Virginia Tech, Blacksburg, VA
Rodriguez Ripley Maddux Motley Architects
The following are some of the most commonly asked questions we get on
general acoustics. See the other FAQ sections for further information. The
FAQ link on our Links-Educational sites page will take you to a broader FAQ file on acoustics.
What is an acoustical consultant?
What do we hear as sound?
How do we quantify sound?
How do we measure sound?
is an acoustical consultant?
After Wallace Clement Sabine discovered the principles of modern architectural
acoustics around 1900, he was sought for his advice on the design of new
buildings and the solution of problems in existing buildings. Fifty years
later, the challenge of acoustical design for a new United Nations headquarters
brought the formation of a firm that became a model for the modern acoustical
consulting firm. Today, approximately 1400 individual members of the Acoustical
Society of America indicate a primary interest in architectural acoustics
or noise. About half are regularly available as consultants. Many of those
involved in noise control are also members of the Institute of Noise Control
Engineering. Most qualified consultants work for one of the approximately
130 member firms of the National Council of Acoustical Consultants.
Acoustical consultants come from diverse academic backgrounds. They use
a combination of scientific theory, experimental data, experience, and judgment
to analyze new problems and provide advice. Sometimes answers to common
problems can be immediate. However, most of the problems we face require
measurements or mathematical analysis or both. The diverse backgrounds,
specialties, and services among consultants can make it hard to find the
best consultant for some projects. Acoustical consultants should be selected
based on a careful comparison of their qualifications and services with
the needs of the project. The work of most acoustical consultants is properly
limited to questions related to acoustics. Acoustical consultants are not
usually fully knowledgeable of requirements in building codes and other
technical matters. Other professionals should be involved in projects as
appropriate and necessary. Back to Top
do we hear as sound?
The human ear is a remarkable mechanism. It can sense and give meaning
to a wide range of sounds varying in loudness and pitch. Perception is
not directly related to the physics of sound.
The effect perceived by the ear as sound is a very small and rapid change
in air pressure. The variations are very slightly above and below the
barometric pressure. The perceived loudness of a sound corresponds primarily
to the size of the variation in pressure. The ear is very sensitive and
can detect, at some frequencies, variations less than 3 billionths of
a pound per square inch (psi). Variations greater than 0.001-.01 psi can
cause pain or discomfort.
For humans to hear the sound, these air pressure variations must be rapid.
They must complete a cycle at least 20 times per second and not more than
20,000 times per second. The repetition rate of these cycles is called
the frequency of the sound. It has units of Hertz (Hz) where one Hz corresponds
to one cycle per second. The bass sounds in music are low-frequency, and
the treble sounds are high-frequency. Back to Top
do we quantify sound?
Because of the wide range of sound pressures encountered, a special scale,
the decibel scale, has been developed. The sound level increases by 3
dB (or decibels) when the actual sound energy present is doubled. Conversely,
half the sound energy must be eliminated to reduce the level 3 dB. A six
decibel change indicates four times as much sound, and a 10 decibel change
10 times the sound. However, the ear does not perceive these changes to
be so large. Most people find a 3 dB change barely noticeable. A 6 dB
change is clearly noticeable, and a 10 dB change is perceived as a doubling
or halving of loudness. This is very important. It means that 90% of a
given sound energy must be eliminated before most people will judge a
sound half as loud.
People do not hear very low-frequency and high-frequency sounds as well
as they hear sounds in the middle speech frequencies. A special electronic
filter is used in sound meters to simulate this characteristic of the
ear. For many environmental sounds it gives a simple, single-number descriptive
level that correlates with the way people judge the loudness of the sound.
This measure is called the A-weighted sound level. The units are usually
written dBA or dB(A). Back to Top
do we measure sound?
The basic instrument for sound measurement is the sound level meter. This
is a microphone connected to a voltmeter with some additional features.
The microphone produces an electrical voltage signal proportional to the
sound pressure variation, and the voltmeter is calibrated to read the
sound level in decibels. Most meters have "fast" and "slow" responses.
The fast response corresponds more closely to the actual fluctuating sound
heard, with the needle sometimes moving very quickly. However, the slow
response is easier to read and commonly used for environmental sound measurements.
It is often useful to know the level of the sound at different frequencies
over the audible range. Frequency analyzers are used for this. The most
common is the octave-band analyzer that divides the audible frequency
range into octave bands (similar to musical octaves) so sound energy in
each band can be measured. These bands are identified by the center frequency
of each band. The center frequency doubles for each higher frequency band.
Narrow-band analyzers provide finer resolution for diagnostic work and
evaluating tonal noises. Frequency analysis data can be A-weighted for
a better indication of the contribution of each frequency to the A-weighted
level. However, they are not normally A-weighted if used for comparison
with criteria based upon unweighted data.
When measuring typical fluctuating environmental sounds over long periods,
it is common to use average and statistical measures. One cannot simply
write down several measured sound levels and arithmetically average the
values. The energy average of 60 dB and 40 dB will be approximately 57
dB, half the higher 60 dB level. The time averaging of sound levels can
be complicated unless one has a special instrument that does it automatically.
The time-average sound level over a given period is also the energy-equivalent
continuous sound level for the period. A steady sound at the average level
has the same energy as actual sound during the period. Time-average level
is often called the "equivalent level," abbreviated Leq or LEQ.
When sound levels vary, some information is lost in expressing the sound
during a period as the average level. Percentile levels can be used to
show the variation in sound levels during the period. For example, the
level exceeded 90% of the time is defined as the L90 value. It is a good
indicator of the background sound level. The higher sound levels are indicated
by the levels exceeded a small percentage of the time. Some ordinances
are based on the L10. L01 indicates the highest levels likely to occur.
The difference between L01 and L90 or L10 and L90 is a good indicator
of variation in sound levels. A measurement period always must be specified
for any statistical values such as L01, L10 or L90 to have meaning.
A special type of average sound level over a 24 hour period has been defined.
The Day-Night Average Level has a 10 dBA night-time penalty added to all
sound between 10:00 p.m. and 7:00 a.m. This is abbreviated DNL, LDN or
Ldn. Most typically, daytime average levels are close to or slightly less
than the day-night level. Night-time levels are usually 5 to 15 dB less
than the day-night level. As with any average, communities with very different
actual noise characteristics can have the same day-night level.
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