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Reverberation - Friend or Foe?

Reverberation is probably a word you have heard many times, but what exactly is it and is it a good thing? Can we get rid of reverberation and if so, how?

Reverberation is probably a word you have heard many times, but what exactly is it and is it a good thing? Can we get rid of reverberation and if so, how? This article will explore all of those questions and more, to leave you clued up on reverberation.

First things first, are reverberation and echo the same thing? They are similar but not quite the same thing. Reverberation is generally a jumbled mix of decaying sounds without discernible syllables (in the example of speech). 

An echo is a completely discernible sound, word or syllable which by definition is a reflection of a single surface with a reflection time long enough to be heard after the source has stopped and can be distinguished as a separate sound.

Sound travels in waves, and like the waves in the sea, these sound waves have energy. When sound waves are generated by a voice, instrument or any other audible source they radiate out from that source and strike objects and walls around them. Hard surfaces reflect most of the sound energy and (acoustically) soft surfaces absorb, transmit and reflect depending on the frequencies and the material. The amount of absorption depends on the structure of the material, but, more on that later.

So what happens when sound is reflected from a surface? Sound waves travel at around 300 metres per second. That seems incredibly fast to us (roughly 670 mph!!) but it is slow when compared to the speed of light, which travels 1 million times faster. That is why we see the flash of fireworks before we hear the ‘bang’ from them.

The result of this is that in a conventional room, the sound energy reflects multiple times off the surfaces in the room, losing energy as it does so. Depending on the (acoustically and usually physically) hard or soft nature of the surfaces, more or less energy will be lost when each of these reflections occurs; the harder the surfaces, the less energy is absorbed and so the longer the sound takes to die away.

(Picture of a graph showing the speed of sound vs light)

What actually happens to sound when it is absorbed?

If you cast your minds back to your high school science classes, you would have been taught that energy never dies, it only changes from one form to another. In the case of sound absorption, as the sound energy passes into and through an absorbing medium, the friction caused by the medium on the movement of the particles strips them of energy, which is turned into minute amounts of heat.

So is reverberation a problem?

In short, it depends on the situation. Reverberation is not necessarily a problem, and sometimes it is a positive boon. When it comes to opera, classical music, organ and choral music, it could be said to be essential to the performance of the genre. Performances like these which are generally acoustic in nature (ie not amplified or performed through a microphone) rely on reflected sound for the performers to hear themselves and to add desirable reflections that sustain and ‘fill out’ the performance.

However, for amplified music and particularly the spoken word, reverberation is positively detrimental. The later reflections are still loud enough to cause the ear and brain difficulties in interpreting what is being said. With the diction of the speaker being smeared by the elongation of consonants especially, an increasing amount of brain power is required to decode and understand. 

Imagine a snare drum beat and instead of being short and sharp, it is elongated so that it hasn't died away completely before the next beat happens. The further away from the source of the original sound you are, the less of it you hear directly and the more you hear the result of the reverberation. The point at which intelligibility dramatically falls away is called the Critical Distance. At this distance, you are hearing an equal amount of direct sound and reflected (reverberant) sound.

How do we measure reverberation?

Reverberation is measured in seconds and this measurement is called the reverberation time (RT). It is defined as the time it takes for the original sound level usually of a short loud sound such as a balloon burst or gunshot, to die away to 1 millionth (-60dB (Decibels)) of the original peak level.

So can you define an ‘ideal’ reverberation time?

The ‘ideal’ reverberation time depends completely on the venue! For cinema and the spoken word an RT (reverberation time) of around 0.5 seconds is the aim, however, the RT is usually much greater unless a room or hall has been designed from the ground up or has had a lot of absorption added.

For contemporary music genres like rock or modern worship songs around 1-1.2 seconds is good.

For classical music, 1.6-2.3 seconds would be considered ideal, and for organ music 2-4 seconds is desirable.

So you can see that if you have a venue with multiple uses there are conflicting demands on the RT. However for multi-use venues generally an RT of around 1.2 seconds is a good compromise for intelligible speech and music that is not too dry. We can always add reverberation electronically but it is not possible to take it away in this manner.

Some venues have sliding panels and heavy drapes to allow for a change in the RT depending on the performance type, but for most venues, these would be seen as too expensive or impractical to achieve. 

(Photo of ideal reverberation times for different events)

How do we reduce reverberation time to improve intelligibility?

Very few public buildings in the UK were designed with more than a passing nod to acoustics, certainly not your average village hall or community building. We have worked in brand-new spaces where the acoustics are terrible and could have been designed with better acoustics in mind. 

The typical treatment is to add absorption to reduce the reflections and therefore reduce the reverberation time. This can be incredibly effective, even adding a few square metres of panel absorbers can turn a difficult space into an acoustically comfortable one.

Panel absorbers are the most popular non-construction solution to excess reverberation. They are generally large flat fabric-covered panels of acoustic-grade mineral wool or other fibrous material. They are available in matching or complementary colours so can be discrete or made into a feature. The sizes are generally chosen to fit in with spaces between windows and other features in the room, although they can also be fixed to the ceiling or ‘flown’.

The number of absorbing panels required is calculated by measuring the volume of the room and the reverberation time to calculate the current amount of incidental absorption. Then these figures are plugged back into the same formula to calculate the required additional absorption to bring the RT time down to the desired level.

In this way, what was once a space where it was difficult to hold meetings, concerts or other gatherings, can be tamed and turned into an acoustically comfortable space. The great thing about acoustic treatment is that it doesn't use any power, doesn't wear out and works all the time, whether you are using amplification or not.

There are other cases where acoustic treatment would be very useful, such as video conferencing rooms, where you can often hear as much of the room as you can the person you are listening to.

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