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  Recording Acoustic Guitar
Introduction
"Recording acoustic guitar is a nightmare! It will drive you crazy! Your recordings will sound bad, and whatever you decide to do differently, it will only make things worse. At the end, when you're sick and tired and have tried it all, you decide to simply do what you did when you started. And it will sound worst of all. Now THAT'S encouraging!"
OK. I've said it.
Those of you who have not lost all their hope at this point, and are still determined to make a good audio recording of their acoustic guitar, well, you may have the right mentality to bring it to a happy end. Read on...
Brainwork
I wrote in another article on this web site how playing guitar, and learning to play new songs, is a highly intelligent process. The same applies to recording acoustic guitar. It demands quite a bit of intelligence too. Unfortunately, in these days of fast and commercialized music, largely based on electronically generated sounds, the fine art of recording acoustic instruments has pretty much gone astray. Over the years it often surprised me how little knowledge there is, even in many professional recording studios, about how exactly you need to record an acoustic guitar, and what it is really all about. That's nothing personal, and I'm sure it is not always the case, but unfortunately it is the way I have experienced it.
Many people will state that the best results will be obtained with:
1. A microphone the price of a Mercedes Benz
2. A zillion bits dynamic range on your digital recorder
3. A sound engineer with psychic abilities who can position the microphone in a 'sweet spot'
In my opinion, none of the above is true. A few hundred bucks (even less) will get you a good enough microphone (I would recommend you get two, so you can make stereo recordings), a 16 bits consumer-type digital recorder can produce great results if you use it right, and the sweet spot is not so crucial anymore once you know what you are doing.
Room Acoustics
There is, however, one thing that is absolutely crucial for good sound recordings. It's called room acoustics. Room acoustics will color the sound of your guitar dramatically, because the sound created by the guitar will reflect from objects in the room, and from the wall, the floor and the ceiling. We all recognize this easily when we are in an empty room. It sounds hollow and has 'echos'. As soon as we put carpet on the floor, hang curtains before the windows, and fill the room with furniture, the effect seems to be gone. But that's no true. Even a living room packed with carpets, curtains, furniture and cushions will still reflect a lot of sound.
Here is an experiment for you. Get a guitar strap, so you can play standing up. Now go out of your house. Somewhere in a garden, far away from any walls is best. Play your guitar and listen carefully. What do you hear? The first thing you will notice is that the volume seems a little low. That is because there is no sound reflecting from walls and furniture. The only sound you can hear, is the sound that goes from the guitar straight to your ears. Now listen more carefully. You'll probably agree with me that your guitar actually sounds pretty good! The sound is balanced, and pure. Much better than what you're used to. If your not standing at this point, stand up now, and walk back into your house while you continue to play your guitar. As soon as you enter the house, you'll recognize the room acoustics as if you switch on an effect pedal. The take home message:
What you are hearing as soon as you enter the house, is not the sound of your guitar, but the sound of your guitar distorted by sound reflecting from everywhere in the room. Unfortunately, 99 out of 100 times, this sound is much worse than the natural sound of your guitar, without reflections. You probably were never aware of this, because you are used to the sound of the room where you always play. Your brains are quite smart, and will compensate your sound perception for the acoustic distortion, and they will make you accept it as a natural thing to happen. However, as soon as you start recording, things get worse. In the next sections, I'll explain why.
Improving Room Acoustics
In order to reduce the effect of reflecting sound waves during audio recordings, professional sound studios are typically packed with all sorts of sound absorbing panels and sound diffusors. The objective is to prevent sound waves to bounce back and forth through the studio (with the absorbing panels) and, in case it still happens, scatter the sound waves all over the place (with the diffusors). You recognize this immediately, as the studio will sound quite 'dead'. That's good - this way you won't have undesired reverberation and echoes on your recording. You cannot remove these once they are recorded, whereas you can easily add them afterwards with an electronic reverb unit.
However, the absorbing panels and diffusors work great for higher frequencies, but not as good for the low frequencies in the audio spectrum. In practice, when you record an acoustic guitar, most of the popular sound absorbing panels will still reflect quite a bit of the tones produced by your low E, A, and even the D string. Simultaneously, diffusors really don't diffuse these notes that much. The effect of these reflections is not as easy to recognize as the 'chatter echo' you can hear in an empty room. Those are, in fact, predominantly the higher frequencies you hear. The lower frequency reflections of your guitar, which are almost impossible to prevent, will cause an uneven bass response, booming bass notes, and an overall degradation of the recorded sound of your guitar.
Fortunately, there are two different remedies to prevent the problems mentioned above:
1. Record your guitar in an open field. Make sure there is no wind, no rain, no cows or other noisy animals (including insects landing on the microphone), and ask someone ranked high in some air traffic control organization to make sure no airplanes will fly over in a radius of some 40 kilometers around you. Nearby roads must be closed, and everybody in a radius of, say, one kilometer must stand still and be silent until you're done.
2. Record your guitar in a a huge building (at least some 20 x 20 meters, larger is better, with a ceiling at about the same height. Most cathedrals will do. However, make sure all walls and at least the floor are covered with carpet, and fill the entire room with large, soft objects to reduce the long reverberation time. A large opera house typically works great too. All you have to do is rent it for the time you need to make your recordings.
In both cases, sound reflections will not bother you, and your recording will sound great with little additional effort. However, in case these two solutions are not practical for you for some reason, you may want to read on.
Standing Waves
I mentioned the 'booming bass' in guitar recordings. It's a infamous problem, and if you ever made acoustic guitar recordings (with a microphone) at home, I'm sure you know what it sounds like. Some bass notes are very dominant, and seem to make your whole audio system resonate. If you try to remove these notes by means of tone control or an equalizer, the best you can do is create a thin, tinny sound, without body.
If you really want to know in detail how this all happens, you should ask a physicist, or read the articles on http://www.kettering.edu/~drussell/. I will only explain the very basics here, because anything more will be way beyond the scope of this article. I will, however, use some of the animated GIF that I copied from the same website.
The first thing you need to know, is that sound waves (or any waves for that matter) can add up. That's simple to understand: two guitars sound louder than one. In the animation below you can see how this happens. Two waves, a large one and a small one, move left and right. Whenever they meet, the add up and create one single, huge wave.
Addition of two separate waves
When two sound waves meet, interesting things can happen. This is illustrated in the animation below. You see two waves on top, one moving to the left and one moving to the right. These waves could be waves of any kind, in water, in the air, whatever. When the waves meet, they add up as illustrated earlier. The blue wave at the bottom sows you what the net effect is when this happens. So the blue wave is simply the addition two waves on top.
Standing waves
Have a look at the right hand dot. You could imagine this is a cork floating on the surface of a pond, when the two waves pass by in the water. The cork starts to bounce up and down, much stronger than when only one wave had been present. This is because the effect of the two waves add up.
But now look at the left dot. It is only at a slightly different position, and despite the fact that it exposed to the effect of the same two waves, it is standing still!
This all is the effect of wave interference, or, in this case, standing waves.
The fact that one of the dots is standing still (or the cork is not moving) can be explained when you recognize that when one of the two waves tries to push it up, the other wave pulls it down by the same amount. And because the two waves have the same shape and are moving at the same speed, this will continue to happen all the time. The other cork, however, thanks to its different position, will always have the two waves work together. When one waves pushes it up, so does the other one. When one waves pulls it down, so does the other.
Standing Waves in the Recording Studio
Now, the cork on the pond will behave identically to the membrane of your microphone. And in the studio, the two waves will be made by the sound wave coming from your guitar, and waves reflected by the walls, floor, and ceiling.
In other words, in all rooms, including sound studios, sound wave interference occurs. When sound wave intereference occurs, the volume of the sound will vary greatly, depending on the position where you listen. In other words, if you play a note on your guitar, the recorded volume will vary depending on where you put your microphone.
Now this would not be too bad. All you would have to do, is put your microphone at a place where the volume is good, right? Unfortunately, this is not the case. Because other sound frequencies have other wave lengths, the interference effect will be different for every frequency. So not only does the volume depend on the position of the microphone, it also depends on the frequency that is recorded! The total effect depends on many things (room dimensions, the objects in the room, damping properties etc.) and is therefore quite complicated and almost impossible to predict.
Basically, what this means is that when you use a microphone to record a guitar in a room, it will sound as if you use an equalizer with many, many channels, with the sliders all over the place! The resulting sound is what we usually call the effect of 'room acoustics'.
Now you will understand why owners of sound studios spend a fortune on sound absorbing panels. These will reduce the reflected sound, hence, reduce wave interference problems. However, as mentioned earlier, the fundamental frequencies corresponding to the low E, A and even the D string of your guitar are still reflected quite well, even on highly absorbing sound panels. This means that in particular the bass strings will cause the largest problems. With any microphone setup, some frequencies will be amplified at the position of the microphone (= right hand dot), whereas other frequencies will be attenuated (= left dot). If you move the microphone or the guitar, just slightly, everything changes, and other frequencies are amplified or attenuated. It's a completely unpredictable process.
Now you will also understand why so often your in-house guitar recordings have these nasty 'boomy basses'. It just means that at the position of your microphone, that particular frequency is amplified, because the wave coming from your guitar and the reflected waves add up. If you reposition your microphone, the same thing will happen, but for another note. It probably won't make things much better, because microphone positioning cannot solve this problem. The wave interference is all over the room! An EQ could help, in theory. The problem is, however, that it requires a very skilled sound engineer and an equalizer with many, many channels (I would say at least 64). Moreover, the distortion of the different frequencies is so complex that it is impossible to completely correct for it by ear. And to make it even worse, the required equalization may vary over time, as you move your guitar, because this will change the sound interference patterns.
What can be done?
The acoustics of any room can be improved by proper acoustic treatment. In fact, may will tell you that rooms can be treated to the point of near perfection. However, as long as you don't have a clear image of what "perfection" means to you, an efficient approach is not straightforward. From another standpoint, rooms can add greatly to the recorded sound. Unfortunately, most rooms in most houses that most of us live in don't.
When I recorded my debut CD, Fingerstyle Guitar Solos, I ran into exactly the same problems I described before. Whatever the sound engineer did, I never liked the sound. It took me quite a while to find out what the problem was, and that's why I decided to write it down here, so that you can spend your time playing gutar instead.
My solution was quite drastic. It was a sort of "generalistic approach", which basically eliminated the acoustics of the studio (or any room for that matter) . First, I selected a clear reference for a "perfect sound". In my case, that was the sound of my guitar recorded in an open field. Next, I performed numerical simulations of the acoustic properties of the studio ( fortunately, I have a strong background in physics), to find those positions where the problems with interference patterns were minimal. In retrospect, this step may have been optional, but I think it did help nevertheless. Then, I first recorded each song while sitting in an open field, using a portable, battery-powered digital recorder. The open field recordings really had a great sound, but were ruined with the chirping of tweeting birds, the wind, flies landing on the microphone and an airplane or two. Besides that, the sound was fantastic! In the studio, we cut out all those disturbed sections, and used the remaining part to create a frequency spectrum of the recording, as shown below.
 Frequency spectrum of audio recording
When we made a spectrum of a studio recording of the same song, the differences became clear immediately. Concentrating on the differences between the two spectra, we then used a 64 channel digital equalizer to shape the spectrum of the studio recording to make it similar to the spectrum of the open field recording. This resulted in a dramatic improvement!
Eventually, I took it one step further, and created a computer program with some elctronic equipment that would modify the studio spectrum into the shape of an open field spectrum (or whatever reference spectrum) during the recording itself. So this was a sort of automated pre-recording approach. The advantage of this approach is that strong amplification or reduction of certain frequencies afterwards may sometimes create additional noise and other funny sounds. And strong equalization was sometimes necessary! The disadvantage was that each song had to be recorded twice: once in an open field, and once in the studio! In the meantime, I could not change the guitar or the strings.
The studio setup was roughly like this:
 Audio recording with computer controlled pre-recording EQ
So whatever the micropone recorded, even if there were standing waves, booming basses and other nasty things, a PC connected to a programmable equalizer shaped the sound spectrum of the recording similar to the open field recording, even before the music arrived at the digital recorder! The resulting recorded sound was very natural and balanced.
Then, at the very end, the recordings were mastered. A little reverberation was added using a high-end unit (Lecixon 960L-S), and equalization was performed. In fact, this step added - a simulation of - the room acoustics (reverberation and frequency filtering). You can hear the results on the short sound samples here.
So, what can you do?
The approach described above is high-tech, and may not be feasible to everyone. The following tips, however, are based on the same principles as described above, and may improve your own acoustic guitar recordings a lot.
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