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Thomann's Cool Online Guides: Large Diaphragm Microphones

9. Technical Specifications

What technical specifications should I look out for?

1. Frequency Response

Manufacturers’ frequency response data, on the whole, is not very reliable. But published response curves or numerical data such as 20 – 20.000 Hz don’t say much about the actual sound of a microphone. Sound is subjective and not easy to translate into objective data. Trust you own ears or ask friends and colleagues about their experiences with a particular microphone.

2. Self-Noise

Self-noise, sometimes called “equivalent noise” or “equivalent sound pressure level” is a figure that gives you a pretty good idea about how much hiss a microphone produces in a typical situation. Self-noise, as the term suggests, is not something the microphone picks up but the noise floor it produces of its own as a by-product of its internal amplifier electronics.

Self-noise varies a great deal on different microphones and is arguably the most important point in a microphone data sheet. The lower the self-noise figure, the better your signal-to-noise ratio, the cleaner your recordings. Low self-noise is particularly important when recording low level sources such as quiet singers or nylon string guitars.

The best large diaphragm condensers produce one-digit self-noise figures. 5 dB-A or 9 dB-A doesn’t matter all that much, as even quiet rooms are noisier than that. Up until about 14 dB-A a microphone is usually perceived as “pretty damn quiet.” At about 20 dB-A you begin to hear some background hiss, and at about 23 dB-A that hiss is already pretty obvious, especially when miking quiet sources. Self-noise figures higher than 23 dB-A are only acceptable in non-critical situations such as recording a loud rock vocalist.

Note that the “A” behind the decibel figure is very important. It denotes that the measurement is “A-weighted”, i.e. the measurement focuses on those frequency ranges that the human ear is particularly sensitive to. A-weighted figures thus give a more realistic impression of a microphone’s noise level than unweighted ones and have become the usual way to represent noise performance. Some manufacturers give weighted and unweighted noise figures. Make sure you pick the weighted figures when comparing to other manufacturers’ figures. Unweighted figures are typically higher than weighted figures, so you can’t compare A-weighted figures with unweighted ones.

Interestingly enough, good tube condensers are hardly any noisier than good solid state (FET) microphones. A first class tube mic can be lower noise than a mediocre FET condenser.

3. Maximum Sound Pressure Level (SPL)

It’s a common myth that condenser mics are damaged when you exceed their max SPL figure. Don’t worry, that’s not true! Maximum sound pressure level is just the sound level at which the microphone begins to distort the signal audibly. 125 dB max SPL, for instance, means that the total harmonic distortion (THD) exceeds the 0.5% limit when the mic is exposed to a sound level of 125 dB-SPL. That’s all. Some manufacturers measure with a limit of 1% THD, which produces – seemingly – better figures for the same mic.

Max SPL figures are not quite as important as many people think. Modern condenser microphones can take a lot of SPL before they begin to distort. Almost any condenser mic can stand more SPL than you are willing to take, let alone your neighbours. And even if you do find a singer that can scream louder than 120-130 dB-SPL, you can simply engage the pad-switch, which will give you another 10-20 dB of headroom.

4. Sensitivity

No, sensitivity does not relate to how much physical abuse a mic can take ;-) Sensitivity means output level per sound pressure level. In other words how “loud” is the microphone, how “hot” its output level? If you placed different mics at the same distance from the same source, they’d produce different output levels. The hotter the output level, the less gain you need from your preamp. Condenser mics usually produce higher output than dynamic microphones. Sensitivity is non-critical for condenser mics; they’re “loud enough.” With any decent microphone preamp, sensitivity does not matter with condenser mics. (Sensitivity does matter with dynamic mics, though!)

Sensitivity are usually given in mV/Pa (millivolts per sound pressure). Some manufacturers prefer to give figures in decibels. In either case, higher figures mean hotter output – but beware of negative numbers: –35 dB is higher (and better) than –37 dB! Unfortunately there are several standards for measuring decibel figures, so you’re never quite sure if decibel figures are comparable or not. Better stick with mV/Pa figures. Those should always be comparable. As a rule of thumb: sensitivity above 8 mV/Pa means you’re safe.

5. Impedance

Rated impedance or output impedance is another non-critical figure. Modern microphones have impedances ranging from 50 to 600 ohms. That’s what is called “low impedance.” The exact figure doesn’t matter much.

(Rated) load impedance is something different: that’s the minimum input impedance your microphone preamp is supposed to have. The suggested load impedance is usually 1000 ohms. If your preamp’s input impedance is 2000 ohms, that’s even better. A preamp input impedance lower than 1000 ohms is not optimal – your mic will still work; but its actual performance may be a little worse than specified.

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Large Diaphragm Microphones overview


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