What Happens When Sound from an Electroacoustic Product Passes Through Openings in the Customer’s Mechanical Structure?
Compared with operation in an open space, the SPL will decrease because of the additional acoustic resistance introduced by the perforated plate in front of the product.
The openings in a perforated plate can be treated as acoustic mass, which generally creates greater acoustic impedance at high frequencies than at low frequencies.
A researcher conducted the following experiment:
Within the frequency range of 1 to 20 kHz, the results show that when all other conditions, including the open-area ratio, perforated plate thickness, and perforated plate diameter, are kept the same:
(1) With the same total perforation area, hole diameter, and number of holes, it was observed that a thicker perforated plate results in greater acoustic loss and a higher f0. (For further discussion of f0, refer to the following study on hole spacing.)
(2) With the same total perforation area and perforated plate thickness, it was observed that a larger diameter for each hole, meaning fewer holes, results in greater acoustic loss and a higher f0.
(3) With the same perforated plate thickness and individual hole diameter, increasing the number of holes, meaning a higher perforation ratio, results in lower acoustic loss and a higher f0.
(4) Under the conditions described above, acoustic loss increases at higher frequencies. This is believed to be caused by the acoustic mass and inertive reactance of the acoustic openings, which provide greater impedance at high frequencies.
Regarding item (2), many people may find the result questionable because it seems counterintuitive. Shouldn’t a larger hole result in lower acoustic loss?
However, the first thing to understand is that the total perforation ratio is kept constant. Under this condition,
as the hole diameter increases, a larger sound pressure difference develops across each opening. Large holes can generate high-pressure air vortices around their edges, resulting in greater acoustic resistance.
When the total perforation ratio is kept constant, larger holes may actually increase acoustic resistance. This conclusion is also supported and confirmed by another phenomenon.
When the total perforation area and number of holes in a perforated plate are fixed, placing all the holes close together makes them behave like one large opening.
In this case, the “effective aperture area after correction” becomes larger than the “original aperture area.” This increases the acoustic mass and the pressure of the air vortices, resulting in greater acoustic resistance and, therefore, greater acoustic loss.
The author of this study also conducted several experiments, as shown below:
The results show that the more closely spaced the holes are, the greater the corrected effective acoustic mass becomes, and the lower the f0 will be.
(To lower the f0 of a vented enclosure, the acoustic mass must also be increased, for example by increasing the vent length or plate thickness. This is consistent with the theories described above.)
Therefore, to reduce acoustic resistance, the holes should be spaced farther apart when the total perforation area and individual hole diameter remain the same.