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The capture of sound refers to methods of recording audio. The first part of this topic covers gain structure and how to capture a clean signal. The second part of this page covers microphones characteristics and suitability. The last section covers microphone techniques.
Specification 👇
Specification 👇
Component 1 Specification
Component 1 Specification
Component 2 Specification
Component 2 Specification
Component 3 Specification
Component 3 Specification
Component 4 Specification
Component 4 Specification
Component 4 Specification Part 2
Component 4 Specification Part 2
Gain Structure
Gain Structure
Edexcel defines gain as “the amount of boost applied to the preamp stage of an audio channel.”
In other words, gain is the increase of power or amplitude of the signal between the input to the output port. Gain is measured in decibels (dB).
Gain staging is the process of setting the gain levels at different points in your signal chain .
Each processor in your signal path will have its own output level which sets the input gain for the next processor in the chain. It is important to check that the input and output levels of a device are at a suitable level when chaining multiple hardware devices together. A 'suitable level' is a level that is low enough that it's not clipping and high enough to that it's possible to work with.
A problem with signal chains is that with every processor that you add to the path, more noise is added to your final output signal. To add to this, the higher you set your gain, the louder your signal will be but also the louder any noise will be.
When gain staging, the main focus should be on maximising the signal-to-noise ratio.
Signal-to-noise ratio is “the difference between the audio that is being captured and noise captured simultaneously.”
Maximising the signal-to-noise ratio means ensuring that the audio that you are intending to capture is louder than any noise that is also captured. There is more information on this topic on the Levels page.
There are three different types of noise that Edexcel require you to know about; clipping, interference and hiss.
Clipping
Clipping
Clipping is the overloading of a signal, so that the top of the waveform becomes squared or is cut off. It can lead to your audio sounding distorted and it is most commonly caused by setting the gain too high at the preamp stage. This can be avoided by lowering the gain.
Interference
Interference
Interference is the destructive combining of sound waves, resulting in sound levels increasing or being cut at certain points, due to phase difference. It can result in nasty crackling or buzzing sounds. It is most commonly caused by the cables connecting your equipment acting as an antenna and picking up nearby radio waves. These can be from computers, radio stations, fluorescent lights, power lines etc. It is always worse when using unbalanced cables, so it is preferable to use balanced cables. There is more information on why this on this leads and signals page.
Hiss
Hiss
Hiss is a broadband noise that spans the entire audible spectrum. Due to this fact, it is impossible to completely remove hiss with EQ. It sounds like how you would expect – like someone hissing or white noise.
It is often caused by electronic devices themselves referred to as self noise or noise floor. Having the gain high can be a reason for the hiss being more prominent, but sometimes this is unavoidable.
Higher end equipment will have lower noise floors, meaning you won’t be able to hear the hiss quite so much. However, when higher end equipment isn’t available, there are ways of dealing with hiss. It is quite common to use plugins that take a sample of the natural room tone and noise floor, and then play it back out of phase with itself so that the sound is cancelled out and you are left with just your clean signal.
Preamp Controls
Preamp Controls
On a preamp, there are a range of controls. Some can help with signal-to-noise ratio and gain staging, others help with making your signal path work.
Not sure what a preamp is? Click here and go to the hardware section.
Phantom power is required to power some devices, such as condenser microphones and DI boxes. By having the phantom power ‘on’ on your preamp, you are sending 48v of power to any device connected as an input. Be careful not to send it to any device that doesn’t need it!
Gain, as described above, refers to the power of a signal. At the preamp stage, gain is used to boost a signal to an operable level, or line level.
Pad is “a switch on equipment that attenuates the gain by a set about to prevent clipping.” If the pad switch is engaged on your preamp, the gain will be reduced by a set amount of decibels (dB), commonly -10dB or -20dB.
A high pass filter is “a type of filter that removes only bass frequencies below a set point (cut-off frequency) and allows high frequencies through unaffected.” More information on filters and frequencies can be found here (EQ page).
Polarity is similar to a 180° phase shift, resulting in phase cancellation, but the waveform starts at its original position. It can be helpful to have a polarity switch on a preamp when recording one sound source with multiple microphones. As the microphones are pointing at the sound source from different angles, they will be out of phase with one another. If you switch the polarity on one input, it can minimise the effects of phasing and result in a thicker sound.
Clip/activity LEDs are a visual representation of the signal level input. They are included on preamps so that you can ensure that you are getting a high enough input level, but also that it’s not clipping. Green lights show a ‘healthy’ signal, yellow lights mean a ‘strong’ signal, red lights mean clipping.
Microphones
Microphones
Types and how they work
Types and how they work
The three types of microphones that you need to know about are dynamic, condenser and ribbon microphones. Below is a breakdown of each type...
Dynamic Microphones
Dynamic Microphones
All microphones use diaphragms to capture the movement of sound waves produced by sound sources. When sound waves hit the diaphragm, it vibrates in response. In a dynamic microphone, this causes a metal coil inside to move backwards and forwards around a magnet. This induces an electrical current in the coil due to the magnet field produced by the magnet. This process is known as electromagnetic induction.
Pros and Cons
Pros and Cons
✔️Good for recording louder sounds/instruments, such as drums or brass instruments. Can handle high SPL (sound pressure levels).
✔️Pretty cheap.
✔️Very robust so are used in live situations.
✔️Don’t require any external power.
✖️Not very sensitive so requires a higher level of gain.
✖️Limited polar patterns (generally only cardioid)
✖️Frequency response tends to drop off at about 16kHz.
✖️Poor transient response, making the recording less accurate.
Examples: Shure SM57, Shure SM58, Audio Technica ATM63
Condenser Microphones
Condenser Microphones
In a condenser mic, there is a diaphragm (front plate) and a backplate, both of which are charged. When the front plate moves, the gap between the front and back plate changes, creating an electrical signal. This phenomenon is known as capacitance. The voltage between the plates is supplied by either a battery in the mic or external phantom power.
Pros and Cons
Pros and Cons
✔️Good for recording quieter sounds. Doesn’t have a high SPL tolerance, so better on voices, string instruments and wind instruments.
✔️Very sensitive.
✔️Can do any polar pattern, and most come with an option to flick between multiple.
✔️Much more accurate frequency response than dynamic mics. Can pick up frequencies higher that human hearing.
✔️Good transient response so recording is more accurate than dynamic mics.
✖️Can get expensive.
✖️Not very robust and can break easily.
✖️Requires 48v of phantom power.
Examples: Neuman U87, Audio Technica AT2020, Rode NT1, Behringer C-1
Ribbon Microphones
Ribbon Microphones
Unlike dynamic and condenser microphones, which capture sound by responding to sound pressure level (SPL), ribbon mics work by reacting to the change in air velocity. Sound waves hit the thin, corrugated strip of aluminium that serves as a diaphragm. The magnets either side have different poles (one south and the other north) so when the ribbon vibrates, it creates a voltage that corresponds to the changes in the air velocity of the sound source. In older ribbon mics, the signal has to be passed through a step-up transformer to boost the signal to a usable level.
Pros and Cons
Pros and Cons
✔️Good for recording vocals, brass and guitar amps. Sometimes used as guitar overheads.
✔️Don’t need phantom power.
✔️Can do a wide range of polar patterns due to natural bidirectional pick-up.
✔️Natural frequency response, similar to the ear.
✔️Very accurate transient response.
✖️Not very sensitive.
✖️Very expensive.
✖️Not robust at all.
Examples: Royer R-121, Coles 4038, Beyerdynamic M 160
Polar Patterns
Polar Patterns
Polar patterns are visual representations of the pick-up range of a microphone. The diagrams on the left are looking at a microphone from a birds-eye view and the coloured in areas around show the polar patterns/where the sound is being picked up. There are four polar patterns that you need to know; cardioid, hypercardioid, bi-directional and omnidirectional.
The cardioid polar pattern captures sound from the front of the microphone and a bit from the sides. An advantage of this polar pattern is that it is directional, which can help to ensure that only the sound that you want to capture is recorded, as extraneous background noises are more likely to be outside of the pick-up range. A disadvantage, however, is that there can be off-axis colouration, meaning that the frequency response at the sides of the pick-up range can be ‘duller’. See the angles section of this page for more info on this.
The hypercardioid polar pattern is similar to the cardioid polar pattern, but also captures some sound from the rear of the microphone. Much like the cardioid polar pattern, it is directional which can be good at focussing the capture of the microphone. A problem of this, however, is that it can create quite an unnatural sound, which is where the hypercardioid polar pattern with its extra pick-up area at the rear of the mic comes in. The rear pick-up area allows for some of the environments reflections to be captured, which can result in a more natural sounding recording. On the other hand, a disadvantage of the polar pattern is that, due to its directional nature, it required very specific, measured placement, which may be tricky when recording vocalists or musicians that move around a lot.
The bi-directional, or figure-of-8, polar pattern captures sound from both the front and the rear of the microphone equally. This polar pattern is quite often used to record duetting vocalists as it means that they can sing together at the same time and potentially capture a better performance. The harsh side rejection also minimises spill and extraneous noise from being captured too. However, with the wrong application, this polar pattern can add a lot of unwanted noise into your recording as it picks up equally from the rear as it does from the front!
The omnidirectional polar pattern captures sound in 360° resolution, ie. all around the microphone. A major advantage of this polar pattern is that it isn’t affected by the proximity effect. Conversely, while it can be really useful for capturing sound sources arranged in the round, it can also introduce a lot of potentially unwanted ambient noise, which can create an unfocussed recording.
Frequency and Transient Response
Frequency and Transient Response
The frequency response of a microphone is its ability to pick up and accurately capture certain frequencies.
When you buy a microphone you will probably come across a diagram like the one to the right. This is the frequency response of an SM58. As you can see, the microphone struggles to pick up frequencies above 10kHz, so you wouldn’t want to use this microphone on an instrument that has a high register if you wanted to get an accurate recording.
Transients are peaks in a waveform of an audio recording. Transient response is the ability of a microphone to react to these changes.
Frequency Response of a Shure SM58 Dynamic Microphone
Frequency Response of a Shure SM58 Dynamic Microphone
Microphone Sensitivity
Microphone Sensitivity
Microphone sensitivity refers to the sound pressure level , or SPL, that the microphone can handle.
A dynamic microphone can handle pretty high sound pressure levels, so it’s suitable for recording trombones or percussion instruments for example. This can be a down side, however, if you are trying to record something with a lower SPL as a higher level of gain will be required, which can reduce the signal-to-noise ratio.
Proximity Effect
Proximity Effect
As defined by Edexcel, proximity effect is “the change in bass response captured by a directional microphone depending on the distance of a sound source from the microphone.”
The closer the sound source is to the microphone, the more the low frequency response increases, making your recording sound more “bassy.”
Proximity effect is only a problem with directional microphones and polar patterns, meaning that microphones with omnidirectional polar patterns do not suffer from this problem.
Microphone Switches and Accessories
Microphone Switches and Accessories
Switches
Switches
The pad switch, as mentioned above “attenuates the gain by a set amount to prevent clipping.” It reduces the sensitivity of the mic so you should only use this when recording very loud sounds/instruments.
Again, as mentioned above, a high pass filter (HPF) is “a type of filter that removes only bass frequencies below a certain point (cut-off frequency).” You might use this on your microphone if you can hear a low-frequency rumble in the signal when monitoring. Sounds like this can come from vibrations travelling through an unsupported floor or through the mic stand.
The function of a polar pattern switch is pretty self explanatory. It allows you to switch between different polar patterns that your microphone might have.
Accessories
Accessories
A pop shield/filter is a device used to prevent plosives ("‘P’, ‘B’, or ‘D’ sounds that cause low frequency pop sound due to strong air movement"). It's made of a kind of mesh material that dissipates the sound and it’s placed between the performer and the microphone.
A suspension cradle/shock mount is “a device that suspends a microphone while on a stand to reduce noise from vibrations and other physical movements.” It is often used in conjunction with a HPF to reduce low rumbles.
Managing Spill
Managing Spill
Spill is “signal that is captured in a microphone unintentionally, usually from another sound source being captured simultaneously."
When recording more than one sound source at a time, like a band for example, microphones can pick up sounds from instruments other than the one that they’re intended to capture. This can be managed however with the use of baffle boards and other acoustic treatment. Placing an absorbent material, such as a baffle board, between two sound sources can reduce the amount of spill. Isolation booths can also help.
Spill can also occur when recording to a backing track, or overdubbing. When recording vocalists, who will almost definitely be listening to a guide track or something similar in their headphones, its important that closed-back headphones are used. If not, sound from the headphones might be audible from outside and therefore may be captured by the microphone.
Microphone Techniques
Microphone Techniques
Microphone Placement
Microphone Placement
Distance
Distance
When it comes to distance, you can either do an ambient recording or a close mic recording.
The ambient option (placing the mic relatively far away from the sound source) will result in a more natural sound because you are capturing the room’s natural reflections as well as the main sound. However, it may be harder to control noise that you don’t want in your recording.
When you close mic, the microphone is placed at close proximity to the sound source. The sound may not sound quite as natural, due to the proximity effect, but you can control unwanted noise more easily.
Angles
Angles
When it comes to angles, you can either place the microphone on-axis or off-axis.
On-axis is when the microphone is positioned directly in front of the sound source, commonly at an angle of 90°.
Off-axis is when the microphone is not positioned directly in front of the sound source, commonly at an angle of less than 90°.
If the microphone is on axis, the sound waves hit the microphone diaphragm evenly.
If it’s off axis, the sound waves hit the diaphragm’s leading edge first and then travels across the diaphragm. As a rule of thumb, an off-axis mic placement results in a lower overall amplitude and worse high end frequency response when compared to an on-axis microphone placement. This means that the sound captured will be duller.
Microphone Configurations
Microphone Configurations
Mono
Mono
A mono microphone configuration requires just one microphone, as it is recording only one channel of audio.
A benefit of a mono recording is that you can record a more focussed sound. There is also no problem with phasing and you can really hone in on what part of the sound of a sound source you want to capture. A drawback however is that you don’t get any width information.
Stereo: Coincident Pair, Spaced Pair and Mid-side Pair
Stereo: Coincident Pair, Spaced Pair and Mid-side Pair
All stereo microphone configurations require two microphones as you are recording two channels of audio.
The coincident pair configuration, also commonly referred to as XY, involves two microphones with a cardioid polar patterns. They are placed so that the capsules are almost touching at an angle of between 90° – 135°.
✔️Phasing isn’t an issue as the sound arrives at both microphones. at the same time, due to them being placed close together.
✖️Stereo image isn’t that wide.
Coincident Pair
Coincident Pair
The spaced pair configuration involves two microphones, one panned left and one panned right. It is common to use either an omnidirectional polar pattern or cardioid. The distance between the microphones can vary massively (10cm to 10m) depending on what you’re recording (a guitar, drum kit or a full orchestra), where you’re recording (small recording studio or large concert hall) and what sound you want (intimate feel or ambient).
✔️Creates a wide stereo image.
✖️Phasing can be a problem due to the microphones being far apart from each other, therefore meaning that sound will arrive at them at different times.
Spaced Pair
Spaced Pair
Mid-side pair consists of two microphones in a similar configuration to coincident pair (at 90° to each other). However, only the mid mic, with a cardioid polar pattern, is on-axis to the sound source; the side mic is off-axis with a figure of 8 polar pattern. In this configuration, you end up with two channels (mid and side).
Mid-side Pair
Mid-side Pair
To hear the full stereo image, the signal from the side mic is duplicated and the copy is given its own separate channel and is phase inverted. Then the original signal is panned hard left and the copy is panned hard right. Changing the volume of the side tracks will determine the width of the stereo field.
✔️Phasing is never an issue as the microphone capsules are close together.
✖️Creates a wide stereo image.
Multi-mic Techniques
Multi-mic Techniques
Some instruments, such as drum kits, grand pianos and orchestras, may benefit from being recorded with more than two microphones.
It is very common to record drum kits in a multi-mic array. You may, for example, have two overhead stereo microphones, two microphones on the snare (one above and one below), a microphone on each of the toms and a microphone in or just outside of the kick drum.
✔️An advantage of using multi-mic techniques is that you can capture a more detailed recording as you can focus in on what parts of the sound of the sound source you want to capture. For instance, if the drum part that you’re recording is quite snare heavy, you can focus attention of the sound to that part of the kit a bit more.
✖️A disadvantage, on the other hand, is that phasing can be a problem. The sound source will arrive at each microphone at different times meaning that the sound waves are likely to be out of phase with one another. This can result in a thin sound unless fixed at the mixing stage.
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