Hello,

Below is an excerpt from:

Yılmaz, N.D., Powell, N.B., “Biocomposite structures as noise control elements”, in “Green Biorenewable Biocomposites: From Knowledge to Industrial Applications”, Ed. Thakur, V.K., Kessler, M., Apple Academics- CRC Press, USA, ISBN 9781771880329, p. 161. , 2015

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There are three major methods to reduce unwanted noise. Primary methods consider modifications at noise and vibration sources. Secondary methods include alterations along the sound propagation path, and tertiary methods engage in sound receivers. Primary methods are restrained by economical and technical parameters to a great extent; while tertiary methods have to deal with each receiving person separately. This situation renders the secondary methods relatively advantageous in a number of applications.3,10 The secondary methods concerning the control of airborne noise include the use of sound barrier and absorbers.11.

Sound absorbers are porous materials. [...] Noise is attenuated in tortuous channels of pores present in the porous materials due to viscosity and heat conductivity of the medium.12,13 Porous sound absorbers can be classified into three groups: cellular, granular and fibrous materials.

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And below is from 

S.R.Jagatap, S.H.Kulkarni, C.D.Shete, R.Kollam, Low Frequency Noise Control of Diesel Generator Sets using Helmholtz Resonators, Proc. of Int. Conf. on Recent Trends in Mechanical, Instrumentation and Thermal Engineering 2012

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Some basic strategies for reducing generator set high frequency noise are reduce the sound level of the source, acoustic barriers, acoustic insulation, isolation mounts, cooling air attenuation, exhaust silencers, efforts to maximize the distance between the generator set and the property line (or people). But low frequency noise remains untreated due to their longer wavelengths in above practices. ANC (Active Noise Control) [2] has been extensively researched but failed to develop a mature technology to be used for complex environments likes power generator acoustic. PNC (Passive Noise Control) system as Helmholtz resonators (HRs) have been used in various applications for low frequency noise [1].

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And below is from :

http://www.advanced-noise-solutions.co.uk/cs-low-frequency-noise.html

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Low frequency noise is particularly difficult to control and reduce. The reason is that the performance of common types of noise control solution such as a silencer, enclosure, screen, barrier, etc. is poor at low frequencies. Because it involves suppression of, or breaking up the noise generating mechanism at source, an advanced noise control solution can provide a high noise reduction at any frequency. Often, it completely eliminates the low frequency noise drone, hum or whine.

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Best Regards,

Nazire

Thank you

Good answer from Nazire. Anyway, there is not a "perfect formule". All solutions depend on the noise source (type, spectral, LW, position, how it is handed), the receiver and how many dB's you need or want to reduce. Do you have any specific noise problem? Let me know. Cheers