Q & A

Sound and vibration measurement, analysis and control can be very technical at times. There are a lot of common misconceptions about how sound and vibration behave, and what works - or doesn't - to control them.  For example, contrary to extremely popular opinion, a row or trees, even several rows, will do nothing practical to reduce sound.  Want to know why?  See below.

Q  What can I do when my neighbor is making too much noise?

Probably the least expensive step it to try talking to the plant manager, store owner or homeowner about the possible problem.  You have to determine whether you'll get a real listener or a hostile attitude, of course.  We encountered one situation where a homeowner complained about a loud swimming pool pump (it really was loud).  He offered to completely pay for a new, quiet pump.  The neighbor refused.  After arbitration, they're now (as of this writing) headed to court.

Contact your local Health Department.  They should know what laws or regulations apply.  Some departments can measure sound levels, but they may not have calibrated equipment or know how to use it well.

Does your state, county or municipality have a noise regulation?  If it is a "nuisance law" ("no unnecessary noise") it may be hard to enforce, especially if the other side fights back.  Noise regulations need a number of elements to be technically valid.  We commonly see a version of one used by many towns who apparently copy it from some other town when they decide they need a regulation.  The principal problem with it is that it calls for measurements to be made in a way that necessitate outdated, no longer available equipment and based on old standards.

We can assist you by making sound measurements ranging from individual readings at various times and locations to setting up monitors to record the sound levels over hours or even days.  We can testify at hearings or in court, if necessary.

Q  What should I do if a person or enforcement entity says we're making too much noise?

Look into it!  Ignoring the matter, even if is "just" a neighbor, won't make the problem go away.  What's the complaint?  What process or equipment might be causing the noise?  Try turning various pieces of equipment off and on to identify the source.  Once you know what's causing the problem you can better deal with it.

If you are accused of making excessive noise we can study and measure the alleged problem, work with your attorney, advise you on control measures (if they're necessary) and testify if it becomes necessary.

Q  What are "decibels?"

We could give you the technical definition but that probably wouldn't help very much!

Surprisingly, a "decibel," or "dB," doesn't necessarily have anything to do with sound.  Vibration can be expressed in "decibels," for example, and because vibration may be measured in terms of acceleration, velocity or displacement there can be a different kind of decibel for each.  Sound pressure (which is what we hear), sound power, and sound intensity, all different, can all be expressed in terms of "decibels."  Is this clear to you now?!

A "decibel" involves a ratio of some physical parameter (e.g., pressure fluctuations for sound, in particular) to a reference quantity (i.e., a reference pressure, for sound).  Because a decibel involves a ratio, it technically has no units or dimensions.  In non-engineering terms, stating that there are X number of "decibels" without stating the reference quantity is really meaningless.  (This, by the way, is a fault with many local noise and vibration regulations.)  For sound decibels ("sound pressure level") the preferred reference pressure is 20 uPa (microPascals), as defined by the American National Standards Institute (ANSI).

Decibels are also a logarithmic quantity.  This means that, say, a 10 dB increase corresponds to 10 TIMES as much of the thing you're measuring or hearing, not just 10 more of it.  To illustrate, consider you have $100 in your pocket.  If someone gives you $10 more you have $110.  But if your $100 is increased by 10 dB you'd have $1,000!  Conversely, if your $100 were decreased by 10 dB you'd have one-tenth that, or only $10 in your pocket, not $90.

As you might imagine, this makes doing "decibel arithmetic" a bit tricky for someone who doesn't understand the mathematics.  Here are a few examples.

A particular noise source produces a sound pressure level of "50 dB" (ignore what we said above about the reference quantity!) at a distance of 100 feet from the source.  If a second, identical source at the same location is turned on, what is the total sound level?  The answer is not 100 dB (50+50=100)!  Decibels add logarithmically, so the answer is 53.

Say we have 10 identical noise sources with a combined sound level of 100 dB.  If one is turned off, what is the new level?  90?No.  Decibels subtract logarithmically, too.  The total sound from the remaining 9 sources is 99.5 dB.  If you turn of a total of 5 of the 10 sources (half) the level will be 97.  With 8 of the 10 off the total is 93, and with 9 of the 10 off the sound from one source is 90 dB.

This leads to an often-quoted "rule of thumb" that if you double or half the number of sound sources the change will be an increase or decrease of 3 dB, assuming the sources are all the same.

By the way, a 3 dB change is inaudible in most cases.  A 5 dB change is slightly audible, and a 10 DB change is generally perceived as a doubling or having of the "loudness" of the sound.

Q  So what is sound?

As most people mean it, sound is simply a fluctuation in air pressure.  Of course, you can hear sound under water or through a solid wall, but mostly people mean airborne sound.

The pressure fluctuations may come from a moving sheet metal cover on a machine, the blades on a fan passing by a stationary point, the pulsations of a compressor, the turbulent flow of air out of a jet engine or many other sources.  In the end it is all pressure fluctuations that we hear.

To be audible the pressure fluctuations must be of sufficient magnitude and frequency.  If they are too faint you won't hear them (0 dB, which is not no sound, but just the amount of minimum sound the typical person can hear in a controlled environment).  Too high a magnitude and the pressure fluctuations can cause pain or quick damage to your ears; this occurs at around 140 dB (relative to the standard reference sound pressure).

Likewise if the fluctuations happen too slowly or too fast our ear (a "transducer," to an engineer) can't detect the change.  For a good ear the frequency over which it can hear sounds (assuming they're of sufficient amplitude or loudness) is 20 cycles per second (now called Hertz, abbreviated Hz) to 20,000 Hz, or 20 kHz.  Depending on your age, work history, illnesses and medication, to name a few factors, your frequency range may not be as broad.  For comparison purposes, human speech lies in roughly the 500 Hz to 3,000 Hz (or 3 kHz) range.

Click here for a link to a page illustrating sound pressure waves.  (Note that the site and its owner are not affiliated with Russell Acoustics, LLC.  He just has some nice graphics!)

Q  What is a "dBA?"

Because the human ear does not hear, as loudness, equal sounds at different frequencies, there are various measurement methods that try to take this into account.  For example, a sound level of, say, 80 dB at a frequency of 250 Hz will not sound as loud as 80 dB at 2,000 Hz.

Probably the single most common measurement of sound uses a setting on a "sound level meter" that has the instrument basically measure the overall sound in a way that corresponds to the ear's way of hearing sounds at different frequencies; the ear's "frequency response."  This is the "A-weighting network," therefore decibels determined this way are labeled "dBA" or "dB(A)."  If "dB" is used by itself (no "A" or other letters) this generally means (at least among professionals) un-weighted measurements, or no adjustments for frequency.

In the U.S. dBA is used by many local and state agencies - it is easy to measure - and five major Federal agencies: OSHA (U.S. Department of Labor), the FAA (Federal Aviation Administration), DoD (Department of Defense), the EPA (Environmental Protection Agency, who is pretty much out of the noise business), and HUD (Housing and Urban Development).  Depending on the circumstances, the measurements may need to be done in different ways based on the agency's criteria.  For example, aircraft noise studies would probably include measurements made over one or more 24-hour periods to determine the "Day-Night Sound Level" ("Ldn" or "DNL") for comparison to FAA criteria.  A developer wishing to construct housing near an industrial site, rail line, airport or highway may want to consider measurements to compare to HUD criteria, even if the project isn't HUD-financed, because it is a measure of what might need to be done to the structures to reduce at least interior sound levels.

"dBA" measurements also turn out to be a pretty good predictor of overall "annoyance" with the general sound level in a community.  That is, measurements of the long-term average sound level, in terms of dBA, correlate well with studies of how people overall rate their community for noise.  Rest assured there are lots of different studies on this, and one can find different opinions on what is "acceptable."

There are a LOT of different ways to measure sound!  "dBA" is just one of them, although very common.

Q  What's the difference between "sound" and "noise?"

Finally an easy one!  Noise is unwanted sound.  Keep in mind that what is "unwanted sound" to one person may be very desirable to others.  The music you play in your condo, at some sound level, may be very undesirable to your neighbor, even though the sound level next door is much lower; and the opposite is also usually true.

In environmental acoustics the magnitude of typical sound levels is usually well below those encountered in industry.  The two problems (industrial noise and environmental noise) are considerably different.  Industrial noise deals with potential hearing loss, which starts occurring (depending on whose studies you read) at 80 to 90 dBA for a full shift over a working lifetime (in other words, exposure time, not just level is a factor).

Environmental noise cases usually involve sound levels in the 40 to 65 dBA range - sometimes higher - as measured over long times, and higher levels over short times.There are many different ways for measuring and setting standards for environmental noise, so it is important to understand the "metric" that applies in a particular circumstance and how it is to be measured.

Q  Can't I stop noise from going into another room or out of a building by putting up a sound absorber - maybe that special egg-crate-looking foam - on the walls?

If you told me your car wouldn't start, and I told you to rotate the tires, would you think my "solution" would fix the problem?

In brief, no, the "special" foam isn't going to do what you think it does, frankly because you don't know or understand what it does.  The wedges on the surface look "acoustical" so they must do something, right?  About the only worse thing you could do - we've seen it done! - is to use sheets of "Styrofoam."  As one person said, "Foam is foam, right?"  Wrong.  For homework, look up the differences between open-cell and closed-cell foam.

A sound absorbing material PREVENTS REFLECTION.  This is described by its "absorption coefficient."  It is not a noise barrier material, whose acoustical performance is expressed in "transmission loss."  Two completely different properties, two completely different materials.

If "absorption" is the prevention of reflection (stay with me on this) then an OPEN WINDOW is a PERFECT ABSORBER (of sound).  How much of the sound hitting the opening reflects back? None!  If you think this is a silly analogy, go read up on the work by Wallace Clement Sabine back in the late 1800's, early 1900's.  (Yes, acoustics HAS been around for a long time!)  You'll see that Sabine actually used open windows for his material tests.

Anyway, if open windows are a perfect (or even really good) sound absorber, how much good do you think it would it do for preventing the sound from getting out of one room or building into another if the walls were open windows?  We hope you might be getting the idea that things don't perhaps work the way "everyone knows" they work?  They don't; there are a lot of misconceptions about sound and vibration measurement and control.

Q  So what about the trees?  Surely three overlapping rows of evergreens between a factory, highway, or shopping center - a neighbor's noisy swimming pool pump! - will significantly reduce the sound I hear?

Sorry to have to say it again, but no.  You're probably repeating another of the "everybody knows" things, not basing it on data - and there are test data available if you know where to get it.  The fact of the matter is it takes (from our measurements; we HAVE made the measurements) on the order of 100 feet of dense woods to reduce the sound an additional 5 dBA (or thereabouts) over what you get from the distance effect.  Trees bounce the sound energy around a bit but really don't attenuate it. Engineers know that something called "conservation of energy" applies, which means if you don't get rid of the sound energy somehow or route it to another place, there is going to be as much noise as there was before.

Not seeing a sound source very likely has a psychological effect (out of sight, out of mind) but does very little for the physical problem.  The difference in sound from three rows of evergreens, all things else being the same, is probably immeasurable; certainly inaudible.  Sorry to disappoint you.

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