Understanding a few basic acoustical terms can make it easier to evaluate room acoustics, sound isolation and acoustic treatment options.
The definitions below provide a practical introduction to several commonly used terms. They are intended as a general guide rather than an exhaustive technical reference.
If you require further clarification, please Contact ACE-PROAUDIO and we will be pleased to assist.
NRC is a single-number rating used to describe the average sound absorption performance of a material.
It is calculated from the absorption coefficients measured at:
250 Hz • 500 Hz • 1000 Hz • 2000 Hz
In general:
Higher NRC = greater sound absorption
Soft and porous materials such as acoustic panels, mineral fibre, fiberglass, fabric and carpet typically have higher NRC values.
Hard materials such as concrete, brick, glass, tile and untreated drywall generally absorb less sound and therefore tend to have lower NRC values.
NRC is useful for making a quick comparison between products. However, when more detailed acoustic performance is required, the individual absorption coefficients at each frequency should also be examined.
The absorption coefficient indicates how effectively a material absorbs sound at a particular frequency.
The value normally ranges from approximately:
0.00 = very little absorption
1.00 = very high absorption
Because acoustic materials behave differently at different frequencies, a material may provide strong absorption at mid and high frequencies but considerably less absorption at low frequencies.
For this reason, acoustic treatment should not be selected based on NRC alone.
When comparing different materials, the test method and mounting arrangement should also be considered because they can significantly affect the published results.
Common laboratory standards include:
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ASTM C423
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ISO 354
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Impedance tube measurement methods
For practical room treatment, reverberation chamber data is commonly used to assess broad-band absorption performance.
STC is a single-number rating used to describe the ability of a wall, partition, door or building assembly to reduce airborne sound transmission.
In general:
Higher STC = better airborne sound isolation
Sound isolation normally depends on several factors, including:
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Mass
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Construction method
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Air cavities
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Insulation
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Decoupling
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Sealing of gaps and penetrations
Dense materials and properly designed multi-layer partitions usually provide better sound isolation than lightweight porous materials alone.
STC is useful for general comparison, but detailed transmission loss data across different frequencies provides a more complete indication of performance.
MTC — Music/Machinery Transmission Class is related to sound isolation performance but places greater emphasis on lower-frequency sound.
Low-frequency noise is particularly important when dealing with:
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Music systems
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Subwoofers
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Machinery
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Mechanical equipment
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Entertainment venues
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Studios
Low-frequency sound is often more difficult to control because longer wavelengths can more easily excite building structures and travel through walls, floors and ceilings.
Decoupling is a construction technique used to reduce the transfer of sound vibration between building structures.
Instead of allowing two surfaces to be rigidly connected, the structures are partially or completely separated.
Typical methods include:
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Double-wall construction
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Air gaps between partitions
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Resilient channels
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Isolation clips
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Floating floors
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Rubber or spring isolators
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Isolated ceiling systems
Decoupling can significantly improve sound isolation when it is incorporated correctly into the overall construction design.
A room mode is a low-frequency resonance created when sound waves interact with the dimensions of an enclosed room.
At certain frequencies, sound waves may reinforce or cancel each other.
This can create areas where bass sounds:
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Too loud
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Too weak
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Uneven
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Boomy
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Difficult to control
Room modes are especially important in spaces such as:
Recording Studios • Control Rooms • Home Theatres • Listening Rooms • Broadcast Studios
There are three principal types of room modes.
Occur between two opposing surfaces, such as:
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Front wall and rear wall
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Left wall and right wall
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Floor and ceiling
Axial modes are normally the strongest room modes.
Involve sound reflecting between four room surfaces.
They are generally weaker than axial modes but can still affect low-frequency response.
Involve reflections between six room surfaces.
They are generally more complex and usually have less individual influence than axial modes.
Room dimensions, loudspeaker placement, listener position and acoustic treatment can all influence modal behaviour.
These two terms are often confused.
Acoustic treatment controls the sound inside a room.
Its purpose may include:
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Reducing excessive reverberation
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Improving speech intelligibility
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Controlling reflections
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Improving music clarity
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Controlling low-frequency problems
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Creating a more balanced acoustic environment
Typical treatments include:
Acoustic Absorbers • Bass Traps • Diffusers • Acoustic Panels
Soundproofing is concerned with reducing sound transmission from one space to another.
Typical methods include:
Mass • Decoupling • Insulation • Airtight Sealing • Floating Construction
Installing acoustic foam or wall panels may improve the acoustics inside a room, but they normally provide only limited improvement in sound isolation.
RT60 is one of the most important measurements in room acoustics.
It describes approximately how long it takes for the sound level in a room to decay by 60 dB after the sound source stops.
Rooms with long reverberation times can sound:
Echoey • Unclear • Boomy • Difficult for Speech
Reducing excessive reverberation can improve:
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Speech intelligibility
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Music clarity
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Listening comfort
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PA system performance
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Overall acoustic quality
The appropriate RT60 depends on the size and intended use of the space.
A classroom, auditorium, sports hall, recording studio and concert hall will not necessarily require the same reverberation characteristics.
Speech intelligibility describes how clearly spoken words can be understood by listeners.
Poor speech intelligibility can result from:
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Excessive reverberation
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Background noise
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Poor loudspeaker placement
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Inadequate sound-system design
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Strong reflections
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Insufficient direct sound
Good acoustic treatment and properly designed sound reinforcement can significantly improve speech clarity.
There is no single acoustic treatment solution that is suitable for every space.
The appropriate treatment depends on factors such as:
Room Size • Room Volume • Construction • Existing Reverberation • Frequency Response • Intended Use • Occupancy • Loudspeaker System • Required Acoustic Performance
For larger or more critical spaces, measurements and acoustic calculations should normally be carried out before deciding on the type, quantity and placement of treatment.
ACE-PROAUDIO provides practical assistance in:
Acoustic Assessment • RT60 Measurement • Acoustic Treatment Planning • Sound Reinforcement • Professional Audio • Broadcast Systems • System Integration
Whether you are dealing with an auditorium, multipurpose hall, studio, meeting room, educational facility or other professional space, the objective is to develop an acoustic solution appropriate to the actual requirements of the room.
Contact ACE-PROAUDIO for further information at +60162798533 (cyril)
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