How Loud is Your Ultrasonic Processor?

The effect of operating an ultrasonic liquid processor (sonicator) on hearing is a question that arises often during discussions with our customers. The noise made by sonicators can be described as intense "hissing", which, if precautions are not taken, can be loud enough to cause significant discomfort or even lead to hearing loss. We performed a series of noise level measurements to determine how loud different types of ultrasonic processors really are.

Understanding Noise

Noise is unwanted sound, which is commonly a superposition of pressure waves of different frequencies (pitches) that travel through a medium (most commonly air) and can be detected by a person's ear. The loudness or intensity of noise is expressed in dBA (weighted decibels), which is a unit that takes into account sound intensities at all audible frequencies, weighted to correspond to the frequency response of human hearing. The decibel is a logarithmic unit, a number that represents how much a sound differs from a reference point. The louder the sound, the greater is its decibel value. A 3 dBA increase in noise is just barely perceptible to the human ear, while a 10 dBA increase generally feels as a doubling of loudness [1].

How Loud is Too Loud?

• The human ear can tolerate sound up to about 120 dBA;
• Prolonged exposure (over 8 hours) to any noise at or above 85 dBA is associated with gradual hearing loss;
• It is recommended that regular unprotected exposure to 100 dBA of noise be minimized to 15 minutes or less;
• Regular unprotected exposure to noise at 110 dBA for more than 1 minute can cause permanent hearing loss.

Common Sources of Noise 

Some common sources of noise are presented in the table below [2]:

Ultrasonic Noise Measurements Performed at Our Facility: Materials and Methods

The experiments were conducted at Industrial Sonomechanics' product testing facility. We tested four ultrasonic processors operating in both batch and flow-through modes at the frequency of approximately 20 kHz. Noise level data were collected at the distances of two and twenty feet from the processors. Ultrasonic amplitude settings of 90 and 40 microns were tested with water, soybean oil and propylene glycol using each processor. 

Results and Discussion

Neither the amplitude setting nor the type of tested liquid had significant effects (< 3 dBA) on the observed noise levels. The results presented in the table below are for water at the amplitude setting of 90 microns. 

This data clearly shows that operating an ultrasonic processor without adequate hearing protection for both the operator and other workers in the vicinity of the equipment is unsafe. Since additional sources of noise may worsen the problem, professional assessment should always be performed in each specific work environment to determine what noise reduction is required for compliance with local regulations.  

Recommended Noise-Reduction Methods