Microphones: Frequency Response

by Pablo Bellinghausen

Even though professional studio and stage mics are compatible with most mic inputs in mixers and audio interfaces, there are hundreds of different models, each with their own sound, and it’s quite hard to know how they will behave without actually trying one. For this reason, most mic brands will provide a data sheet with technical specs, either inside the box or as a free download on their website. Most of the time they will supply a few diagrams, the most common one being for the product’s frequency response.

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Reading the Graphs

Although they may seem daunting at first, frequency response charts are the best way to get a sense of the tonality of the microphone, and are really a pretty basic concept:

Frequency-Response-Explanation

As can be seen in the picture above, the horizontal axis is the pitch of the sound, going from the lowest pitch we can hear (about 20 Hz) to the highest one (20,000 Hz). This will vary slightly between individuals but it’s a good starting point. The fundamental notes of a piano will contain a good portion of that range.

The vertical axis is the loudness of the sound, in a relative scale called the decibel. This closely matches our perception of loudness, so that an increase of 6 dB roughly equals twice the perceived volume.

Every microphone will naturally pick up certain frequencies better than others, which will sound louder than the rest. By looking at its loudness versus frequency curve, we can figure out to which frequencies the microphone is more sensitive.

Large, smooth bumps or grooves in said curve are normally there by design and, like an inbuilt tone control, they can be used to complement (and compliment!) the original sound.

NT1-A-Freq-Response

For example, we can see by the official frequency response graph of the RØDE NT1-A above that ithas a small boost around the 12 kHz area, which gives an impression of extra detail and “air” to the sound. In contrast, narrow spikes in the frequency response are resonances somewhere inside the mic and when too large they can make the sound harsh, indistinct, or metallic.

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Published vs. Real-World Measurements

As can be seen in the below pictures, the Sennheiser e 614’s published front frequency response (in red) closely matches a third-party measurement; there are some small spikes but it’s otherwise quite close. This is to be expected from a top brand product.

Sennheiser-e614-OfficialFRSennheiser-e614-MeasuredFR

The e 614 is a supercardioid microphone, which means it rejects a majority of the sound coming from the sides and back (more on that on our next instalment). It is however practically impossible to have a smooth frequency response from the back of a directional microphone. In practice, the more effective the rejection is, the more jagged the frequency response at the back will be.

The measured back frequency response (in green) shows much larger spikes, and a much bigger variation from the official one. This is actually a pretty good measurement for this kind of model, but it clearly shows the practical limitations of directional mics. This is one of the many reasons as to why both microphone quality and placement are so important. It also shows one of the marketing tricks most manufacturers will use, which is to smooth out the frequency response so as not to show any unsightly spikes.

It is a fact of life that taming those resonant peaks requires a lot of research and development, as well as very high manufacturing standards, all of which drives up the cost of the product significantly. Budget microphones will tend to have a more jagged response at all angles, no matter what their published graphs say; whenever a frequency graph looks suspiciously even (particularly at the cheaper end of the scale), chances are it’s been smoothed out considerably.

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Manufacturing Tolerances

Another good reason to buy higher-quality microphones, and to use “matched pairs” when recording stereo signals, is that the frequency response will vary from one mic to the other, even for the same model. The below image shows the response graph of 85 Behringer ECM8000 measurement mics, supposedly almost ruler-flat:Behringer-Measurement-Mic-Official

Behringer-Measurement-Mic-02

Part of the production process of higher-quality manufacturers is to discard microphone capsules that have a frequency response outside a set tolerance (for the best brands that can be just a couple of dB throughout), but many companies will skip that step for cost reasons. Some premium models will even come with a personalised frequency measurement in the box which, although often smoothed out, is always a great addition.

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Additional Notes

It is important to point out that an uneven frequency response isn’t necessarily a bad thing; it is one of the things that impart tone or “character” to a recording, and when done right it can bring a pleasant quality to it; many classic models are cherished precisely because of this. Like carefully blended spices on a good meal, a coloured frequency response can enhance an otherwise dull sound; it won’t sound as realistic, but in many artistic applications, subjective quality is even more important than objective precision. To design a nice-sounding colouration and then impart it consistently across a production line is however far from easy.

The fact remains that although it’s hard to gauge the real sound of a microphone from its frequency response graphs, these remain a very useful guide when purchasing and comparing different models. Next time we will be looking at microphone directionality and polar patterns, which is the microphone’s frequency response at different angles. Until then, happy recording!

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Graphs and other data measurements have been faithfully redrawn based on various sources from manufacturers or online/print publications.
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