Acoustic Definition and Meaning: What is Acoustic?

Acoustic Definition

a·cous·tic (-kstk)

adj. also a·cous·ti·cal (-st-kl)

1. Of or relating to sound, the sense of hearing, or the science of sound.


a. Designed to carry sound or to aid in hearing.

b. Designed to absorb or control sound: acoustic tile.

3. Music

a. Of or being an instrument that does not produce or enhance sound electronically: an acoustic guitar; an acoustic bass.

b. Being a performance that features such instruments: opened the show with an acoustic set.

acoustic [əˈkuːstɪk], acoustical


1. (Physics / General Physics) of or related to sound, the sense of hearing, or acoustics

2. (Physics / General Physics) designed to respond to, absorb, or control sound an acoustic tile

3. (Music / Classical Music) (of a musical instrument or recording) without electronic amplification an acoustic bass an acoustic guitar

Noun 1. acoustic - a remedy for hearing loss or deafness

curative, cure, therapeutic, remedy - a medicine or therapy that cures disease or relieve pain

Adj. 1. acoustic - of or relating to the science of acoustics; "acoustic properties of a hall"



adj acoustic [əˈkuːstik]

having to do with hearing or with sound This hall has acoustic problems.


1 noun plural the characteristics (eg of a room or hall) which make hearing in it good or bad.

2 noun singular the science of sound.

Explanation of Acoustics

The laws of sound physics (acoustics) - How does it work?

The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the so called “audio range”). 20 Hz is a very low frequency while 20 kHz is a very high frequency. Examples of low frequencies in our surroundings are the bass in music or the low humming that a fan generates. Examples of high frequencies are the treble in music or the high notes of a small child. Both speech and music are a mixture of high to low frequencies.

When sound is made, may it come from human speech or a HiFi speaker, the room is filled with it. And if it is not absorbed then it bounces around until drained of energy. In a room with hard walls this bouncing can go on for quite some time. These echoes, or reverberations, greatly impacts our daily lives – may it be during conversations, HiFi listening or working in an open office environment. 

Imagine the following two examples:

Example 1: You are talking to a friend in a stairwell. The stairwell is made out of concrete and has a lot of parallel walls where the sound bounces back and forth, creating a lot of echoes. You will find that both of you need to raise your voices considerably in order to make yourselves understood. And, although loud but understandable, the experience is not very pleasant and nothing you would like to endure for a longer period.

Example 2: You are talking to a friend outside, on a lawn. No walls or anything around you that can create any echoes or reverberations. You will find that you can hear your friend perfectly even when he/she is talking quite softly and that what you hear sounds very pleasant.

Example “2” is naturally the one to strive for, is it not? What if every environment we are in could be like this? According to many scientists our lives would be a lot healthier and less stressful if they were. And the way to get there is to either constantly be on that lawn or to absorb the echoes at home or at the office in order to create a similar acoustic environment. This can be done through the use of sound absorbers.

All materials have some sound absorbing properties. Incident sound energy which is not absorbed must be reflected, transmitted or dissipated. A material’s sound absorbing properties can be described as a sound absorption coefficient in a particular frequency range. Materials like polyester fiber and glass and mineral wool are known to be good absorbers. A thick concrete wall is an example of a very poor sound absorber.

What one does not want to do is to remove only one range of frequencies. Very many solutions on the market today do just that: They are only effective on high frequencies. This means that they remove part of the echo (reverberation) in the room but leaves the rest bouncing back and forth: Creating a very uneven acoustic environment. In an open office environment, for example, this leads you to constantly hear a low murmuring in the air, since the low frequencies are still bouncing back and forth – creating a bad environment for you and your co-workers.

High frequencies are quite easy to remove and they are removed with a curtain or a thin piece of felt on the wall. Thus many products are made this way, while claiming to be sound absorbing. Which, in a way they are, but they only absorb part of it – leaving the rest unabsorbed. In the best of scenarios they do not change much at all while in the worst of scenarios they actually make things worse since they create a very uneven acoustic environment. Compare it with removing all the salt from your food: You still have food but what is left is not as tasty as before.

Another thing we need to know about acoustics is that the absorption of low frequency sound increases with the thickness of the absorber. The absorption will be more effective where the particle velocity is high. Close to the boundary of the room the particle velocity will be zero and so this is not an ideal location for sound absorption. The absorption furthest away from the backing surface will be the most effective and this is why thick layers absorb at lower frequencies. An alternative to having super thick sound absorbers is to place the sound absorbing material where it does the most good: Away from the wall. Tests have shown that having an air column close to the wall is almost as good as having sound absorbing material there, when it comes to absorbing low frequencies. The best thing here is of course to have both a lot of sound absorbing material and an air column close to the wall. Most products on the market have neither but the products from Sounds Of Science naturally has both.

How Does Acoustical Absorption Work?

Walk outside on a cold winter day just after the first big snowfall and you can hear the hush in the air. Everything sounds different because that hush in the air is the snow absorbing sound. Now, compare that to walking into a gym where the sound bounces around and lingers in the room. Also, have you wondered why some hotel rooms are “soundproof” and some seem to have walls that are paper thin?

When sound hits a material three things can happen. First, the sound can be reflected like in the gymnasium where it bounces off the hard walls and is redirected. Secondly, the sound can be transmitted through the material like in the thin walls of a hotel. Finally, the sound can be absorbed. What happens to the sound when it is absorbed? It gets trapped in the material and converted into a very small amount of heat. Imagine a college football stadium filled with screaming fans charged up for the state rivalry game. If all of the sound from the screaming fans could be absorbed into one cup of coffee in the middle of the field, there might just be enough to energy to heat the coffee.

The types of materials that absorb sound are porous like the snow. The air gets trapped between the little snow crystals or fibers and turned into heat. One of the most common materials used to absorb sound is fiberglass. We also often get asked about Styrofoam. Although Styrofoam may act as a good thermal insulator, it is very poor acoustical absorber.

A very important thing to note is that acoustical absorption is frequency dependent. That means that two materials may look the same but one might absorb high frequencies and another may absorb mid or low frequencies. Typically, as the thickness of the material increases, so will the absorption of low frequency sound.

At Acoustics By Design, it’s our job to engineer acoustical solutions that control how the sound is reflected, transmitted, or absorbed based on the type of space. If maximum absorption were the goal, we’d treat every project like a movie theater or even like an anechoic chamber. But in the real world, different spaces have different acoustical needs, and our job is to engineer acoustical solutions that are custom fit to meet those needs.

Acoustics is the science of sound. There are many kinds of sound and many ways that it affects our lives. We use sound to communicate and you might also know that acoustics is important for creating musical instruments or concert halls or surround sound stereo or hearing aids. But sound is also used to find oil and gas, to study earthquakes and climate change, and to make sure that the baby in a mother's womb is healthy. Some animals, like bats and dolphins, use sound to find their food.

There are many different areas of study and practice within Acoustics. On this site we have condensed materials into 8 content areas:


Material in this category covers the science of sound and waves. How sound is created, how it travels and how it is received. In addition how materials react to different types of sounds.


Animal Bioacousticians study how animals make, use and hear sounds. Animal bioacoustics also includes the use of sound to study and detect the presence of animals and their behavior, the sounds they make, the effects of man-made noise on animals, and the use of SONAR to monitor the presence of plankton and fish.


Architectural acousticians study how to design buildings and other spaces that have pleasing sound quality and safe sound levels. Architectural acoustics includes the design of concert halls, classrooms and even heating systems.


Doctors and medical researchers study and use acoustics to diagnose and treat different types of ailments. The study of medical acoustics includes the use of ultrasound and other acoustical techniques to learn how different types of sound interact with cells, tissues, organs and entire organisms.


Musical acousticians study the science of how music is made, travels and is heard. Including architects, musicians and instrument designers.


Hearing specialists and educators are interested in how our ears sense sounds, what types sounds can damage our ears and how best to educate people about the dangers of loud sounds. Hearing specialists also study how to help people with hearing impairments.


Speech Scientists study how speech is made, travels and is heard.


Underwater acousticians study natural and man-made underwater sounds, how sounds are made and travel in the underwater environment.


Noise specialists are mostly concerned with making our world a quieter place. They study natural and man-made noise, especially from machinery and transportation, and how people respond to noise. Knowledge produced by these scientists can be used to redesign noisy machinery, or to recommend ways of shielding the noise, or to help lawmakers and public officials create rules for limiting exposure to noise.

Acoustics is a branch of physics. However it has wide connections with engineering such as mechanical engineering, electrical and electronic engineering, ocean engineering, civil engineering, geophysical engineering etc. Usually people would think of acoustics as engineering rather than physics. Acoustics is a huge and broad field and can be divided into many subfields such as : physical acoustics, audio and electroacoustics, speech and hearing, noise and vibration, industrial ultrasonics, biomedical ultrasonics, infrasonics, nonlinear acoustics, underwater acoustics, musical acoustics, building and architectural acoustics etc. The applications of acoustics are broad and important such as to nondestructive evaluation, materials research, study of cavitation, medical ultrasound imaging, application of ultrasound to drug delivery , and gene therapy, sonars, geophysical exploration, seismic studies, noise and vibration control, acoustical designs of theatres, opera houses, buildings etc. With the arrival of nanotechnology and biotechnology, the role of ultrasonics is gaining even more prominence. Acoustics is the study of the physical characteristics of sound. It deals with things like the frequency, amplitude and complexity of sound waves and how sound waves interact with various environments. It can also refer casually and generally to the over-all quality of sound in a given place. Someone might say in a non-technical conversation: "I like to perform at Smith Hall; the acoustics are very bright."


Most of our music making is carried out indoors. In such a situation, the listener's experience is formed almost as much by the room itself as by the instruments. For a successful performance (or recording), the concert space (or studio, or living room with recorded sounds) must fulfill the following:

  • The audience must clearly hear all of the music with the proper balance between instruments, and the proper tonal balance for each instrument.

  • The performer must clearly hear himself and the other performers.

  • Reverberation should be appropriate to the style of the music.

  • Extraneous sounds must be inaudible in the concert space.

  • The sound of the concert should be inaudible outside of the concert space.

These goals are more or less in order of importance. The last requirement will not affect the concert itself, but may affect the possibility of holding future concerts. With these criteria in mind, we will examine the important structural factors of the room which control them.



Fig.1 Direct sound and early reflections

Figure 1 shows the paths taken by the sound as it travels from the performer to the listener. (The wavefronts of the sound are not shown, they would be perpendicular to the lines drawn.) The heavy line, number 1, shows the shortest path, the direct one. The other paths all involve one reflection, so must be longer than the direct path, although their relative lengths will change as the performer and listener move about the room. Since sound travels at a steady 1 foot per millisecond, the sound of a single event is going to arrive at the listener's ears several times as determined by the different path lengths. We can chart the arrival times on a graph:

Fig. 2 Arrival times of a single sound

The amplitude of a particular reflection is determined by the path length and the efficiency of the wall in reflecting sound. That efficiency is described as the coefficient of absorption (any sound not reflected is absorbed). The coefficient of absorption is a number between 0 and 1, with 1 representing total absorption (an open window) and 0 representing total reflection.

We are very used to hearing sounds indoors, so we have learned not to be confused by the multiplicity of sounds arriving from various directions. We almost always realize the sound comes from the direction of the first arrival. (The whole issue of localization is too involved to get into here. It depends a lot on the number and shape of our ears.) Any reflections that arrive within 20 milliseconds of the first add to the impression of loudness of the sound. Any reflections that arrive more than 40 milliseconds after the first may be heard as a distinct echo, but are usually accepted as reverberation. Reflections that arrive between 20 and 40 milliseconds after the direct sound can be confusing and interfere with understanding if the sounds are speech.


Sound does not stop at the listener's ears of course, it continues and is reflected again by the other walls of the room. If the coefficient of absorption is low, a sound may bounce several dozen times before it fades away.

Fig. 3 More reflection paths

This drawing would be solid black if all of the possible reflections were shown. The arrival time graph is more informative:

Fig. 4 Time and amplitude of sounds at listener's ear

This shows the complete picture of what is heard if a single, short sound is produced in a room. Most of the sound energy that is reflected twice or more is heard as reverberation, a sort of stretching of the sound event. The actual amplitude of reverberation is not very important (unless it is strong enough to obscure following sounds) but the time that it persists is. Short reverb times (a half to a full second) are comfortable for speech, whereas moderate times (1 to 3 seconds) work well with various kinds of music. Some music was written for very reverberant environments such as large stone churches, and should be heard that way.


Reverberation time is the most often quoted description of a performing space, but it is not really the most important. The frequency response of the reverb should be reasonably flat, or slightly low pass, which is sometimes described as "warm reverb". That means that low partials of sounds will persist a little longer than high components, matching the decay characteristics of most instruments. The opposite effect, where high pitched sounds linger, can be very annoying. This is the situation in many indoor swimming pools.

The envelope of the reverberation should match that in figure 4, a fairly even decay, with no "lumps" of sound. A rectangular room with flat walls will not provide such an envelope; the reverberation will occur in bursts, often with distinct echos ("slap-back). To provide even reverberation, the shape of the walls should be complex, but not very regular. A regular structure, such as a staircase, will often produce a series of echoes called flutter echo.


Control of reflections and reverberation can satisfy the first three goals on our list. Isolation is a matter of the materials and techniques used to build the room. The walls must be heavy and solid, and for really excellent isolation, all walls, doors, floor and ceiling must be doubled; literally one room within another. Attention must be paid to such details as air ducts and holes for electrical cables, for sound can leak through any opening. Once an adequately isolated structure is finished, noise generating devices must be kept out. Light fixtures, (especially fluorescent), heaters, and backstage equipment can all create noise and must be chosen for quiet operation.

Adequate isolation is almost impossible to achieve after construction if it was not built in in the first place, but since it is an issue that is very important to low budget recording and electronic music, here are a few things that can be tried.

  • First, find the leaks that sound follows between the studio and the outside world. Edges of doors, vent ducts, electrical outlets are all suspect. They can be treated with the materials sold for heat insulation, if the heavy, expensive versions are used.

  • Direct attachment of sound sources to walls, floors or ceiling should be avoided. Swing speakers from ropes or mount them on stands. Put three layers of carpet on the floor, or set things on the canvas part of camp stools.

  • Hang absorptive materials. Heavy curtains or rugs from floor to ceiling work well, as does four inch thick fiberglas insulation. (Thinner fiberglas has poor frequency response) There are plastic foams designed for this purpose, but they are expensive and a fire hazard. Egg carton material has a nice shape for diffusion, but is not particularly absorptive. If the above procedure makes the room too dead, hang some light hard panels in front of but not touching the absorption

Building for good acoustics

A small concert hall was given acoustical treatments in a recent renovation. Here are the visible features that were added:

Fig. 5 Some structures to control reflections and reverberation

The diffusers smooth out the reverberation and make the sound reasonably uniform at different seats. The absorptive curtains allow the reverberation time of the room to be adjusted to control the loudness of ensembles of various sizes. Movable panels behind the performers serve to group the early reflections into the "sooner than 20ms" range and also (probably more important in this small hall) help the performers hear each other.

The issue of architectural acoustics is very complex, and often not handled well. It seems that most concert halls are constantly being tinkered with and occasionally rebuilt at fantastic costs; perhaps our expectations are unrealistic now that we are used to hearing every note and nuance in our living room.
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