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For optimum performance the sonic horn manufactured by Primasonics® is exponential in shape, in other words it is designed in the shape of a 'bell' section. The reason for this shape is quite simple; the acoustic sound wave frequencies produced by each of our six selected frequency models are extremely pure in tone and powerful in performance due to this design. To understand better the need for the exponential, or bell shape of the sonic horn, a short interesting history of the bell may be helpful.

Firstly the definition of the bell as defined in the Encyclopaedia Britannica - "a special hollow vessel usually of metal, but sometimes of horn, wood, glass, or clay, struck near the rim by an interior clapper or exterior hammer or mallet to produce a pure ringing tone. Bells may be categorized as idiophones, instruments sounding by the vibration of resonant solid material, and more broadly as percussion instruments."

The early purpose of bells were to create clear power frequency tones which would travel long distances and therefore the exponential shape became established as the only way that all these goals could be met. For sonic horns, now referred to in industrial terms as acoustic cleaners, the important part in this definition is the vibration of resonant solid material, the bell shape resonates sound far more effectively.

The bell is primarily a musical instrument, designed to be used as part of a musical piece, or rung to invite parishioners near and far to their local Sunday service and as well as this the bell has been used to warn of impending danger.

The most famous bell in the United Kingdom is Big Ben and is housed in the tower below on the Houses of Parliament. Big Ben is 9'-0"/2.74m diameter, 7'-6"/2.3 high, and weighing in at 13 tons 10 cwts 3 qtrs 15lbs (13,760 Kg).

On the stroke of midnight every New Year's Eve people wait patiently to hear Big Ben chime in the New Year.

Pictured left is the Westminster Parliament Building and below right is the actual bell 'insitu'.

This bell has been consistently emitting true frequency tones across London since 31st May 1859, when the bells of the Great Clock of Westminster rang across London for the first time.

It is perhaps in the church where the bell is most recognisable. They have been used here for centuries to announce the beginning of the services that take place.

On a Sunday morning in villages all over the UK, bells can be heard ringing, the pure sound frequencies drifting through the air bringing the unmistakeable sound of the bell to the local people over a wide area.


Another famous bell is the Liberty bell. First chiming out true tones of frequency from the tower of Independence Hall it summoned the local citizens to be present for the first public reading of the Declaration of Independence by Colonel John Nixon on the 8th July 1776.

In actuality, bells have been used since more or less the beginning of civilisation. Amongst other things bells were used to frighten away unwanted and evil spirits and change the weather. By the time of the Middle Ages, the understanding of the mechanics of the bell improved, they were better crafted than their crude ancestor bells and even given their own towers.

The bell pictured is a Meneely made bell. The Meneely Company were the main manufactures of the bell in 19th and 20th century USA.

It is important to be aware that the shape of the bell has remained constant for centuries and centuries. This is because this shape, the exponential shape, is ideal for obtaining maximum acoustic power over long distances and at pure selected frequencies. Primasonics® utilised this bell shape and the bells ability to offer such clear tones at pre-selected wavelengths and at maximum power in the design of all their special range of acoustic cleaners. All our acoustic horn designs have pure fundamental frequencies of between 350 Hz and 60 Hz.

In the diagram below is shown the Primasonics® exponential sonic horn bell section on the left. Compare this with the cheaper, less effective conical bell section on the right. We would never use this straight conical design as we believe it to be detrimental to achieving both pure selected sound frequencies and also in utilising the maximum power of the particular Primasonics® Acoustic Cleaner.

To summarize, the correct design of the bell sections are critical. The history of the bell has proved that the exponential bell shape is the only true spare.

Thus the exponential shape of the bell sections in Primasonics® Acoustic Cleaners or Sonic Horns is vital in order to achieve both pure, powerful selected frequency tones and to cover a much larger area. Primasonics® Design Engineers have learned a powerful lesson from history which is now transposed into providing the best range of sonic horns available.

Primasonics® range of Sonic Horns are precicely fabricated from 316 grade stainless steel as standard The key point to make here is that it is vitally important for the internal surfaces of the bell sections to be smooth so as to achieve optimum power and performance.

This is not always the case for other sonic horn manufacturers. With a cast metal bell section the internal surfaces are very rough and to highlight the difference between a smooth fabricated sonic horn and a cast iron horn we can look at a golfing analogy.

If you are a golfer, some days golfing is a real pleasure, other days it is a real pain. Your goal always is to obtain optimum performance from both club and ball in order to reach the pin in the most direct and sweetest manner.

However if the ball ends up landing in the rough, instead of on the smooth fairway or green, it is clear that the next shot from the rough will lack the control and precision the smooth fairway or green shot can offer.

Visualise the smooth internal surface of the Primasonics® horns as the even fairway or suave green - offering increased focused control with maximum power conversion.

Compare this with the rough internal finish of cast iron which related to being in the rough where you have to employ greater energy with less power conversion and poorer control.

Below is some golfing advice on rescuing your shot from the rough and finding your way back to the fairway.

In terms of Primasonics® horns, you will not require rescuing as you will have a sonic horn that is second to none in performance and ability.

"Pitching is the more lofted shot around the green. Usually, you need to carry the shot 75 percent of the way to the hole. Pitching should be attempted with a sand wedge or lob wedge to allow the softest shot possible.

Your setup should be a narrow stance with the majority of your weight on your front foot. By keeping your weight forward, it allows you to make a steeper swing to cut through the heavy rough.

A narrow stance prevents lower body sway that frequently results in "skulling" the ball across with indifferent results." In summary, the smooth shaped interior of all Primasonics® sonic horns allows the acoustic sound wave the opportunity to develop maximum power at truer selected frequencies and once emitted, to travel in a powerful, effective cyclonic motion - providing maximum power with maximum performance!

Please find below a Glossary of Terms relating to Acoustic Cleaning which we have compiled for the benefit of all those interested in understanding the principles of the science and its associated, often industry-related terminology.


The removal or erosion of material from the surface of a solid due to friction imparted by the movement of another gas, liquid or solid.


The science of sound, including its production, transmission and effects


The state in which two materials are held together by forces existing between the surfaces.


The adherence of one substance on the surface of another.


A technique that combines small or powdered materials into larger particles, sub-clusters and clusters.

Airborne sound

Sound transmitted through air as a medium rather than through solids or the structure of a building

Ambient Noise

The background noise in an area, generally measured without noise of particular interest


A space which is almost totally free of reflection over a wide range of frequencies. An anechoic chamber gives close to free field conditions.

Background level

The normal sound level present in an area.

Bag filter

Process plant containing one or more cloth bags for recovering or removing particles from dust laden gas or air.


A process in which two or more materials are mixed together so the parts are intermingled with one another.


A high temperature water heater used for the generation of steam or cooling of gasses.


The process of making beer, ale or other malt beverages by boiling mashed malt to produce a wort, flavoring with hops, fermenting this mixture with yeast, and drawing off the fermented wort for distribution in barrels or bottles.

Carbon Black

Carbon black is a powdered form of carbon. It is used for its mechanical properties and pigmentation effects in many automotive products as well as rubbers inks and dyes.


A powder made from silica, alumina, lime, iron oxide, and magnesia which solidifies when mixed with water; this is used as an ingredient in mortar, concrete self levelling solutions etc.


Solid materials made by firing of non-metallic minerals, used in the manufacture of such products as tile, plaster refractories or brick.


A process in which a substance of individual particles comes together to form a coherent mass.


The act or process of pressing together substances either through gravity or applied pressure, which can form a denser substance or cohesive product.


The transfer of heat by molecular collision. This process is more efficient in metals and other thermal conductors and poorer when combustion products build up on heat exchange surfaces.


A device designed to regulate the fuel, air, water, steam, or electrical supply to the controlled equipment. It may be automatic, semi-automatic or manual.


The addition of some unwanted substance to a product or intended mixture of products.


Removal of kinetic energy in an oscillating medium by converting it to heat using frictional or viscous forces.


A division of a uniform scale based upon 10 times the logarithm to the base ten of the ratio of sound field intensities being compared

Diffuse sound field

A sound field in which the energy density is the same everywhere and sound waves are likely to be travelling in any direction with equal probability.


Concentration of acoustic energy within a limited location in a room as the result of reflections from concave surfaces.

Free field

A region in which no significant reflection of sound happens

Fundamental frequency

The frequency with which a periodic function reproduces itself.


An enclosed chamber where high temperature reactions or combustion take place.


The process of converting solid or liquid products into a gaseous fuel through heating in the absence or reduced presence of oxygen.


Calcium sulfate dihysrate, CaS04.2H2O, used in wallboard manufacture, and fertilizers.


A sinusoidal component in a complex periodic wave of frequency, which is an integral multiple of the fundamental frequency of the wave.


A substance that absorbs moisture from the air.

Induced draft fan

A fan which pulls a gas or air stream usually used for high temperature gasses. .


The rate of sound energy transmitted in a specified direction through a unit area

Kiln Dust

Dust produced during cement or lime processing.


A covering, usually of insulating material, on pipe or ductsLoudness
The subjective judgement of the intensity of a sound.

Natural frequency

The frequency at which a resiliently mounted mass when set into vibration would vibrate under the influence of gravity alone with no additional forces or constraints.


Solids that reflect light of certain wavelengths, without producing appreciable luminescence these are used in solids and paints to achieve a desired colour.


Chains of molecules formed by the chemical combination of two or more identical combining units called monomers.


Materials suitable for use at high temperatures; usually used for thermal insulating or thermal barriers composed of aluminas, silicas, etc.


The persistence of sound within a space after the source has ceased, due to repeated reflections at the boundaries of the space (walls)

Reverberation time

The time it takes for a reverberant sound of a given frequency to decay by 60dB after the source is cut off.


A means of converting Nox gasses (nitrogen oxides) with the aid of a catalyst into Nitrogen and water. SCR's use ammonia as the reducing agent. This prevents Nox gasses entering the atmosphere where they can combine with cloud moisture to produce the strong inorganic acid – Nitric Acid.

Slag cements

Cements made from blast furnace slag.

Sound Power Level (SPL)

A value equal to 10 times the logarithm to the base 10 of the ratio of total acoustic power emitted by a source to a reference pressure normally 2 x 105 N/m2.

Spray Drying

The production of a solid product by the atomization of a liquid solution into a heated vessel which evaporates the liquid. The dry particles fall to the bottom of the vessel.

White Noise

Noise of a statistically random nature having equal energy at every frequency between set limits.

Wood Pulp

Wood which has been broken down, to a fine powdered substance called pulp through mechanical or chemical processes.

The particular pitch of the frequency emitted by each model of sonic horn within the Primasonics® range is important with respect to the products successfully either preventing particulate build up or ensuring maximum mass material flow. That is indeed why the Primasonics® frequency range is greater than other rival companies, resulting in improved cleaning efficiency within a wider range of applications and industries. The Primasonics® Audiosonic™ Acoustic range covers a sound wave spectrum from 420 Hz down to 60 Hz - but why?

In short, the lower the frequency, the longer the wavelengths and the further the sound will travel and the more effective cleaning will be achieved over the greatest area such as a very large silo (60 Hz). The opposite is also true, in that the higher the frequency (350 Hz) the shorter the wavelengths which means that the same level of sonic energy is contained within a small area to powerful effect such as the discharge area of a silo.

Let's explain it like this. Imagine if your neighbours were enjoying a lively night in playing their favourite music loudly. What is it you hear, the singing, the piano or guitar? No. You hear the rhythm, the beat. You hear the drums. The deeper/lower the sound frequency, the further the sound travels. Another example in practice is the sounds created by the traditional Scottish and Irish marching bands.

The sounds made by these bands as they approach can be heard from some distance as they parade through the streets.

What marks the coming of the band, what can the eager listener hear first. The bass drum is the first sound heard because it has the lowest sound frequency which travels the furthest distance. It is only when the band is close to you that you begin to hear the high pitched instruments such as the bagpipes because of their much higher, much shorter length of sound waves.

As mentioned, the reason the bass drum is heard so far away is because it has the lowest acoustic frequency. The lower the frequency the further distance the sonic sound waves travel.

This premise is employed in the design of the Primasonic Sonic Horns which are employed within very large silos or boilers where the powerful acoustic energy has to remain effective over great distances.

They operate at a lower frequency than other acoustic cleaning systems and therefore can be more effective at keeping material on the move and preventing side wall build up.

To reiterate the bass drum sends out vibrations across a larger distance than other instruments due to the lower frequency.

So both the bass drum and the larger, lower frequencies Primasonics® Acoustic Cleaners operate at lower frequencies.

The converse is also true in that the flute with its high frequency and short sound waves acts in the same way as a Primasonics® 350 Hz sonic horn.

In summary, Primasonics® employ a wide range of frequencies 350 - 230 - 120 - 75 - 60 Hz in order to provide maximum sonic cleaning and de-bonding power for each individual application.

What is Acoustic Cleaning?

Acoustic Cleaning is the use of high energy - low frequency sound waves which eliminate particulate build up and maintain material flow throughout a wide range of "dry processing" industries and applications.

What is Sound?

Sound may be best described as the rapid passage of pressure fluctuations through bonded material by means of a vibrating source and transmission medium.

What happens to changes in frequency?

We have selected a range of key fundamental frequencies between 60 and 420 Hz for a very particular reason. At the 420 Hz frequency the wavelengths are much shorter than at the 60 Hz frequency level. Therefore where high intensity, short distance material debonding is required (for example at the discharge of a silo) we would employ a suitable higher frequency Audiosonic™ Acoustic Cleaner. The opposite is true for say large silos; here to prevent material build up of a sidewall a suitable low frequency Audiosonic™ Acoustic Cleaner is employed to provide long-distance debonding power.

How do Audiosonic™ Acoustic Cleaners work?

Particles with different masses and clusters of particles are all hit by the alternating sound waves as the particles have different masses they travel slightly different distances, when this is repeated between 60 and 420 times per second the particles start travelling out of phase with each other and break apart.

How is this "sounding" activated?

The sounding is created when normal plant compressed air enters the Wave Generator this forces a diaphragm which acts a little like a reed to allow pulses of air into the horn section. The frequency of the sound is then dependant on the size and shape of the horn section.

Does the shape of the Bell Section matter?

Very much so, imagine a church bell and why it is shaped in the manner it is. This is for two reasons, firstly to provide as pure a tone (or fundamental frequency) as possible and secondly to cover an effective range. Could you imagine a square, straight-sided bell sounding properly - no of course not and that is why all our bell sections are precision spun in this exponential round bell shape.

What is the key difference between Audiosonic™ Acoustic Cleaners and vibrators?

Vibrators by their very nature and location have to first pass all their vibrations through the wall of the vessel. This results firstly in severe loss of power and secondly the damaging transmission of vibration through the vessel which can result in metal fracture. With Audiosonic™ Acoustic Cleaners - 100% of the debonding power goes into the material without any risk of damage to any structure or material of construction.

What is the key difference between Audiosonic™ Acoustic Cleaners and Air Cannons?

Air Cannons employ a unidirectional "blast" of high-pressure air to try and remove blockages that have already built up. That is why you always see groups of Air Cannons on any part of an application. Audiosonic™ Acoustic Cleaners prevent the build up from occurring in the first place. Sound waves travel in a 360° radius at a speed of 344 metres per second, therefore a single Audiosonic™ Acoustic Cleaner can be much more effective.

What is the relationship between frequency and dB?

Both are very important, the frequency provides the correct number of debonding pressure fluctuations and the dB provides the necessary energy to successfully complete the job.

Will Audiosonic Acoustic Cleaners damage my equipment?

Primasonics® Audiosonic™ Acoustic Cleaners are designed by tuning the wave generator and shape of the horn so that sound is reflected from solid surfaces rather than passing into them this helps in three ways, equipment is not vibrated and damaged, sound is increased inside the vessel where cleaning is required and as the sound is contained in the vessel this reduces noise nuisance.

What are the compressed air requirements?

The Audiosonic Acoustic Cleaners operate using normal plant compressed air provided three important factors exist.

  • Compressed air piping up to the cleaner - 25 mm diameter
  • Compressed air pressure - 4.8 - 6.2 bar/70 - 90 psi/ 480 - 620 kPa
  • Compressed air volume* - 21.25 l/s 45 SCFM @ 5.5 bar/80psi

* - but remember the Audiosonic™ Acoustic Cleaner is only "sounded" for a few seconds over usually between every 3 - 30 minute period depending on the application.