If a tree falls in the forest and no one hears it, does it make a sound?
You can philosophize all you want, but the correct scientific answer is "no."
Sound is a sensation caused by vibrations in the air that are picked up by the ear and transmitted to the brain by the auditory system.
If there are no ears to sense the waves, there is no sound.
Sound vibrations vary in amplitude (strength or volume) and pitch (tone or frequency). (See the "Pitch versus amplitude" sidebar to the right).
Amplitude is expressed in decibels (dB).
The decibel is a measure of the sound pressure level. Decibels are the most widely used measure of sound volume. (See "The decibel" sidebar on page 26).
Noise defined
What is noise? Noise can be generalized as any unwanted or undesirable sound.
The word noise comes from the Greek word "noxia," which means "hurt or injure." What one person thinks is a pleasing sound may be noise to another.
For example, you have probably seen one of the bumper stickers that read, "Loud pipes save lives." They refer to the fact that a motorcycle with a loud exhaust is more obvious to other motorists and therefore safer for the biker.
One has to wonder if loud pipes are for safety, or is safety just an excuse for having loud pipes?
It depends on whom you ask, but I feel it safe to say that most people consider loud motorcycle pipes to be somewhat annoying. But to a biker, the rumble and roar of a motorcycle may sound beautiful.
What about your truckmount?
You may feel the same kind of pride that a biker does when he fires up his Harley.
Hopefully, when the neighbors hear your truckmount, some will look out the window and call the number on the side to get a quote.
Maybe we need a bumper sticker that reads, "Loud truckmounts attract business."
Regardless of what you think, the sound your truckmount makes is just noise pollution to most people.
Noise pollution
Noise pollution is displeasing sound that is disruptive to humans or animals.
Traffic and aircraft are common sources of noise pollution, as are car alarms, office equipment, factory machinery, construction work, grounds-keeping equipment, barking dogs, appliances, power tools, lighting hum, audio entertainment systems, loudspeakers, noisy people and unfortunately, truckmounts.
Exposure to noise pollution can lead to ear discomfort, hearing damage, inability to concentrate, speech interference, reduced productivity and sleep interference, as well as emotional and psychological stress. The noise generated by our truckmounts must be considered as more than just a nuisance.
Truckmount noise
Where does truckmount noise come from?
The engine that runs the machine is one source of noise.
Most slide-in truckmounts utilize a small engine that can range from noisy to deafening, depending on its size and the effectiveness of the muffler attached to it.
With a direct-drive or PTO truckmount that utilizes the vehicle engine, the exhaust goes through the vehicle muffler, dramatically quieting the exhaust sounds.
The real culprit
The positive displacement blower is the primary source of truckmount noise.
There are several factors that contribute to how disruptive or unpleasant the noise from a truckmount blower is.
The blower produces pulsations of the air that moves through the blower.
Each revolution of the blower produces a certain number of pulses, depending on the number of lobes on each rotor of the blower. Each of those pulsations moves a specific volume of air. The bigger the blower, the more air that is displaced with each pulse and therefore the louder the noise.
The standard positive displacement blower is the dual-lobe design.
Each of the two rotors has two "paddles" or lobes that push a certain amount of air with each rotation.
Since there are two rotors, and each rotor has two lobes, there are four air pulses with each complete rotation of the blower.
If you turn a dual-lobe blower at 3,600 revolutions per minute (RPM), you will get 14,400 pulsations per minute.
Divide that by 60 seconds in a minute, and you get 240 cycles per second or a 240-Hz tone. If the blower is rated at 450 cubit feet per minute (CFM) at that speed, there are 1.875 cubic feet of air per pulse.
Some truckmount manufacturers have been using blowers with three lobes per rotor.
With these tri-lobe blowers, each complete blower rotation will produce six pulses of air.
If the blower is turning at 3,600 RPM, you will get 21,600 air pulses per minute. Divide that by 60 seconds in a minute, and you get 360 pulses per second, or a 360-Hz tone.
If this blower is also rated at 450 CFM at that speed, there are 1.25 cubic feet of air per pulse. Since each of these pulses will contain 33 percent less air than an equivalent dual-lobe blower, you will have lower sound pressure and the noise level will be reduced.
Take control
What are some ways to control truckmount noise? Change the pitch.
Human hearing is more sensitive to certain frequencies of sound.
Within the middle range of human hearing (300 to 5,000 Hz), higher frequencies tend to be more irritating, especially at increased volumes.
Therefore, changing the speed of a blower can dramatically change the perceived noise.
By spinning the blower at a lower speed, you can lower the pitch.
Of course, you also reduce blower CFM or airflow.
Also, as previously discussed, changing the number of lobes or the shape of the lobes can make a dramatic difference in the tone of the blower.
Since decibels represent logarithmic scale and each increase of 10 decibels represents 10 times the sound pressure, reducing the sound of a blower by only five decibels makes a huge difference in noise levels.
Here are some ways manufacturers reduce the noise of a truckmount:
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Blower size: Use a smaller blower spinning at the same speed. This will yield a quieter machine, but vacuum performance will drop, of course.
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Blower case configuration: There have been advances by blower manufacturers in the configuration of the blower case. A thicker blower case will resonate less noise. Some manufacturers also have made changes to the ports on the blower that cancel some of the noise.
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Blower lobe configuration: Tri-lobe blowers, helical rotors and other advances are available. Each has an affect on the price of the blower. The quieter the blower, the more it usually costs.
Mufflers and silencers
Many people think that mufflers and silencers are the same thing.
Although the objective is the same, the method of noise reduction differs between the two.
Mufflers work through backpressure and resonance. Sound waves and pressures are reflected back toward the inlet, canceling out some of the vibrations. This results in reduced noise.
Mufflers are relatively inexpensive, but they still come at a price.
Increasing flow through the muffler results in greater backpressure. Excessive backpressure puts additional strain on the engine, increasing fuel usage and possibly resulting in premature engine failure.
Silencers are more expensive than mufflers, but they result in little — if any — backpressure.
Silencers work by moving the airflow through internal chambers and tubes. This results in sound waves that cancel each other out. They must be tuned for a specific range of frequencies.
Silencers may also contain sound-absorbing materials to reduce other frequencies. Properly selected silencers last longer than mufflers and do not create additional engine strain or excessive fuel consumption.
Enclosures
Larger blowers may be encased in a shell that is made of acoustically dead material.
This means the material absorbs sound rather than radiating it. Enclosures are not usually practical for truckmounts unless they are inside a large box truck or trailer.
If you already own a truckmount, there are some things that you can do to reduce noise pollution.
Reduce exposure time
Run your machine only when you are actually doing work.
Don't let the machine run idle for very long. Reduce run time to a minimum, especially if you are near occupied structures.
Redirect exhaust
Any person in the direction the blower exhaust is pointed will obviously hear the most noise.
Think about which way you park your truck.
Try to direct sound toward the street and away from people and occupied buildings. This includes the building or home in which you are working.
Increase distance
You can also increase the distance from occupied structures.
In a residential neighborhood, you may want to park your truck in the street and run your hoses to the house.
Distance makes a great deal of difference in sound. Just as ripples in a pond get smaller as they radiate from where you dropped the pebble, sound decreases in strength as you get further from the source.
Time of day or week
Be courteous.
Even the quietest truckmount sounds loud in a residential neighborhood at 3 a.m.
It is a good idea to refrain from running your machine in residential areas before 8 a.m. during the week and 9 a.m. on Saturdays — and wait until 10 a.m. on Sundays.
Try to finish up your last job by 7 p.m. if possible.
Check local codes and noise ordinances to make sure you are in compliance.
Exhaust/silence systems
Keep your exhaust and silencing systems in good repair.
Even a small exhaust leak can result in a huge increase in noise levels.
Install sound deadening mats
This can help reduce vibrations in waste tanks and on vehicle panels.
You can also install insulation on van or truck walls.
Rerouting the exhaust
You can sometimes reroute the exhaust straight down through the vehicle floor.
This has the effect of dissipating some of the sound energy and dispersing it in all directions instead of just one.
Before you modify any portion of your truckmount exhaust system, check with the manufacturer.
A local muffler shop can probably make custom pipes for you.
Close the doors
This will not work with a slide-in machine, but most PTO or direct-drive truckmounts can be closed up with the hoses running out through a small opening in the door.
In a recent experiment, I found that a popular direct-drive truckmount produced 110 dB at the machine. When I closed the door, the level dropped to just 96 decibels.
That is a reduction of more than 20 times the sound pressure level.
The big picture
The truckmount has been the workhorse of our industry for more than 30 years.
Through advances in technology and manufacturing, these machines have become more powerful and effective than ever before.
As cleaning and restoration professionals, we must remember that the machine is not the "cleaner," and the carpet is not the customer.
We are people serving other people.
The less intrusive we are, the better for our customers and our industry as a whole.
Bruce DeLoatch is an Institute of Inspection, Cleaning and Restoration Certification (IICRC)- approved instructor. He is an IICRC Certified Master Textile Cleaner and Master Restoration Technician. Bruce is co-founder of The Cleaner's Coach, a marketing and consulting firm, as well as marketing director for Truckmounts and Cleaning Solutions in Norcross, GA. His e-mail address is bruce@cleanerscoach.com.
