The open speaker box is also referred to as the "open back speaker." It has a long history and can be said to be the originator of all kinds of modern speakers. The radio has been used almost all kinds of speakers since its birth.
When we don't install a "baffle" on the woofer, the speaker in the free-radiation mode, the lower-band sound emitted by the woofer cancels each other out during the air compression and sparse process on both sides of the diaphragm, so there is no sound radiation. It is called "sound short circuit".
The easiest way to avoid an acoustic short is to mount the speaker on the baffle. The shape of the baffle can be various. The larger the baffle, the richer the bass.
If we have an "infinite baffle" on hand, we can get the ideal low-frequency radiation without acoustic short circuit.
However, in actual use, we can only get the "limited size of the baffle", but this does not matter, as long as the size is large enough, greater than half the wavelength of the frequency we care about, it can be considered to be infinite.
If the baffle we use is circular and the speaker is installed in the center of the baffle, at a certain frequency, the frequency below it will produce a short circuit of sound. This frequency is called the cutoff frequency.
The open speaker box is widely used in radios, tape recorders, and television sets. The speaker box is formed by a combination of a small baffle and a sound pipe. It is the same principle as the planar baffle. The sound waves emitted from the front and rear of the speaker are anti-phase, and the subsequent sound waves can radiate to the front without offsetting.
Since there is a cutoff frequency, a large speaker is required in order to reproduce the bass.
Different from the flat baffle, due to the shape of the box and a certain depth, there may be a simple vibration mode caused by the reflection of each side of the speaker, which will form a series of peaks and valleys.
Therefore, the size and shape of the cabinet and the installation position of the speaker must be carefully considered to prevent the normal vibration from being excited, especially to avoid the "merger" of the normal vibration, which would result in severe acoustic dyeing. The specific method is that the size ratio of the cabinet should be based on the principle that it cannot be integral multiples, and the center of the speaker should be offset from the center of the front panel.
In order to avoid resonance caused by the standing wave in the box during design, the box plates should be kept as parallel as possible, and the sound absorbing material should be attached to the box.
In the modern speaker design, even in some international famous brand products, we can often see the trace of the open speaker box.
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