A Guide to Reverberation and Better Sound

A Brief Explanation of Reverberation

Reverberation is a form of sound energy that ‘bounces’ around a space and is measured by the duration of time it takes for that sound to decay. Two examples most of us will be familiar with is a swimming pool or a sports hall, where you can hear a voice or any other sound much longer after it has been conducted – these spaces are often very reverberant.

Depending on the room’s material composition or geometry, these can influence the sound and its path as it reflects, where there are hard surfaces such as brick or concrete, these can reflect the sound, soft furnishings such as sound absorption materials, curtains and carpet can absorb the sound, reducing it’s reflection back into the room and turning the sound energy into small amounts of heat that are partially transmitted into the material.

Room Geometry can also be another factor in the internal acoustics, if the room has an odd shape such as a pitched roof example, this can often change the path of the sound and often will create odd acoustic effects on the sound.

A typical example of Reverberation that leaves a negative effect is a sports hall; it is often constructed on a concrete floor with perhaps a wooden or lino flooring which can be highly reflective, walls would often be a painted breeze block, again, highly reflective, and a ceiling area which could be anything from wood panelling to a metal roofing – reflective again. There is often little to no soft furnishings that help absorb that sound energy bouncing around, these may be curtains, carpeted flooring, soft seating and in the rare occasion sound absorption materials.

This high level of reverberation effects the quality of speech, which in the industry is referred to as Speech Intelligibility.

A room, depending on the space and it’s requirements, may actually need reverberation to help amplify or compliment a specific sound. Churches were built in their grand and expansive fashion to accommodate large choirs along with large audiences. If we had a completely absorptive space, the natural resonance and the room’s ability to amplify sound would be lost – no longer achieving the choral effect.

Reverberation adds quality, warmth and cohesion to a wide range of music – it is added digitally in most recorded music through a range of processing techniques.

We also depend on early reflections as a way of echolocation, for those with visual disabilities – they will depend on this greatly to audibly scan a space for objects, simply clicking can be a good way of indicating the proximity of an object or the size of space.

There are some interesting examples in Trevor Cox’s book “Insert Excerpt” where a search for the world’s longest reverb time is influenced by both what the structure is made of and the shapes of that structure.

If the topic of reverberation interests you, we have put together a blog “A Brief History Of Reverberation” as part of our ‘A Brief History Of’ blog series.

Reverberation Terms

Absorption Coefficient

This gives a specific material a value of how acoustically absorptive it is, for example; concrete is very dense and reflective and would have a low Absorption Coefficient, and for a heavy set of curtains it would have a higher Absorption Coefficient.

The scale of an Absorption Coefficient is between 0-1, and broken down across a range of octaves which is broken down in decimal points between 0-1 for each one.

We would most commonly put this as a percentage as an overall, unless a specific frequency requires more attention.

125hz	250hz	500hz	1000hz	2000hz	4000hz	NRC
0.48	0.85	1.07	1.04	0.96	0.89	1.00

(Taken from our MuteSpace Pro 50mm)

Decay

This refers to the time it takes for the sound to decay after it has been conducted, often expressed as an RT60 when measuring reverberation time – this means we are measuring how long it takes to drop below 60dB. It does vary with frequency, with the lower frequencies often containing an increased amount of sound energy against higher frequencies.

Early Reflections

These reflections are defined as being heard directly after the sound, and often not heard as separate, this would put it in the region between 50-80 milliseconds, anything beyond 80 milliseconds we tend to define them as Late Reflections, please see below. Early Reflections help to reinforce the direct sound.

Late Reflections

These reflections are heard as a long, decaying sound after the direct sound and early reflections. The sound energy will continue to bounce around in rooms that consist of hard surfaces.

Speech Intelligibility

From the speaker to the listener, we require a suitable acoustic environment to clearly hear speech. We measure the clarity of speech using the Speech Transmission Index.

Speech Transmission Index

The Speech Transmission Index helps to grade a specific environment for the quality of speech transmission, this is measured on a scale between 0-1, with a range of nominal qualification bands from A+ – U – it is an objective measurement, so may not be applicable to all those who may use the space.

Category	Nominal STI Value	Type of Message Information	Examples of Typical Uses	Comment
A+	>0.76		Recording and Broadcasting Studios	Optimal intelligibility, this level would not always be achievable in the majority of environments.
A	0.74	Complex messages and unfamiliar words	Theatres, speech auditoriums, courts, assistive hearing systems (AHS)	High level of speech intelligibility
B	0.7	Complex messages and unfamiliar words	Same as above	High level of speech intelligibility
C	0.66	Complex messages and unfamiliar words	Theatres, speech auditoriums, teleconferencing, courts	High level of speech intelligibility
D	0.62	Complex messages and familiar words	Lecture theatres, classrooms and concert halls	Good level of speech intelligibility
E	0.58	Complex messages, familiar context	Concert halls and modern churches	High quality PA system required
F	0.54	Complex messages, familiar context	PA systems in shopping centres, voice alarm systems and cathedrals	Good quality PA system required
G	0.5	Complex messages, familiar context	Shopping centres, voice alarm systems	Target value for all areas requiring a voice alarm system
H	0.46	Simple messages, familiar words	VA and PA systems in all difficult spaces	Lowest recommended limited for voice alarm systems
I	0.42	Simple messages, familiar words	VA and PA systems in very difficult spaces
J-U	<0.38			Unacceptable

FAQs

Sound absorption aims to improve the sound quality within a room, such as improving the clarity of speech or creating a perfect listening environment. Soundproofing looks at reducing noise levels between rooms – if you looking to reduce noise from another room or neighbour for example, you can see the guides below for sound proofing ceilings, walls and floors.

The measurement of reverberation is taken with what is called the ‘Interrupted Noise Method’, where pink noise is played at *dB and is immediately cut off after *seconds. You then measure the time it takes to decay below 60dB – this is the RT60 measurement.It can also be done as a subjective measure, just through listening you can hear how reverberant the space is, it is often difficult to conclude with a solution to reach a targeted acoustic value but can be perfectly acceptable in office refurbishments for example.

Another way of estimating reverberation levels is through modelling software, which takes average absorption ratings per building material and bases the measurement on a simple cube or cuboid room, so this won’t consider the room geometry, but will give some indication on more basic rooms.

An absorption coefficient is how acoustically absorptive a material is, for example; a heavy drape or curtain would have a higher rating than a plank of wood – if you spoke directly into a curtain, it would have a muffled reflection, if you spoke into a plank of wood, you would find that a lot of the sound is reflective (try it!).The measurement is from 0-1, from little to no absorptive qualities to very high respectively. Below is a short list of examples.

iKoustic can help in a range of ways, we often use specific acoustic modelling software to help give a gauge of the reverberation levels in your space for general improvements. If you are aiming to meet a certain target we would offer our Acoustic Testing to give us a more accurate measurement.We have a wide range of sculpted, baffle, panel range of sound absorption products to not only resolve the reverb issues, but also provides a beautiful, sleek and modern look to your space in a range of colours.

We do! We use rendering software with virtual renders of your ideas within the space state-of-the-art cutting technology to create precision cuts when we have agreed the design. We take your future or current colour scheme into account to create these bringing your ideas to life.

What options do I have available?

Our range spans from discreet sound absorption panelling to adventurous bespoke absorption in a huge range of colours. They can be hung, adhered to the wall or ceiling and even free standing on the floor. You can create quiet working pods, vocals booths or even shape your logo out of them.

We go beyond to create stand out feature pieces for your project, if you have a vision – we can help you get there.

We have included some example videos below for you.

What we do for you

Free and friendly over the phone advice on your project
Free material design and product guidance on what best to use
Free quotation and quote alterations
Visual rendering of your space and proposed products with a range of options (this may be chargeable)
State-of-the-art cutting for precision
Site Visits (this may chargeable and will reflect your location and project)
Acoustic Design
Experienced Acoustic Installation Team (this will be chargeable and reflect your location and project)

Where to now?

You have made it this far, do get in contact with us we would love to hear from you and how we can help.

To help us, you will want to gather;

Any plans you may have had of the room including dimensions
Any colour scheme or ideas you have in mind
Any images, this can help us visualise the space

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