Topic 3: Microphones and Audio Recording

Introduction to Microphones:

Stereo Microphone Techniques

Before reading about microphones read this introduction to audio cables:

Introduction

The selection and placement of microphones when recording audio can have a major influence on the final sound quality of a recording, be it acoustic instrument recordings, voice-overs or audio effects. Choosing the wrong microphone and using the wrong microphone placement technique can have disastrous consequences for the quality of an audio recording. On the other hand, a skilled musician, playing a quality instrument, properly miked, makes for a smoother mixing process, and always results in higher quality audio recordings. This often means you will need fewer or no corrective audio effects to get your audio sounding “right”.

The final audio quality in all music depends on how well the audio was recorded. However, the sound quality at the recording stage is more important for certain genres of music than it is for others. Classical music recordings, for example, depend almost exclusively on the space they were recorded in, what type of microphone was used and on the microphone placement and technique used. It is, therefore, logical to say any serious recording musician, especially in modern times, should have at the very least a basic understanding of microphones and audio recording techniques.

This article will look at the basics of stereo audio recording, this includes: different microphone types, mono vs stereo and stereo miking techniques. Hopefully, the content in this article will help give the reader a better understanding of audio recording and microphones.

Microphone Types

A microphone is an instrument that works in many ways like the human ear that is a microphone converts sound (vibrations) into electrical energy which can then be amplified, transmitted or recorded. A microphone allows a sound engineer to record acoustic instruments into his or her computer via a mixing desk, audio interface and or computer. Before using a microphone, be sure to know its frequency response (which frequencies it picks up). This information can be easily found with a Google search. This is important because some microphones are better at picking up lower frequencies, while others are better at picking up higher ones. A good example is a kick drum mic which is well suited to kick and electric bass amp recordings, but not violin string sections.

There are generally three types of microphones: Dynamic, Condenser and Ribbon.

Ribbon microphones are harder to come by and almost exclusively used in high-end studios. Therefore, we will be sticking to dynamic and condenser microphones in this article.

Dynamic Microphones

Dynamic Microphones are durable and designed for live use and “close miking” This makes them good for recording loud instrument sounds like guitar amps, drums and even trumpet. The Shure SM57 is a classic example of a good and affordable dynamic microphone that is used by professional musicians around the world for recording drums, guitar amps and horns.

Image: Shure SM57 Dynamic Microphone

Condenser Microphones

Condenser microphones are capable of picking up a wider range of sound frequencies than dynamic microphones. They're more sensitive than dynamic microphones and because of this are mainly used in the recording studio. Condenser microphones can be used to record a wide variety of different sounds and instruments, but are more suitable for individual instrument recordings and vocals. They can also be used for stereo instrument recording and ambient miking. Condenser microphones are usually placed on a stand as they are quite sensitive.

Image: AKG C214 Condenser Microphone

Large Diaphragm vs Small Diaphragm

Once you begin recording and using microphones you will undoubtedly come across the terms “large diaphragm” and “small diaphragm”. But what do they mean? The diaphragm is simply the part inside the microphone that detects sound waves. When the diaphragm vibrates, it causes other components in the microphone to vibrate. These vibrations are converted into an electrical current which becomes the audio signal. Different manufacturers use different classifications and measurements systems to classify what is large diaphragm and what is small diaphragm, but these distinctions don’t matter too much.

Generally speaking when a microphone is described as having a small diaphragm it usually means that it has a lower frequency response and is less sensitive to high SPL (Sound Pressure Levels).

Large-diaphragm microphones, on the other hand, have a higher frequency response and are more sensitive to SPL, meaning they are better suited for room miking, recording studio vocals and recording “softer” instruments that have a wide dynamic range such as drum overheads, strings, choir groups, piano and so on.

Image : Small Diaphragm Condenser Microphone

Image: Large Diaphragm Condenser Microphone

Condenser Vs Dynamic Microphones

The biggest difference between condenser microphones and dynamic microphones is that condenser microphones need something called +48V Phantom Power to work. +48V Phantom Power is usually supplied by a power button/switch on the audio interface/mixer that the microphone is connected to. So if you want to record with condenser microphones, make sure your interface has a 48V option, otherwise you will not be able to record.

Polar Patterns

All microphones have what is called a ‘polar pattern’. The microphone’s polar pattern determines how the microphone will pick up the sound being recorded. Having an understanding of polar patterns will help you choose the appropriate microphone(s) for the job, allowing you to capture the sound you want while reducing the chance of capturing unwanted sound.

The most common polar patterns found on microphones are: cardioid, omnidirectional and bidirectional.

Cardioid Polar Pattern

Microphones with a cardioid (heart shaped) polar pattern will only pick up sounds that are directly in front of the microphone. This means they are very useful when there are many other sounds or instruments in the space you are recording in. Simply point the microphone at the source while making sure all unwanted sounds are behind the microphone.

Good For: Vocals, miking up drum kits, live performance, recording in a noisy or untreated room.

Drawbacks: A problem with cardioid mics is that they are susceptible to a phenomenon called the “proximity effect”. The proximity effect is caused by extreme close miking and results in a boost in bass frequencies of the sound source. The proximity effect is a common issue for inexperienced vocalists, especially in live music settings. To counter this try to record with the singer at least a couple inches away from the microphone.

Variations of the cardioid microphone include Super-Cardioid and Hyper Cardioid; each of these has a slightly altered pick up pattern, allowing the microphone to pick up slightly more sound from the back and sides.

Image:Cardioid Polar Pattern

Omnidirectional Polar Pattern

Microphones with omnidirectional (Omni) polar patterns pick up sound equally from all sides of the microphone. Microphones with this type of polar pattern can be used in studios or in environments where there isn't much background noise or where the acoustic quality of the room is pleasant.

Good For: Recording the sound of a room(ambient miking), when recording orchestra, choir or grand piano, recording a musician that moves a lot, also good for stereo pair recordings.

Image: Omnidirectional Polar Pattern

Bidirectional Polar Pattern

Microphones with a bidirectional (figure of eight) polar pattern pick up sound equally from the front and back of the microphone, while rejecting sound from the sides. Microphones with this type of polar pattern are good for stereo recording and for capturing two or more instruments at the same time.

Good For: Stereo recordings such as M/S and Blumlein Pair, isolating other instruments, recording dual instrumentalists/singers.

Take a look at the three main microphone polar patterns below, to see how their polar patterns differ from one another.

Image: Bidirectional Polar Pattern

Mono vs Stereo

Before we take a look at some stereo microphone techniques it is important for us to define what mono and stereo signals are.

Mono: In monaural sound one single channel is used. It can be reproduced through several speakers, but all speakers are still reproducing the same exact copy of the signal. In a microphone recording this means that only one microphone was used to record a single source. There are ways that we can create a “fake” stereo image using a mono signal, but that is a tutorial for another time. Most modern pop music is created using mono sound sources which are then used to create a “false” stereo image.

Stereo: A stereo recording is created by capturing sounds with two or more microphones of one source, taken from different locations. The mono signal from each source is then panned either left, right or centre to create a sense of width in the recording. When the left and right channel are combined it creates the “stereo effect”. The stereo effect can also be used to make sounds much wider than they could possibly be in a real world setting. However, traditionally speaking, stereo recordings aim to create a more natural and authentic musical experience.

There are two main ways that width is created in “true stereo” recordings:

1.) Difference in Timing: Placing two microphones in different locations while recording one source, results in the sounds reaching the microphones at slightly different times. This time difference (usually only by a couple of milliseconds) is what creates the stereo image.

2.) Difference in frequency balance: This stereo method is achieved by placing two or more microphones at the same point and facing them in different directions in relation to the instrument. This results in the microphones picking up different frequency balances, which results in what we hear as a stereo effect. The greater the angle, the more pronounced the stereo image will be.

Stereo Microphone Techniques

When modern music is recorded, engineers usually put a mic on every instrument to record it, then pan each individual sound left right or centre. We then hear where each instrument is coming from in a “false stereo" field as a whole. This method is sometimes appropriate, other times it is not.

Unlike mono recordings, a two-microphone “true stereo” recording captures the whole ensemble playing together in a shared space. This method of recording is often more desirable as it adds lifelike realism to a recording by capturing: the left to right position of each instrument, the distance/depth of each instrument, the ambience of the space recorded in and the timbres of the instruments as heard by the audience. Stereo recordings can therefore offer a truer musical experience, especially for classical music.

Stereo recordings work best when there is a reasonable amount of space between the instrument(s) and the microphones; getting too close defeats the purpose. But at the same time, don’t let that stop you from trying new things. Lastly, these are only three stereo recording techniques outlined here. There are many more in existence. Research them.

Before you go down the road of stereo recording its important to ask yourself if your recording will actually benefit from being recorded in stereo. While stereo miking a soloist playing classical guitar might makes' a lot of sense, there is not much point in stereo miking a saxophone in a full jazz band. So make sure there is a reason for using a stereo recording. Don’t do it just for the sake of it.

Below are some commonly used stereo microphone techniques.

A/B Spaced Pair Stereo Recording

The A/B spaced pair microphone technique is an example of a miking technique that uses a time delay between two signals to create a stereo image.

Microphones Used: Two paired cardioid, or omnidirectional microphones.

Position: Point both microphones at the instrument. It is often a good idea to capture the “different sounds” the instrument produces. So for a guitar, one microphone could point at the fretboard, while the other could be pointed at the sound hole or nut.

Mix: One signal is panned completely left and the other signal is panned completely right (you can play around with this) and the result should be a recording with a nice wide stereo image.

A drawback of the spaced pair technique is that when the recording is summed to mono, you may experience some phasing issues due to the fact that the two signals are slightly out of sync. To counter any overt phase you may need to use a 3:1 spacing ratio. This means that the second mic is placed three times the distance from the first mic, relative to how far the first mic is from the source.

Image: A/B Stereo Configuration

*Example: If the first mic is 1 foot from the source then the second mic should be 3 feet from the first mic.

X/Y Stereo Recording

The X/Y technique is an example of a miking technique that uses a difference in frequency balance between signals to create a stereo image.

Microphones Used: Two paired small diaphragm condenser microphones.

Position: Arrange the two microphones at a 90 degree angle, with both microphone capsules positioned at a single point. Arrange them so one is above the other, making sure they're as close as possible without actually touching. This should form something reminiscent of an X shape.

Mix: As per usual, pan one microphone signal completely left and the other completely right. You should get a stereo image, although it won’t be as wide a stereo image as the spaced pair.

Although this microphone technique won’t give you a stereo image as wide as the spaced pair technique because it uses a frequency imbalance to create a stereo image instead of time. It also means that you are less likely to have phase issues when your audio is summed to mono.

Image: X/Y Stereo Configuration

*Note: Widening the angle of the two microphones will create a wider stereo image.

M/S Stereo Recording

The Mid Side stereo recording technique, or M/S stereo recording technique, is another example of a stereo recording technique that uses a difference in frequency balance between signals to create a stereo image. This method is slightly more complicated, but it will give you more control over stereo width.

Microphones Used: One small diaphragm cardioid condenser microphone and one large diaphragm figure 8 condenser microphone.

Position: The figure 8 microphone is positioned sideways, so that it is at a 90 degree angle to the instruments(s). The figure 8 microphone will pick up sound on the sides. The small condenser microphone is placed above the figure 8 microphone (or below) and points directly at the sound source. This microphone serves as the middle pickup.

Pic 6

Mix: This part seems complicated, but it really isn’t. To create the M/S Stereo effect, simply:

Duplicate the channel recorded by the figure 8 microphone (side) on to a new and separate channel. Copy the audio onto the new channel.

Pan both side channels completely left and completely right.

Next, invert (polarise) the newly duplicated side audio channel, this will cancel out the two side channels, and help reduce phase.

Make sure that the two side channels volume levels are linked equally, so that if you move one the other moves too.

You should now have three audio channels: one middle, one panned completely left and one panned completely right.

Mix the middle channel with the sides to adjust width. The higher the volume level of the sides compared to the middle the greater the stereo width. Mix to taste.

This technique is a great stereo technique because it gives you more control over how much of a stereo image you can use, without having to re-record. You could even use automation to change your stereo spread as your song/composition progresses. Additionally, this method is mono-compatible, meaning you are less likely to encounter phase issues when the audio is summed to mono.

Image: M/S Microphone Configuration

Conclusion

Although stereo recording is a great tool for recording a variety of instruments in a vast array of settings, sometimes it is best to stick to mono. Certain instruments almost always sound better in mono than they do in stereo. This is especially true for lower register, individual instruments such as kick drums, bass instruments, snare drums and even vocals.

Keep in mind that there are many more industry-standard stereo recording techniques, the ones here are just to get you started. Through some basic reading and research, you can learn a lot more about microphones and audio recording techniques. Once you get comfortable with these techniques you can then begin experimenting with techniques of your own.

If you take anything away from this, it should be that microphone technique and placement is an art in itself, and art cannot be rushed or forced. Use what you need because it makes for a better recording, rather than because you think you have to use it. In addition to this, listen to the artist(s) playing, while taking a walk around the room, listen for the “sweet spot”, and place your microphones there. Experiment, take time to learn what different microphones can and can’t do. Match the instruments being recorded with a microphone that has an adequate frequency response. But most of all, try to have fun with it.