Since the invention of the loudspeaker, people have been working hard to extend its frequency range to both ends. The upper end of the high frequency is now better solved by means of small-diameter light diaphragm and other means, but the reproduction of the lower end of the low frequency still needs help In the bulky box cavity. The sound pressure level of the reproduced sound at the low-frequency end is related to the amount of air that the speaker diaphragm can push. The volumetric flow velocity is the product of the diaphragm's radiation speed and the area, so if the smaller diaphragm has a longer moving distance --- —Stroke, the same low-frequency sound pressure level as a large cone can be obtained, and a deep and powerful bass can be emitted. In order to obtain better bass performance, the low frequency speaker needs to use a cabinet to work normally. The appearance of the sound box is varied, the most common ones are rectangular, and the box structure mainly includes several types such as closed box, reflecting box, transmission line, passive radiator, coupling cavity and horn.
Closed enclosure (Closed Enclosure) is the simplest speaker system. It was proposed by Frederick in 1923. It consists of a speaker unit installed in a fully sealed cabinet. It can completely isolate the forward and backward radiated sound waves of the speaker. Due to the existence of the closed box, the rigidity of the resonance of the motion mass of the speaker is increased, and the lower resonance frequency of the speaker is increased. The sound and color of the airtight loudspeaker is a bit deep, but the bass analysis is good. When using ordinary hard folding ring loudspeakers, in order to obtain satisfactory bass reproduction, a large box with large volume is required. The new airtight loudspeaker uses the enclosed box The elastic effect of the compressed air quality, although the speaker is installed in a smaller box, the air cushion behind the cone will exert a counter-driving force on the cone, so this small airtight speaker is also called an air-cushioned speaker.
Bass-Reflex Enclosure (Bass-Reflex Enclosure) is also called Acoustical Phase Inverter, invented by Thuras in 1930. In its load, there is a sound outlet opening on a panel of the cabinet. The opening position and There are many shapes, but most of them are equipped with acoustic tubes in the holes. According to the Helmholtz resonance principle, the relationship between the internal volume of the box and the sound duct hole produces resonance at a certain frequency, which is called the maximum vibration frequency. The sound waves radiated from the rear of the loudspeaker are inverted through the duct, and then radiated from the sound outlet to the front, and superimposed in phase with the sound waves radiated from the front of the loudspeaker. It can provide a wider bandwidth, higher sensitivity, and smaller Under ideal conditions, the lower limit of the low-frequency playback frequency can be as much as 20% lower than the speaker resonance frequency. This type of speaker can reproduce rich bass with a smaller cabinet, and is currently the most widely used type.
Acoustic resistance enclosure (Acoustic resistance Enclosure) is essentially a deformation of an inverted enclosure. It is filled with sound-absorbing materials or structures in the sound outlet duct, which acts as a semi-enclosed box to control the phase inversion and buffer it. Reduce the maximum vibration frequency to broaden the bass replay frequency band.
The transmission line speaker (Labyrinth Enclosure) is named after the transmission line of classical electrical theory. There is a sound tube made of sound-absorbing wall panels on the back of the speaker, and its length is one-quarter or eighth of the wavelength of the low-frequency sound that needs to be boosted. one. In theory, it attenuates the sound wave coming from behind the cone, preventing it from being reflected to the open end and affecting the sound radiation of the woofer. But in fact, the transmission line speaker has a mild damping and tuning effect, which increases the sound output of the speaker near or below the resonance frequency, and reduces the amount of stroke while enhancing the bass output. Usually the sound ducts of this kind of sound box are mostly folded in a labyrinth shape, so they are also called labyrinth or tortuous.
Passive radiating speaker (Drone Cone Enclosure) is a branch of bass reflex speaker, also known as empty cone speaker. It was published by Olson and Preston in the United States in 1954. Its opening sound outlet was replaced by an empty paper cone (passive cone) without a magnetic circuit and voice coil. The radiated sound generated by the vibration of the passive cone and the forward radiated sound of the speaker In the in-phase working state, the composite sound and passive cone quality formed by the air in the box and the passive cone support elements are used to form resonance to enhance the bass. The main advantage of this kind of speaker is that it avoids the unstable sound produced by the reflected sound hole, and it can obtain good sound radiation effect even if the volume is not large, so the sensitivity is high, which can effectively reduce the working range of the speaker, and the standing wave affects the sound. Clear and transparent.
The coupled cavity speaker is a cabinet structure between the closed type and the bass reflex type. It was published by Henry Lang in the United States in 1953. Its output is output by the sound hole driven by one side of the cone, and the other side of the cone is connected with a Closed-box coupling. The advantage of this kind of speaker is that the amount of air pushed by the speaker at low frequencies is greatly increased. Since the coupling cavity is a tuning system, when the cone movement is restricted, the output of the sound outlet does not exceed the sound output of a single cone, which broadens the low frequency weight. Amplify the range, so the distortion is reduced and the withstand power is increased. In 1969, the A·S·W (Acoustic Super Woofer) speaker published by Yukihiko Kawashima of Lo-D in Japan is a coupled cavity speaker, suitable for reproducing bass without distortion with small-diameter long-stroke speakers.
Horn type enclosure (Horn type Enclosure) for household type, mostly adopts the form of folded horn (Folded Horn), its horn mouth is coupled with a larger air load at the mouth, and the driving end has a small diameter. The back of this type of speaker It is fully sealed, and the pressure in the chamber is as much as the back of the speaker cone. In order to keep the pressure balance between the front and rear of the cone, an inverted horn is installed in front of the speaker. The folding horn speaker is a derivative of the phase-inverted speaker, and its sound effect is better than the closed speaker and the general bass reflex speaker.