Case Study No.1: Microphones

The Sontronics Sigma VS The Neumann U87Ai

Hi Everyone!

This post is based on more of the technical side of miking up in such a way to minimise bleed. Which microphones, position and all the technical goodness! So let’s get to it!


A singer/guitarist would like to record their song as a live performance in the studio. Two microphones shall be used for this job. The desired outcome is to eliminate as much bleed as possible between the vocal and the guitar. Better quality of the separation will allow for many more options within the mixing process.

Polar pattern is a very important choice to make. One of the first decisions to make as it then helps determine which microphones you choose to use. Which Polar pattern is best for this job? Omni-directional, Cardioid or figure of eight?

Omni-directional captures all around it through all frequency bands, meaning bleed would be highly prominent in both microphones. So therefore Omni is not the way to go.

Cardioid, does reject sound when off axis. However, at certain frequency bands an example of this is that at low frequencies the polar pattern changes from Cardioid to Omni-directional, as evidenced by the illustration from Neumann website showing polar response over 125K.

u87ai cardioid omni


Therefore using a microphone set to a Cardioid polar pattern would not be viable in doing this job. As the resonance from the guitar and the human voice from the chest have the potential to effect the microphones polar pattern causing more bleed than we want.

The tight rejection characteristics of a figure of eight polar pattern make it the perfect candidate for avoiding bleed. These null points around the microphone can be used very specifically to minimise bleed, and furthermore this tight off axis rejection by the microphone is consistent over all frequencies. Not susceptible to lower frequency bleed like Cardioid is.

However you have to take into consideration that the back side of the figure of eight shall have bleed from reflections of the room. This can be made into a very useful tool, when used in right way to taste of the song that is being recorded.  Changing the tone of what is being recorded.

For instance for a brighter tone to the recording having the back of the microphone pointing to a hard wooden floor or a window forces reflections of high frequencies. Whereas having the back of the microphone looking at a woollen carpet, the carpet absorbs the high frequencies, getting a “warmer” or “duller” tone. In this case one is changing the tone by changing the high frequencies content through reflections.

Null point diagram IMG-20140512-00339

The picture above shows the position of the artist and the microphones. Showing how I achieved this technique, using two figure of eight microphones. As you can see the artist was sat down throughout the recording process, this was not only for the artist’s comfort. Having the artist sat down limits how much they move away from the microphones. In this situation, meaning that the artists would not moving in and out of the null points on each mic causing more bleed that we want.

Positioning the mics correctly is fundamental to this technique and getting minimal bleed. The null points of the microphones go all around the side, top and bottom of the mic. These are the points that you need to take advantage of. On the vocal mic positioning the null points towards the artists guitar, and in this instance you can see that bottom of the microphone is looking at the guitar.

The bottom microphone in the picture, was used to record the guitar. It is looking at twelve fret on the fret board of the guitar. Whilst the null points are facing out to the sides but also, the top null point of the mic is facing directly towards the artists mouth. This is very effective of reducing bleed. The drawn diagram of the guitarist shows the null points in red dotted lines. That is how the mics should be set up with null points facing the correct direction to minimise bleed.

The minimising of bleed, separating the microphones and NOT seeing this technique as a stereo pair! These are individual microphones suit to taste the material being recorded. The two mics of choice do not and preferably should not be the same. But why do we need this separation in our recording?

Having a minimal amount of bleed and a clear separation between the microphones and the recording means more than you think it would. It means that within the mixing process many more possibilities are available to you. You can EQ, compress or even add different reverb to each track!  You could compress the vocals one way then compress the acoustic guitar completely differently! As well as many more options during the mixing process, it also makes the process much easier on yourself. Say if there’s an issue with the guitar that issue will not be highly prominent in the vocal track because you’ve used this technique correctly, and also taking advantage of the microphones specifications correctly.

The weapons (microphones) of choice were the Neumann U87Ai and the Sontronics Sigma. The U87Ai is a legendary multi-pattern condenser microphone, and the Sigma is set figure of eight ribbon microphone.

These mics were chosen for this job as they complement each other within frequency response. Where the Neumann U87Ai has a boost of  approx ?dB at 4K – 7K, the Sigma starts to cut the highs from around 5K upwards, being 6db down after one octave (10K) thus showing a roll off slope of 6dB/oct. This 4K – 7K boost also adds clarity/definition of the human voice, for instance the consonants at the beginning of words. The “K’s”and the “S’s” etc. Without this the vocals would be very muffled and hard to understand, so using this to your advantage on the vocals is very effective. The Neumann U87Ai and Sontronics Sigma frequency response graphs can be seen below.

u87 cardiod 1k


The U87Ai would preferably used on vocals in this instance as the artists voice was prominent within the  4K – 7K region. Meaning that relevant boost would pronounce the voice within that mic where the Sigma has its high end cut limiting its response to the artists voice. Helping distinguish between each microphone as well just placing them in the appropriate positions for the job. This leaves the Sontronics Sigma to capture the acoustic guitar, whilst the front was looking at the guitar the back of the mic was looking up towards the ceiling. getting lots of reflections from the room. The Sigma has a high end cut from 5K upwards eliminating the high frequencies, ,leaving the Sigma with a “warm” or “dull” tonality.

Living up to the Specifications?

When you read the specification data given to you by the manufacturer, you can almost be swayed into buying a microphone but do these two mics: The Neumann U87Ai and the Sontronics Sigma live up to their specs give to us by the manufacturer?

The Specifications of each microphone can be seen below.

u87 specs sigma specs

Both these microphones have an Equivalent Noise Level (A-weighted) of 14dB-A. The Equivalent Noise Level is the level of noise the microphone produces itself. Typically it is never audible but it is always there. Both of these microphones were not noisy by themselves or within recording.

The higher the sensitivity the more efficient the microphone and the less the gain will have to cranked up on the desk. The Sontronics Sigma has a sensitivity of 18mV/Pa whereas the Neumann U87Ai has a sensitivity of 22mV/Pa.

Do these mics stay true to their frequency response graphs? Indeed they do!

The high end cut on the Sigma is very audible in the microphones overall tonality. My initial thought when using the Sigma on an acoustic guitar was “Would this high end cut remove the high frequency transients that are a common within an acoustic guitar?”

For the Sound Pressure Level (SPL) I conducted a little test. Measuring the SPL of the loudest instrument in the studio. The snare. The SPL meter was held above the snare as a microphone would usually placed above a snare when recording. The overall SPL measurement came to 110dBSPL, the snare was forced to its saturation point and could not get any louder. The Maximum Sound Pressure Level (SPL) for Total Harmonic Distortion (THD) 0.5% for the Neumann U87Ai is 117dB and the Sontronics Sigma is 125dB. Both of these microphones could easily record a snare drum without distorting.

The SPL was also measured of the acoustic guitar recorded, with the U87Ai and the Sigma. The SPL meter was placed net to sigma to get an accurate measurement of the amount of SPL the Sigma would be picking up. The acoustic guitars SPL was measured at 78 dbSPL, a very low level and inevitably in the capability of the U87Ai and the Sigma’s abilities without any unwanted distortion.

Delicacy of microphones is not down to the SPL but from dropping them or blowing air into microphones capsule.

A Tasteful Conclusion

When listening back to the recording, there is a very noticeable separation between the U87Ai and Sigma. There is an 8dB difference in amplitude between the guitar in U87 and the Sigma. I measured the guitars peak using the built-in plug-in Gain on Pro Tools.  This 8dB difference is very significant and can be heard when the tracks are panned in the opposite directions.

gain protools

All in all this is a very successful outcome, meaning more option available to us within the mixing process. Not just making out job easier but getting it right from the source and that is evidence of a job done properly and professionally!



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