How to Measure Room Acoustics

A room can look well designed but still be difficult to use if its acoustics are wrong.

Speech may be hard to understand. Background noise from ventilation may become distracting. Music can sound flat, harsh or confused. In larger spaces, announcements may be audible without being clear.

Measuring room acoustics helps establish why these problems are occurring and what can be done about them.

A professional assessment may consider reverberation, speech intelligibility, background noise and the way sound changes across different parts of the space. The right measurements depend on how the room is used and what it needs to achieve.

What are room acoustics?

Room acoustics describe how sound behaves within an enclosed space.

When someone speaks, or a loudspeaker produces sound, some of that sound travels directly to the listener. The rest reaches walls, floors, ceilings, furniture and other surfaces, where it may be absorbed, reflected or scattered.

These reflections affect what people hear.

Too many strong reflections can cause:

• Excessive reverberation
• Poor speech intelligibility
• Echoes
• Noise build-up
• Reduced concentration
• Poor musical clarity
• Difficulty understanding announcements

Too much absorption can also cause problems in spaces designed for music or performance. A room may feel acoustically flat and lack the reflections needed to support performers and audiences.

Good room acoustics are therefore not simply about making a space as quiet or absorbent as possible. The acoustic conditions need to suit the room’s intended use.

Which spaces need good room acoustics?

Room acoustics can matter in almost any occupied building, but they are particularly important in spaces where listening, communication or concentration is central to the experience.

These include:

• Classrooms and lecture theatres
• Offices and meeting rooms
• Restaurants and cafés
• Auditoria and concert halls
• Theatres
• Recording and broadcast studios
• Conference rooms
• Sports halls
• Community halls
• Places of worship
• Healthcare spaces
• Transport terminals
• Reception areas
• Open-plan offices

The preferred conditions will vary.

A classroom needs clear speech and controlled reverberation. A concert hall needs reflections that support music. A restaurant may need enough acoustic treatment to control noise build-up without removing its atmosphere.

What is measured during a room acoustic assessment?

There is no single measurement that describes every aspect of a room’s acoustic performance.

A typical assessment may consider:

• Reverberation time
• Speech intelligibility
• Background noise
• Sound level distribution
• Acoustic clarity
• Echoes or strong reflections
• Differences between areas of the room
• Noise from mechanical and electrical services

The consultant selects the measurements that are relevant to the room, its intended use and any applicable criteria.

Reverberation time

Reverberation is the sound that remains within a room after the original source has stopped.

Reverberation time describes how long it takes for the sound level to decay by 60 dB.

A long reverberation time means that sound continues within the room for longer. This can make the space feel echoey and cause one word, note or sound to overlap with the next.

A shorter reverberation time generally supports clearer speech, but the ideal result depends on the space.

ISO 3382-2 specifies methods for measuring reverberation time in ordinary rooms. ISO 3382-1 covers reverberation and other acoustic parameters in performance spaces.

Why reverberation time matters

Excessive reverberation can:

• Make speech harder to understand
• Increase overall noise levels as people raise their voices
• Make teaching and meetings more tiring
• Blur musical detail
• Reduce the clarity of public announcements
• Make a room feel busy and uncomfortable

A very low reverberation time may be suitable for a recording or speech-focused space but less appropriate for live music.

There is no single ideal reverberation time for every classroom, meeting room or concert hall. Suitable targets depend on factors including:

• The room’s purpose
• Its volume
• The type of activity
• The number of occupants
• Whether sound amplification is used
• Relevant standards or client requirements
• The needs of people using the space

For schools in England, acoustic performance standards are set out in Building Bulletin 93, which supports the acoustic requirements of the Building Regulations and school premises regulations.

How is reverberation time measured?

A suitable sound is produced within the room and its decay is measured at a number of microphone positions.

This can be done using methods such as:

• An interrupted noise signal from a loudspeaker
• An impulse sound
• An acoustic measurement system that calculates the room’s impulse response

Measurements are normally taken across different frequency bands because a room may perform differently at low, middle and high frequencies.

The number and location of sound source and microphone positions depend on the room and the purpose of the test. Measurements should represent the space as a whole rather than one unusually favourable or unfavourable position.

The results can then be compared with:

• A design target
• A project specification
• Relevant guidance
• Earlier measurements
• An acoustic model
• The performance required for the room’s use

Speech intelligibility

Speech intelligibility describes how clearly spoken words can be understood.

A person may be able to hear that someone is speaking without understanding what they are saying.

Speech clarity can be affected by:

• Excessive reverberation
• High background noise
• Poor loudspeaker coverage
• Echoes
• Distance from the speaker
• The direction of the sound system
• The shape and finishes of the room
• Competing conversations
• Noise from building services

This matters in classrooms, conference spaces, transport buildings, public-address systems, places of worship and other rooms where spoken information must be understood.

What is the Speech Transmission Index?

The Speech Transmission Index, usually shortened to STI, is an objective method of rating speech intelligibility.

It considers how the room, sound system and background noise affect the transmission of speech-like signals.

STI can be measured using a suitable test signal played through a loudspeaker or sound system and recorded at relevant listening positions.

IEC 60268-16:2020 defines the STI model, test signals and measurement and prediction methods. It also explains limitations that need to be considered when interpreting the result.

When might STI testing be used?

STI testing may be appropriate for:

• Public-address systems
• Voice alarm systems
• Transport terminals
• Auditoria
• Lecture theatres
• Conference spaces
• Places of worship
• Large reception areas
• Spaces where announcements are safety-critical or operationally important

The result should be interpreted against the requirements that apply to the particular system and space. The STI standard itself does not set certification criteria for every possible application.

Background noise

Background noise is the sound present within the room when the activity being assessed is not taking place.

Possible sources include:

• Ventilation and air-conditioning systems
• Fans and pumps
• Electrical equipment
• External road, rail or aircraft noise
• Activity in neighbouring rooms
• Building services in ceiling or floor voids
• Rain noise
• Open-plan office activity
• Nearby machinery

Background noise can mask speech, music and other important sounds.

Even where reverberation is well controlled, a room may still perform poorly if its background noise is too high.

How is background noise measured?

A calibrated sound level meter is used to measure the sound level and, where needed, its frequency content.

The assessment may consider:

• The overall sound level
• How the level changes over time
• Frequency-band levels
• Whether the noise is tonal
• Whether it is steady or intermittent
• The difference between occupied and unoccupied conditions
• Which building services are operating
• External influences during the survey

Measurements need to represent normal operating conditions.

A ventilation system measured while running at an unusually low setting may not represent the conditions experienced when the room is occupied.

The results may be compared with project-specific criteria, relevant standards or building-services noise limits.

Other room acoustic measurements

Some rooms need a more detailed assessment than reverberation time, STI and background noise alone.

Acoustic clarity

Clarity parameters can help describe the balance between sound arriving soon after the source and later reverberant energy.

This can be relevant in performance and speech spaces.

A room may have a broadly appropriate reverberation time but still suffer from poor clarity because of its shape, layout or strong late reflections.

ISO 3382-1 includes methods for measuring reverberation and other acoustic parameters in performance spaces.

Echoes and strong reflections

An echo is a reflected sound that is heard separately from the original.

Possible causes include:

• A distant rear wall
• A curved or domed surface
• A large glazed area
• Parallel reflective walls
• A balcony or ceiling feature
• An untreated surface opposite a stage or loudspeaker

An acoustic consultant may use measurements, listening tests and acoustic modelling to identify where the reflection is coming from.

Sound distribution

In larger rooms, the acoustic experience may vary substantially between positions.

People near the front may hear clearly while those at the back struggle. One side of a room may receive more sound from a loudspeaker than the other.

Measurements at several positions can establish:

• Differences in sound level
• Changes in speech intelligibility
• Variations in reverberation
• Gaps in sound-system coverage
• Effects caused by balconies, screens or room geometry

Open-plan office acoustics

Open-plan offices present a different challenge from enclosed rooms.

The aim may be to support communication over short distances while limiting how far distracting speech travels.

ISO 3382-3:2022 specifies methods for measuring room acoustic performance in unoccupied open-plan offices. It focuses on the spatial decay of speech rather than only the time it takes sound to decay.

An assessment may consider:

• How speech levels reduce with distance
• The distance over which speech remains distracting
• Background noise
• Workstation screens
• Ceiling absorption
• Layout and furniture
• The effect of enclosed meeting spaces
• Speech privacy requirements

What equipment is used to measure room acoustics?

The equipment depends on the measurements being carried out.

A professional room acoustic assessment may use:

• A calibrated sound level meter or measurement system
• Measurement microphones
• An acoustic calibrator
• An omnidirectional loudspeaker
• A power amplifier
• STI test equipment
• Audio interfaces
• Specialist analysis software
• Tripods and positioning equipment

Sound level meters are commonly classified as Class 1 or Class 2 under IEC 61672-1.

These classes relate to the instrument’s performance tolerances. Class 1 has tighter tolerances than Class 2. It is not simply a description of one meter being more sensitive than another.

The appropriate equipment and class depend on the purpose of the assessment, the required standard and the level of measurement uncertainty that is acceptable.

Can you measure room acoustics with a phone?

A phone app may help demonstrate that a room sounds echoey or that noise levels change when equipment is switched on.

It may also be useful for informal comparisons.

However, a phone does not normally provide a reliable basis for:

• Formal compliance testing
• Detailed frequency analysis
• Professional reverberation measurements
• STI testing
• Verification against an acoustic specification
• Diagnosing complex room acoustic problems

Phone microphones and apps vary, and the device may not have a traceable calibration.

Professional measurements use suitable equipment, controlled procedures and enough measurement positions to represent the room properly.

What happens during a professional room acoustic survey?

A typical assessment may follow these stages.

1. Establish how the room is used

The consultant will discuss:

• The activities taking place
• The number of people using the room
• Whether speech, music or concentration is the priority
• Any current complaints
• Planned changes to the room
• Applicable project criteria

2. Review the room

The consultant considers:

• Room dimensions and shape
• Wall, floor and ceiling finishes
• Furniture
• Glazing
• Occupancy
• Sound systems
• Mechanical services
• Doors and openings
• Adjacent spaces

3. Agree the measurements

The appropriate tests are selected.

These may include:

• Reverberation time
• Speech intelligibility
• Background noise
• Frequency analysis
• Sound level distribution
• Impulse-response measurements
• Open-plan office parameters

4. Carry out the survey

Measurements are taken at suitable source and receiver positions.

The conditions during the test are recorded so that the results can be understood and repeated where necessary.

5. Analyse the results

The results are reviewed against the room’s purpose, relevant criteria and any design targets.

6. Recommend improvements

The consultant explains:

• What is causing the problem
• Which areas are most affected
• What level of improvement is realistic
• Which treatments or design changes may help
• Where treatment should be positioned
• Whether further modelling or testing is needed

How can poor room acoustics be improved?

The right solution depends on the cause of the problem.

Add sound absorption

Absorptive finishes can reduce reflections and reverberation.

Options may include:

• Acoustic ceiling tiles
• Suspended rafts
• Wall panels
• Baffles
• Acoustic plaster
• Upholstered finishes
• Curtains
• Specialist absorptive products

The required quantity and positioning matter.

Adding a small panel in an arbitrary position may make little difference. Treatment should be selected according to the room volume, frequency range and required performance.

Review the room layout

Changes to the layout may improve:

• Distance between speakers and listeners
• Sound-system coverage
• Separation between noisy and quiet activities
• Speech privacy
• Exposure to strong reflections

In an office or restaurant, furniture, screens and zoning can form part of the acoustic strategy.

Control building-services noise

Where ventilation or mechanical equipment is creating excessive background noise, possible measures may include:

• Selecting quieter plant
• Reducing air velocities
• Adding attenuators
• Isolating vibrating equipment
• Improving ductwork design
• Relocating grilles or equipment
• Adjusting control settings
• Reducing breakout noise

The solution should maintain the required ventilation and thermal performance.

Improve the sound system

Poor speech intelligibility is not always caused by the room finishes alone.

A public-address or presentation system may need changes to:

• Loudspeaker positions
• Loudspeaker direction
• Coverage
• Delay settings
• Signal processing
• Operating levels
• The number or type of loudspeakers

Treating the room and improving the sound system may both be necessary.

Use diffusion or alter reflective surfaces

Some rooms benefit from scattering reflections rather than absorbing all of them.

This can be particularly relevant in music and performance spaces.

Changes might include:

• Diffusive surfaces
• Angled panels
• Shaped ceilings
• Altered wall geometry
• Specialist reflectors

The design should be based on the desired acoustic character rather than a general aim to remove reflections.

Room acoustics and sound insulation are different

Room acoustic treatment controls sound within a space.

Sound insulation controls sound passing between spaces or through the building envelope.

Adding wall panels may reduce reverberation inside a meeting room, but it may not prevent conversations being heard next door.

Likewise, upgrading a wall may improve sound insulation without making speech inside the room clearer.

Where noise is entering or leaving the room, the project may need an airborne or impact sound insulation assessment rather than, or in addition to, a room acoustic assessment.

Solutions such as heavier partitions, isolated linings, improved doors and sealed penetrations may then be relevant.

Why measure before choosing acoustic treatment?

Acoustic products are often sold using broad descriptions such as “soundproof”, “noise reducing” or “acoustic”.

These terms do not establish whether the product will solve the particular problem.

Measurement can help determine:

• Whether reverberation is actually excessive
• Which frequencies need attention
• How much absorption is required
• Where treatment should be installed
• Whether background noise is the main issue
• Whether the sound system is contributing
• Whether sound insulation is also required
• What improvement is realistically achievable

This reduces the risk of installing treatment that looks appropriate but has little effect.

Should room acoustics be considered during design?

Yes.

Room volume, geometry, finishes, services and sound systems all affect acoustic performance.

Considering acoustics early gives the design team more options. Treatment can be integrated into the ceiling, walls, lighting and interior design rather than added later as a corrective measure.

Early acoustic input can also help avoid conflicts between:

• Acoustic absorption and interior finishes
• Ventilation and background noise
• Sound insulation and building services
• Acoustic panels and fire requirements
• Ceiling treatment and lighting
• Room layout and sound-system coverage

Retrofitting can still improve an existing room, but it may be more disruptive and limit the available solutions.

Talk to auricl about room acoustic testing

If a room is echoey, noisy or difficult to understand speech in, tell us how the space is used and what is going wrong.

We can measure its acoustic performance, explain the results clearly and recommend practical ways to improve it.