Acoustics 101

This glossary of terms has been provided with a brief description, for the most part in non technical terms in an effort to remove some of the mystery surrounding Acoustics. While the explanations may not be totally correct in their literal interpretations it is hoped that the plain language approach will provide a better understanding of the terminology frequently used in the field of acoustics.

ACOUSTICS: The science of Sound. Its production, transmission and effects.

ACOUSTICAL: The properties of a material to absorb or reflect Sound (adjective) Acoustically, (Adverb).

ACOUSTICAL ANALYSIS: A review of a space to determine the level of reverberation or reflected sound in the space (in seconds) influenced by the building materials used to construct the space.  Also the amount of acoustical absorption required to reduce reverberation and noise.

ACOUSTICAL CONSULTANT: A professional usually with an engineering degree who is experienced in providing advice on acoustical requirements, and noise control in a variety of situations.

ACOUSTICAL ENVIRONMENT: The acoustical characteristics of a space or room influenced by the amount of acoustical absorption, or lack of it in the space.

AIRBORNE SOUND: Sound that reaches the point of interest by propagation through air.

ARCHITECTURAL ACOUSTICS: The control of noise in a building space to adequately support the communications function within the space and its effect on the occupants. The qualities of the building materials used determine its character with respect to distinct hearing.

ARTICULATION CLASS: A single number rating used for comparing acoustical ceilings and acoustical screens for speech privacy purposes. AC values increase with increasing privacy and range from approximately 100-250. This classification supercedes Speech Privacy Noise Isolation Class (NIC) rating method.

ARTICULATION INDEX (AI): A measure of speech intelligibility influenced by Acoustical Environment rated from 0.01 to 1.00.The higher the number the higher the intelligibility of words and sentences understood from 0-100%.

ABSORPTION: The properties of a material composition to convert sound energy into heat thereby reducing the amount of energy that can be reflected.

AREA EFFECT: Acoustical materials spaced apart can have greater absorption than same amount of material butted together. The increase in efficiency is due to absorption by soft exposed edges and also to diffraction of sound energy around panel perimeters.

ASSISTIVE LISTENING DEVICE: An electronic device that provides amplification of sound to a hearing impaired person. Device include personal hearing aids, magnetic induction loops, FM radio systems and infrared systems. All have advantages and disadvantages and some may be dependent on good acoustical environment for optimal performance.

ATTENUATION: The reduction of sound energy as a function of distance traveled. (See also Inverse Square Law).

A WEIGHTING: An electronic filtering system in a sound meter that allows meter to largely ignore lower frequency sounds in a similar fashion to the way our ears do.

AMBIENT NOISE/SOUND: Noise level in a space from all sources such as HVAC or extraneous sounds from outside the space. Masking sound or low-level background music can contribute to ambient level of sound or noise.

BACKGROUND NOISE: The sum total of all noise generated from all direct and reflected sound sources in a space that can represent an interface to good listening and speech intelligibility. (Hearing impaired persons are especially victimized by background
noise).

BAFFLE: A free hanging acoustical sound absorbing unit. Normally suspended vertically in a variety of patterns to introduce absorption into a space to reduce reverberation and noise levels.

BARRIER: Anything physical or an environment that interferes with communication or listening. A poor acoustical environment can be a barrier to good listening and especially so for persons with a hearing impairment.

BEL: A measurement of sound intensity named in honor of Alexander Graham Bell. First used to relate intensity to a level
corresponding to hearing sensation.

BOOMINESS: Low frequency reflections. In small rooms acoustical panels with air space behind can better help control low frequency reflectivity.

CLOUD: In acoustical industry terms, an acoustical panel suspended in a horizontal position from ceiling/roof structure.  Similar to a baffle but in a horizontal position.

COCKTAIL PARTY EFFECT: Sound in a noisy crowded room generated mostly by conversation. Levels rise and fall as people compete with one another to be heard. Perception of speech can be nearly impossible in high levels of noise.

COCHLEA: A snail shaped mechanism in the inner ear that contain hair cells of basilar membrane that vibrate to aid in frequency recognition.

CYCLE: In acoustics, the cycle is the complete oscillation of pressure above and below the atmospheric static pressure.

CYCLES PER SECOND: The number of oscillations that occur in the time frame of one second. (See FREQUENCY.) Low frequency sounds have fewer and longer oscillations.

DAMPING: The dissipation of vibratory energy in solid media and structures with time or distance. It is analogous to the absorption of sound in air.

DECIBEL (dB): Sound level in decibels as a logarithmic ratio. Sound intensity described in decibels. i.e.: Breathing 5 dB, office
activity 50 dB, Jet Aircraft during takeoff at 300′ distance 130 dB.  (See submenu TABLES under Acoustics for a table on Sound Source of Environment).

DEFLECTION: The distance an elastic body or spring moves when subjected to a static or dynamic force. Typical units are inches or mm.

DEAF: Loss of auditory sensation with or without use of assistive listening device. Loss of hearing more severe than is generally
characterized as “Hearing Impaired”.

DIFFUSION: is the scattering or random reflection of a sound wave from a surface. The directions of reflected sound is changed so that listeners may have sensation of sound coming from all directions at equal levels.

EAR: An incredible hearing mechanism consisting of outer, middle and inner ear segments that cause sound pressures to be picked up by the ear that are transmitted through auditory nerves where signals are interpreted by brain as sound.

ECHO: Reflected sound producing a distinct repetition of the original sound. Echo in mountains is distinct by reason of distance of travel after original signal has ceased.

ECHO FLUTTER: Short echoes in a small reverberative spaces that produce a clicking, ringing or hissing sound after the original sound signal has ceased. Flutter echoes may be present in long narrow spaces with parallel walls.

EQUAL LOUDNESS CONTOURS: Curves represented in graph form as a function of sound level and frequency which listeners perceive as being equally loud. High frequency sounds above 2000 Hz are more annoying. Human hearing is less sensitive to low frequency sound. (See also PHON.)

FLAME SPREAD: Classification indicating propagation of flame across a sample compared to flame propagation across concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.

FLANKING: The transmission of sound around the perimeter or through holes within partitions (or barriers) that reduces the otherwise obtainable sound transmission loss of a partition. Examples of flanking paths within buildings are ceiling plena above partitions; ductwork, piping, and electrical conduit penetrations through partitions; back-to-back electrical boxes within partitions, window mullions, etc.

FREE FIELD: Sound waves from a source outdoors where there are no obstructions.

FREQUENCY: The number of oscillations or cycles per unit of time.  Acoustical frequency is usually expressed in units of Hertz (Hz) where one Hz is equal to one cycle per second.

FREQUENCY ANALYSIS: An analysis of sound to determine the character of the sound by determining the amount of sounds at various frequencies that make up the overall sound spectrum. i.e.: higher frequency sound or pitch vs. low frequency.

HEARING IMPAIRMENT: A degree of hearing loss, temporary or permanent due to many causes.  Hearing loss can be caused by illness, disease, or by exposure to excessively high noise levels.  Affects 25-50 million people in USA of all ages.  Hearing impairment as generally used means a hearing loss of a mild, moderate, or severe degree as apposed to “Deafness” which is generally described as little or no residual hearing with or without the aid of an assistive listening device.  Hearing Impaired persons are particularly victimized by long reverberation times.

HEARING RANGE: 16-20000 Hz (Speech Intelligibility)
600-4800 Hz (Speech Privacy)
250-2500 Hz (Typical small table radio)

HERTZ (Hz): Frequency of sound expressed by cycles per second.  (See CYCLE).

IMPACT SOUND: The sound produced by the collision of two solid objects. Typical sources are footsteps, dropped objects, etc.., on an interior surface (wall, floor, or ceiling) of a building.

INTENSITY: (See LOUDNESS).

INVERSE SQUARE LAW: Sound levels fall off with distance traveled.    Sound level drops off 6 dB from source point for every doubling of distance.

LIVE END/DEAD END: An acoustical treatment plan for rooms in which one end is highly absorbent and the other end is reflective and diffusive.

LOUDNESS: The average deviation above and below the static value due to sound wave is called sound pressure. The energy expended during the sound wave vibration is called intensity and is measured in intensity units. Loudness is the physical resonance to sound pressure and intensity.

MASKING: The process by which the threshold of hearing of one sound is raised due to the presence of another.

MASS: The fundamental property of a material relevant to sound transmission loss through that material. Generally, the more massive the material, the greater the sound transmission loss.

MOUNTING: Standards established by ASTM to represent typical installation for purpose of testing materials. i.e.: a mounting test specimen mounted directly to test room surface. D mounting furred out to produce air space behind.

NOISE: Unwanted sound that is annoying or interferes with listening.  Not all noise needs to be excessively loud to represent an annoyance or interference.

NOISE CRITERIA (NC): Noise criteria curves used to evaluate existing listening conditions at ear level by measuring sound levels at loudest locations in a room. NC criteria can be referred to equivalent dBA levels. NC curves are critical to persons with hearing loss.

NOISE ISOLATION CLASS (NIC): A Single number rating of the degree of speech privacy achieved through the use of an Acoustical Ceiling and sound absorbing screens in an open office. NIC has been replaced by the Articulation Class (AC) rating method.

NOISE REDUCTION (NR): The amount of noise that is reduced through the introduction of sound absorbing materials. The level
(in decibels) of sound reduced on a logarithmic basis.  (See TABLES submenu under Acoustics for Sound Pressure Level Changes).

NOISE REDUCTION COEFFICIENT (NRC): The NRC of an acoustical material is the arithmetic average to the nearest multiple of 0.05 of its absorption coefficients at 4 one third octave bands with center frequencies of 250, 500, 1000, 2000 Hertz.

OCTAVE BANDS: Sounds that contain energy over a wide range of frequencies are divided into sections called bands. A common standard division is in 10 octave bands identified by their center frequencies 31.5, 63, 125, 250, 500, 1000, 2000, 4000 Hz.

OTO: Pertaining to the ear.

OTOLOGIST: A doctor specializing in the structor, disorders and treratment of the ear.

OTOLARYNGOLIST: A doctor specializing in disorders and treatment of the ear nose and throat disorders.

PHON: Loudness contours.  A subjective impression of equal loudness by listeners as a function of frequency and sound level (dB). An increase in low frequency sound will be perceived as being much louder than an equivalent high frequency increase.

PITCH: The perceived auditory sensation of sounds expressed in terms of high or low frequency stimulus of the sound.

PRESBYCUSIS: The loss of hearing due primarily to the aging process High frequency loss is frequently a result of early hearing loss.

REFLECTION: The amount of sound wave energy (sound) that is reflected off a surface. Hard non porous surfaces reflect more sound than soft porous surfaces. Some sound reflection can enhance quality of signal of speech and music. (See Echo).

RESONANCE: The emphasis of sound at a particular frequency.

RESONANT FREQUENCY: A frequency at which resonance exists.

REVERBERATION: The time taken for sound to decay 60 dB to 1/1,000,000 of its original sound level after the sound source has stopped. Sound after it has ended will continue to reflect off surfaces until the wave loses enough energy by absorption to eventually die out. Reverberation time is the basic acoustical property of a room which depends only on its dimensions and the absorptive properties of its surfaces and contents. Reverberation has an important impact on speech intelligibility.

REVERBERATION TIME: Sound after it is ended at the source will continue to reflect off surfaces until the sound wave loses energy by absorption to eventually die out.

SABIN: A unit of sound absorption based of one square foot of material. Baffles are frequently described as providing X number of sabins of absorption based on the size of the panel tested, through the standard range of frequencies 125 – 4000 Hz. The number of sabins developed by other acoustical materials are determined by the amount of material used and its absorption coefficients.

SABINE FORMULA: A formula developed by Wallace Clement Sabine that allows designers to plan reverberation time in a room in advance of construction and occupancy. Defined and improved empirically the Sabine Formula is T=0.049(V/A) where T = reverberation time or time required (for sound to decay 60 dB after source has stopped) in seconds.
V = Volume of room in cubic feet.
A = Total square footage of absorption in sabins.

SEPTUM: A thin layer of material between 2 layers of absorptive material. i.e.: foil, lead, steel, etc. that prevents sound wave
from piercing through absorptive material.

SIGNAL TO NOISE RATIO: Is the sound level at the listeners ear of a speaker above the background noise level. The inverse square law impacts on the S/N ratio. Signal to Noise Ratios are important in classrooms and should be in range of 15 to 20 dB.

SMOKE DEVELOPED INDEX: Classification that relates to a comparison of smoke development of a particular material compared to concrete panels and red oak. Results are obtained through an ASTM E84 or UL723 test.

SOUND: Sound is an oscillation in pressure, stress particle displacement, particle velocity in a medium – (in room temperature.
In air speed of sound is 1125’/second or one mile in 5 seconds.) Sound produces an auditory sensation caused by the oscillation.

SOUND ABSORPTION: is the property possessed by materials, objects and air to convert sound energy into heat. Sound waves reflected by a surface causes a loss of energy. That energy not reflected is called its absorption coefficient.

SOUND ABSORPTION COEFFICIENT: The fraction of energy striking a material or object that is not reflected. For instance if a material reflects 70% of the sound energy incident upon its surface, then its Sound Absorption Coefficient would be 0.30.

SOUND BARRIER: A material that when placed around a source of noise inhibits the transmission of that noise beyond the barrier. Also, anything physical or an environment that interferes with communication or listening. For example, a poor acoustical environment can be a barrier to good listening and especially so for persons with a hearing impairment.

SOUND LEVEL: A subjective measure of sound expressed in decibels as a comparison corresponding to familiar sounds experienced in a variety of situations. (See TABLES submenu under Acoustics for Sound Source of Environment table.)

SOUND PRESSURE: The total instantaneous pressure at a point in space, in the presence of a sound wave, minus the static pressure at that point.

SOUND PRESSURE LEVEL: The sound pressure level, in decibels, of a sound is 20 times the logarithm to the base 10 of the ratio of the sound pressure to the reference pressure. The reference pressure shall be explicitly stated and is defined by standards.

SOUNDPROOFING: Building materials that makes structures impervious to sound or insulates against sound.

SOUND LEVEL METER: A device that converts sound pressure variations in air into corresponding electronic signals. The signals are filtered to exclude signals outside frequencies desired.

SPEECH: The act of speaking. Communication of thoughts and feelings by spoken words.

SPEECH PRIVACY: The degree to which speech is unintelligible between offices. Three ratings are used, Confidential, Normal
(Non obtrusive), Minimal.

SOUND TRANSMISSION CLASS (STC): This is a rating for doors, windows, enclosures, noise barriers, partitions and other acoustical products. The rating is in terms of their relative ability to provide privacy against intrusion of speech sounds. This is a one number rating system, heavily weighted in the 500Hz to 2000Hz frequency range where speech intelligitibility largely occurs.

SPECTRUM: The description of a sound wave´s components of frequency and amplitude.

TIME WEIGHTED AVERAGE (TWA): The yardstick used by the Occupational Safety and Health Administration (OSHA) to measure noise levels in the workplace. It is equal to a constant sound level lasting eight hours that would cause the same hearing damage as the variable noises that a worker is actually exposed to. (This hearing loss, of course, occurs over long-term exposures.) Same as LOSHA.

ULTRASOUNDS: Sounds of a frequency higher than 20,000 Hz. The frequency region containing these frequencies is called the ultrasonic region.

VIBRATION: A force which oscillates about some specified reference point. Vibration is commonly expressed in terms of frequency such as cycles per second (cps), Hertz (Hz), cycles per minute (cpm) or (rpm) and strokes per minute (spm). This is the number of oscillations which occurs in that time period. The amplitude is the magnitude or distance of travel of the force.

VIBRATION ISOLATOR: A resilient support that tends to isolate a mechanical system from steady state excitation.

VOLUME: The Cubic space of a room bounded by walls, floors, and ceilings determined by Volume = Length x Width x Height of space.  Volume influences reverberation time. (See How to Compute Cubic Volume under ACOUSTICS in menu for formulas of more complex volumes.)

WAVELENGTH: Sound that passes through air it produces a wavelike motion of compression and Parefaction. Wavelength is the distance between two identical positions in the cycle or wave. Similar to ripples or waves produced by dropping two stones in water. Length of sound wave varies with frequency. Low frequency equals longer wavelengths.

Sound is propagated in air, much like blowing up a large balloon, which expands equally in all directions. (Fig 1)

For sound to be generated and heard it must have a source, a medium through which to pass and a receiver.

For purposes of this discussion we will assume that we are talking about normal speech communications. The source is the speaker’s voice, the medium through which it is transmitted is air and the receiver is the listeners ear.

As sound is generated by the speaker’s speech, the speakers voice acts like a diaphragm which causes the molecules in the air to pulsate back and forth while moving in all directions, at a speed of 1130 ft per second (770 mph).

A single segment of a sound wave may be characterized as pressure compressions and rarefactions.

The distance between the sound compressions and rarefactions is known as wavelength. Low frequency sounds have a long wavelength and are perceived as low-pitched sounds such as the rumble of a truck. High-pitched sounds have very short wavelengths such as sound emitted from a whistle. In the field of music a piano can generate sounds ranging from 20 cycles per second (Hz) all the way up to 4600 cycles per second.

(FIGURE 4) shows several wavelengths in feet and inches as a function of frequency.

Loudness: (Amplitude or Intensity) is measured by the strength of the sound, depicted as a sine wave above and below the normal atmospheric pressure. (Figure 5) shows the combination of sine waves the sounds at different loudness levels or amplitudes. For the sake of simplicity the sound intensity or pressure levels are measured in decibels.

Our ears do not perceive all sounds equally at the various frequencies or sound intensities. (FIGURE 6) shows an equal loudness contour chart. The sound levels for a particular sound as defined by the level at 1000 Hz will find the same for any given frequency along the curve. For example a 20-decible sound at 1000 Hz would be perceived as the same sound level of 50 decibels at 100 Hz. This indicates that our ears are less sensitive to low frequency sounds than mid to high frequencies. As will be seen later, this sensitivity difference will feature repeatedly in the control of noise.

The deviation of sound above and below the atmospheric pressure levels is called Sound Pressure. The energy expanded in the the process of sound propagation is labeled intensity (loudness) and is measured in energy units. At this point the science of sound can be a little more complex and intimidating since placing a numerical value on sound is very difficult due to the extraordinary sensitivity of the human ear. Our ears can detect deviations in atmospheric pressure in the order of 1,000,000 to 1 and sound intensities of over a trillion to one.

In order to make the measurement, calculation and perception of sound more manageable, a compact scale has been devised incorporating the decibel (dB). A decibel is a logarithmic unit measure of sound pressure.

(FIGURE 7) Shows sound levels of recognizable sound in decibels with a subjective evaluation from “very faint” to “deafening”. it shows the logarithmic values of intensity of energy units and the relative loudness as perceived by the human ear. Obviously, it is much easier to comprehend the decibel levels.

(FIGURE 7)

The Relative Loudness levels are important insofar as they demonstrate that a 10-decibel increase will be perceived as twice as loud as the pervious level or conversely, a decrease of 50% from the previous higher level. It is less important to understand the physics of this relative difference as much as to accept it as an acoustical phenomenon.

Note: (FIGURE 7) expresses the sound pressure levels as single number levels in the A weighted scale. The A weighted scale uses the equal loudness contours to provide a single number value in the same manner as our ears perceived sound. The A weighting discounts the low frequency sound level perception (This will be discussed further under Sound Level Meters).

Another very important but little known acoustical phenomena is the Inverse Square Law. As a sound wave propagates spherically, the sound energy is distributed over the ever-increasing surface diameter of the wave front surface. The Inverse Square Law teaches us that for every doubling of the distance from the sound source in a free field situation, the sound intensity will diminish by 6 decibels.

Under ideal conditions a free field could be represented by a sound signal being generated from a mountain peak. In real life situations however, rooms bounded by walls, floors and ceilings will interrupt the inverse square law at a distance in tan average 30′ square room at approximately 10-12 feet from the sound source. Nevertheless it is important to accept the notion that sound will diminish in intensity with distance. For example, in a typical classroom with a teachers voice signal of 65 decibels at a three-foot distance from the teacher; at 6 feet away the sound intensity will be 59 decibels and at twelve feet it will diminish down to 53 decibels. (This is important to remember as we discuss the Signal to Noise Ratio S/NR later on)

(FIGURE 8) shows a segment of the sound wave front surface area increasing with distance.

(FIGURE 8)

In the angle shown in Figure 8, the
same sound energy is distributed over
the spherical surfaces of increasing areas a
s d is increased. The intensity of the sound
is inversely proportional to the square of the
distance of the wavefront from the signal source.

Example:

1d = 1
2d = 4
3d = 9
4d = 16

When a sound source ceases in a space the sound waves will continue to reflect off the hard wall, floor and ceiling surfaces until it loses enough energy and dies out. The prolongation of the reflected sound is known as reverberation. Reverberation Time (RT) is defined as the number of seconds it takes for the reverberant sound energy to die down to one millionth (or 60dB) of it’s original value from the instant that the sound signal ceases.

Reverberation is dependent only on the volume of a space and the acoustically absorptive quality of the rooms finishes. Hard surfaced rooms will have a longer reverberation time than rooms finished with sound absorbing materials.

When a sound wave travels outward in all directions and encounters an obstacle such as a wall, floor or ceiling surface the direction of the sound will be changed or reflected, The direction of travel of the reflected sound will be at the same angle as the original sound striking the surface as shown in (FIGURE 10)

The immediate effect of multiple reflections is an increase in the sound intensity caused by the reflections. A listener will hear the direct sound arriving at the ear along with all of the multiple reflections. Thus the combined loudness of the direct sound and the reflected sound will be greater than the direct sound alone.

When the direct sound diminishes in intensity in accordance with the inverse law the multitude of reflective sound intensities are combining to produce as increase in the reflected sound levels to a point where the reflected sound can be higher than the direct sound. A typical example of this phenomena would be a voluminous, hard surfaced gymnasium that can experience a significant build up of reflective sound intensity.

Where the direct sound and reflected sound are about equal is called the critical distance. In a typical classroom critical distance is about 12″ from the source. Beyond the critical distance the sound reduction will be less than 6 dB.

All building materials have some acoustical properties in that they will all absorb, reflect or transmit sound striking them. Conventionally speaking, acoustical materials are those materials designed and used for the purpose of absorbing sound that might otherwise be reflected.

Sound absorption is defined, as the incident sound that strikes a material that is not reflected back. An open window is an excellent absorber since the sounds passing through the open window are not reflected back but makes a poor sound barrier. Painted concrete block is a good sound barrier but will reflect about 97% if the incident sound striking it.

When a sound wave strikes an acoustical material the sound wave causes the fibers or particle makeup of the absorbing material to vibrate. This vibration causes tiny amounts of heat due to the friction and thus sound absorption is accomplished by way of energy to heat conversion. The more fibrous a material is the better the absorption; conversely denser materials are less absorptive. The sound absorbing characteristics of acoustical materials vary significantly with frequency. In general low frequency sounds are very difficult to absorb because of their long wavelength. On the other hand, we are less susceptible to low frequency sounds, which can be to our benefit in many cases.

For the vast majority of conventional acoustical materials, the material thickness has the greatest impact on the material’s sound absorbing qualities. While the inherent composition of the acoustical material determines the material’s acoustical performance, other factors can be brought to bear to improve or influence the acoustical performance. Incorporating an air space behind an acoustical ceiling or wall panel often serves to improve low frequency performance.

Simply stated, noise is unwanted sound. Intrusive sound that interferes with speech intelligibility or speech privacy is characterized as noise. Noise is a relative term and can range from low levels of intrusive sound in a quiet environment to very loud sounds in a already noisy environment.

Reflected sounds that are not absorbed will cause as increase in overall sound levels in a space. When the space is acoustically treated the sound build up is eliminated or reduced. The sound level differences between the untreated space and the acoustically treated space is described as the Noise Reduction (NR).

Sound measured on one side of a wall and then on the opposite side will reflect the sound transmission blocking characteristics of the wall. The difference between the two measurements is described as the Noise Reduction characteristics of the wall.

Treating a noisy reverberant space with acoustical materials can reduce the reflected sound build up that occurs due to the reflective hard surfaces.

In a”before treatment” and “after treatment” comparison the decibel level can be calculated. This diagram can be used to compare how the sound reduction will be perceived.

Example: An existing room with 500 sabines of absorption prior to acoustical treatment that adds an additional 1500 sabins will have a absorption ratio of

Treated, Untreated=4

The treated room will be perceived to be about 37% quieter than the untreated room.

The deviation of sound above and below the atmospheric pressure levels is called Sound Pressure. The energy expanded in the the process of sound propagation is labeled intensity (loudness) and is measured in energy units. At this point the science of sound can be a little more complex and intimidating since placing a numerical value on sound is very difficult due to the extraordinary sensitivity of the human ear. Our ears can detect deviations in atmospheric pressure in the order of 1,000,000 to 1 and sound intensities of over a trillion to one.

In order to make the measurement, calculation and perception of sound more manageable, a compact scale has been devised incorporating the decibel (dB). A decibel is a logarithmic unit measure of sound pressure.

(FIGURE 7) Shows sound levels of recognizable sound in decibels with a subjective evaluation from “very faint” to “deafening”. it shows the logarithmic values of intensity of energy units and the relative loudness as perceived by the human ear. Obviously, it is much easier to comprehend the decibel levels.

(FIGURE 7)

The Relative Loudness levels are important insofar as they demonstrate that a 10-decibel increase will be perceived as twice as loud as the pervious level or conversely, a decrease of 50% from the previous higher level. It is less important to understand the physics of this relative difference as much as to accept it as an acoustical phenomenon.

Note: (FIGURE 7) expresses the sound pressure levels as single number levels in the A weighted scale. The A weighted scale uses the equal loudness contours to provide a single number value in the same manner as our ears perceived sound. The A weighting discounts the low frequency sound level perception (This will be discussed further under Sound Level Meters).

When background or ambient noise levels are sufficiently high enough, the background noise can mask the sound levels of speech that wish to be heard. Restaurants can often be classic examples of excessive noise interference due to lack of sufficient quality or quantity of sound absorbing materials that prevent excessive noise buildup. Diners have to speak louder and louder to be heard and in doing so compete with one another, thereby increasing the sound levels to even greater levels. Appropriate acoustical treatment will prevent the reflected noise buildup and significantly reduce the necessity for diners to speak louder to enjoy conversations with one another.

Speech Intelligibility is defined as the percentage of speech heard correctly by the listener. The emphasis is on “correctly” rather than simply “heard”. Speech intelligibility is influenced by reverberation time, distance of the listener from the speaker and background noise level.

Of these elements, reverberation time and background noise are influenced by the architecture of the space which, suggests that the acoustical environment should be the focus of greater design attention.

(Figure 12) depicts a classroom setting in which the instructors voice signal is determined to be approximately 65 dBA at a distance of 3 feet from the teacher. The background noise level is 45 dBA and assumed to be fairly constant through out the room. The Inverse Square Law teaches us that the sound level of the instructors voice signal will be 59 dBA at a 6-foot distance, 53 dBA at a 12-foot distance and 47 dBA at a 24-foot distance.

A good Signal to Noise Ratio should be not less than +15 dB, that is to say the signal strength should be at least 15 decibels above the background noise level in order to achieve good Speech Intelligibility by the listener. From (Figure 12) it can be seen that beyond a 12-foot distance from the instructor the S/NR is far below what it should be.

In a typical classroom the ambient or background noise level in the unoccupied classroom should not exceed 35 dBA with the HVAC system running.

Speaker orientation can be an important factor in classrooms and meeting rooms in particular. As indicated in the diagram, there is a difference of about 10 decibels (dBA) in speech levels between the front and back of the speaker. This amounts to about 1-1/2 decibels per 30 degrees of rotation from the head on direction of the speech signal.

The average sound level of a male speaker is about 65-64 decibels at a distance of 3 feet from the speaker. The average speech level of a female speaker is about 2-4 decibels lower at a 3-foot distance from the speaker.

Orientation of the speaker to the listener is an important consideration. This is particularly so when considered in context of the Inverse Square Law, which teaches us, that for every doubling of the distance the signal will diminish in intensity by approximately 6 decibels.

Speech privacy in an “open office” for example is the ability to maintain confidentiality of conversational speech from the source to adjacent areas. In the open plan office speech privacy is accomplished by introducing masking sound by way of white noise, or background music to prevent other employees nearby from understanding what the speaker is saying.

Most commonly used and necessary in the open plan office layout, sound masking systems involve the generation of the generation of random noise fed through speakers uniformly distributed though out the office to produce a subtle and uniform masking sound level. Sound masking systems provide the means to maintain speech confidentiality as well as a masking protection against other intrusive and distracting noises in the office environment.

Contrary to the popular notion that sound passes through a structure, such is not the case. Sound generated on one side of a wall will energize the wall structure and set it in motion, much like a diaphragm. The wall itself becomes the transmitter of the sound energy which can be heard on the opposite side of the wall by the listener.

When sound waves strike a wall assembly for example, the sound is comprised of a wide spectrum of frequencies. The wall may react to the various frequency energy in different degrees, based on the characteristics and component make up of the wall.

The accompanying diagram depicts 2 walls, one an 8” concrete block wall and the other a steel stud and gypsum board wall. Each have the same single STC value of 47 but they each have different sound isolation characteristics.

The drywall stud wall does not perform as well at the low frequencies compared to the block wall. While the block wall does not provide as high isolation at the mid range frequencies it maintains a fairly uniform increase in performance across the entire frequency spectrum. Note the drop in performance of the drywall partition between 2000 Hz and 4000 Hz. This drop is known as the “coincidence dip” which reflects the resonant frequency of the wall assembly.

Rather than rely on a single number STC rating, it can often be useful and more effective to look at the total performance data and select a wall, floor or ceiling assembly that will provide the most effective performance based on the anticipated sound generated through the structure. Different structures or construction assemblies have different resonant frequencies

––––––––– Metal stud – gypsum board
––––––––– 8 inch concrete block wall

The Sound Transmission Class is a single number rating of the effectiveness of a material or construction assembly to retard the transmission of airborne sound. The sound transmission loss between the source and receiving rooms are plotted on a graph by frequency and sound level in decibels. The STC curve is a sliding contour that is fitted to the performance data plotted in a manner that will allow no more than 32 deficiencies below the appropriate contour. The maximum deficiency at any given frequency shall not exceed 8 decibels

Once the appropriate contour has been selected the STC is determined by the decibel value of the vertical scale at 500 Hz. The STC is expressed as a single STC number (Example STC 32)

Note that the STC contour is similar to the inverse of the equal loudness contour, insofar as it discounts the lower frequency sounds to reflect how our ears perceive the lower frequency sounds.

Airborne Sound can be isolated by containing the sound from intruding beyond the source area by blocking it’s potential transmission. Sound isolation can be accomplished by creating appropriate barriers and reducing the reflected sounds in the source area by acoustical absorption within the source area.

EFFECTS OF SOUND LEAK ON PARTITION SOUND INSULATION

From the gap we can see that even a small crack can significantly compromise the sound isolation performance of a wall, floor /ceiling assembly to be as airtight as possible. Caulking compounds or acoustical sealants are indispensable for effective noise control.

In an unoccupied space, sounds can be heard from a variety of sources. Careful scrutiny of the room can lead to identifying the intrusive sources. The diagram illustrates a few of the most common sources of noise.

Many acoustical test reports and other data is frequently quantified in metric terms. The accompanying metric conversion chart may help make the appropriate conversions to imperial measurements.