusoidal, and the maximum, minimum, and relative phase vary depending on the length of the tube and the driving frequency.

Referring to Figures 3, 4 and 5, when the length l of the tube at the driving frequency is smaller than 1/4 wavelength, between 1/4 and 1/2 wavelength, and 1/2 wavelength The velocity standing wave pattern for the case is shown. Tube length means effective tube length including end effects. The + and - signs indicate relative phase along the length of the tube. FIG. 3 shows that the particle velocity Us at the open end 34 of the tube 33 is much greater than the velocity at the source end speaker 32, and the phase at both ends of the tube is the same. Increasing the driving frequency causes the tube length to become slightly larger than 1/4 wavelength, producing the standing wave pattern shown in FIG.
There are points in the tube where the velocity is zero, and the particle velocity at the open end 34 of the tube 33 is out of phase with the source velocity of the speaker 32. However, the velocity of the open end is much greater than the velocity of the source end speaker 32. In this range of frequencies, tube 33 produces a large velocity gain.

As the drive frequency increases further, the length of the tube 33 becomes 1/2 wavelength at the drive frequency, resulting in the standing wave pattern shown in FIG. The particle velocity at open end 34 is of the same magnitude and in opposite phase as the source velocity of speaker 32. As the frequency increases further, the length of the tube increases by 3/
When there are four wavelengths, the pattern is similar to that shown in FIG. 3, but the particle velocity at the open end 34 of the tube 33 is opposite in phase to the velocity at the speaker 32 at the source end. As the drive frequency increases further, the tube length becomes one wavelength, and the particle velocity at the open end 34 becomes approximately the same in magnitude and phase as the velocity at the source end speaker 32.

Tube 33 acting as a low loss acoustic transmission line
gives a velocity gain that repeats and a phase that inverts with frequency. In the ideal lossless case, the gain is proportional to the secant of 2πl/λ. Here λ
is the wavelength of the acoustic energy within tube 33 at the drive frequency.

In the embodiment of the invention shown in FIG.
The back side of 2 drives the back tube, which connects the top opening 28 to the speaker 22. This rear tube is driven out of phase with the front surface of the speaker 22.
If there is no tube connecting the front side of the speaker 22 to the bottom opening 31, in which case the front side of the speaker 22 will be directly exposed to the outside of the cabinet, the back tube connecting the back side of the speaker 22 to the top opening 28 will be in phase. An inversion should occur so that the front face of the tube speaker 22 and the open end 28 are in phase and together act to deliver a substantial wave of energy to the listening area. This condition is met if the length of the back tube is between 1/4 and 3/4 of a wavelength. At the frequency where the length of the tube is 1/2 wavelength,
Volume velocity of the front surface of the speaker 22 and the upper open end 2
The volume velocities of 8 are nearly equal in phase and magnitude, which increases sensitivity by 6 db compared to the same speaker with infinite buffer. At frequencies where the tube is 1/4 or 3/4 of the wavelength, the tube coupling the speaker 22 to the open end 28 provides a significant velocity gain and also greatly increases the sensitivity of the speaker system.

Just above the frequency where the tube is 3/4 wavelength long, the velocities of the front and top open end 28 of the loudspeaker 22 are out of phase. As the frequency increases such that the velocity gain provided by the tube decreases towards unity, the front surface of the loudspeaker 22 and the top aperture 28 act similar to an acoustic dipole. speaker 2
At frequencies where the length of the tube coupling 2 to the open end 28 is one wavelength, the front surface of the cone of the loudspeaker 22 will be approximately the same magnitude and out of phase as the particle velocity at the top aperture 28, and the response of the loudspeaker system will be It will be the lowest.

Referring to FIG. 6, the general shape of the response of a speaker system driving a tube proximate the back of the speaker cone is shown. For a frequency range slightly larger than three octaves, a loudspeaker system with a single tube acting as an essentially lossless acoustic transmission line provides considerable gain over a loudspeaker system consisting of an infinite buffer of the same loudspeakers.

Referring to FIG. 7, a graphical representation of the acoustic power output as a function of frequency for the embodiment of FIG. 1 having a front tube coupling the front surface of diaphragm 22 to lower opening 31 is shown. In this configuration, the longer tube fills the notch for a range of frequencies one wavelength long. The front tube is capable of inverting the phase of the volume velocity imparted by the front surface of the cone of the loudspeaker 22 in the range of frequencies where the front tube at the lower opening 31 is between 1/4 and 3/4 of the wavelength. Thus, achieving this result. Furthermore,
This front tube provides a velocity gain and has the advantage that the overall system sensitivity is much higher than a back tube from the back of the speaker 22 to the top opening 28.

By making the front tube 1/3 the length of the back tube, at frequencies where the back tube is 3/4 wavelength, the front tube will be 1/4 wavelength, and both tubes will provide significant gain and their Reverse the phase when the frequency is exceeded. Thus, the output of both tubes is 5/4
They are added in phase until the back tube changes phase at a frequency that is equal to the length of the wavelength. Addition of the front tubes increases the effective bandwidth of the two tubes by at least 50
%increase. The resulting null in which both tubes have the same volume velocity magnitude and phase occurs at the frequency where the length of the back tube is 3/2 of the wavelength.

The present invention further takes advantage of properties that are normally considered disadvantageous. The acoustic impedance presented to the cone of speaker 22 by each tube places significant loads on the cone, and speaker 22 is not the ideal velocity source described above in connection with the simplified analysis. The cone velocity at frequencies where the tube has significant gain is much smaller than if the speaker were at infinite buffer. Thus, the required displacement of the cone is
Infinite Buzz is reduced compared to similar speakers.

The gain of the tube is not greater than that mentioned above.
The reason is that although the losses in the tube are kept as low as practical, there are some losses within the tube and the tube has some real component of air loading. The mechanical impedance of a lossless tube is defined as the force divided by the velocity and is represented by the cone of the speaker 22 as:

Z _T = Z _p [A _c /A _T ] ² A _T exp (jωl/c) + Γ
exp(−jωl/c)/exp(jωl/c)−Γexp(−jωl
/c) Here, Z _p is the characteristic acoustic impedance of the tube,
A _c is the effective area of the speaker 22, A _T is the cross-sectional area of the tube,
Γ is the reflection coefficient at the open end 34 of the tube, and c is the speed of sound within the tube. Substituting the ratio of the area of the tube to the area of the cone (ATCR=A _T /A _c ), we get Using a general speaker model, the expression for cone velocity is given as follows.

Vc/E(jω)=Bl/Re 1/G+jωM
_n +j (-1)/ωC _n +Y _T1 +Y _T2 where, V _c is the cone speed, E is the speaker 22
The voltage applied to the voice coil, Bl, is the electromechanical turns ratio of the speaker 22, which is proportional to the magnetic flux density B in the voice coil gap and the length of the voice coil in the gap G. G =
(1/(Re/Bl ² )) + (1/Rm), where Re is the resistance of the voice coil, Rm is the mechanical response of the speaker 22, Mm is the mechanical mass of the voice coil and cone assembly, Cm is the mechanical compliance of the speaker 22, Y _T1 and Y _T2 are the admittance of the front and back tubes, and from the above equation, the speaker 22
is derived for the cone of

Now that the operation has been described, the selection of parameters for a practical system will now be considered. The longer the length l of tube 33, the lower the frequency at which the system response rolls off. Generally, the effective tube length (including end effects) l is preferably equal to 1/4 of the speed of sound in the tube divided by the desired lower roll-off frequency of the system. For a cutoff of 60Hz, its length is approximately
It is 1.4 meters.

The distance S between the two tube openings 28 and 31 (or
When using a single tube, the distance between the speaker cone and the opening of the tube is preferably about 1/8 to 1 of the length of the longer tube. If S is very small, the null at the frequency where the length of the longer tube is equal to 3/2 of the wavelength (or one wavelength for a single tube system) will be very deep. By increasing S, this zero depth can be made almost inconspicuous. However, if S is too large, the system response will degrade at mid and low frequencies. In the embodiment shown in FIG. 1, apertures 28 and 31 are spaced apart for practical purposes in the front panel, yet close enough to avoid significant degradation of mid- and low-frequency response.

For a constant ratio (Bl) ² /Re, the tube area to cone ratio (ATCR) is the magnitude of the system response peak at frequencies where the tube length is an odd multiple of the quarter wavelength of a single tube. control. FIG. 11 shows the frequency characteristics when the ratio of the cross-sectional area of the tube to the cone area is changed. The vertical axis indicates the relative level (dB) to 1 watt. Curve A shows the case where ATCR is 1, and the peak is relatively large. When ATCR is 0.67, i.e. the area of the cone is 1.5 times the cross-sectional area of the tube,
As shown by curve B, the system response is relatively smooth. ATCR is 1/2, i.e. the area of the cone is more than twice the cross-sectional area of the tube, for example curve C (ATCR is 0.33)
In this case, the system response will be degraded. The reason is that the tube presents an increased load on the speaker cone.

It was found that tube bending did not significantly change the system efficiency in the operating band. The tube of the embodiment of FIG. 1 has three 180° turns and one 90° turn. Sharp bends can be a source of audible turbulence.
In the embodiment of FIG. 1, the sinusoidal excitation produces audible turbulence, but the turbulence noise is not audible with the musical excitation. Also, the system response in the high frequency region is
It has been found that more uniformity can be achieved by designing the folded tube with a practical number of different length linear segments.

There should be negligible compliance (air volume) between the speaker cone and the tube. Thus, in the embodiment of FIG. 1, the cone of the loudspeaker 22 forms part of the wall of the tube that connects it to the upper opening 8 and the lower opening 31.

The free air resonant frequency of the loudspeaker can be chosen such that the length of the longer tube is 1/2 wavelength, thereby causing the response that can be generated by resonance between the reactive elements of the loudspeaker and the tube. Fluctuations can be reduced. Desirably, the loudspeaker is overdamped to prevent unwanted resonance between the loudspeaker and the tube.

Increasing the Bl product increases the response peak at the ends of the band (for odd multiples of the tube length 1/4 wavelength), similar to the effect of ATCR. Thus, a low ATCR is partially offset by using a high Bl product. Furthermore, larger
The Bl product reduces sensitivity in the intermediate band where the longer tube is 1/2 wavelength long. The Bl product is preferably chosen to make the response uniform. For a given shape of cone and tube, Bl is selected such that the response at frequencies corresponding to λ/4 of the larger tube is comparable to the response at frequencies corresponding to λ/2 of the larger tube. is desirable.

Referring to FIG. 8, a schematic diagram is shown illustrating an embodiment of the present invention that uses multiple speakers to provide a relatively large effective cone area. This example is a BOSE802 (trade name) with 8 speakers on the front.
This is a modification of the speaker system. In this embodiment, the opening 42 is formed by a tube bent into a rectangular cross section.
It is a single tube unit with the back of the cone of the speaker 41 coupled to. Arranging one or more elongated vertical panels extending partially or wholly from the front panel to the rear aperture in a plane perpendicular to the front panel to provide separation between the speakers and facilitate interaction in the event the speakers are unbalanced. This allows one or more loudspeakers to operate out of phase with other loudspeakers. 1st
In the illustrated embodiment of the invention, the cabinet is 17 inches (43.2 cm) wide and 8 1/4 inches (21 cm) wide.
cm) high and 6 inches (15.2 cm) deep, it can be small enough to be used as a portable cassette AM-FM receiver cabinet, and the BOSE802(trade name) speaker with a pair of 3-inch tweeters.
A cabinet that uses a single 4 1/2 inch (11.4 cm) speaker of the type used in the system.
Can drive a 15 watt battery operated power amplifier. BOSE802 (trade name) has a pair of tweeters, one on the left and the other on the right.
It is isolated above the Hz crossover frequency and emits significant levels of bass without audible distortion. In this embodiment, each of the apertures 28 and 31 has a radius of 5
It is inches (12.7cm) and 1 1/4 (3.2cm) high.
Batsuful 25, 26 and 2 extending from the front to the rear
Each of 7 is 11 1/2 inches (29.2 cm) long. Vertical buffles 21 and 23 are 6 inches (15.2 cm) and 4 1/2 inches (11.4 cm) long, respectively. All exterior parts are made of Lexan 1/2" (1.27cm) thick and all interior parts are 1/4" thick.
Constructed from inch (0.64 cm) PVC, it provides an essentially lossless acoustic transmission line with rigid walls that have minimal displacement for strong pressure peaks that can occur as a result of standing waves in the tube. .

Non-uniformities in the system response are reduced by the equalization circuit, allowing the overall system response to have any desired characteristic curve. It is desirable to use an equalization circuit to insert a notch below the frequency at which the tube length of the system response is one-quarter wavelength. The response of tube speaker systems is below this frequency. By placing the equalization circuit with this notch in front of the power amplifier driving the speaker, the power amplifier transfers less power to the speaker in this frequency band. This feature reduces power amplifier consumption (required capacitance) and reduces speaker diaphragm displacement and distortion. This feature is also useful for other speakers, such as speakers with ports.

Referring to FIG. 9, a circuit diagram of a preferred notch circuit embodiment for certain parameter values is shown. 1st
Referring to Figure 0, the frequency response characteristics of this notch circuit are shown, with the notch frequency just below 40Hz and a significant response at 50Hz. An important feature of this circuit is that the response drops off sharply just below the system's low cutoff frequency and maintains the response relatively low below the cutoff frequency. Thus, a circuit that provides a response at the notch frequency of 6 dB below the low frequency cutoff is satisfactory. Also, an equalization circuit having a complex conjugate pole and zero pair near the notch frequency is desirable. Additionally, this notch filter can be combined with other out-of-band roll-off filters for even greater efficiency.

As can be seen from Figure 10, the notch frequency is approximately 37Hz and the cutoff frequency (3dB drop point) is approximately 37Hz.
It is 47Hz. That is, the notch frequency is about 1/3 octave lower than the cutoff frequency (one octave from the notch frequency is approximately 37 Hz higher than the 37 Hz notch frequency).

Although it is desirable to use equalization circuitry in a speaker system according to the present invention, it is possible to construct a system without electronic equalization. Parameters without electronic equalization are generally chosen for optimal bandwidth without excessive variation. In electronic equalization, parameters are preferably chosen for a relatively smooth response over a relatively wide bandwidth, thereby making it easy to electronically equalize and gives an almost uniform response.

In summary, there has been described a novel device and system which provides an improved and economical loudspeaker system capable of accurately and efficiently reproducing signals extending into the very low sound range in a compact construction that is relatively simple and inexpensive to manufacture. Explained the technology. Although the invention has been described with particular reference to a loudspeaker system, the principles of the invention may be applied to other systems that provide energy coupling between a vibrating surface and a medium in which pressure waves are propagated. Accordingly, the principles of the invention can be applied to sonar and ultrasound systems that use vibrating surfaces from or coupled to a medium that propagates pressure waves, or to microphones. It will be apparent to those skilled in the art that various modifications can be made to the embodiments and techniques described above without departing from the scope of the invention.

[Brief explanation of the drawing]

Figure 1 shows a portable entertainment
FIG. 2 is a front view of an embodiment of the invention that produces very low sound with a very small central cabinet; FIG. 2 is a schematic diagram showing a speaker placed at one end of a hollow hard tube acoustic transmission line. Figures 3, 4, and 5 show the results when the length of the tube is shorter than 1/4 wavelength, between 1/4 wavelength and 1/2 wavelength, and 1/4 wavelength, respectively.
This shows the standing wave pattern when there are two wavelengths. Figure 6 shows
The frequency response of a typical tube speaker is shown. FIG. 7 shows the frequency response of the embodiment shown in FIG. FIG. 8 is a schematic diagram illustrating an embodiment of the present invention using multiple similar speakers within a cabinet. FIG. 9 is a circuit diagram of the notch circuit. FIG. 10 is a graph showing the frequency response of the notch circuit of FIG. FIG. 11 shows the frequency characteristics of the speaker when the ratio of the cross-sectional area of the tube to the cone area is changed. (Explanation of symbols), 11: Speaker system, 1
2: Top panel, 16: Front panel, 17: Back panel, 18: Bottom panel, 21, 23: Vertical panel, 22: Speaker, 24, 25, 26, 2
7: Horizontal panel.

Claims (1)

[Claims] 1. A device for transmitting pressure wave energy by a pressure wave propagating medium, comprising: a conversion means having a vibrating surface for converting one of the pressure wave form and the electrical form into the other; and at least one low loss pressure wave transmission line means for transferring energy between the medium and the vibrating surface, one end of the pressure wave transmission line means proximate the vibrating surface and the other end proximate the vibrating surface. The low loss pressure wave transmission line means is proximate to the vibrating surface and has an effective length substantially corresponding to a quarter wavelength at the lowest frequency of pressure wave energy transmitted between the medium and the vibrating surface. consists of a hollow tube with a non-absorbing inner wall having a cross-sectional area smaller than the area of , pressure wave energy transfer device. 2. the vibrating surface and the medium are characterized by pressure wave impedances that typically result in a mismatch between them, and the low loss pressure wave transmission line transmits low frequency energy between a characteristic impedance and the medium and the vibrating surface. 2. A device according to claim 1, characterized by a length that effectively couples the . 3. The method according to claim 1, wherein the distance between one end and the other end of the low-loss pressure wave transmission line means is smaller than the length of the pressure wave transmission line means and larger than the total length of the vibration surface. Device. 4 The area of the vibration surface is 1.5 to 2 of the cross-sectional area.
2. A device according to claim 1, which is of the order of double. 5. The device according to claim 1, wherein the medium is air. 6. The apparatus of claim 5, wherein the hollow tube comprises a plurality of overlapping sections connected in series between the vibrating surface and means defining an opening proximate to the medium. 7. The device of claim 6, wherein the hollow tube comprises sections of different lengths.