Acoustics: Capturing Sound

Reverberation is caused by the direct signal reflecting off surfaces.

Fletcher Munson Curves

Loudness, being a psychological quantity, is difficult to measure, so Fletcher Munson averaged their results over many test subjects to derive reasonable averages.

The lowest equal-loudness contour represents the quietest audible tone and is also known as the absolute threshold of hearing. The highest contour is the threshold of pain.

Equal-loudness contours were first measured by Fletcher and Munson using headphones (1933).

In their study, listeners were presented with pure tones at various frequencies and over 10 dB increments in stimulus intensity.

For each frequency and intensity, the listener was also presented with a reference tone at 1000 Hz. The reference tone was adjusted until it was perceived to be of the same loudness as the test tone.

Dynamic Vs Condenser

The two main types of microphones are Dynamic and Condenser. They are both used for studio applications but have different characteristics studio engineers should be aware of before recording.

AKG D112

Dynamic Microphones

- Robust

- Used for live applications

- Low Frequency response

- Usually Cardioid Polar Pattern

- Does not require external power source

- Less sensitive to low level sound

- Handles high sound pressure levels

Condenser Microphones

- Used for studio applications

- Sensitive to high frequencies

- Requires external power source: Phantom Power

- Sensitive to low level frequencies

- Easily damaged if dropped

Cross-Section of a Condenser Microphone

Condenser microphones use a pair of charged metal plates, one fixed and one movable (diaphragm), forming a capacitor.

When a sound wave hits the diaphragm, the distance between the two plates changes.

This produces a change in an electrical characteristic called capacitance.

It is the variation of the spacing, due to the motion of the diaphragm relative to the back plate, which produces the electrical signal corresponding to the sound picked up.

In a dynamic microphone, sound waves hit a thin metallic diaphragm that is attached to a coil of wire.

The diaphragm vibrates the coil in response to the sound wave. A magnet that is positioned inside the coil produces a magnetic field.

It is the motion of the coil in the magnetic field which generates the electrical signal. The principles are the same as those used to generate electricity.

Part of a microphones specification includes its Polar Pattern

Polar patterns indicate the area around the diaphragm where sound is most sensitive

Some frequencies within the diaphragms range are more sensitive than others.

Studio engineers should also be aware of the Proximity Effect whereby there is an increase of low frequency the closer the sound source.

Another technical specification of a microphone will be its frequency response. Some microphones will be more sensitive to certain

frequencies than others and it is this factor that determines which microphone to use for certain applications.

The frequency response of the in-built microphone of different makes of iPhones. The iPhone 3G has the best low frequency response.

Frequency response for the AKG D112. This microphone is commonly used for kick drums and low frequency instruments.

Frequency response for the Neumann U47. This microphone is commonly used for recording vocals.

XLR Cables are the most common microphone cable. They are Balanced, which allows them to carry Phantom Power.

Balanced and Unbalanced Jack plugs.

Often they are called TS (tip/ring) or TRS (tip/ring/sleeve) plugs.

Female and Male ends to XLR cables.

FURTHER READING Modern Recording Techniques 7th Edition

How Music Works by John Powell

Sound on Sound (magazine/website)

Music Tech magazine

TeachMeAudio.com

recordproduction.com