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Room acoustics is a subfield of
acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...
dealing with the behaviour of
sound In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...
in enclosed or partially-enclosed spaces. The architectural details of a room influences the behaviour of sound waves within it, with the effects varying by frequency. Acoustic reflection,
diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
, and diffusion can combine to create audible phenomena such as room modes and standing waves at specific frequencies and locations, echos, and unique
reverberation In acoustics, reverberation (commonly shortened to reverb) is a persistence of sound after it is produced. It is often created when a sound is reflection (physics), reflected on surfaces, causing multiple reflections that build up and then de ...
patterns.


Frequency zones

The way that sound behaves in a room can be broken up into four different
frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...
zones: * The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone, sound behaves very much like changes in static air pressure. * Above that zone, until
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s are comparable to the dimensions of the room, room
resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...
s dominate. This transition frequency is popularly known as the Schroeder frequency, or the cross-over frequency, and it differentiates the low frequencies which create standing waves within small rooms from the mid and high frequencies. * The third region which extends approximately 2
octave In music, an octave (: eighth) or perfect octave (sometimes called the diapason) is an interval between two notes, one having twice the frequency of vibration of the other. The octave relationship is a natural phenomenon that has been referr ...
s is a transition to the fourth zone. * In the fourth zone, sounds behave like rays of light bouncing around the room.


Natural modes

For frequencies under the Schroeder frequency, certain
wavelengths In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same '' phase'' on ...
of sound will build up as resonances within the boundaries of the room, and the resonating frequencies can be determined using the room's dimensions. Similar to the calculation of
standing waves In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect t ...
inside a pipe with two closed ends, the modal frequencies (f_) and the sound pressure of those modes at a particular position (p_(x,y,z)) of a rectilinear room can be defined as f_ = \frac\sqrt p_(x,y,z) = A\cos\Big( \fracx \Big)\cos\Big( \fracy \Big)\cos\Big( \fracz \Big) where m,n,l = 0,1,2,3 ... are mode numbers corresponding to the x-,y-, and z-axis of the room, c is the speed of sound in \frac , L_, L_, L_ are the dimensions of the room in meters. A is the amplitude of the sound wave, and x,y,z are coordinates of a point contained inside the room. Modes can occur in all three dimensions of a room. Axial modes are one-dimensional, and build up between one set of parallel walls. Tangential modes are two-dimensional, and involve four walls bounding the space perpendicular to each other. Finally, oblique modes concern all walls within the simplified rectilinear room. A modal density analysis method using concepts from
psychoacoustics Psychoacoustics is the branch of psychophysics involving the scientific study of the perception of sound by the human auditory system. It is the branch of science studying the psychological responses associated with sound including noise, speech, ...
, the "Bonello criterion", analyzes the first 48 room modes and plots the number of modes in each one-third of an octave. The curve increases monotonically (each one-third of an octave must have more modes than the preceding one). Other systems to determine correct room ratios have more recently been developed.


Reverberation of the room

After determining the best dimensions of the room, using the modal density criteria, the next step is to find the correct reverberation time. The most appropriate reverberation time depends on the use of the room. RT60 is a measure of reverberation time. Times about 1.5 to 2 seconds are needed for opera theaters and concert halls. For broadcasting and
recording studio A recording studio is a specialized facility for Sound recording and reproduction, recording and Audio mixing, mixing of instrumental or vocal musical performances, spoken words, and other sounds. They range in size from a small in-home proje ...
s and conference rooms, values under one second are frequently used. The recommended reverberation time is always a function of the volume of the room. Several authors give their recommendations A good approximation for broadcasting studios and conference rooms is: :TR  kHz= .4 log (V+62)– 0.38 seconds, with V=volume of the room in m3. Ideally, the RT60 should have about the same value at all frequencies from 30 to 12,000 Hz. To get the desired RT60, several acoustics materials can be used as described in several books. A valuable simplification of the task was proposed by Oscar Bonello in 1979. It consists of using standard acoustic panels of 1 m2 hung from the walls of the room (only if the panels are parallel). These panels use a combination of three Helmholtz resonators and a wooden resonant panel. This system gives a large acoustic absorption at low frequencies (under 500 Hz) and reduces at high frequencies to compensate for the typical absorption by people, lateral surfaces, ceilings, etc. Acoustic space is an acoustic environment in which sound can be heard by an observer. The term ''acoustic space'' was first mentioned by
Marshall McLuhan Herbert Marshall McLuhan (, ; July 21, 1911 – December 31, 1980) was a Canadian philosopher whose work is among the cornerstones of the study of media studies, media theory. Raised in Winnipeg, McLuhan studied at the University of Manitoba a ...
, a professor and a philosopher.


Nature of acoustics

In reality, there are some properties of acoustics that affect the acoustic space. These properties can either improve the quality of the sound or interfere with the sound. * Reflection is the change in direction of a wave when it hits an object. Many acoustic engineers took advantage from this. It is used for
interior design Interior design is the art and science of enhancing the interior of a building to achieve a healthier and more aesthetically pleasing environment for the people using the space. With a keen eye for detail and a Creativity, creative flair, an ...
s, either use reflections for benefits or eliminates the reflections. The sound waves usually reflect off the wall and interfere with other sound waves that are generated later. To prevent sound waves reflecting directly to the receiver, a diffusor is introduced. A diffusor has different depths in it, causing the sound to scatter in random directions evenly. It changes the disturbing
echo In audio signal processing and acoustics, an echo is a reflection of sound that arrives at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the lis ...
of the sound into a mild reverb which decays over time. *
Diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
is the change of a sound wave's propagation to avoid obstacles. According to Huygens
principle A principle may relate to a fundamental truth or proposition that serves as the foundation for a system of beliefs or behavior or a chain of reasoning. They provide a guide for behavior or evaluation. A principle can make values explicit, so t ...
, when a sound wave is partially blocked by an obstacle, the remaining part that gets through acts as a source of secondary waves. For instance, if a person is in a room and shouts with the door open, the people on either side of the hallway will hear it. The sound waves that left the door become a source then spread out in the hallway. The sounds from the surroundings might interfere with the acoustic space like the example given.


Uses of acoustic space

The application of acoustic space is very useful in architecture. Some kinds of architecture need a proficient design to bring out the best performances. For example, concert halls, auditoriums, theaters, or even cathedrals. * Concert Hall – a place that is designed to hold a
concert A concert, often known informally as a gig or show, is a live performance of music in front of an audience. The performance may be carried by a single musician, in which case it is sometimes called a recital, or by a musical ensemble such as an ...
. A good concert hall usually holds around 1700 to 2600 audience. There are three main attributes of a good concert halls: clarity, ambiance, and loudness. If the seats are well positioned, the audience will hear clear sound from every single seat. For more ambiance,
reverberation In acoustics, reverberation (commonly shortened to reverb) is a persistence of sound after it is produced. It is often created when a sound is reflection (physics), reflected on surfaces, causing multiple reflections that build up and then de ...
times are designed as preferred. For instance,
romantic music Romantic music is a stylistic movement in Western Classical music associated with the period of the 19th century commonly referred to as the Romantic era (or Romantic period). It is closely related to the broader concept of Romanticism—the ...
usually requires an amount of reverberation time to enhance the emotions, therefore, the ceilings of the concert hall should be high. * Theater – a place that is designed for live performances. The first priority for sound design in a theater is speech. Speech has to be heard clearly, even if it is a soft whisper. The reverb is not needed in this case, it interrupts the words spoken by the
actor An actor (masculine/gender-neutral), or actress (feminine), is a person who portrays a character in a production. The actor performs "in the flesh" in the traditional medium of the theatre or in modern media such as film, radio, and television. ...
s. The
intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude * Level (disambiguation) * Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) *Field strength of electric, m ...
has to be increased, in order to enlarge the acoustic space, to cover the theater without disrupting the dynamic. In large theaters, amplification must be used. *
Cathedral A cathedral is a church (building), church that contains the of a bishop, thus serving as the central church of a diocese, Annual conferences within Methodism, conference, or episcopate. Churches with the function of "cathedral" are usually s ...
(and church) have an area called a
choir A choir ( ), also known as a chorale or chorus (from Latin ''chorus'', meaning 'a dance in a circle') is a musical ensemble of singers. Choral music, in turn, is the music written specifically for such an ensemble to perform or in other words ...
, usually located near the
transept A transept (with two semitransepts) is a transverse part of any building, which lies across the main body of the building. In cruciform ("cross-shaped") cruciform plan, churches, in particular within the Romanesque architecture, Romanesque a ...
, where the tower is located in most cathedrals. The choir is for the
choir A choir ( ), also known as a chorale or chorus (from Latin ''chorus'', meaning 'a dance in a circle') is a musical ensemble of singers. Choral music, in turn, is the music written specifically for such an ensemble to perform or in other words ...
to sing. This kind of singing needs a soft cloudy sound for ambiance and emotion. The height of the cathedral does not only show religious pride but also improves the acoustics. There is more reverb when the source generates a sound in the space


Planning the acoustics of the room

The acoustic impression of a room is determined by: * fraction of direct sound in the total sound level * the time delay and direction of early reflections and their contribution to the overall sound level * the delayed onset and spatial distribution of the reverberation, as well as its proportion in the overall sound level and its temporal development (reverberation time) The task of room acoustics is to influence these parameters by designing the room in such a way that the acoustic properties of the room are maximized for its intended use. However, not all venues are designed with acoustics in mind. In this case, speaker placement will play a decisive role in the movement of sound waves, affecting clarity, loudness and overall sound quality. The goals of acoustical room design can be: * when recording sound in recording studios, there should be no undesirable influence of the room: it is important that the reflectivity is as low as possible so that the recorded sound is mainly determined by the direct sound and the acoustic character of the recording room does not adversely affect the recording. * as much speech intelligibility as possible in classrooms, lecture halls and theaters: the proportion of direct sound should be high and early reflections should come mainly from the speaker's side with a relatively short onset delay to increase the speaker's volume and aid localization. * to achieve the best possible spatial music perception in concert halls: the proportion of direct sound must be balanced with reflections - large enough to make the music perceived clearly and transparently, but not too large so as not to impair the spatial impression. Early reflections should make up a large proportion of the total sound and be distributed as evenly as possible in direction. Since the acoustic properties of rooms for different applications are almost incompatible, it is hardly possible to create a universal room that combines good speech intelligibility and good spatial music perception.


See also

* Acoustic board *
Acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...
* Anechoic room *
Architectural acoustics Architectural acoustics (also known as building acoustics) is the science and engineering of achieving a good sound within a building and is a branch of acoustical engineering. The first application of modern scientific methods to architectur ...
* Digital room correction *
Noise control Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors. Overview The main areas of noise mitigation or abatement are: transportation noise control, a ...
* Sound proofing * Whispering gallery


Notes


References


External links


Understanding Acoustic Treatment

Does Acoustic Treatment Make a Difference?
{{Authority control Acoustics Building engineering