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Our mission: To promote, protect and preserve hunting and the shooting sports.
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Designing and Operating a Range to Minimize Sound

Designing and Operating a Range to Minimize Sound

By John Joines, Range Engineer
National Rifle Association

(This article is reprinted from the Third National Shooting Range Symposium, 1996 with permission from International Association of Fish and Wildlife Agencies, Wildlife Management Institute and U.S. Fish and Wildlife Service.)

As range owners and operators, we continuously have to be concerned about sound.

Sound defined

First of all, let's define sound. Sound is a vibratory disturbance in the pressure and density of a fluid or the elastic strain of a solid, with a frequency in the range between 20 and 20,000 hertz (Hz). Sound can be measured using electronic instruments called sound meters.

Sound waves have a variety of characteristics. The two most important are amplitude (the highest value or the height of the sound wave) and frequency (the number of complete cycles of a periodic process occurring per unit time). When a firearm is discharged, both high and low frequency sound waves are created. These characteristics come into play when barriers are used.

Sound can be sensed by the auditory nerves of the human ear. Sound has varying degrees of intensity or sound pressure levels. These levels are measured in decibels.

Sound continuously radiates in ever-widening spheres. One needs to understand this in order to know how to control sound. However, each range is unique, and what works at one range may not work at another.

Sound comes to a point source emitter and travels to a receptor or something that can record or hear sound. To visualize sound waves, picture waves on a lake that are created when you drop a pebble in it. The waves move out in ever-widening circles. However, in the case of sound, it does not travel along a two-dimensional surface like the surface of a lake, but in a three-dimensional sphere. Also, like the waves on a lake, sound waves can be reflected. Over distance, sound waves will reduce in intensity.

Noise is defined as something that is loud, disagreeable, unwanted. Noise can be categorized as hazardous, a nuisance or objectionable. Laws sometimes will define hazardous and nuisance sound levels, but there is no definable level for objectionable sound levels.

Hazardous levels may be defined as sound pressure levels of 90 decibels or greater for an 8-hour period.

Nuisance levels often are quantified by local regulations depending upon the laws or sound ordinances. In some cases, nuisance levels may be as lows as 45 decibels. Other ordinances may set a nuisance level between 65 and 100 decibels, and during a specific time (e.g., 11 p.m. to 7 a.m.)

Objectionable levels of sound usually are not quantified, for there is no way to assign a meaningful decibel rating. Objectionable sound is merely pervasive, unwanted, untimely and annoying. Water dripping from a faucet in the middle of the night is a good example of an objectionable noise.

The following brief description should give a general idea about the intensity of sound.

Normal speech is measure between 63 and 65 decibels. This is not very loud. However, if I clap my hands, the sound pressure levels increase to approximately 80 decibels. Firing a pistol generates approximately 110 decibels. A rifle shot generates approximately 120 decibels. All these measurements may be taken by a sound meter.

In order to understand sound, one must first understand the Inverse Square Law.

For sound generated in an open field without obstructions (grass, shrubs, trees or buildings), there are only the point source emitter and the receptor (in this case, the sound meter).

The Inverse Square Law says: For every doubling of the distance away from the point source emitter, the sound pressure levels will be reduced by 6 decibels. For instance, consider an initial measurement of 116 decibels and 10 feet away from the point source emitter. When the distance is doubled from 10 to 20 feet, the sound pressure level is reduced to 110 decibels. When the distance again is doubled from 20 to 40 feet, the sound pressure drops another 6 decibels to 104 decibels. At 80 feet, the level drops to 98 decibels.

Effects of grass and shrubs

Now we should take a closer look at excess attenuation gained by the effects of grass and shrubs. Excess attenuation is the attenuation beyond that caused by wave divergence or distance from the source over a free field. A free field is defined as a flat plane with no obstructions. If grass is added to the free field, for a source and receiver height of 8 feet, excess attenuation of 5 to 10 decibels over a distance of 824 feet has been observed in the frequency range from 100 to 6,300 Hz. In the range of 300 to 600 Hz, excess attenuation may go as high as 50 decibels.

The excess attenuation over thick grass and shrubbery is higher still. The excess attenuation at 1,000 Hz can be as high as 23 decibels per 300 feet with an increase of 5 decibels per 300 feet for each doubling of frequency. These figures are approximate and depend upon the type of vegetation and shrubs.

Effect of trees

Trees in some cases will allow you to get between 3 and 23 decibels in additional sound reduction per 300 feet.

For example, sound emitted from a range that is situated next to a hardwood forest with a high canopy and an open understory will be reflected by the tree trunks and attenuated only about 3 decibels per 300 feet. However, if the forest consisted of evergreens with foliage growing from top to bottom (such as with Canadian hemlock), then excess attenuation can be as high as 23 decibels per 300 feet. Trees can be very helpful depending upon species and location in relation to the firing line.

Depressed or elevated ranges

Some ranges are built in old strip-mining areas. When the range is constructed below the level of surrounding ground, measurements show that depressing a range by 12 feet yields an excess attenuation of 7 to 10 decibels at all distances from the range. For an elevated range, excess attenuation of 2 to 10 decibels is found within 300 feet of the range. Beyond 300 feet, the noise radiated by elevated and ground-level ranges is the same.

Effect of walls

Nonporous walls of sufficient mass (minimum of 4 lb/ft2), if interposed between source and receiver, can result in appreciable noise reduction, because sound can reach the receiver only by diffraction around the boundaries of the obstacle. To achieve this, the wall must be free from cracks or holes. A quarter-inch hole can defeat the best sound-deadening materials. This type of wall will yield approximately 8 decibels of excess attenuation.

Walls made of more substantial concrete (mass of 20 lb/ft2) can result in excess attenuation up to 22 decibels.

Effects of buildings

Data shows that intervening buildings introduce an excess attenuation of 15 to 20 decibels. Surprisingly, this excess attenuation is essentially independent of the number of intervening structures. In other words, the first shielding structure reduces the sound level, and subsequent structures do not produce further reductions. In addition to this excess attenuation, the sound pressure level measured behind a wall is decreased by about 3 decibels per doubling of distance owing to cylindrical spreading of the sound wave.

 

What to do if you receive a sound complaint

Please heed the following considerations and questions should your range receive a sound complaint.

1. Who is the complainant? Can it be worked out? Can you give the complaining person a club benefit? Do they want to belong to the club? Can you give them free membership?

2. Is the club violating a noise ordinance? Is the shooting in early morning or at night? What are your Sundays and holidays schedules?

3. The following information applies to when a noise ordinance is involved.

A. What does the preamble say? For example, does the ordinance control certain types of noises from a particular emitter zone only during certain hours?

B. Are there exemptions that fall under the category of "recreation, shooting and training"? Please note the importance of your range name. Mention ranges or shooting in your range/club's purpose statement or bylaws or include words such as "...for all other purposes consistent with law...."

C. Are quantitative, described levels specified? Without a specific level, an ordinance is too vague or subjective.

D. If quantitative levels are not established, does the ordinance define such words as hazardous, objectionable or nuisance? Be careful when subjective words are used. Some people apply their own value judgements to ranges. When this happens, difficulties arise in defending sound issues.

E. Does the ordinance specify the scale, mode, location for measurement, hours of the day, time weighted average (TWA), noise zones or other information pertinent to decibel testing?

What to do when no noise ordinance exists

Since there is no violation of law, range officials who might otherwise conduct testing are at a disadvantage. Without standards prescribed by ordinance, range officials may not know:

1. what mode or scale to use on the test meter.

2. what type of meter to use.

3. who to share the results with once they have conducted the test.

4. whether the results will be interpreted as a/an:

a. hazardous level.

b. nuisance level.

c. objectionable level.

d. area where a Time Weighted Average applies.

In the absence of a noise ordinance, it is easier for the range operator to question the protocol and/or methodology of the complainant than it is for the range operator to defend his protocol and/or methodology.

Conclusion    

The bottom line is that sound needs to be carefully managed. Sound tests must be taken at various locations on and around the range. Once sound levels are measured, a plan can be developed using natural or manmade objects to assist in attenuating sound.

Remember when dealing with complainants about noise: In the absence of a noise ordinance, it is easier for the range operator to question the protocol and/or methodology of the complainant than it is for the range operator to defend his protocol and/or methodology.