Posts Tagged ‘decibels’

Sound and Exposure Standards

Sunday, November 14th, 2010

     How many crickets clicking their legs together in unison would it take before we would suffer hearing loss at the sound exposure? Would we need to sit in a garden filled with millions of them all night long, only to discover in the morning that we could no longer hear the tea kettle’s whistle? The chart below may not provide the answer to this question, but it does provide some very good examples of different sounds and the point at which they become hazardous.

     So how do we know where we’re at safety-wise with sound pressure levels and exposure times? This question wasn’t pondered until the 1950s, when the military, specifically the Air Force, provided the first standards in this regard in 1956. This initial action was followed up by numerous studies and standards committees wrestling with the issue. It wasn’t until 1981 that the Occupational Health and Safety Administration (OSHA) required employers to implement hearing conservation programs for employees in certain noise-filled environments.

     What surprised many of the first scientists studying the impact of sound is that sounds don’t necessarily have to be initially perceived as “too loud” in order to cause hearing loss. Many sounds that we perceive as easily tolerated can in fact cause hearing damage if exposure is long enough.

     So what’s “long enough?” Title 29 of the Code of Federal Regulations, Section 1910.95, lists the OSHA permissible sound exposure durations at various sound levels, as shown in Table 1.

Duration of Exposure (Hrs.) Sound Level (dB)
8 90
6 92
4 95
3 97
2 100
1.5 102
1 105
0.5 110
0.25 or less 115

Table 1 – OSHA Permissible Noise Exposures

     Just to put things into perspective, a small chain saw tearing into a log typically produces sound at 90dB, or 90 decibels, which you will recall from last week’s article is the measuring unit used for sound. And that noisy truck clattering down your street, the one that your dog can’t help but bark at, can produce 100dB. The guy standing on the airport tarmac directing your plane into the gate can be exposed to as much as 150dB. There’s a good reason he’s wearing ear protection.

     Let’s take a closer look at the information provided in Table 1. It states that you most likely will not suffer hearing loss if you spend up to 8 hours in a place where the sound level does not exceed 90dB. Comparing that information to Table 2, which is specific to noises produced at a power plant, we see that this sound level is produced by the typical steam turbine.

     One thing to keep in mind is that when we are exposed to various sounds throughout the day, we can compute a time-weighted average noise, or TWAN, to help us determine if our overall environment poses a threat to our hearing. This method of assessing the gross impact of many different sound exposures is represented by the formula:

TWAN = (C1 ÷ T1) + (C2 ÷ T2) + (C3 ÷ T3) + …

where C represents the total time of exposure at a measured sound level, and T represents the total time of exposure. T, which in our example stands for “hours,” is found in the left column of Table 1. Based on scientific studies of sound’s effects on the human ear, if the TWAN is greater than 1.0, then the exposure exceeds safe limits.

     Let’s find out if a worker in a coal fired power plant is at risk of losing his hearing during the course of a typical eight hour workday. Table 2 shows the different noises he has to contend with during that time.

Duration of Exposure (Hrs.) Location Sound Level (dB)
0.5 Steam Turbine Basement 90
2.5 Air Compressor Room 95
0.25 Forced Draft Fan Gallery 110

Table 2 – Example Exposure in an 8 Hour Day

     Now let’s find out if his OSHA recommended sound exposure limit has been exceeded. The values for C, or total time of exposure, are given in the left column, and the corresponding sound level in dB’s is shown in the right column of Table 2. Using these numbers as a reference, we now correlate them with the information contained in Table 1 which cites the OSHA standards. Plugging in the numbers, we find that this worker’s TWAN would be:

TWAN = (0.5 hours ÷ 8 hours) + (2.5 hours ÷ 4 hours) + (0.25 hours ÷ 0.5 hours)

= 1.18

     Since 1.18 is greater than 1.0, we see that the worker’s noise limit would indeed be exceeded. He would need to either wear hearing protection or limit his exposure time in order to comply with OSHA regulations and protect his hearing.

     Next time we’ll discuss options open to us to control sounds in our environment.


Sound and the Decibel

Sunday, November 7th, 2010

     Hearing is one of our most valued senses.  It, along with sight, is how we acquire most of the information available to us in our environment.  But have you ever considered what sound actually is, or how it is perceived?

     Sound, at its simplest, is a series of pressure waves traveling through a medium, whether that medium be a solid, liquid, or gas.  Now let’s focus for a moment on the medium we’re most familiar with, which, unless you live underwater, is air. 

     The last time you turned on the radio, I’ll bet you didn’t consider what makes it possible to hear sound coming from that box.  How does that magical fete happen?  Well, turning the knob on the box results in the amplification of electrical impulses produced by incoming radio waves broadcast from the radio station.  These impulses power the speakers, causing them to vibrate.  The vibrating speakers in turn create pressure waves in the surrounding air, and those waves will travel until they crash into an obstruction, such as the workings of your inner ear.  At this point the magnificent human body comes into play, skin, bones, and fluid working together to convert the action of the pressure waves from your radio’s speakers into nerve impulses which find their way to your brain via your nervous system. Your brain interprets these nerve impulses and perceives them as music.

     If sound pressure waves are strong enough, they can crash against the workings of our inner ear and create damage.  Damaging pressure waves can be created by things like sitting too close to the massive speakers at a rock concert, or a jet’s engine, as well as by many types of machinery present in a factory.  In most cases hearing can be preserved by wearing appropriate protection, coupled with limiting the amount of time you are exposed to the loud source of sound.  Both of these factors are important, as we’ll discuss later.

     Thankfully, there is high-tech equipment capable of determining whether sound pressure waves are capable of inflicting damage on our delicate organs, equipment which compares the source’s measured sound pressure to the sound pressure associated with the threshold of human hearing.  By threshold, I mean the sound pressure at which our ear begins to perceive sound, which was determined by scientists to be 0.00002 Newtons per square meter, or N/m2.  The Newton is a unit of measurement within the metric system used to quantify force, much as pounds quantify units of force/weight within the English system, and since pressure is force divided by area, we’ll be dealing with units expressed in N/m2. The sound pressure measurement is then converted to a sound pressure level, expressed in units of decibels, or dB, according to this formula:

Lp = 20 × Log(p ÷ p0)

where Lp is the sound pressure level in dB, p is the measured sound pressure, and as discussed previously, the threshold sound pressure, p0, equals 0.00002 N/m2.   If you will remember from previous blog discussions, Log, a standard function on most calculators, is used to scale down the sound pressure level numbers, which tend to be quite large, into smaller, more evenly spaced numbers, making them easier to plot on a graph.

     Okay, so suppose you want to determine if a piece of factory equipment is dangerously noisy to those working around it.  Using a sound level meter you would first measure its sound pressure, p, to be 20 N/m2.  Applied to the formula given above, we calculate the sound pressure level to be:

Lp = 20 × Log(20 N/m2 ÷ 0.00002 N/m2) = 120 dB

     Now bear in mind that 20 N/m2 is the level at which most people’s ears will start to hurt.  Prolonged exposure at this level can cause damage to the ears and hearing loss.  Depending on your exposure time to a sound source, even much lower sound pressure levels can lead to hearing loss. 

      So how do we know where we’re at safety-wise with sound pressure levels and exposure times?  Next week, we’ll find out when we look at exposure standards set up by the Occupational Health and Safety Administration (OSHA).