Reduction of noise in ultrasonic cleaners
The noise levels produced by ultrasonic cleaning systems are very high and annoying, with a characteristic whistle that is clearly distinguishable, even at a distance in an environment which is itself noisy.
The manufacturers must therefore adopt all the known and most up to date techniques in order to minimise the noise caused by the systems; however, it is the customer who must select those systems that produce less noise, also because the responsibility for the safety of the systems falls completely on the employer, alone, where the unit has been installed.
Italian Legislative Decree no. 277/91 – which required the obligation of performing a noise risk assessment, and the preparation of a preventive instrument, the Assessment Report – is repealed for the parts related to noise. With the entry into force of Italian Legislative Decree no. 195/06, it was established that the noise risk assessment, an integral part of Legislative Decree 626, as enshrined in subsequent article 49/d remains an obligation on the part of the employer that must be performed in accordance with, and by the means provided for in art.4
The new law establishes a limit to noise exposure and two action values:
Daily exposure to noise (Lex/8h)in db(A) |
Peak sound pressure |
|
Lower action value |
80 |
112 Pa equal to 135 db(C) |
Upper action value |
85 |
140 Pa equal to 137 db(C) |
Exposure limit |
87 |
200 Pa equal to 140 db(C) |
In fulfillment of the new law, the employer must eliminate the risks at the source or reduce them to a minimum and, in any event, to levels not exceeding the exposure limit values.
The law, in establishing three noise thresholds (80, 85 and 87 dbA), allows for the identification of four classes of noise exposure for workers.
Average daily value (LEQ/G) IN DB (A) – Risk classification
| LESS THAN 80 | NO RISK |
| BETWEEN 80 AND 85 | MODERATE RISK |
| BETWEEN 85 AND 90 | HIGH RISK |
| ABOVE 90 | SERIOUS RISK |
As we have said, ultrasonic cleaning systems are objectively quite noisy, so it is easy to exceed the limits set out above. Therefore it is in the customer’s interest to purchase a quieter system which offers the same levels of performance.
Each manufacturer declares values, sometimes certified, but the surprises that are revealed at the time of commissioning are almost always unwanted and the solution, with the system already installed, if even possible, is always expensive. It is the employer who is responsible for the costs of the unit which was installed.
It is therefore appropriate that a clause is added to the purchase agreement stating that, at the time of commissioning and testing, a noise analysis is also carried out by a third party, i.e.: an independent specialist company. Today, there are numerous in existence and the cost of a similar intervention is a few hundred Euros.
This analysis is in any case, a legal obligation, the law states it must be carried out before the system is accepted and payment is made.
However, when manufacturing ultrasound cleaning systems, one should observe some direct methods that should immediately steer the buyer when making their first selection:
Frequency
A higher resonance frequency results in less noise produced by the system.
A young person, in good health perceives frequencies up to a maximum of 10-12 thousand Hz., so what is that loud noise that is heard when near an ultrasonic cleaning system? It is because of the inertial mass: the vibration waves that, at their origin only have an ultrasonic frequency that is beyond the audible threshold, are lowered and distorted by the mass of the cleaning liquid and the immersed pieces to be washed.
In addition, there are the harmonic waves. These are produced as we are in an elastic environment, upper and lower harmonics, according to the laws of acoustics which have been codified for thousands of years and, unlike these last ones, are audible!!! Fortunately, the first harmonics are attenuated, the second even more, and so on. But since the power of the carrier is very high, even if attenuated, the first one can also be heard quite well, the second one with significant powers. This is the noise level which is heard.
It therefore follows that the higher the carrier frequency is, even the first and second harmonic are beyond the audible threshold, and the noise that is heard is produced by the third or even the fourth harmonic which, as amplitude, power, is much lower than that of the carrier.
Tank geometry
A square tank is noisier than a rectangular tank.
In fact, the wave propagation lattice that is formed by bouncing off walls with equal dimensions, has vectors oriented in the same direction, therefore the amplitude of the wave is increased and the carrier frequency inversely decreases, which determines the noise.
Positioning the transducers
Transducer elements positioned on the bottom produce much more noise than when placed on vertical walls.
Evidently, the waves that propagate from the bottom immediately reach the external environment, and only a well insulated cover can dampen the noise. In addition, the cleaning performance is lower because the acoustic lattice does not form, allowing the internal parts of the pieces to be cleaned.
Normally, when the powers are significant, the transducers are positioned on the vertical walls of the tank.
Tank support frame
The tank must rest on, not be welded to, a framed tubular structure. The structures achieved using shaped sheet metal tend to vibrate together with the tank, but at a lower frequency than the load bearing structure. With a tubular structure, it is easier to isolate the tank/structure contact points using vibration damping devices. During the design stage, it is necessary to establish the placement of the external fairing as far as possible from the walls of the tank. The structure must rest on the floor with adjustable feet secured inside the tubular frame using sound absorbing plastic.
Insulation
There are two types of insulation for a well designed tank:
- Volcanic stone wool which lines the tank to contain the heat is indispensable, but has no sound absorbing properties.
- Lead sheets and specific sound absorbing materials, which must be bonded to the external fairing, on appropriately formed panels. The thickness of this coating has obvious importance to achieving maximum efficiency.
Magnetostrictive transducers
They are much noisier than the piezoelectric transducers for the reason, already explained, that they work at a frequency close to the audible threshold.
Therefore the first sub-harmonics are fully within the threshold and only their isolation in special cabins allows the use of the system. This is why there is an almost total predominance of piezoelectric systems today.
