Ask the Tech Expert: acoustics and reverb. The problem of the cathedral sound.

We ran an open question on the website and Facebook recently inviting you to submit your questions in relation to common problems with church sound for the crack team at SFL Group to solve.

The challenge of building acoustics is a common one and something that Randy raised:

“Our church organist is pushing to have our sanctuary carpet replaced with hardwood or tile flooring in order to create the “cathedral” effect. What are the pros and cons?”

What are the pros and cons of carpet vs hardwood flooring?

Acoustics is something that we are always harping on about at SFL and we cover the topic extensively in our Sound Tech and PA Training for Churches Course. We often share a concept which we call the “hierarchy of importance”, this helps us to understand which elements of an audio system are most fundamental to sound quality. The very foundation of the hierarchy however is not a piece of technical equipment, but is the acoustics of your venue. In short, acoustics are really, really important – get them wrong and even with tens or hundreds of thousands of pounds/dollars of technical equipment and the most competent of sound engineers you will never achieve good quality sound.

Acoustics is a very complicated field, but for Randy’s question I will just focus on one acoustic property, reverberation.

Any time we create sound in an indoor environment we can split what we hear into two specific categories: the direct sound which travels straight from the source to the listener, and the reverberant sound which is made up of all the copies of that sound which reach the listener after having bounced off one or more surfaces in the venue. Depending upon the size of the venue and the materials it is made of reverberation may die away very quickly, or it may linger and last a long time.

Reverberation is not a bad thing in and of itself; it is a natural product of creating any sound in an enclosed space. In fact if you ever find yourself in an enclosed environment without any reverberation (such as an anechoic chamber, used for conducting precise scientific measurements of sound sources) it can feel quite unnatural and uncomfortable. Most of the time we are not consciously aware of reverberation, it is something we naturally expect to hear and our auditory systems process it entirely subconsciously. However sometimes reverberation can cause us issues if it is not appropriate to the venue or application.

Problems with reverberation can go one of two ways:

Too much reverberation will smear all the short, sharp transient sounds which are critical to speech intelligibility (think about the T’s and P’s) and make it harder to understand what is being said. Likewise percussive or strummed instruments which are central to many forms of contemporary worship have very short transient components that will lose clarity and impede the clear expression of rhythm, making music feel cluttered and messy. At the same time, the length of the reverberation time means that sound energy stays in the venue for longer and builds up to greater levels, this can result in worship feeling overly “noisy” and is a common cause of the “it’s too loud” complaint. In a reverberant environment more of the stage noise will bleed out into the front of house sound so it also becomes harder for the sound person to control overall volume levels from the mixing desk.
In contrast, too little reverberation will lead to a very crisp, clear sound, but makes the venue feel very dry and uninspiring to sing in. This may be ideal for venues which are focussed purely on speech or contemporary music performance, but in churches where the worship is a participatory experience your congregation will begin to feel very exposed and have a tendency to hold back from fully engaging in the worship. Because of the participatory nature of church services we generally aim for a slightly longer reverberation time in church venues than we would in other contemporary performance oriented spaces, it is particularly important if you are engaging an acoustic consultant that they understand this – for example a consultant whose experience may lie predominantly in theatres or concert venues may not be the ideal person to design the acoustics for your church.

At SFL we believe that around 1.4-1.6 seconds is about ideal for most churches, though this is venue and application specific so is not a concrete rule. Slightly longer reverberation times can usually be tolerated in larger venues or venues where more traditional forms of musical worship are the dominant mode. As some interesting references, many classical music venues are designed for slightly longer reverberation times in the order of 2-2.5s, whilst building regulations for classrooms and lecture theatres in the UK call for a reverberation time of no greater than 0.8s. Given the diversity of programme material in the typical church meeting the 1.4-1.6s target sits quite comfortably in the centre ground of these figures.

There is a commonly held belief in certain circles that cathedrals and other large church buildings have the ideal acoustic. I’ve lost count of the number of times I’ve been told that a church has a “fantastic acoustic”, which usually means it has quite a long reverberation time. Cathedrals in particular tend to have very long reverberation times as they are both very large spaces and tend to be constructed out of lots of very hard and reflective materials such as solid wood and stone. According to figures from the University of Salford, St Paul’s Cathedral in London has a reverberation time of a whopping 13s. This appreciation of acoustics – that the “cathedral-like” reverberation is the ideal – is not necessarily wrong, but it is overly simplistic. The long reverberation times experienced in cathedrals would have been very helpful for historical forms or worship which were often choral or organ led, in particular a more reverberant environment will help unamplified voices carry further. However for a church trying to deliver more contemporary worship styles or relying upon electro-acoustic amplification rather than purely on the acoustic to aid propagation of the spoken voice the “cathedral” feel will ultimately form an impediment to sound quality and listener comfort.

Without knowing more about Randy’s example it is hard to give specific advice, but unless the organ is the sole source of musical accompaniment I would probably recommend keeping your carpets. Materials like carpet and curtains can be a great way to add some acoustic absorption to your venue as a bi-product of other aesthetic design choices and are both cheaper and more subtle than the more extreme scenario of resorting to specialist acoustic treatment.

Our Church Sound Tech Course (on DVD, download and via our subscription site) is designed as a very comprehensive guide to church sound – you could watch it as a group over a series of training evenings. Watch a sample section on the “hierarchy of importance” for free here.

For those of you near London or the UK South East, Pat is running a day of Sound Tech training for Musicademy on 19 March.

Do add your own thoughts to the above article in the comments thread below. We’ll post the next in the series soon. And if you have any other PA and Sound Tech questions, please post them below so we can add them to the list for future articles.