INSULATION
A well-designed tank must offer 2 different tyoes of insulation:
1) Volcanic rock wool it surrounds the tank in order to contain heat, but it has no sound-dampening effect.
2) Lead panels piombo and sound-dampening materials, which must be fixed on the external fairing on taylor-made panels. optimal efficacy will depend on the thickness of such panels.
MAGNETOSTRICTIVE TRANSDUCERS
They are much louder than piezoelectric transducers due to their lower frequency.
As a direct consequence of lower frequency, the first set of harmonic waves is well below the audibility threshold. Therefore, the plant can only be used if installed in a special cabin. This is why, today, ultrasonic plants are almost exclusively fitted with piezoelectric transducers.
Undeniably, ultrasonic cleaners generate very loud noise, more acute than the background sound that is usually heard in factories.
This is why manufacturers must employ the most up-to-date techniques to ensure the maximum reduction of noise; however, those held responsible for any safety issue concerned with noise are the employers that buy and install ultrasonics plant: this is why they should carefully check the effective level of noise control provided by the plant they plan to buy.
The legislative decree April 10th, 2006 n. 195 , published on the Italian Official Gazette on May 30th, 2006, integrated the Legislative Decree 626/94 D.Lgs. 626/94, with a new Comma V-bis: "Minimum requirements for the protection of workers against health and security risks connected to noise exposition during work hours, in particular risks connected to hearing” which actualizes the measures specified in Directive 2003/10/CE concerning workers’ exposure to risks deriving from physical agents (noise).
The sections concerning noise included in the previous Legislative Decree 277/91 (the first law to regulate noise risk) are therefore abrogated. With the enforcement of legislative decree 195/06, employers are responsible for evaluating the risks connected to noise, as specified in D.Lgs. 626, Article 49/quinquies, according to the modalities specified in Article 4.
The new law establishes exposure limits and two types of action:
Moreover, according to the new law employers must eliminate or reduce risks below exposure limits. By fixing 3 noise thresholds, the law establishes 4 different typologies of exposure risks for workers:
However, as ultrasonic cleaners are very noisy by their own nature, the above mentioned limits are very difficult to comply to. It is therefore in your best interest to buy the plant that offers the maximum noise control without undermining its performance.
Manufacturers claim and in some cases certify a certain noise level for their plants. However, it is only when the machine is installed and tested that the real noise level can be evaluated - and once the plant is installed, it is often impossible to modify it, and if it is possible it is definitely very expensive.
Therefore, the purchase contract should provide that an independent, specialised company will perform a noise assessment (which is a law requirement and, as such, must be commissioned sooner or later) when the plant is tested and before you complete the payment.
It's an analisys that must be done, according to italian laws, so it's better to do it before accepting and apyment of the plant.
However, there are some building criteria that can help buyers make the right choice in this complex field:
FREQUENCY
The higher is the resonance frequency, the less noisy is the plant.
Normally we can hear frequencies up to 10-12,000 Hz. The reason why we perceive such a strong noise when close to an ultrasonic cleaner is the inertial mass. At their point of origin, vibration waves have an ultrasonic (therefore non-audible) frequency. As the cleaning process is performed, however, their frequency is decreased and distorted by the mass of the cleaning liquid and of the pieces to be cleaned.
In addition, the tank walls have their own inertial mass: when they start vibrating, they generate very loud superior and inferior harmonic waves. Although we perceive the first and the second sets of harmonic waves as weakened, due to the power of the working frequency, or "carrier wave", they are still very loud.
As a consequence, if the frequency of the carrier wave is sufficiently high even the first and second sets of harmonic waves are beyond the audibility threshold. All that we hear, therefore, is the noise produced by the third or even the fourth set of harmonic waves, which is much less powerful and loud than the carrier wave.
TANKS GEOMETRY
Square tanks are louder than rectangular tanks
The wave propagation pattern created as waves bounce on equal-size walls present vectors orientated in the same direction. Therefore, the wave length is increased and, conversely, the frequency of the carrier waves decreases.
TRANDUCERS POSITION
When positioned on the bottom, tranducers produce much more noise than when positioned on the side walls
When propagating from the bottom, waves immediately reach out to the outer environment, so that only a well-insulated lid can weaken the noise they produce. Besides, their cleaning efficacy is undermined because they cannot clean the inner parts of the objects.
Usually, in high-power plants the transducers are positioned on the vertical tank walls.
TANK SUPPORT
Tanks must rest on a tubular structure, though they must not be welded to it. A tubular strucure allows to insulate with anti-vibration materials the contact points between the tank and the supporting structure. The external fairing must be placed as far as possible from the tank walls. The structure must rest on feet attached to the tubular structure with insulating material.
