Acoustic Treatment

An illustrated introductory guide to the various methods of acoustic treatment for improving room acoustics and reducing sound transmission, and some simple methods for identifying acoustic problems

Acoustics is the study of the behaviour of sound in different environments. It investigates the interaction between sound waves and the physical environment, including their reflection, transmission and absorbtion by different materials, as well as the interaction between the sound waves themselves at different volumes, frequencies and relative phase. It is a complex field, but even at a basic level can illuminate some of the issues arising in designing and using recording and listening spaces, and offer solutions of varying complexity and cost.

Acoustical Analysis

Full acoustical analysis of a room requires expert knowledge and expensive reference-grade equipment. Test signals - including bursts of white or pink noise (see 'Colouration, below) and other impulses - are generated in various locations within the room by reference speakers, and the response of the space is gathered with measurement microphones and analysed with spectrograms and frequency plots. This is the kind of method used by studio designers, acoustic architects and engineers.

While these methods offer the best results, they are unfortunately not realistically accessible to most project studio users. There is however some analysis and calculation which you can do using simpler tests and measurements, which may suggest some starting points for improving your workspace's acoustics.

Sound reflection

Smooth hard surfaces reflect sound waves in a similar way to a mirror reflecting light. In a critical listening environment this can be a major problem, with signals traveling directly to the listener from the speaker becoming 'smeared' and phase shifted when combined with their acoustic reflections. Conversely, an 'anechoic', or completely dead space with no echo at all (as found in measurement studios, and having walls comprised of vanes and spikes designed to soak up sound - see picture below) can be a disconcerting experience, as the brain expects some acoustic feedback from its environment. An ideal listening space is not completely free of echoes, but neither does it echo or 'ring' excessively.

Omni-directional speaker in an anechoic chamber - Czech Technical Univerzity

Photograph by Quentar, Michal Starosta, Tomáš Solár - Licensed under Creative Commons

To identify potential trouble spots which should be high on your list of candidates for acoustic treatment (eg a foam tile or acoustic panel, a diffuser, or a thick curtain) you can use this simple technique: either you or a colleague sits in the preferred listening position, and the other goes round the walls of the room, holding a mirror flat against the wall at various points - especially smooth sections. If the listener can see a speaker in the mirror then you should consider putting something to absorb or diffuse the sound at that position, as sound from the speaker will follow a similar path of reflection.

Phase Coherence and Comb Filtering

When a sound wave is combined with a copy of itself delayed by a microscopic amount (caused for example by a very short echo), its various harmonic elements will appear either louder or softer, depending on their wavelength and the delay time. Peaks and troughs in these two 'phase shifted' soundwaves, when superimposed on each other, will combine or cancel each other out at different wavelengths to produce exaggerated peaks and troughs at a set of related frequencies throughout the audio spectrum. This phenomenon is known as 'comb filtering', where groups of harmonically related frequencies are amplified or attenuated, with resultant sound colouration.

Understanding exactly how this occurs is not necessarily important at the level of this introductory guide, but be aware that even very short echoes can have adverse effects on sound quality which will not necessarily be immediately evident as audible 'reverberation' in the familiar sense.

Colouration

If the aim of an ideal sound reproduction and listening system is to reproduce all frequencies evenly and without bias, then its output could be thought of as analagous to pure white light, where all wavelengths are represented at equal amplitude. Wavelengths of sound can be thought of as being like wavelengths of light, with long wavelengths (low frquencies) corresponding to red, and short wavelengths (high frequencies) to blue and violet - this is what is meant by the audio 'spectrum'. Any emphasis of a particular frequency will result in a corresponding change of response from an even spectrum to a more 'coloured' sound.

To elaborate, the familiar term 'white noise' means an audio signal with equal sound amplitude at all frequencies. Pink noise attenuates some higher frequencies, and red noise consists only of bass register frequencies. By extension, blue noise sees higher frequencies increasing in amplitude.

This analogy is taken to its logical conclusion when measuring and plotting audio spectra of equipment and acoustic spaces by analysing their response when fed pure white noise and other test signals. The term 'colouration' is useful to describe the degree to which a monitoring system and its environment will affect the evenness of its reproduction of the audio spectrum, or 'spectral purity'.

Colouration can be caused by an uneven response at any stage of the reproduction chain - from encoding software and audio interface to amplifier and speakers - and also the acoustic nature of the listening space. Room modes will cause 'bumps' in the spectrum, and the phasing caused by short echoes can lead to comb filtering, as described above.

Acoustic Treatment

In the absence of an acoustically designed listening space you will need to make the best of your chosen room. In general, a smaller space will be easier to optimise than a large one, and less resonant at troublesome lower frequencies; a good all-purpose room will be quite 'dead', with as little echo as reasonably possible.

Irregularly shaped rooms are also often preferable, with square spaces usually being the most difficult to treat, due to their tendency to resonate strongly at a set of related frequencies (see 'Standing Waves')

Hard, smooth surfaces are the most acoustically reflective, and anything you can hang on walls (curtains, drapes etc) will cut down high frequency resonance. Several ready-made acoustic kits - comprising high-density foam wall panels, blocks and 'flutter walls' are available for different sizes of room, and can make significant improvements to 'live' sounding rooms. Specialist acoustic fabrics are also available for covering surfaces and panels, which offer superior sound absorption properties.

Acoustic Treatment Solutions:

Materials

Many acoustic treatments use foam blocks or tiles as part of (or all of) their solution. If making your own acoustic treatment materials, choosing the right type of foam is important, as different types can have quite different acoustic properties. Rigid high-density foam has better sound absorbtion properties than more 'open cell' and flexible types.

Compressed fibreglass and rock wool can also be good materials for constructing absorbers, and there are several commercial products offering superior acoustic performance for incorporation into small acoustic projects.

1 Sound-Absorbtive Panels

One of the most common simple acoustic treatments involves the installation of foam panels at trouble spots, to absorb sound energy. These will mostly affect the high and midrange frequencies; low frequency soundwaves have too long a wavelength to be much altered by this technique

As mentioned above, the type of foam used will significantly affect the effectiveness of absorbtive panels, and many commercial variants are available. In general, high density foam will absorb far more sound energy. Some panels also have crenellated surfaces to add an element of high frequency diffusion:

 

A foam tile with crenelated surface and 'jigsaw puzzle' slots for building larger sections

The thicker the panels the greater the bandwidth of signals they will absorb, as the greater distance until a reflective surface (eg wall) is reached, the greater range of wavelengths which will be dampened. Similarly, the density of the foam will determine the degree at which sound penetrating it is attenuated.

Using some of the techniques suggested later to identify potential reflection points will help in initial positioning of panels, but as a rule of thumb good candidates for initial panels will be behind the speakers and behind the listener, as much of the sound from the monitor speakers will be directed and reflected along this axis. Also the points on adjacent walls at which the sound of the speakers is reflected directly to the respective ear can be particularly problematic.

Alternatively, if planning your own acoustic treatment try to find a commercial 'kit' designed for the same size of room as you have to get an idea of some possible solutions.

Some professional studios feature moveable and/or reversible panels with different textures on each side to allow a variation of the room acoustic:

Moveable panels fitted to the walls of the live room at Christchurch Studios

2 Diffusers

Another approach to reducing the problems caused by sound reflection is not to absorb, but to diffuse the sound waves. Diffusers present a carefully randomised surface to the approaching wavefront, and reflect it back unevenly or in different directions, which reduces the space's ability to resonate at particular frequencies. They are useful if a natural 'live' sound is preferred, but without the colouration caused by standing waves and first-order phase combination.

Wall mounted diffuser

Close-up of a section of the diffuser, covered with rough fabric and showing different depth cavities for uneven reflection

3 Flutter Walls

As noted, large, flat, hard surfaces are the best acoustic reflectors - not a good thing. The walls (and/or windows) directly behind the monitor speakers are often particularly problematic in this respect, offering multiple parallel reflective surfaces, which will provide a multiple or 'flutter' echo. Breaking up these surface in texture, orientation and profile will help reduce the flutter echo effect, improving intelligibility in voice recording and accuracy in monitoring.

A flutter wall is often placed behind and between the speakers, and usually consists of a set of differently angled and textured absorbtion and diffusion materials.

4 Corner Absorbers

The corners of a listening room are often an unobtrusive location for larger blocks of absorbtion material, and several companies produce 'corner bass traps' - generally large blocks of acoustic foam. Bass wavelengths are however too long to be very effectively treated by diffusion or absorbtion by this method - the size of bass diffusers and thickness of absorbtion material necessary both being prohibitive. Additionally, though these 'bass traps' are often placed in the corners of rooms, the ideal position for bass absorbtion is actually at a distance of one quarter of the wavelength from the wall, in the direction of travel of the bass mode, as this is where the particle velocity is at its maximum, and foam cell absorbtion therefore most effective.

Corners are however an ideal position for broad band absorbers, as many different modes can accumulate there, and these blocks can help to suppress midrange resonance and reverberation. Additionally, Helmholtz bass absorbers or membrane absorbers (see below) can work well when positioned in room corners.

5 Suspended Panels/Blocks

As well as walls and floors, ceilings often present the largest smooth flat surface in a room. Hanging absorbtion panels and irregular foam shapes (for absorbtion and diffusion) from the ceiling will help break up this resonant area, with the added advantage of raising the acoustic treatment materials above the workspace, thereby leaving more room where space is at a premium.

6 Scatter Blocks

If full absorbtion treatment isn't required, or if you want to retain some 'liveness' in the room then scatter blocks can be used, which essentially are small acoustic foam blocks covering parts - but not all - of the surface.

7 Room Treatment Kits

There are several companies making 'out of the box' kits designed to treat different sizes of room, which help to make them more acceptable as recording and critical listening spaces. These kits will usually comprise several of the above elements, and are combined in appropriate ratios to suit the needs of generic home or office sized rooms. They usually require a selection of foam blocks and panels to be glued to the walls of the room in predetermined positions.

In our experience this is an effective step in improving your acoustic environment, especially if you do not have the time or expertise to analyse and develop your own solution, and have a fairly standard rectangular work room requiring simple treatment.

8 Perforated Plate Absorbers

Wood or plastic panels perforated with small holes or 'microslits' can help form absorbtion chambers which work in similar ways to Helmholtz resonators (see below), when used as the surface panels of cavity wall surfaces, resonant compartments, or screens. They can be particularly effective in targetting lower frequencies, which foam absorbers can find awkward.

Perforated plate absorbers on the walls and ceiling of Christchurch Studios main room

For maximum effectiveness this type of absorber should be designed to target your room's problem frequencies, but there are several respectable DIY guides to how to build them if you know what you are aiming for. 

9 Acoustic Screens

Screens made from sections of foam which slot together, or plywood screens filled with rockwool or other absorbtion material, and/or covered with acoustic fabric, can help isolate a performer and/or microphone within a larger space. These free-standing units can be placed between performers who are being recorded with different microphones to give better separation between them, or can be used to create different spaces within larger rooms. Surrounding a performer or workstation either partly or wholely with screens will cut down on the amount of acoustic reflection from the rest of the room, and the amount of 'bleed' to nearby microphone(s).

Music producer Matthias Strassmüller at work partially surrounded with foam screens and diffusers

Moveable isolation screens with glazed section 

10 Soft Furnishings

Armchairs, sofas and cushions all help absorb sound waves. Large foam-filled pieces, placed slightly away from room edges and corners will help especially to absorb some low and midrange resonances.

11 Acoustic Fabrics

Acoustic fabrics are specially designed to help reduce the acoustic reflectivity of surfaces with which they are covered. They are useful for covering walls, acoustic screens, suspended foam blocks, bass traps, and even furniture and tabletops, to reduce room resonances.

They usually have a rough or uneven texture, and materials such as hessian, wool, rough cotton etc can have similarly beneficial effects if used to cover screens, panels and diffusers (see photographs throughout).

Textured acoustic fabric used to line a double-skinned door enclosure 

12 Acoustic Plaster & Plasterboard

Resembling traditional plaster, acoustic plaster or plasterboard lends additional acoustic absorbtion properties to plastered walls and surfaces where traditional plaster or plasterboard would otherwise be used.

13 Helmholtz Resonant Absorbers

Helmholtz resonant absorbers are one of the most effective ways of controlling very specific bass and midrange frequencies. Working on a similar principle to how a tone can be generated by blowing across the neck of a bottle, they use a resonant space with either a single tubular opening which will resonate at a particular frequency, and thus absorb sound at that frequency, or a surface perforated with many holes to offer more broad band absorbtion.

They can additionally be filled with absorbtion material to increase high frequency attenuation without compromising their mid/bass absorbtion characteristics. If you have a spectrogram showing the problem frequencies of your room, Helmholz absorbers can be made to target very narrow bands. Accurate acoustical analysis is however recommended, as well as specialist knowledge in their construction.

14 Membrane Absorbers

A membrane absorber combines a flexible membrane with either a resonant space - very like a Helmholtz resonator - or a damped piston to dissipate vibrational energy - similarly to the tympanic membrane (eardrum) and ossicles of the ear.

Unlike foam cell absorbers they are most effective on reducing sound pressure, rather than particle velocity, and therefore are best placed at the extremities of the space (ie built into the walls) where sound pressure is at its highest. [In the same way, while foam or fibreglass absorbers are often fixed to surfaces, they are actually most effective when placed a small distance away from walls and ceilings.

15 Egg Boxes

There is an often-held belief that egg boxes, with their crenelated surface which resembles that of some acoustic tiles, can diffuse or absorb sound in a similar fashion; unfortunately this is not the case. Their resemblance to proper acoustic materials is merely cosmetic, and while they can have a marginal effect on sound reflection they will achieve no more than hanging a heavy curtain over the same area. Not recommended!

Temporary Solutions

If you lack a dedicated audio workspace, or the resources to apply fixed acoustic treatment to your studio space, you may want to consider some temporary or moveable solutions to add some acoustic damping during a recording or mixing session. When recording spoken word especially, cushions, blankets, duvets or heavy fabrics arrayed around the sides and back of the microphone can have a beneficial damping effect on reverberation, and are easily removed once the session is over.

An improvised 'vocal booth' consisting of cushions, curtain and chairs

For a similar solution which is a little less 'make-do-and-mend' and a little more professional looking, there are small acoustic screens available which are designed to be fixed to a microphone stand which will partly enclose the microphone with acoustic materials, and help isolate it from the surrounding environment and lessen 'slapback' echo from any surfaces behind the microphone.

Another advantage of these units is their portability, enabling you to record in a variety of locations whilst still achieving an element of consistency in sound quality.

The SE Electronics Reflexion Filter (sic)

Standing Waves/ Room Modes

Between any two parallel surfaces, there will exist a set of frequencies whose corresponding wavelengths are exact divisions of the distance between these surfaces, and which will therefore resonate more strongly. In a rectangular room these are called the axial modes, and exist for the three dimensions (Length, Width and Height) of the room.

For example, if you have a room 5 metres long, then along that room axis, soundwaves with a wavelength of exactly 5 metres will bounce back in such a way that they will compound the volume of harmonic content at this frequency (34Hz): this is a 'standing wave'. Similarly, the same surfaces will accentuate waves of length 5/2, 5/3, 5/4, 5/6 metres etc. and their corresponding frequencies - 68Hz, 102Hz etc.

Sound waves obviously move three dimensionally, and sound reflection can result in many different patterns which will lead to reinforcement of wave amplitude at different frequencies. These frequencies are called the room 'modes', all characterised by standing waves. The more reflections a mode requires, the less significant it is acoustically, as the faster the reflections will decay. Thus axial modes (as above) will be the strongest, tangential modes (below) less powerful, and the various oblique modes weaker still, depending on how many reflections they take to 'double back' on themselves.

Calculating the axial modes of a rectangular room is relatively straightforward.

The main mode between two parallel walls is calculated by this simple equation:

F = c/(2*D)

where c = speed of sound (metres/second) and D = distance between surfaces (in metres)

At sea level, the speed of sound is typically 340m/s; thus the simple version of the room mode calculator is:

F = 170/D

Exact multiples of this frequency will behave similarly. As an example, I have measured out my office, and calculated the room modes for each dimension (length, width and height)

Length 7.0m : F=24.3Hz
Width 4.4m : F=38.6Hz
Height 2.6m : F=65.4Hz

Using multiples of these frequencies, and then rounding to the nearest whole number, we derive this table of modes, with frequencies listed in Hertz (Hz)

Mode	Length (7.0m)	Width (4.4m)	Height (2.6m)
F	24	39	65
2F	49	77	131
3F	116	196
4F	97	155	262
5F	122	193	327
6F	146	232	392
8F	194	309	520
10F	243	386	650
12F	292	463	780
16F	389	618

 

The biggest potential problem frequencies of this room are around 195Hz and 390Hz (highlighted in bold) as there are modes clustered around them in all three dimensions. Additionally these frequencies are an octave apart (ie one frequency is double the other), so their audible effect may be cumulative and significant.

Although changing the dimensions of your room will rarely be an option, an awareness of likely problem frequencies will help inform your choice of acoustic treatments and monitors, and the positioning of monitors and furniture etc.

Square rooms will have particularly troublesome modal characteristics, as the same frequencies will resonate on two axes, and cubic rooms are worse still. Calculating room modes for these types of room will be doubly (or triply) informative and useful for identifying problem frequencies.

Sound Proofing

While acoustic treatment aims to improve the acoustic properties of a recording or listening room, there is also the concern of external noise penetrating into these spaces, or of course sound escaping through walls, doors, windows and ceilings and annoying the neighbours!

If you are designing a purpose-built facility, there are several methods of reducing structural sound transmission - floating rooms, triple glazing, sound proofing materials built into cavity walls, multi-skinned doors etc. - but these realistically need to be designed and installed by specialists. [If you are in this enviable position then please contact us for advice on planning your project, as the requirements go beyond the scope of this paper]. On a smaller scale you may have the opportunity to specify secondary glazing, new doors or other improvements for your room.

There are several recognized test standards for measuring the attenuation performance of construction and acoustic materials: 

Noise Reduction Coefficient (NRC) is an arithmetic value average of sound absorption coefficients measured at frequencies of 250, 500, 1000 and 2000 Hz, and indicates a material's ability to absorb sound.

The NRC of a material is a single number between 0 and 1.0 which reflects its sound absorption performance - the higher the number, the better the acoustic performance. As a rule, NRC numbers are cited when aiming to reduce noise levels within an interior space.

Sound Reduction Index (SRI) is a laboratory measurement of the acoustic transmission properties of a material.

It is central to the various acoustic standards of the International Organisation for Standardization ISO140 - Acoustics - Measurement of sound insulation in buildings and of building elements.

Ceiling Attenuation Class (CAC) is the measure for rating the performance of a ceiling system as a barrier to sound transmission between adjacent spaces.

Guide values:

• <25 = Poor performance
• >35 = High performance 

Soundproofing  Methods 

Acoustic Tiles

There are tiles available in a variety of materials to line studio walls and ceilings and cut down on the amount of sound which passes through them. Depending on the design these tiles can be attached to the walls or ceiling, or dropped into an existing ceiling grid. These tiles may be solid or multi-layered, and made of materials such as fibreglass or cork.

Floating Room

Within the outer room, an inner room is built which is mounted on a suspension system to decouple it from the main structure. Additional soundproofing material is mounted in the cavity created. Double doors in the style of an 'airlock' are required. Many professional studio rooms are constructed in this way, as it is the most effective solution to structure-borne noise, but also the most complex and by far the most costly.

Multiple Glazing

In the same way that domestic double glazing can dramatically reduce noise transmission, studio windows will benefit from double or triple glazed units, especially if they have a larger than normal space between the panes. Glass sliding doors between recording and monitoring spaces will be similarly improved by double or triple glazing and use of multiple layers of glass with spaces between them.

A dual double glazed doorway, lined with acoustic fabric, separates two studio rooms 

A multiple-glazed studio window, also showing the separation of the floating inner room 

Cavity Fill

Lifting floors, and filling cavities with wadding, rockwool, fibreglass panels or other specialised soundproofing materials will give various levels of attenuation of sound transmitted through floors and ceilings. It is however a quite labour-intensive and disruptive job, but will help as part of a thorough soundproofing strategy.

Multi-skinned doors

Similarly to multiple glazed windows, doors with multiple 'skins' separate their structure into several cavities, which additionally can be filled with soundproofing materials, to reduce sound penetration. It is not unusual for recording studio doors to be six inches thick, and fit hermetically into their frame. Some - especially floating structures (see above)- have two doors arranged like an 'airlock' with a small cupboard-sized space in between.

Standards

The International Organization for Standards (ISO) defines many standards for acoustic measurement, environmental noise and the acoustic properties of materials. Details of the current requirements and stipulations of these standards can be obtained from

ISO Standards - TC43 - Acoustics

Further Reading

The Physical Principles of Sound

The Master Handbook of Acoustics

Thanks

University of Bristol Music Department Studios, Christchurch Studios and Digital Village Bristol for allowing us to photograph some of their treated spaces.

 

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