Noise
The noise in ultrasonic washing systems
The issue of noise in ultrasonic washing is quite common. All washing systems typically produce a distinctive whistling sound that is audible even at significant distances and in already noisy environments.
To minimize the noise emitted by their systems, manufacturers must employ specific techniques. Customers should then choose those systems that are actually less noisy. This is also important because the responsibility for the safety of the systems rests entirely and solely with the employer of the production unit where they are installed.
The Legislative Decree of April 10, 2006, No. 195, published in the Official Gazette on May 30, 2006, introduced into Legislative Decree No. 626/94 the new Title V-bis “Minimum requirements for the protection of workers against risks to health and safety arising from exposure to noise at work, particularly regarding hearing,” implementing the measures provided for by Directive 2003/10/EC on the exposure of workers to physical agents (noise).
Legislative Decree No. 277/91, which initially established the obligation to assess noise risks and prepare a preventive tool, the Risk Assessment Report, is repealed in its parts concerning noise. With the entry into force of Legislative Decree No. 195/06, the risk assessment for noise, as an integral part of Legislative Decree No. 626, remains an obligation of the employer to be fulfilled in accordance with the provisions of Article 4.
The new law establishes a limit value for noise exposure and two action values:
| Daylight exposure level (Lex/8h) in db(A) | Peak acoustic pressure | |
|---|---|---|
| lower action value | 80 | 112 Pa pair to 135 db(C) |
| top action value | 85 | 140 Pa pair to 137 db(C) |
| exposure limit | 87 | 200 Pa pair to 140 db(C) |
In fulfilling the new law then the employer eliminates the risks at source or minimizes them, and in any case at levels not exceeding the exposure limit values. The law setting three noise thresholds (80, 85 and 87 dbA) allows to identify four noise exposure classes for workers:
MEDIUM DAILY VALUE (LEQ/G) IN DB (A) – RISK CLASS
| MINOR 80 | RISK FAILED |
|---|---|
| BETWEEN 80 and 85 | SLIGHT RISK |
| BETWEEN 85 and 90 | RISK CONSISTING |
| OVER 90 | SERIOUS RISK |
Since ultrasonic washing systems can be quite noisy, it is very easy to exceed the aforementioned limits. For this reason, the customer should consider purchasing the least noisy ultrasonic washing system possible, while ensuring that it meets the required performance standards. Every manufacturer declares values, sometimes certifying them, but the surprises that are encountered during the start-up phase are almost always very unpleasant. Implementing a remedy, when possible, is always costly and falls under the responsibility of the employer.
Therefore, it is advisable to include a clause in the purchase contract that provides for a noise analysis to be carried out during the commissioning phase, preferably by a specialized and independent third-party organization. The cost of such an intervention is truly only a few hundred euros, but it is an analysis that, by law, must be performed. It is best to have it done before accepting and paying for the system.
However, there are some parameters in the construction of ultrasonic washing systems that should be considered in the selection process. Here are the main ones:
Frequency
The more the resonance frequency is high and the noise is less loud. In fact a healthy person perceives frequencies up to a maximum of 10-12 thousand Hz. The noise you hear when you are near an ultrasonic washing system is due to the inertial mass. The vibration waves, which initially have an ultrasonic frequency beyond the audible threshold, are lowered and distorted by the complex mass of the washing liquid and the immersed parts to be cleaned.
Geometry of the vasks
A square bathtub is noisy than a rectangular shape. The wavelength propagation pattern that is formed with bounce on walls with equal dimensions, has vectors oriented in the same direction. Thus increasing the wavelength and thus inversely decreasing the carrier frequency.
Positioning of transmitters
Transducer elements positioned at the bottom produce much more noise than when positioned on the vertical walls. Obviously, the waves propagating from the bottom come immediately to the outside environment and only a well-insulated cover can attenuate the noise. Moreover, the yield in the wash is less as the acoustic pattern does not form that allows washing in the inner parts of the pieces. Normally when the powers are relevant, the transducers are positioned on the vertical walls of the tub.
Vaccine mounting frame
The tub must rest but not weld on a tubular structure. Structures made with sheet metal tend to vibrate along with the tub and with a frequency lower than the carrier. With a tubular structure, it is easier to isolate the tub / structure contact points with anti-vibration materials. In planning, you should consider placing the outer fairing as far as possible from the walls of the bathtub. The structure should rest on the floor with levelling feet that are stuck inside the tubular with soft absorbing plastic material.
Insulation
In a well-designed tank there are two types of insulation:
- With volcanic rock wool that strings the tub in order to contain the heat, indispensable, but has no sound absorbing effect.
- With lead plates and specific sound absorbing materials, which must be fixed on the outer fabric, on suitably shaped panels. The thickness of this coating is obviously important for maximum effectiveness.
Magnetostritive transducers
They are much louder than piezoelectric ones for the reason, already largely explained, working at a lower frequency. So the first sub – harmonics fall fully into the audible threshold and only the segregation in special cabs allows the use of the implant. That is why today’s predominance of piezoelectric plants is almost absolute.
