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Acoustics: The Most Misunderstood Field in Sound Reinforcement

I just want to say up front that I am NOT an acoustician. Having said that however, I am very experienced in audio design, touring, and installation application in a wide variety of acoustic spaces. I have worked with some of the best acousticians in the country. I am also experienced in virtual acoustics but, that is a subject for another time.

Just what are good acoustics? I hear this question all the time. I also hear many people say that this or that“Hall” has the best acoustics they have ever heard. I always want to know what they are listening to when I hear this statement. Some think that a beautiful gigantic cathedral made of concrete and marble will have wonderful acoustics. Well maybe so. Some think a good concert hall will have good acoustics. Possibly. Some may say I love rock and roll concerts outside where they can hear everything without the sound bouncing all over the place. Hmmmm.

When designing a sound system, consideration the acoustic RT of the room as well as the acoustic gain and control the direct energy so it does not bounce everything all over the room before it gets to the seats where the people are.

Why do some musical formatslead to sounding better in a particular environment and others don’t? Well, that is a very good question. It all has to do with the length of the RT or reverb time and the length of a particular note that is being produced. I know this all sounds scientific and complicated but, bear with me for a moment.

We will start with a cathedral. What do we hear mostly in a cathedral? Pipe organ or choir. The pipe organ has very long drawn out notes that will reverberate throughout the space while giving time for each note to decay before the next note is played. Sometimes it can seem like the final chord played on the organ is still reverberating in the air while the organist has left the building and is driving home. The same with a choir. The long RT gives an “added bonus”to the fullness of the note or chord. It seems to envelope us and surround us because it is doing just that.Now let’s take a modern rock &roll band with a live drummer that wishes to play a concert in that same cathedral. Now, we are in for a totally different experience and one that is probably not good. The long RT of the space will not lend itself to the very staccato dynamic notes of the drums along with the acoustic gain being produced.

Now the notes are indeed coming back at you from many different directions and your brain can’t decide which one to listen to first. They are coming faster than the previous notes have had time to decay. This is like being surrounded by 100 voices coming at you from different directions all at the same time.

So now that we have demonstrated that there must be different acoustic environments for different genres of music. How do I decide what is right for my particular church venue? This question we will address in a moment.

If I am designing a sound system, I should take into consideration the acoustic RT of the room as well as the acoustic gain. I also want to control the direct energy so it does not bounce everything all over the room before it gets to the seats where the people are. Unfortunately, many audio designers don’t pay enough attention to the room acoustics and just want to hang a box in the air. Then, later they will put acoustic panels everywhere totry to tame the offending frequencies that are bouncing off every wall in the room. Well, while this might be good for taming some of the high frequencies it is not doing muchfor the mid and lower bands. I donot subscribe to this “I’ll fix it later” method, as you tend to lose thosehigh frequencies that you will wantto “buy back” later somewhere in the audio chain. There are other waysto tame those frequencies and theyare called diffusers. For the long bass frequencies, there are also bass traps. All frequencies have different wave lengths and they all require different ways to tame them. For instance, a 1000hz wave is around 1 ms long and approximately just short of foot long. About 11 inches or so. That means a 10,000 hz wave is on 10th that long or around an inch and a 100hz wave is around 11 ft. While these are all things you should keep in mind, there are other ways to think about acoustics in your space without making your head explode with math.

The very first thing anyone should ever think about is “Direct Energy” and where it is going. You can take care of most of your acoustic reflection issues with just seeing how your energy is directed. Simple, right? This, is why there are so many speaker types and why you shouldn’t even consider buying a system without knowing your direct energy requirements.

The first thing most churcheswant to look at is budget rather than what is right for the design criteriaof their space. Always look at the design criteria first and then budget accordingly. There is always someone out there willing to give you a cheap system, but more often than not they have no idea of the science behind what makes a system work or fail.

God has set up the laws of physics for a reason and not just because he had nothing better to do. Obey them and you will always win. Try cheating them and you will always lose.

There are many ways to direct energy and it does require thinking somewhat out of the box. I always look at the pure physics of a room before anything else. Just because someone else puts boxes in the air does not necessarily mean that is the right solution for your room.

Ok, now to answer the “What is right for me” question. There are charts available that show the proper RT of a room for just about any performance criteria. If you are a very traditional church you can get away with and most likely want a longer RT foryour space. If you are a modern worship house then shorter isbetter. For modern performances,we like to have a 1.2-1.4 acousticgain for a room. You can consult an acoustician for the room acousticsor do a lot of the research yourself.It is all available on the internet. Remember though, that while acousticians are for the most partgreat at what they do, they are often not audio designers. Always get an opinion from someone experiencedin the field of direct energy. Theyare usually found in the touringand performance part of the audio industry. We like to partner withgreat acousticians to create the best solutions for a specific space.

Once you have the proper RT for your room, and the correct loudspeakers have been installed,a high-quality DSP (Digital Signal Processor) should be in place in order to tune the system. Utilizing measurement tools (SOA uses SIM and SmaartLive V8), the system can now be tuned by incorporating filters to maximize performance of the frequency response, and time align for optimal performance. This is where experience is invaluable.I have witnessed many people attempt to tune a system, butthey are married to the trace onthe screen, spending many hours experimenting with certain filters to get the trace to look exactly like they want to see it on the screen. However, the system sounds terrible. On the flip side, I havealso seen some attempt to tune the system with their ears only, lacking the reference offered by proper measurements that confirm what you’re hearing, and allow you to act accordingly. By using both the tools of measurement, as well as ones’ own ears, the system can be brought to the highest standards, specific to the space.

In regard to acoustics, there are many ways to treat this particular area. Virtual acoustics is one, beam steeringis another. We will tackle beam steering in the next article. With the right beam steering and/or virtual acoustics, you may be able to have the best of both worlds.

Daryl Porter is the Vice President/Co-owner SOA (www. secondopinionaudiollc.com), providing performance audio, video & lighting systems to the highest standards of excellence for over 35 years.