The purpose of this application note is to provide basic instruction in the fundamentals of sound measurement to aid in the successful enforcement of noise ordinances. The main topics to be covered are sound and its characteristics and how they relate to the sound level meter.
When you blow up a balloon you are using your lungs to force air into the balloon. This causes the balloon skin to expand into its stretched out shape. The air in the balloon is now under pressure. If we squeeze the balloon in the middle, what happens? The balloon bulges out at the ends and the pressure inside the balloon increases. When the balloon is released it pops back to its original shape at its original pressure.
Suppose the balloon were very long and someone squeezed it at one end. What would we observe at the other end? First we would notice that nothing happened for a short period after it was first squeezed, then, just like the small balloon, the pressure would increase. What is happening is that the excess pressure caused by the squeeze is traveling down the tube at a speed of about 1200 feet per second. This excess pressure is the sound wave. If the squeeze were released, a decrease in pressure would travel down the tube in the same manner. To convince your self that these actions actually produce sound waves, burst the balloon with a pin.
How do we describe sound? Consider something that appears not to have anything to do with sound at all: a weight hanging from a spring.
If we pull the weight down a certain distance from the point it naturally hangs, then release it, the weight starts returning toward the rest position. But it goes through the rest position until it reaches a point as high above the rest position as it was pulled below it. The weight then starts down again to a new lowest position, where the process repeats over and over. Since the energy source of the person that initially pulled down on the weight is gone, the weight rises and falls a smaller distance each time, eventually coming to rest once again. The maximum displacement from the "at-rest" position is called the amplitude, and the time it takes to go through one complete cycle (from down to up to down) is called the period of the vibration. The number of periods that occur in one second is called the frequency. The units of frequency were once called cycles per second, but are now called Hertz and abbreviated "Hz". So what’s the correlation between a weight on a spring and sound in air? Look at a stereo speaker emitting a single tone. As the speaker cone moves forward and backward like the spring, it alternately compresses and expands the air in front of the cone. The compression and expansion then moves out away from the speaker as a sound wave.
Single Frequency Sound
There are a number of common sources of sound that act much like the spring because they cause a single frequency sound to be produced. The keys of a piano are a good example. Pressing the middle C key causes its string to vibrate about 260 times per second. The vibrating string and soundboard cause the air adjacent to it to compress and expand with the same frequency. Just like the balloon, the changing pressure moves outward as a sound wave. Other examples of tones are the hum of a motor (60 Hz) and the sound of a police whistle (3500 Hz).
Suppose instead of just pulling the spring down and releasing it, there is an invisible hand that randomly either pulls or pushes on the weight at different times. Sound can behave in this random manner as well – think of music. So how would you describe this motion? Certainly there is no single frequency or amplitude with which to describe the motion as in the previous case. Fortunately noise such as this can be shown to consist of many single frequency components, each having its own amplitude.
Sound With Many Frequency Components
As an example of sound with many frequency components, consider motorcycles and automobiles. The amplitude of sound from motorcycles is typically greater than for automobiles. Motorcycle sound also contains components that are higher in frequency than an automobile. These are two reasons why motorcycles annoy people more than automobiles.
How do we describe the volume of sound we hear in everyday life? Because the sound levels we encounter in daily life can vary over such a wide range, talking about sound pressure in units such as pounds per square inch would be unwieldy. To remedy this situation we define Sound Pressure Level (SPL) as:
SPL = 20 x logarithm10 (measured sound pressure / reference sound pressure)
The reference pressure used for environmental noise turns out to be the lowest level sound that a person with normal hearing can detect. The unit of SPL is called the decibel (dB). Does all this complicated jargon mean that an enforcement officer will have to have a degree in mathematics? NO! All enforcement equipment is calibrated directly in decibels, so no calculations are involved.
Sound Level And Distance From The Source
Most people know that noise levels increase as you get closer to a sound source and decrease as you move away. It is important to note that the sound pressure level rises at a faster rate as you move closer and at a slower rate as you move away. Think about what happens when you drop a stone in water. The waves that are created are closer together and higher (amplitude) nearer the point of impact and further apart and lower as you move away from where the stone entered the water. Sound pressure behaves in the same manner. The importance of this observation is that officers should make sure he/she is at least as far away from the source as your ordinance requires when taking sound measurements. It is better to be a little too far than a little too close.
Combining Sound Pressure Levels
Suppose we have two identical sound sources, each alone producing the same dB level. So what is the SPL of the combined sources? It is not the sum of the two. We cannot simply add decibels directly to get the overall effect. The correct answer is obtained by using the following rule: Each time the number of identical noise sources is doubled, the SPL increases by 3dB; each time the number is halved, the SPL is decreased by 3dB. This rule is called 3dB doubling or 3dB exchange rate.
How does the 3dB rule help you? Suppose your noise ordinance has an 80dB noise limit. You cite a violator for causing an 89dB noise level and the case comes to court. The judge asks you how loud 89 dB is. Knowing this rule of thumb, you are able to tell the judge that 89 dB is the same noise level that would be generated by 8 identical vehicles, each producing the maximum allowable sound level of 80 dB. Case closed!! Be sure to apply the doubling rule contained in your particular ordinance.
Effects Of Additional Noise Sources
A third factor to remember when measuring sound involves the contribution to the overall level of all the other noise sources present at the time a violator is cited. This extraneous noise is called the ambient level. The violator might ask, "There were a number of other loud noise sources present when you cited me, so how do you know that they didn’t cause the readings to be too high?" The rule that applies here is: a violator should not be cited unless the level measured when the violation occurs is at least 10dB above the ambient noise level immediately before the violation. If this condition is met, then the additional noise caused by all the other sources producing noise will add less than 0.4dB to the level produced by the violator.
The last factor to remember is the effect of large objects on sound reflection. Think of the stone in the water again. If the water waves encounter an obstacle as they move away from where the stone entered the water, you will see part of the wave reflected back in the direction it came from, modifying the height of the waves, which is equivalent to the sound pressure in air. The same phenomenon occurs in air when measuring sound. The rule of thumb is to remain at least as far away from any large reflecting objects as you are from the source being measured. What about reflection from the ground? The noise level limit stated in the ordinance should take into account the fact that the noise heard by the receiver consists of sound that is reflected from the ground to the receiver as well as the direct wave. Normally there should be no concern. The exception is when the sound level meter is close to the ground. All measurements should be made with the microphone at least three feet above the ground.
The Sound Level Meter
The most common device used in noise ordinance enforcement is the sound level meter (SLM). The SLM performs three basic operations. It uses a microphone to convert the energy in the sound into an electrical signal. An electronic circuit then conditions the signal to provide meaningful results. Finally, the SLM communicates the results to the operator in one or more ways.
Before we address the specifics of various kinds of meters, we should address the most basic question of all, “How should I hold the SLM?” Should the microphone be pointed at the noise source or should the face of the microphone be oriented at some other angle such as at a right angle to the sound wave? The answer depends on the type of microphone being used. There are three different types of microphones available: free-field, random incidence and pressure. Free-field microphones should typically be pointed directly towards the noise source. Random incidence microphones should typically be held at a 70° angle to the source. Pressure microphones should typically be held at a right angle to the noise source. The rule here is to follow the manufacturer’s recommendations with respect to microphone orientation. Generally, low frequency sounds are not affected by the microphone orientation as much as high frequency sounds. Again, this depends largely upon what type of microphone element is used in the SLM.
The Basic SLM
Features vary considerably from meter to meter and from manufacturer to manufacturer. Perhaps surprisingly, so can performance and accuracy. No matter what type of SLM is used, at least two requirements of the meter should always be met. These include some method for performing a field calibration of the SLM and an independent certification that the SLM meets Type I or Type II standards of performance and all other applicable SLM standards in your locality. Your noise ordinance should include a statement of standards that must be met by the meter.
In its most basic form, the SLM will provide the operator with an indication of the instantaneous SPL being detected. Often a basic meter will also provide an indication of the maximum SPL encountered as well. Results from a Basic SLM’s are almost certainly limited to presentation through the display of the meter. Rarely are there capabilities for these meters to output results to a printer or computer. There may or may not be provisions in the meter to allow the operator to change certain characteristics of the SLM’s signal conditioning circuits. These characteristics in a basic meter may or may not include the weighting network and the response time constant. Your noise ordinance should include a specification as to which weighting network and response time constant is to be used.
Weighting networks most common today consist of "A", "C" and "Z" weighting. Each of these weighting networks is a "standard" that dictates how the SLM will recognize the amplitude of the SPL based on the frequency of the sound. For instance, "A" weighting circuits simulate how the human ear responds to sound. We know that humans can hear within a fixed range of frequencies and humans perceive that sound is louder or softer as frequency changes.
Response time constants define how quickly an instrument must be able to recognize and process changing SPL’s. The most common options today are "Fast", "Slow", "Peak" and "Impulse" time constants. If it were not for the existence of frequency weighting and response time constant standards, results from meter to meter and manufacturer to manufacturer would almost certainly vary widely and prohibit the effective measurement and enforcement of noise limits.
Depending upon the requirements of your noise ordinance, you may need an SLM that computes the average SPL over a prescribed amount of time. These types of SLM’s are referred to as Integrating SLM’s because they automatically calculate the average SPL. All Integrating SLM’s calculate this result based on a given doubling or exchange rate, as discussed earlier. Some SLM’s may be fixed for a specific exchange rate at the factory. Others may include provisions for setting the exchange rate in the field. In either event, it is important to note which exchange rate the SLM is using and that it matches the requirements of your noise ordinance. Since it is possible for ordinances to change, it is always more favorable to have an SLM that allows the exchange rate to be changed by the user without requiring factory modification, or worse yet, replacement. Integrating SLM’s may include provisions for printing results or uploading them to computer. Generally speaking, unless the meter also documents the performance of a field calibration in its output, the value of the hard copy results is greatly diminished.
After integration, the next mostly commonly sought after capability in an SLM is datalogging. Datalogging SLM’s provide much more detail of the noise-testing event. This can include a minute-by-minute profile of the sound source’s SPL levels. At a minimum, these kinds of meters should provide hard-copy and computer upload of test results correlated to the real-time and date of the event.
Octave Band SLM’s and Real Time Analyzers
At the top end of the spectrum for SLM’s you will find devices that are capable of determining and reporting the SPL and average SPL at various frequencies. These meters are rarely used for noise ordinance enforcement since ordinances rarely specify noise limits as a function of frequency. Generally speaking, once frequency content of the sound is a concern, specialists in acoustics are required to perform these tests.