Fact: Attenuation performance depends on source characteristics, so it isn’t a single value for all sources
The performance of any attenuation system varies with different frequencies. This means that the amount of attenuation achieved depends on the frequency of the sound being attenuated. In most cases attenuation performance is less at lower frequencies than for high frequency sound. Therefore an attenuation system’s performance will be less for a noise source that produces sound with predominately low frequency than for one producing higher frequency sound.
A standard weighting system is used to provide single-figure attenuation data so that the performance of different materials can be compared. However, while being useful for its intended purpose, these single figure Rw values can cause confusion and difficulties if they are then assumed to be the amount of attenuation that the material will provide for a specific application, without appropriate corrections being applied for frequency content, or surface area, for example.
The attenuation actually achieved by an acoustic enclosure is also affected by other factors, such as vibrational energy, which can then be re-radiated as sound from the body of the enclosure.
In reality, the amount of attenuation provided by an acoustic enclosure will depend upon several factors, including the frequency content of the sound to be attenuated, the amount of vibrational energy the source produces, and even the level of attenuation that is expected.