Loudspeaker system

Details Loudspeaker system Loudspeaker system

2000-03-03

2004-12-21

Harvey, Minsun Oh (Department: 2747)

Electrical audio signal processing systems and devices

Including frequency control

Having crossover filter

C381S098000, C381S402000, C381S184000, C381S186000, C381S399000, C181S144000, C333S132000

Reexamination Certificate

active

06834113

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to a loudspeaker system, and in particular to a system with the ability to create a homogeneous sound over large distances.
RELATED ART
In the field of sound reproduction the audio signal has to pass through a number of different devices, such as microphone, mixer table, amplifier, crossover filter and loudspeakers, before it reaches the listener. In some of these devices the processing only deals with an electrical signal, such as in the mixer table and in the amplifier, and this processing does not significantly affect the quality of the signal. However, the two devices that carry out the conversion from sound waves to an electrical signal and back again, i.e. the microphone and the loudspeaker, must include mechanical parts, and therefore these devices represent the weakest elements in the sound reproducing chain. To improve the sound quality of a sound reproduction system, it is usually best to put effort in improving these two devices, especially the loudspeaker.
It is known that the loudspeaker is the weakest element in the sound reproducing chain. This is especially true in the field of public address (PA) systems, namely systems for addressing many people over large distances, i.e. at big events such as rock concerts, sports events and the like. It can also involve the transmission of verbal information to customers in a mall, or passengers at an airport or a railway station.
One important aspect when constructing a PA-system, is to make sure that all frequencies (20 Hz-20 kHz) reach the ear of a listener simultaneously and at the same level, wherever in the audience he or she is located. This aspect is generally overseen in systems of this kind, as loudspeakers that reproduce different parts of the frequency range often are placed at a large distance from each other. An example of this, is that the low frequency loudspeakers normally are placed on the stage, while mid and high frequency loudspeakers are arranged hanging above the stage. Arrangements of this kind further represent a large array of point sources, with large interference problems as a result. This is especially true for the common fan-shaped arrangement, used to increase the horizontal coverage, but such an arrangement suffers from severe deviations in time, phase and frequency response.
Due to the large distances and large areas at concerts, the car will experience the time and phase deviations as annoying. Thus, incorrect placement of the loudspeakers causes large deviations in frequency and phase, which in conjunction with the loudspeaker distortion result in that the sound gets tiring for the human ear.
Common technology is furthermore limited when it comes to producing high sound-pressure over long distances. It is known that the sound-pressure from a conventional PA-system drops dramatically after about 50 meters, because of interference. The normal approach used to compensate for this is to increase the efficiency of the loudspeaker, for example by using horns, but that only generate a higher sound-pressure close to the speaker, and not necessary at large distances.
A known technique to achieve directed sound energy, is to use a line-source, which is described more in detail in “Multiple-array loudspeaker system.”, E. J. Jordan, Wireless World, March 1971, pp 132-134 and “Audio cyclopedia”, H. M. Tremaine, Mar. 29, 1977, pp 1153-1156. If a number of speaker units (three or more) are mounted to form a linear array with minimum spacing between the speaker units, the sound energy emanating from the speaker units tend to be directed perpendicularly to the long axis of the array. Thus, in a speaker-system, if the speaker units are arranged in a vertical array, vertical dispersion of the sound is minimized and the sound can be concentrated in the direction of the listeners. The vertical direction characteristics of a (vertically orientated) line-source
1
are shown in
FIG. 1
, wherein the solid line represents an idealized distribution (
2
a
). In practice, due to the fact that the radiating area is not a continuous line but is made up of discrete units, at frequencies where the wavelength is comparable to the physical spacing between the speaker units, the vertical distribution splits up into lobes. The main forward facing lobe
2
b
becomes excessively sharp and upward and downward lobes (
3
a
,
3
b
) appear (broken lines in FIG.
1
). The common method of overcoming this is to grade the electrical power fed to the speaker units, so that the centre speaker receives the maximum power, the adjacent speaker units above and below receive say √2 of this power and so on. Another way to minimise the unwanted lobes at high frequencies, is by frequency grading. Thereby, the centre speaker receives the full frequency range and the high frequencies are progressively reduced for units away from the centre. A system of this land is disclosed in U.S. Pat. No. 3,138,667.
This method then only uses the advantages of the line-source configuration at lower frequencies, whereas it represents a conventional single point source at high frequencies, with the result that the directional effect is gradually lost in the higher frequency range.
If the distances between the speaker units in a line-source gradually are made smaller, the line-source eventually becomes a continuous line-source, which can be seen upon as an idealised line-source. There are several known types of speaker types that could be used to construct an idealised line-source, such as ribbon, electrostatic, magnetostatic. However, these speaker types are typically limited in the low frequency response.
One way to overcome the limitations with the line-source arrangements described above, is to combine a low frequency line-source and idealised line-source capable of reproducing high frequencies. One system of this kind is V-DOSC™ by Heil Acoustics, France. Information concerning this system can be found at the homepage of Coxaudio and in the preprint #3269 “Sound fields radiated by multiple sound source arrays” presented at the 92
nd
Audio Engineering Society (AES) convention in Vienna.
The underlying theory which is presented in preprint #3269, defines the acoustic coupling conditions for successfully arraying individual sound sources, including wavelength, the shape of each source, their surface areas and their relative distance. Briefly, the coupling conditions according to this theory can be summarized as follows:
An assembly of individual sound sources arrayed following a regular step distance on a planar or curved continuous surface is equivalent to a single sound source leaving the same dimensions as the total assembly if one of the following two conditions is fulfilled:
1. Frequency: The step distance (the distance between the acoustic centers of individual sources) is smaller than the wavelength.
2. Shape: The wavefronts generated by individual sources are planar and together fill at least 80 percent of the total radiating surface area.
The V-DOSC™ system is a modular line-source system where two or more sub-units have to be arranged on top of each other to create a line-source. However, this system has a limited horizontal coverage of 90°, and due to the construction with two line-sources mounted in a V arrangement with a high frequency horn in between, the system suffers from phase and time deviations. Due to that the line-source and the horn produce sound with different compression levels, the system is not capable of producing a linear frequency response at large distances.
Another big problem that conventional systems, as well as the V-DOSC™ system, suffers from if they are not placed correctly, is that the highly directed sound produces a great deal of early refections, or if unavoidable obstacles are present.
SUMMARY OF THE INVENTION
According to the present invention there is provided a loudspeaker system for creating a homogenous sound over large distances with a wide horizontal distribution comprising at least one line-source of acoustical radiation,

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