ACOUSTIC

Acoustic Related To Architectural / Construction

INTRODUCTION

Acoustics is the interdisciplinary science that deals with the study of all 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 while someone working in the field of acoustics technology may be called an acoustical engineer .

The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.

ACOUSTICS RELATED TO  ARCHITECTURAL  DESIGN / CONSTRUCTION

The Acoustic Nature of Materials

When we choose the materials that will make up the structure of a building, we are making decisions that will affect the nature of sound within the building. Many are the times that an acoustical designer has been called in to “fix” the acoustics after the building has been completed, and we always wish that we had been consulted before ground was ever broken.

It is possible to make improvements after the fact, but when the building has been built we’ve lost the ability to affect the acoustics in two important ways.

The first is having the ability to shape and dimension the rooms, and this we covered in the handout for the previous class “Acoustical and Musical Scales in Sound Room Design”.

The other major problem with starting to build before designing the acoustics is that little or no consideration is given to the acoustic nature of the materials that make up the structure.

The following are some commonly used building materials and their acoustic properties, and ways these materials can be used for sound isolation and acoustic treatment. Sound isolation is the branch of acoustics that deals with keeping sound where you want it – in or out of the building, for instance, or keeping sounds in one room from invading another room.

Sound treatment, on the other hand, is the branch of acoustics concerned with the perfecting the quality of the sound we hear, and using the proper combinations of materials and shapes to create pleasing, musically accurate sound.

1. Concrete, stone, and other masonry materials

Masonry materials are great for sound isolation, especially when used in floors and walls where the masonry material is quite thick. A solid concrete wall 1 ft. thick will rarely cause clients to complain about sound isolation, for two reasons.

One is the material’s rigidity, meaning that it will not flex and create sound waves on the quiet side of the wall. The other is concrete’s mass.

Nothing stops sound waves quite like massive materials, and they are especially capable of stopping the critical low frequencies that are so hard to stop with less massive materials.

2. Stone and brick are very similar to concrete in mass, and concrete masonry units, although they are lighter, can do a very good job when they are fully filled with concrete, instead of just filling the cells that contain the rebar.

3. Concrete slabs also do a good job of isolating sound between floors – something that is very difficult to do any other way.

4. Wood, and wood products: Wood is much less dense than masonry, and provides much less in the way of sound isolation for that reason.

Wood products like MDF, on the other hand, are somewhat more massive, and are sometimes used in interior walls to add mass. OSB is less dense than MDF, but can be useful as well, as part of an integrated system. Plywood comes in varying densities, and again can contribute something to the equation in a multi-layer wall.

Wood’s real beauty lies in its ability to reflect sound in a pleasing way, meaning that it is a useful material for sound treatment. Since wood resonates easily, it has a way of absorbing some of the sound energy as it vibrates, letting some of the sound pass through to the other side, and reflecting some of the sound back from whence it came.

This genteel quality of wood is one reason it is widely used in the making of musical instruments, and wood has a major role to play as an interior finish material in good sounding rooms.

5. Steel: Steel is a quite dense material, but because of its expenseve it is rarely used as a sound isolation material. Steel’s density actually becomes a liability in structural uses where its dense nature causes it to carry sound vibrations for long distances. If you strike an I-beam with a hammer and place your ear to the other end –let’s say 24 ft. away, you’ll see that the sound carries quite well through the steel. This type of sound transfer is called structure-borne vibration, where sound is carried through some material other than air for a time. The other main type of sound transfer is air-borne vibration.

6. Drywall and plaster: Drywall is the poor man’s masonry, and for interior walls can provide a lot of mass for the money. But one ½” layer doesn’t do all that much. Multiple layers are used in sound studios and broadcast facilities where high mass walls are needed.

7. Roofing: Asphalt shingles are fairly massive, as you know if you hauled them up to the roof, but they are also thin. Installation with a large overlap, heavy felt, and even double layer sheathing can help quite a bit. Ceramic and clay tiles are more massive than wood shakes by far, and can do a reasonable job in residential applications. Metal roofing has mass but is thin, and requires that the underlying structure be fairly massive.

8. Glass and other transparent materials: Glass is quite massive – about three times as massive as drywall. So in a sound wall with three 5/8” layers of drywall on one side, one layer of 5/8” glass maybe inserted to create a window on that side , provided that it is properly sealed. A corresponding piece of glass would be required on the other side of the wall, at the appropriate thickness.

9. Insulating materials (fiberglass, foam, rock wool, etc.): Insulating materials have little mass, so they have limited uses for sound isolation. However, fiberglass has good sound absorption characteristics, and is very useful as a sound treatment material for sound room interiors.

Fiberglass and rock wool, which has similar acoustic properties, absorb sound by slowing the velocity of the air particles carrying the wave. Wood, on the other hand, absorbs sound best when in the pressure zone of a sound wave.

10. Fabrics and other soft materials: Fabrics, carpets, and other soft materials can be useful for sound treatment. Heavy stage type curtains are much more effective than thin fabrics. Carpets, although sometimes better than nothing, can soak up too much mid and high frequency sound while leaving boomy lower frequencies untreated. As part of an overall plan, carpet can be put to good use, but area rugs are much more versatile and adjustable.