NOISE LEVEL OSHA COMPLIANCE & SOUND ORDINANCE (DNL – DAY NIGHT LEVEL) SURVEYS (DATA LOGGING / IMPULSE)
Environmental noise is a form of acoustic pollution which has the potential to disrupt human communication, create annoyance, damage hearing and is regulated by the federal OSHA (Occupational Safety and Health Administration) Noise Standard 29 CFR 1910.95 along with other state agencies. In some cases it can also interfere with sensitive equipment such as that used in the nanotechnology and semiconductor industry.
It is also regulated in many residential areas by municipalities such as Fort Worth to provide comfort and peace to residents.
If you are receiving complaints from workers, or have been contacted by OSHA regarding a potential excessive noise issue, ScanTech can evaluate the environment for continuous (data-logging over time) and impulsive noise on the dB(A) and dB(C) scales to determine compliance. We can also fit workers with a continuous dosimeter to track sound pressure levels as they move into different work areas at different times of day. (meeting ANSI & IEC Type II standards)
Our sound level meters are able to average noise at slow, (1 second per sample) fast (0.125 seconds per sample which corresponds to the response time of the human auditory system) or custom sampling rates as fast as 0.035 seconds which are used for impulsive (sudden, short) noise measurements. Impulse sounds are generally defined as acoustic events lasting less than 1 second with a repeat interval of greater than 1 second.
The data logging function also enables the measurement of reverberation which is the measurement of time it takes for the sound level to drop 60 dB or a factor of 1 million, and characterizes how “reflective” a room is to acoustic vibrations.
Because sound pressure levels are measured on a logarithmic scale, (power ratios of 10) an increase in decibels beyond a certain point means that the energy impacted on the ear can quickly pass a threshold where damage can occur. As shown in the chart below, even an increase of 5 dB can greatly reduce the duration before hearing loss can occur.
Note that sound levels above 70 dB begin to interfere with voice communication, while 75 dB and above are generally characterized as annoying. (but dependent on frequency as shown below)
The general effects can be studied in the response table below:
Fortunately, sound pressure levels also decrease with the square of the distance, so moving twice as far away means that the power level decrease by a factor of 4. Bear in mind that 3 decibels (dB) reflects a change in power level by a factor of 2, with +3 dB being twice as loud and -3 dB being half as loud.
Also, the human ear does not have a “flat” frequency response – so frequencies at the extreme edges of hearing perception (from 20 Hz to 20,000 Hz) are not perceived to be as loud as those centered around 1 KHz – 3 KHz which is in the range of normal human speech.
Therefore, different measuring scales on sound meters are used to distinguish between “human” hearing (dBA) and “machine” hearing (dBC) which has a flatter frequency response.
In general, interference with speech communication occurs when intrusive noise exceeds about 60 dB (Federal Interagency Committee On Noise 1992). Indoor speech interference can be expressed as a percentage of sentence intelligibility among two people speaking in relaxed conversation approximately 3 feet apart in a typical living room or bedroom (U.S. Environmental Protection Agency 1972).
The percentage of sentence intelligibility is a non-linear function of the (steady) indoor background A-weighted sound level. Such a curve-fit yields 100 percent sentence intelligibility for background levels below 57 dB and yields less than 10 percent intelligibility for background levels above 73 dB. The function is especially sensitive to changes in sound level between 65 dB and 75 dB.
As an example of the sensitivity, a 1 dB increase in background sound level from 70 dB to 71 dB yields a 14 percent decrease in sentence intelligibility. The sensitivity of speech interference to noise at 65 dB and above is consistent with the criterion of DNL 65 dB generally taken from the Schultz curve. This is consistent with the observation that speech interference is the primary cause of annoyance.
Typically, homes in the United States provide 15 dB of sound attenuation with windows open and 25 dB with windows closed and air conditioning operating. Commercial buildings offer anywhere from 30 – 45 dB of sound attenuation depending on construction and geometry with respect to noise point sources.
Some guidelines on judging sleep interference. The EPA identified an indoor DNL (Day-Night Average Sound Level) of 45 dB as necessary to protect against sleep interference (U.S. Environmental Protection Agency 1978). Assuming a very conservative structural noise insulation of 20 dB for typical dwelling units, this corresponds to an outdoor day-night average sound level of 65 dB to minimize sleep interference.
ScanTech Technical Consulting also has experience in doing “on-paper” studies for environmental impact and Environmental Survey Assessments (ESA) including the DNL influence of major roadways, railways, airports, etc. including the CEQR noise level standards of New York City. There are DNL ordinances in cities like Fort Worth and Grapevine (Section 55 Part A 3) which is shown. Dallas does not give numbers, but situational guidelines for noise in the following link:
Grapevine Section 55 Part A 3 DNL Ordinance Example:
NOISE REGULATION: This ordinance shall apply to all sound originating within the limits of the City of Grapevine, Texas. It applies 24 hours of everyday of the week and year around. This ordinance does not apply to any moving vehicle or aircraft nor does it apply to sound emitted from any emergency warning device. This ordinance does not apply to the following activities as long as they are conducted in daytime hours as a normal function of a permitted, conditional or special use and the equipment is maintained in proper working condition:
· Lawn and yard maintenance
a. General Provisions
1. A person may not conduct a use that creates a sound which exceeds the sound levels established in Table 1 or that exceeds the background sound levels by 5 dBA., whichever is greater.
2. A sound level meter that meets the standards of ANSI with Type 2 or greater precision must be used to determine whether the level of sound violates those established in this section. The instrument must be maintained in good working order. A calibration check should be made prior to and following any noise investigation.
3. The sound levels must be measured at the bounding line between properties at a level five feet above ground level.
4. A sound level meter shall be used to determine the Leq in dBA over an 8 minute time period.
5. The sound receiving property with the most stringent requirements will apply.
6. Traffic, aircraft, and other background sounds are not to be considered in measuring sound levels except when the background sound level is being determined.
7. All outdoor speakers in non-residentially zoned properties shall face away from contiguous residential zoning districts.
Corrections for Character of Sound Applied to Table 1. values are as follows:
· If Sound is Impulsive in Character – (hammering, popping, exploding, etc.) Subtract 5 dB from Maximum Permissible Sound Levels
Note that for the purpose of this ordinance, an impulsive sound shall exist when the sound changes at rate greater than 10 dB per second. The sound level meter should use Fast Response for this evaluation.
· If Sound is Periodic in Character – (hum, buzz, screech, etc.) Subtract 5 dB from Maximum Permissible Sound Levels
Note that for the purpose of this ordinance, a pure tone shall exist if the one third octave band sound pressure level with the tone exceeds the arithmetic average of the sound pressure levels of the two contiguous one-third octave bands by 5 dB for center frequencies of 500 Hz and above and by 8 dB for center frequencies between 160 and 400 Hz and by 15 dB for center frequencies less than or equal to 125 Hz. A one-third octave band spectrum analyzer instrument will be required to make these evaluations.
END NOISE LEVEL ORDINANCE EXAMPLE
SOUND & ACOUSTICAL LINKS INCLUDING HEARING TESTING
OSHA Compliance Tools (including lookup of violations)