NOISE

All ultrasonic cleaning systems are very noisy and annoying, as the noise they produce is very particular with a hiss that is distinctly distant even at a distance in a noisy environment in itself.

Manufacturers must then adopt all state-of-the-art techniques to limit the noise produced by their plants to a maximum, but customers must also choose those plants that are actually noisy, because all the responsibility for the safety of the plants falls completely and only on the employer of the production unit where they are installed.

Legislative Decree no. 195, published in the Official Journal of 30 May 2006, introduced Legislative Decree 626/94, new Title V-bis “”Minimum Requirements for the Protection of Workers against Health and Safety Risks resulting from Exposure to noise during work and in particular hearing “”by implementing the measures provided for in Directive 2003/10 / EC on the exposure of workers to the risks arising from physical agents (noise).

Legislative Decree no. 277/91 – which had first envisaged the obligation to assess noise hazards and the preparation of a preventative instrument, the Evaluation Report – is abrogated for the parts related to noise. With the entry into force of Legislative Decree No.195 / 06, the noise risk assessment, part of Legislative Decree 626, as stipulated in the new Article 49 / quinquies, remains an obligation on the employer to be made second and in the ways provided for in art.4.

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Limits to noise exposiure:

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

We have said that ultrasonic cleaning systems are intrinsically very noisy and therefore it is easy to overcome the limits above and that it is therefore of all interest to the customer to purchase the plant less loud for the same performance.

Each manufacturer declares values, sometimes certifies them, but the surprises that are noted in the act of commissioning are almost always very unpleasant and the remedy with the installation already installed, if not impossible, is always very expensive and the employer work of the unit where you are working.It is then necessary to require that a clause be added to the purchase contract that, at commissioning and testing, a noise analysis be carried out and made by a third party, that is by an independent specialist firm.

There are now so many, and the cost of such an intervention is really a few hundred euros.

It is an analysis that is required by law to be done, so much so is done before the acceptance and payment of the plant.

However, there are some disparities in the construction of ultrasonic cleaning systems, which should immediately guide the buyer to a first selection in this very confused area and not always conducted by serious professionals:

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., And then what is the noise that he feels when he is near an ultrasonic washing plant? It is all the fault of the inertial mass: the vibrational waves that originally have an ultrasonic frequency, that is beyond the audible threshold, are lowered and distorted by the mass complex of the washing liquid and the pieces immersed just to be washed.

Moreover, there are harmonic waves, produced as we are in an elastic, harmonic superior and harmonic subdivision according to the laws of acoustics encoded for thousands of years, and the latter others that are audible!!!!

The first harmonics are luckily attenuated, the second more even and so on. But since the power of the carrier is very high, however attenuated, one can feel the first, the second with significant powers. Here is the noise that is heard.

It follows that with the carrier frequency being higher, the first and the second harmonics are also beyond the audible threshold, and the noise that is heard is produced by the third or even the fourth harmonic which is much less than the amplitude that of the carrier.

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:

1) With volcanic rock wool that strings the tub in order to contain the heat, indispensable, but has no sound absorbing effect.

2) 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.