Creating a balanced acoustic environment in small rooms often requires the strategic placement of sound absorbers. Among the various options available, porous fibrous materials have gained popularity for their ability to provide broadband absorption. In this blog, we will explore the depths of porous fibrous sound absorbers and understand their connection to low-frequency absorption.
Low-frequency sounds pose a unique challenge for absorption due to their longer wavelengths. To comprehend the relationship between absorber depth and low-frequency absorption, let's dive into the physics of sound.
Sound waves exhibit a characteristic known as wavelength (λ), which represents the spatial distance between successive points of similar displacement. The wavelength is inversely proportional to the frequency (f) of the sound wave, following the formula: λ = c/f, where c is the speed of sound in air (~343 meters per second).
For instance, consider a low-frequency sound with a frequency of 50 Hz. Using the equation above, we can calculate its corresponding wavelength as follows: λ = 343/50 = 6.86 meters.
This means that the sound wave will complete one full cycle of compression and rarefaction over a distance of approximately 6.86 meters.
To effectively absorb low-frequency sounds, the theoretic depth (D) or thickness of the porous fibrous absorber should be comparable to or greater than 1/4th of the wavelength of the targeted frequencies. In our example, the recommended depth would be 1/4th of 6.86 meters, which is approximately 1.72 meters - ridiculous, right?
However, in real-world practice, smaller fractions, such as 1/16th of the wavelength, have been found to yield satisfactory results coming out of the fact of incidence angle of sound, air resistance, and total dissipation of reflected sound before reaching the listener.
But to be on the safer side, and ensure reliable performance, it is safe to use 1/12th of the wavelength as the depth of the absorber, which translates to approximately 57 centimeters. For most of you, I'm sure this small fraction is way too much compromise on real estate. But that's physics. You can't outrun it. This exactly is the reason why we build our commercially available Standard Porous Velocity Absorbers no shallower than 18 centimeters which we have tested to observe great absorption down to 180 Hz and substantial performance can be observed even in the lower range. However, we always recommend our clients to go for at least the 33-centimeter deep one if their budget and space allow.
All of our products are based on precise calculations and tests, and below here is the absorption graph of our 18 cm deep Velocity Absorber - Gray(TM)VELO S:
The primary mechanism by which porous fibrous absorbers work is through friction and viscous losses. As sound waves encounter the porous structure, they penetrate the material and interact with its fibers. The depth of the absorber directly influences the amount of interaction between the sound waves and the material. In the case of low-frequency sounds, the longer wavelengths require a greater depth to ensure sufficient interaction and absorption. With a thicker absorber, the sound waves must travel through a more extended path within the material. This increased path length provides additional opportunities for energy dissipation through friction and viscous losses, resulting in improved absorption. However, it is important to note that the relationship between absorber thickness and low-frequency absorption is not linear. While increasing the thickness enhances absorption to a certain point, beyond that threshold, further increases in thickness may not yield significant improvements. Striking the right balance between practical considerations and desired absorption performance is essential.
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