Patent Grant
7894614
The present invention relates to a system and method for achieving extended low frequency response and output sound pressure level capability at low frequencies.
In loudspeaker systems, especially high quality audio systems that are intended to produce the full range of audible signals, particularly those at low frequencies, a major design challenge lies in achieving adequate low-frequency extension, both in terms of low frequency response and maximum achievable sound pressure levels (SPL) at low frequencies. This challenge is further increased when this performance must be achieved in a small enclosure, or with small loudspeaker drivers, or both.
One of the major challenges in loudspeaker system design, in terms of low-frequency performance, is to achieve a frequency response that extends to low frequencies in or below the 30-50 Hz range. A more difficult challenge is to achieve high output sound pressure levels (SPL) at these same low frequencies, owing to the need to move large amounts of air in order to achieve high sound pressure levels. Because the maximum cone excursion of the driver determines the amount of air that can be moved (in combination with the driver's effective cone area), this limitation is referred to as Excursion-Limited SPL, or ELSPL. The ELSPL of a driver is a function of frequency, and typically decreases at lower frequencies because at low frequencies a correspondingly larger amount of air must be moved to achieve a given SPL.
Most conventional loudspeaker system designs representative of the prior art fall into one of two broad categories. Sealed systems, often called closed-box systems, or acoustic suspension systems, provide a second-order high-pass frequency response. They suffer from higher low-frequency −3 dB cutoff frequencies (f3) and low ELSPL. The low frequency cutoff frequency of a sealed system can be reduced, but at the expense of a much larger box. Alternatively, the f3 of such a system may be reduced by employing a heavier cone, which reduces the resonant frequency of the system. Use of this latter technique usually results in much reduced electro-acoustic efficiency. In either case, however, low-frequency ELSPL is not increased.
Ported systems, also known as vented systems or bass reflex systems, add a port to the box in which the driver is mounted, forming a Helmholtz resonator. When properly designed, the box-port Helmholtz resonance produces a lower f3 and also produces a higher ELSPL at low frequencies. In such systems, the box-port Helmholtz resonant frequency is referred to as fb. These systems provide a fourth-order high pass frequency response. As frequency is reduced from higher frequencies down to f3 and then to frequencies below f3, the frequency response begins to fall off very sharply, at a rate approaching 24 dB/octave. The steep rolloff typically begins at frequencies below the box tuning frequency fb. The steep low frequency rolloff tends to cause group delay distortion and poor transient response. Although ported systems provide increased ELSPL at frequencies above f3, the ELSPL of ported systems falls off severely at frequencies below fb, providing virtually no useful output at such frequencies. Ported systems actually produce LESS ELSPL than that of a comparable sealed system at frequencies below fb of the ported system.
One commercial example of a low-frequency sealed system designed for extended low-frequency performance is a subwoofer implemented by Carver Corp. (U.S. Pat. No. 6,566,960). It is essentially a brute-force sealed system that employs a special driver with very large cone mass and very large cone excursion. The design results in very low efficiency and requires extremely high drive power. The very high cone mass also compromises transient response.
A small number of sealed systems employ equalization in order to achieve an extended low frequency response with a reduced f3. This approach does not suffer from the approaches mentioned above wherein larger cabinets or reduced electrical efficiency is required. Such equalization is most often done with an active filter placed in the signal path prior to the power amplifier that drives the loudspeaker. These equalizers typically provide a biquadratic filter function that includes a pair of zeros and a pair of poles. The pair of zeros is typically placed at or near the same frequency as the pair of poles produced by the unequalized sealed system. The pair of biquadratic poles is placed at a lower frequency corresponding to the desired equalized f3 of the system. Such an equalizer is also well known to those familiar with the prior art as a Linkwitz Transform.
This technique, referred to here as an Equalized Sealed System (ESS), is very effective at improving the frequency response of the sealed system loudspeaker. However, it also does nothing to improve or increase the low-frequency ELSPL. Therefore, in order to be practical, and to have an ELSPL commensurate with the extended low frequency response afforded by the ESS technique, such systems typically must employ a large driver with a very large excursion capability. Such systems may typically employ equalization to move the system f3 down by about one octave. This corresponds roughly to 12 dB of equalization, which in turn corresponds to an increased power of 16 times at the f3 of the equalized system. This is a direct consequence of the greatly reduced efficiency of a sealed system at frequencies below its unequalized f3. As a result, large power amplifiers are often required for use with such systems.
The Bag End ELF system (U.S. Pat. No. 4,481,662) is a commercial example of an equalized sealed system. This system comprises essentially a double integrator equalizer placed in the input signal path of a sealed system. This is an alternative to the above-mentioned Linkwitz Transform, and has all of the same shortcomings. In particular, this approach does nothing to improve ELSPL. Yet another equalized sealed system is described in Russell U.S. Pat. No. 3,715,501.
Ported systems can in principle be equalized, but in practice they virtually never are equalized. This is partly due to the greater difficulty of accurately equalizing a fourth-order system. More importantly, however, is the fact that it makes little sense to equalize a conventional ported system to achieve a lower f3, since the fb of a conventional ported system usually lies near the box tuning frequency, and the ELSPL drops off severely at frequencies below fb. For these reasons, it has heretofore usually been impractical to equalize ported systems.
One example of combining an “equalizer” with a ported system is claimed by Bose Corp. (U.S. Pat. No. 4,154,979). This is merely a variant of the well-known 6th order Chebeychev vented alignment originally described by Theile. This approach provides a small amount of bass extension at the expense of a much worse transient response. The active filter in this approach is essentially a second-order high-pass filter, unlike the low-pass equalizer characteristic of the present invention. This approach also does little for low-frequency SPL capability.
Known approaches and arrangements for achieving extended low-frequency performance are thus sub-optimal in one or more of the performance metrics that include f3, ELSPL, efficiency, box size and transient response. All of the above-mentioned approaches, techniques and inventions fail to realize the combined benefits of the present invention.
The present invention addresses the above limitations of known methods for providing low-frequency sound from loudspeaker systems. The present invention is directed to aspects relating to achieving extended low-frequency response and SPL capability in a loudspeaker system. It was discovered that a ported system acted much like a sealed system in regard to frequency response shape and rolloff slope over an extended band of low frequencies when the box tuning frequency was substantially lower than that commonly used with a given driver-box combination. It was further discovered that the low-frequency SPL capability of such a system was greatly improved, as compared with that of a similar sealed system, even at frequencies well below the 3 dB frequency response point of the driver-ported box combination. It is understood by those with ordinary skill in the art that the “3 dB frequency” is the frequency where the amplitude response is down 3 dB from a reference response or nominal response amplitude. It is further understood that this frequency is the frequency down to which a loudspeaker frequency response is said to be “flat”. Hereinafter we refer to such a ported system that has a frequency response similar to that of a sealed system as a Quasi Sealed System (QSS). We further define a Virtual Sealed System (VSS) as a sealed system design whose box volume and driver parameters have been manipulated so that its frequency response accurately models that of a Quasi Sealed System over the frequency range of interest.
In accordance with one aspect of the present invention, there is provided a method of achieving extended low-frequency performance in a loudspeaker system. The method comprises mounting a loudspeaker driver in a ported box, tuning the ported box to an unconventionally low box tuning frequency fb, and equalizing the resulting frequency response to become a desired frequency response that extends to lower frequencies than would be the case without the step of equalization. The combination of the driver, box, port and tuning frequency comprises a QSS as described hereinabove. One example of such a QSS has a frequency response that is accurately modeled by a second-order high-pass frequency response over a frequency range of interest that includes frequencies that are at least one-half octave below the box tuning frequency. The example QSS is further characterized by a frequency response at the box tuning frequency that is at least 6 dB down from the frequency response of the QSS at frequencies well above the box tuning frequency. The example QSS is still further characterized by a frequency response that is equal to within 1 dB of the frequency response of a second order high-pass function over frequencies extending to at least one-half octave below the box tuning frequency. The example QSS has a frequency response that is much more like that of a sealed system than that of a ported system. Applicant points out that not all QSS for use in the Invention necessarily have all of these example characteristics.
In a preferred embodiment, the tuning step of this method further comprises setting the box tuning frequency such that the frequency response of the driver-box combination at the box tuning frequency is substantially below the reference response level (e.g., −6 to −12 dB). More preferably, the resulting frequency response is a good approximation to a second order high-pass frequency response down to frequencies at least one-half octave below the box tuning frequency. In this preferred embodiment the combined driver, box, port and tuning frequency comprise a Quasi Sealed System (QSS) as described hereinabove. In this preferred embodiment, the step of equalization includes providing at least one biquadratic filter function providing at least two poles and two zeros in its frequency response. Preferably, the step of equalization further includes the step of computing the equalizer parameters in accordance with proper equalization of the Virtual Sealed System whose frequency response accurately models that of the Quasi Sealed System. The equalizer is characterized by a maximum equalization gain. The equalizer alters the frequency response of the QSS in such a way as to make the resulting frequency response flat to substantially lower frequencies than those to which the QSS is flat. This embodiment of the present invention, including the step of equalization, is therefore referred to hereinafter as an Equalized Quasi Sealed System (EQSS).
In yet another aspect of the present invention, there is provided a loudspeaker apparatus, comprising a loudspeaker driver for producing sound in response to an electrical signal, a box of a given enclosed volume for housing the loudspeaker driver, a port for tuning the box to a box tuning frequency, and an equalizer for altering the frequency response of the loudspeaker apparatus so as to achieve the desired frequency response. Preferably, the box tuning frequency is set so that the driver-box-port-tuning frequency combination comprises a Quasi Sealed System (QSS) as described hereinabove. One example of such a QSS has a second-order high-pass frequency response characteristic over a frequency range extending to frequencies that are at least one-half octave below the box tuning frequency. The example QSS also has a frequency response at the box tuning frequency that is down by at least 6 dB from the reference level discussed hereinabove. More preferably, the equalizer is a biquadratic filter providing at least two poles and two zeros in its frequency response. Stilt more preferably, the frequency response shape of said equalizer is such that, if it were applied to equalize a Virtual Sealed System that accurately models the Quasi Sealed System, it would yield the desired overall system frequency response, the desired frequency response being flat to substantially lower frequencies than those frequencies to which the OSS is flat.
For purposes of illustrating various aspects of the invention and to provide a further understanding of the method and system of the invention, together with the detailed description, the drawings show forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein:
Referring now to the drawings, wherein like numerals indicate like elements,
The first system, whose frequency response 21 is denoted by triangles in the graph, is a conventional sealed-box system typical of the prior art. It has a second-order frequency response rolloff with decreasing frequency at a rate of approximately 12 dB per octave. Its frequency response 21 is down 3 dB at approximately 110 Hz (not shown), relative to a reference level of 0 dB at higher frequencies. At a much lower frequency of 35 Hz, its response is down approximately 16 dB from the reference level.
The second system, whose frequency response 22 is denoted by squares, is a ported system also typical of the prior art. It includes a port that tunes the box to a box frequency fb of approximately 65 Hz. The ported system has an extended low-frequency 3 dB response as compared to the sealed system just described. Its response is down 3 dB at approximately 68 Hz. However, its response at the much lower frequency of 35 Hz is down about 19 dB, having a weaker response at this lower frequency than the sealed system. It has a fourth-order frequency response rolloff with decreasing frequency at a rate of approaching 24 dB per octave. Such a frequency response tradeoff between sealed and ported systems of the prior art is typical.
The third system, whose frequency response 23 is denoted by diamonds, is a system based on the EQSS method and apparatus of the present invention. It is a Quasi Sealed System (QSS) implemented with the same driver in the same box volume as the sealed system described hereinabove, but with a port added whose diameter and length cause the box to be tuned to a Helmholtz box frequency fb of approximately 37 Hz. The QSS arrangement has a second-order rolloff like that of the sealed system for most of the frequency range, but it has increased low-frequency response as compared to the sealed system. Its frequency response 23 is down 3 dB at 100 Hz as compared to 110 Hz for the sealed system. At the much lower frequency of 35 Hz, its frequency response is down only 13 dB, as compared to the conventional sealed system whose response is down 16 dB at the same frequency. The QSS system thus exhibits a 3 dB increase in efficiency at 35 Hz as compared with the sealed system. Although the QSS system is ported, it can been seen that its frequency response is much more like that of a sealed system than a ported system. It is for this reason that it is referred to as a Quasi Sealed System. Note also that the QSS response 23 is fully 6 dB stronger than the response 22 of the ported system at 35 Hz.
The frequency response of the QSS system is accurately modeled by a so-called Virtual Sealed System (VSS) consisting of a sealed box of volume 12 Liters and a 5.25-inch driver with the following Thiele-Small parameters: Vas=20 L; fs=43 Hz; Qts=0.33. The virtual sealed system is characterized by a critical frequency of 70 Hz, a 3 dB frequency f3 of 98 Hz, and a Q of 0.54.
Based on these observations, it should be understood by one of ordinary skill in the art that the Quasi Sealed System 9, although ported, acts like a sealed system, but with increased efficiency at low frequencies. It should also be understood that the frequency response of the QSS may be equalized in the same way as the Virtual Sealed System, using the same biquadratic filter function, since their frequency responses are essentially the same. If the response of the Quasi Sealed System is equalized to become a more desirable one, the EQSS apparatus of the present invention will be the result.
The second system, whose ELSPL 32 is denoted by squares, is a ported system also typical of the prior art, and identical to the ported system used in
Based on these observations, it should be understood by one of ordinary skill in the art that ported systems are not satisfactory for reproducing deep bass at frequencies below the box tuning frequency f3. Conventional ported systems with very low box tuning frequencies generally cannot be implemented in small boxes with small drivers.
The third system, whose ELSPL 33 is denoted by diamonds, is a QSS arrangement based on the EQSS method and apparatus of the present invention, and is identical to the one used in
Referring now to the ELSPL capability at frequencies ranging down to as low as 35 Hz, it is evident that the Quasi Sealed System is capable of an ELSPL of at least 91 dB SPL over this frequency range. This is 13 dB better than the 78 dB ELSPL capability of the sealed system at 35 Hz. This is a remarkable 21 dB better than the 70 dB ELSPL capability of the ported system at the same 35 Hz frequency.
It should be noted that a typical application of the 5.25-inch EQSS woofer apparatus in the above example might include a stereo pair of loudspeaker systems, each with two such 5.25-inch woofers. It is well known by those familiar with the art that each doubling of the number of identical drivers in a speaker system results in a 6 dB increase in ELSPL for the total system. The typical application described here involves two such doublings, for a total of four 5.25-inch drivers, resulting in a 12 dB increase in system ELSPL. Returning to
Further to the typical example above employing a total of four 5.25-inch drivers is the matter of system sensitivity and required amplifier power. Each 5.25-inch driver in the example arrangement has a reference efficiency of 90.3 dB SPL @ 1 Watt/1 Meter. In the EQSS arrangement, the unequalized frequency response and efficiency of the QSS are down 13 dB at 35 Hz, resulting in an operating efficiency of 77.3 dB SPL @ 1 Watt/1 Meter at a frequency of 35 Hz. It is well known by those familiar with the art that each doubling of the number of identical drivers in a speaker system results in a 3 dB increase in efficiency for the total system. The typical application described here involves two such doublings, for a total of four 5.25-inch drivers, resulting in a 6 dB increase in system efficiency as compared to that for a single driver. Therefore, the typical four-driver application cited by example here will exhibit a total system efficiency of 77.3+6=83.3 dB SPL @ 1 Watt/1 Meter at a frequency of 35 Hz. The system will therefore be capable of reaching its ELSPL of 103 dB at 35 Hz with slightly less than 100 Watts total input power, or 50 Watts per channel from a stereo amplifier.
Based on all of these observations, it should be understood by one of ordinary skill in the art that the EQSS method and apparatus of the present invention provides a very advantageous system-level tradeoff heretofore unavailable in low frequency loudspeaker design. Specifically, it permits the achievement of a higher ELSPL at low frequencies for a given combination of box size and loudspeaker driver than either a sealed system or a conventional ported system. Moreover, as is evident from
For purposes of illustration, and without limiting the scope of the invention, the subwoofer loudspeaker system 70 may be implemented with a 10-inch woofer 5 having the following Thiele-Small parameters: Vas=78 Liters; fs=34 Hz; Qts=0.32; Xmax=5.5 mm; effective diameter=21.5 cm. The subwoofer system 70 may further be implemented with a box of available volume Vb=28 Liters and a port that provides for a box tuning frequency fb=30 Hz. This combination of driver, box and port forms a Quasi Sealed System (QSS) whose frequency response is accurately modeled by a Virtual Sealed System (VSS) comprising a box with a volume of 25 Liters and a driver with the following Thiele-Small parameters: Vas=78 Liters; fs=30 Hz; and Qts=0.34. The model is accurate to within 1 dB down to frequencies as low as 17 Hz. The Quasi Sealed System is capable of producing 105 dB SPL or more down to frequencies as low as 30 Hz. The unequalized QSS has a frequency response that is down 3 dB at 60 Hz. With a proper biquadratic equalizer 3 providing a maximum boost of 12.3 dB, the frequency response of the complete EQSS arrangement thus formed is down 3 dB at 30 Hz with a system Q of 0.7. In preferred embodiments, the subwoofer crossover 71, the equalizer 3, and the power amplifier 4 would be implemented together inside the subwoofer enclosure on what is normally known as a subwoofer plate amplifier module 72.
It is notable that the woofer 5 employed in the subwoofer illustration above is a conventional woofer not specifically designed for a subwoofer application. For example, it has a value for Xmax of only 5.5 mm, whereas drivers designed specifically for the subwoofer application often have Xmax values in the range of 10-20 mm. Drivers designed to have large Xmax usually require much longer voice coils, causing less of the voice coil to reside in the magnetic gap at any given moment. This, in turn, results in reduced sensitivity and driver efficiency. A typical conventional subwoofer driver has an Xmax of 15 mm and an efficiency of 85.5 dB SPL @ 1 Watt/1 Meter. In contrast, the driver of the present EQSS subwoofer example of
The efficiency comparison at a low frequency of 40 Hz is also advantageous to the subwoofer operating in accordance with the present invention. The conventional subwoofer in a 28 L sealed enclosure has a response that is down 4.3 dB at 40 Hz, resulting in a 40 Hz sensitivity of 85.7−4.3=81.4 dB SPL @ 1 Watt/1 Meter. In contrast, the subwoofer designed in accordance with the present invention has an unequalized QSS response that is down 8 dB at 40 Hz, resulting in a 40 Hz sensitivity of 91.5−8=83.5 dB SPL @ 1 Watt/1 Meter, fully 2.1 dB better than the conventional subwoofer. The subwoofer designed in accordance with the present invention requires only 141 Watts of driving power from amplifier 4 to produce its ELSPL of 105 dB SPL at 40 Hz. This is a very modest amount of required amplifier power for a subwoofer housed in an enclosure that provides only one cubic foot of available volume. This demonstrates yet another advantage of the EQSS method and apparatus of the present invention, namely higher efficiency.
Referring now to
Although preferred systems have been described hereinabove, other combinations of equipment can be used without deviating from the scope of the present invention. For example, the method and apparatus of the present invention can be applied to two-way self-powered studio monitors. It should also be clear to those skilled in the art that all of the steps of equalization pertinent to the present invention may be accomplished with digital signal processing techniques without deviating from the scope of the present invention.
Yet another application of the method and apparatus of the present invention is that of automobile subwoofer systems, wherein the advantages of extended low frequency response, high SPL capability at low frequencies, and high efficiency afforded by the present invention are all of great value. Larger automobile subwoofer systems with a long box dimension of 24 inches or more especially benefit from the EQSS method because of the ease with which they can accommodate a long port of adequate diameter, providing for low box tuning frequencies.
Still yet another application of the method and apparatus of the present invention is that of Home Theater subwoofer-satellite speaker systems. The principles of the present invention are especially advantageous to such an application because the small satellite speakers in such systems often have very poor low frequency response as a result of their very small size, thus requiring the subwoofer to operate at frequencies higher than normal for subwoofers (e.g., upwards of 200 Hz). An EQSS subwoofer built in accordance with the principles of the present invention has improved high frequency response in comparison with conventional subwoofers because the loudspeaker driver of an EQSS subwoofer does not have to be optimized for a subwoofer application, meaning that its high-frequency response need not be compromised by use of, for example, a heavy cone with large excursion capability.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
This application is based on and claims the benefit of U.S. Provisional Application Ser. No. 60/516,803, entitled SYSTEM AND METHOD FOR ACHIEVING EXTENDED LOW-FREQUENCY RESPONSE IN A LOUDSPEAKER SYSTEM, filed Nov. 3, 2003, the entire disclosure of which is incorporated by reference herein.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3715501 | Russell | Feb 1973 | A |
| 4154979 | Barker | May 1979 | A |
| 4481662 | Long et al. | Nov 1984 | A |
| 6566960 | Carver | May 2003 | B1 |
| 20020061114 | Croft, III | May 2002 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20050094829 A1 | May 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60516803 | Nov 2003 | US |
