Conflictos ecológico-distributivos

Example 3.14: A kick drum recorded with a general-purpose dynamic mic.

Example 3.15: The same kick drum recorded with a low frequency instrument dynamic mic.

3.16 SENSITIVITY

Microphone sensitivity is a measure of a mic’s electrical output, when it is placed in a sound field of a specific SPL, usually 94 dB SPL. Most manufacturers quote sensitivity as the number of millivolts the mic produces for this reference level (which can be expressed as mV/Pa, or mV per 10 µbars). When sensitivity is expressed in one of these formats, higher numbers are usually more desirable – the higher the quoted number, the less pre-amp gain will be required to bring the mic’s signal up to line level, minimizing any noise potentially introduced by lower quality pre-amps. A mic with a spec of 29.8 mV/Pa has a higher output than a mic with a spec of 5.6 mV per 10 µbars.

Some manufacturers express sensitivity in a “−dB” format, and others in the older level standard of a 74 dB SPL input level, 1 µbar, or 0.1 Pascal – those three units are identical. The 94 dB equivalent is expressed as dB re 1V/Pa, and the 74 dB equivalent as dB re 1V/µbar.

Expressed these ways, microphones with greater electrical outputs will have smaller negative numbers – so for recording very quiet sound sources, a mic with a rating of −31 dB re 1V/Pa would be preferable to one with a rating of −45 dB re 1V/Pa.

To compare two mics, when one quotes sensitivity as “−dB” to a 94 dB equivalent, and the other as “−dB” to a 74 dB equivalent reference level:

94 dB reference equivalent = 74 dB equivalent – 20 dB

74 dB reference equivalent = 94 dB equivalent + 20 dB

3.17 SELF-NOISE AND EQUIVALENT NOISE RATING (ENR)

Self-noise and ENR are the same thing – a measure of the background noise (hiss and rumble) that a mic makes itself. Specifically, it is a measure of the sound pressure level required to create an output level the same as the noise the mic outputs when it is surrounded by silence.

Self-noise is usually quoted as a dBA figure. Good quality modern mic pre-amps and digital recording systems can be very clean and quiet, and are able to resolve the smallest sonic details, so self-noise can be very important – particularly if quiet sounds are being recorded, because the self-noise correlates to the lowest extreme of a mic’s dynamic range.

Self-noise is only quoted for condenser microphones, where it is a measure of the noise created by the head-amp in the mic itself. This noise is usually greater than the noise intro-duced by high quality console or outboard pre-amps, and can be audible when the gain is turned up high to record quiet sound sources. Most of the noise in dynamic and ribbon mics is created by the electrons moving within the coil or ribbon. Because the output voltage of these mics is much lower and more gain is needed on the pre-amp or console, the noise generated by the pre-amp is louder than the self-noise of the mic itself. This makes self-noise irrelevant, but makes having good quality, quiet, high gain pre-amps very important.

Self-noise figures can be as low as 5 dBA for modern large diaphragm condenser mics.

Small diaphragm mics tend to be a little noisier, at 12 to 18 dBA. Very small diaphragm mics are usually noisier still, with specs between 22 and 27 dBA. Self-noise in the upper 20s can definitely cause problems when recording quiet sources, or recording from large distances.

If you know that you are going to record quiet sources, or do classical orchestral or choral recording, low self-noise figures are important, and mics with lower self-noise figures are definitely preferred. But for most rock, pop, and loud instrument recording, self-noise figures are less important than the actual sound of the mic. Using noisier mics as drum overheads for example, would present no problem – not a lot of gain is required due to the high SPL of the sound source, so the self-noise would be insignificant.

3.18 SIGNAL-TO-NOISE RATIO

Directly connected to self-noise, the S/N ratio, or SNR of a microphone is a measure of the amount of signal output, compared to the level of mic-induced noise present, when the mic is in a 94 dB SPL sound field. Higher numbers indicate better performance.

The following equations are useful to convert between self-noise and SNR, if you are trying to compare a mic that has only a self-noise spec with a mic that has only an SNR spec:

SELF-NOISE = 94 – SNR

SNR = 94 – SELF-NOISE

3.19 MAXIMUM SPL

If a mic is overloaded by being put in front of a sound source that is too loud, it will distort.

Maximum SPL is a measure of the SPL that produces a slightly distorted output from the mic – usually quoted for 0.5 percent THD or 1 percent THD (total harmonic distortion). Although it is dangerous to listen to sounds above 100 dB SPL for any length of time, and a drummer may sit in a sound field of greater than 110 dB SPL when playing, the actual SPL a couple inches from a drum can be in excess of 130 dB SPL! So high SPL capable mics are definitely necessary!

Condenser mics often have pads (labeled “−10 dB,” “−20 dB,” etc.), which attenuate the mic’s sensitivity so they can be exposed to louder sound sources and not overload their internal electronics or the pre-amp they are plugged into. Maximum SPL figures are quoted for all pads on the mic engaged.

Some pre-amps and mixing consoles have pad buttons, but with a mixer or pre-amp pad it is still possible to overload the microphone itself. If you are recording a very loud singer (or other sound source) with a condenser mic and are hearing distortion, but all your pre-amp, input and output levels look good – it’s probably the mic distorting. Engaging the mic’s pad should fix the situation – as long as the sound source is not in excess of the mic’s maximum SPL level.

You should be very cautious when using ribbon mics in loud sound fields, or close to sound sources you don’t think are too loud – an SPL louder than the recommended maximum can destroy the ribbon by stretching or tearing it.

3.20 DYNAMIC RANGE

A microphone’s dynamic range is the span of sound pressure levels over which it will effec-tively pick up sound. It is the difference between sound being buried and masked by the mic’s self-noise, or the diaphragm’s inability to resolve low level sonic impulses, and the onset of distortion at the mic’s maximum SPL. Greater dynamic range specs are usually desirable.

sient details include percussion instruments (drums and pianos) and plucked string instruments (particularly acoustic guitars). To record these sounds with the most accuracy, transparency and “zing,” a mic with good transient response is desirable. The diaphragm of the mic needs to be able to move fast and change direction easily to respond to positive and negative transient peaks, and the subtle nuances contained within them.

Dynamic mics, with their heavier, more sluggish diaphragms, cannot respond fast enough to accurately capture initial transients, so the attacks of sounds become compressed and subtly distorted. In terms of accuracy this is undesirable, but compressing the amplitude of the initial and loudest peaks of a sound means that pre-amp gain needs to be turned up more to achieve good signal levels – increasing the sound’s average level. This results in a less transparent, but bigger, phatter, and more powerful sound that can be desirable for rock drums, electric guitars, and basses.

Manufacturers making condenser mics with ultra-small diaphragms usually do so because their design philosophy emphasizes the extended high frequency response and improved transient response that is only possible with ultra-small diaphragms. Many (regular) small diaphragm mics are respected for their transient response. There is no specification for tran-sient response though. It’s not as simple as “the smaller the diaphragm, the better the trantran-sient response” – the head-amp electronics in a mic also have an effect on transient response. Some manufacturers purposefully design and promote the “fast response” of their mic’s electronics.

If you are unable to audition a mic before purchasing it, the best way to judge its transient response is to read non-biased, reputable professional reviews.

Transient Response @

Example 3.16: Acoustic guitar, recorded with a small diaphragm condenser mic known for its good transient response.

Example 3.17: The same acoustic guitar, recorded with a small diaphragm condenser mic with less good transient response.

3.22 POP FILTERS, WINDSCREENS, AND DEAD CATS

Pop filters are essential when recording the human voice (singing or speaking) from within a couple of feet, with the talent aiming at the microphone, as shown in Figure 3.17.

Plosives are created when a burst of air associated with B, D, G, K, P, and T sounds leaves the mouth. If this air current hits a mic’s diaphragm, it can cause loud pops and distortions, and potentially damage the diaphragm. Studio condenser mics do not usually have protection from these stray air currents. A pop filter is an acoustically transparent material or perforated metal screen positioned between the talent and the mic. It diffuses the plosive’s airflow, and prevents it from directly hitting the diaphragm. In order to work properly, a pop filter needs several inches distance between it and the talent, and it and the mic.

Most musical instruments do not propagate plosive air currents, so pop filters are unneces-sary. If a studio vocal mic is positioned at least 6 inches above or below the singer’s mouth, and at least a foot away, making sure the singer does not sing “into” it but “across the top” of it, the plosives are directed away from the diaphragm and a pop filter may not be necessary.

Commercial pop filters with gooseneck mounts can be purchased relatively inexpensively.

Alternatively a pop filter can be made for minimum expense using pantyhose and a wire coat hanger.

WINDSCREENS

A windscreen is a foam cover that is put directly around a microphone’s capsule, as shown in Figure 3.18. In addition to the boom of plosives, loud low frequency rumbles are produced when a wind current flows over or into a mic’s diaphragm. Depending upon their severity, a windscreen will reduce or eliminate these artifacts. Windscreens should be used when a mic is unavoidably positioned in the air stream from an HVAC duct, or when using mics outdoors where wind is unavoidable.

Fi g u r e 3 .17 A correctly positioned pop filter, with a few inches of distance between the singer and the filter, and the filter and the microphone.

Windscreens do affect a mic’s high frequency response, so should only be used when necessary in the studio. When recording singers in a recording studio, pop filters are preferred over windscreens, because they are more acoustically transparent.

WIND MUFFS/DEAD CATS

Wind muffs, or dead cats, are used in high wind conditions in the motion picture, television, and broadcasting industries. They are commonly seen on the end of mic booms and fishpoles.

A highly directional condenser mic is mounted inside the muff, sometimes in its own foam windscreen within the muff. The muff is made of artificial fur, and designed to minimize wind noise but remain acoustically transparent.

In the music production industry, about the only time wind muffs are used is on exposed condenser mics at outdoor events in windy conditions – not in studio recording situations.

3.23 SHOCK MOUNTS

Vibrations traveling through the floor (a singer’s foot movement, mechanical HVAC vibra-tions, trucks driving by, low frequency sound waves, etc.) can travel up microphone stands and into the mic where they manifest themselves as low frequency rumbles and booms. In a live recording or concert stage situation, a mic stand can get knocked or bumped, causing a loud boom. Shock mounts, as shown in Figure 3.19, are elastic suspension mounts that mechanically isolate the mic from the stand. Most vibrations traveling through the mic stand are absorbed by the elastic material, and do not reach the microphone. Shock mounts are generally specific to the microphone in use, and are essential for serious studio and location recording.

Fi g u r e 3 .18 A mic with and without a windscreen.