In audio equipment like microphones，sound box for toys or speakers, acoustic cavities play a crucial role in shaping the sound characteristics. They can enhance or attenuate certain frequencies, affect resonance, and contribute to the overall timbre of the sound produced.
The same sound source, the same speaker in different sound cavity in the playback effect of the tone may vary greatly, some more pleasant, some are more monotonous. Reasonable sound cavity design can make the music sound more pleasant.
The sound cavity design normally includes the following five parts: rear sound cavity, front sound cavity, speaker holes, airtightness, dust mesh and so on.
Let’s talk the effects of these components on sound one by one.
First the effect of the rear sound cavity on the sound.
In speakers, the rear sound cavity, often referred to as a port or bass reflex chamber, helps to reinforce lower frequencies. This design allows the speaker to produce more pronounced bass sounds by utilizing the air pressure from the back of the speaker cone to enhance the output of lower frequencies.
Changes in the shape of the rear sound cavity have little effect on the frequency response curve. However, if a certain part of the rear sound cavity is flat, thin or long, then that part may generate standing waves in a certain frequency band, which will deteriorate the sound quality drastically, therefore, it must be avoided in the design of the sound cavity.
Note: When designing the rear cavity, it must be ensured that the rear sound hole is free from air, i.e. the distance of the rear sound hole from the nearest baffle should be greater than 0.8 times the diameter of the rear sound hole. The volume of the rear cavity should be as large as possible .
Second, the influence of the front voice cavity on the sound
The front voice cavity has little effect on the low frequency range and mainly affects the high frequency part of the musical sound. Too large or too small a preamplifier cavity will have an unfavourable effect on the sound. At the same time, due to the sound hole area on the high frequency also has a greater impact, so the design of the front cavity, need to consider the area of the sound hole. In general, the larger the front cavity, the larger the area of the sound hole should also be.
When the front sound cavity is too small, it will also cause a problem, that is, the position of the sound outlet hole has a sharp increase in the degree of influence on the high frequency, which may cause some difficulties in the appearance of the design.
Combined with the actual situation of the design, it is generally hoped that the thickness of the gasket of the front sound cavity after compression will be between 0.3 and 0.5mm.
Third,the influence of the area of the speaker holes.
The area of the sound hole (i.e. the total projected effective area on the front of the speaker) has a great impact on the sound, and the location and distribution of the holes also have a certain impact on the sound, the extent of which is very much related to the volume of the front sound cavity. In general, the larger the front sound cavity, the smaller the influence of the location and distribution of the holes on the sound.
The area of the sound hole has an effect on all frequency bands of the frequency response curve, and under different conditions, the degree of influence on different frequency bands varies. When the area of the sound outlet hole is smaller than a certain threshold, the SPL value of the whole frequency response curve will drop sharply, i.e. the loss of sound intensity of the music sound is very large, which must be prohibited in the design. When the area of the sound outlet hole is larger than a certain threshold, as the area increases, the high frequency wave peak and low frequency wave peak will move to the right, but the degree of change in high frequency is much larger than that in low frequency, and the change in low frequency is very small, i.e., the change in the area of the sound outlet hole mainly affects the high-frequency performance of the frequency response curve, and does not have much effect on the low-frequency performance.
It should be emphasised that if the outlet holes are more evenly distributed and over-centred within the projection of the front sound cavity, then a smaller value can be taken, otherwise a value on the larger side should be taken. It is recommended that, in general, the limit of the effective range should not be taken.
Fourth,the effect of rear sound covity airtightness on sound
Whether or not the rear sound chamber is effectively airtight has a big impact on the low-frequency portion of the sound.
When the rear cavity leaks, the low frequency will be attenuated, causing damage to the sound quality, the extent of its impact and the leakage area, location have a certain relationship, mainly refers to the internal composition of the cabinet cavity or leakage holes on the Speaker's performance.In general, the larger the leakage area, the greater the low frequency attenuation.
In summary, it is recommended that the structure should be designed in such a way as to ensure that the rear sound chamber is as airtight as possible, otherwise the sound quality may be seriously affected.
Fifth the effect of dust nets on sound
Compared to several other factors, the dust net affects the sound to a lesser extent, it mainly affects the low-frequency peaks and high-frequency peaks of the frequency response curve, with a greater impact on the low-frequency peaks.
The extent to which a dust screen affects sound depends mainly on the acoustic resistance of the screen and the size of the low-frequency and high-frequency peaks. In general, the larger the peak, the greater the degree of attenuation by the dust net.
Dust nets have two main functions, to prevent dust and weaken the low-frequency peaks to protect the speaker.
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