INTRODUCTION
What is Acoustic Noise? Acoustic noise is made up of pressure waves traversing through gases, such as air. Acoustic energy that can be heard is commonly referred to as sound. Human voices, vehicle traffic, wind, pumps and other machineries are the main sources of acoustic noise.
EFFECTS OF ACOUSTICS AND AIR CURRENTS ON IMAGING INSTRUMENTS
Acoustic noise can turns into structural vibration by coupling into an instrument or any mechanical structure. Hence, it is difficult to differentiate the effects of environmental acoustic noise from structural vibrations.
Instruments are affected by acoustic noise in different ways, based on their sensing method and degree of precision. Acoustic noise can cause blurred images or saw-tooth patterns along feature edges for imaging instruments such as AFM and SPM, and can decrease measurement accuracy for quantitative instruments. It can even completely affect the measurement process in extreme cases. Moreover, it can decrease the repeatability of measurements because of the fluctuation in noise levels throughout the day.
CHALLENGES OF ACOUSTIC ISOLATION
As vibrations occur through the structure or through the ground, they can be prevented by detaching the sensitive equipment from the noise source or by placing an isolator in between the two. However, sound waves come from all directions and there is always a chance of acoustic transmission if an air passage is available. Hence, the ideal acoustic barrier is airtight, which leads to challenges with respect to access, visibility and usability when enclosing the equipment that is in use.
Noise may cause reverberation, which is the persistence of sound due to the reflection of sound waves off of surfaces even after the removal of the original sound. The size, shape and presence of objects in a location affect its acoustic characteristics. Hence, it will be difficult to predict the behavior of sound in any given environment and to design an acoustic solution suitable for all environments.
Although the audible range is between 20 Hz and 20 kHz, troublesome noise can exist below or above this range. The application of research grade microphones, data analysis software and site survey equipment for acoustic data analysis is the ideal method for measuring noise in a given location. However, together with vibrations, it is a challenge to predict how an acoustic isolator will perform during its use with specific equipment in any given location.
SOLUTIONS FOR ISOLATING ACOUSTIC NOISE
Eliminating the noise source, i.e., turning off the offending part of the equipment or introducing a low noise policy while performing measurements, is the ideal approach of reducing the impact of acoustic noise. However, some acoustic noise sources are intrinsic to the environment, such as a building’s air conditioning equipment or external wind. In some cases, where it is impractical to eliminate acoustic noise sources, the testing setup itself has to be isolated.
Acoustic curtains or basic baffles are economical options as they are capable of deflecting acoustic energy transmitted from an adjacent source. However, only minimal reduction is possible using these partial enclosures. Constructing an acoustic box utilizing Styrofoam, cardboard, and plywood is another option. These low-end solutions are effective for less sensitive applications.
Although active noise cancellation has not yet been widely adopted for analysis equipment, these systems have been empolyed in other areas, such as noise cancellation headphones. The application of active noise cancellation on larger scales is less effective because of the challenge in separating multi-directional sounds, which are complex to characterize.
Providing an enclosure is the most efficient method to isolate research equipment from offending noise as it reduces the highest level of noise over the broadest spectrum. High performance acoustic enclosures provide a virtually airtight seal around the equipment to prevent the path of transmission of acoustic energy.
When assessing a soundproof hood, the most important thing is for operators to know how they are going to use their equipment, to ensure that all the requirements for the particular environment are met.
