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Microphone Design and Analog Tape

From recent discussions with a mentor of mine, I have started to question and research the relationship between analog tape and the historic design of microphones – specifically condenser microphones. As we have transitioned from the age of analog tape into the modern realm of digital, it seems most modern recordings suffer from an excessive amount of top end frequencies, and I beleive this is due to the conventional design of condenser microphones.

Before we discuss the design conventions of condenser microphones, I’d like to explore ‘reel to reel’ magnetic tape recording and how the speed of the tape historically altered the frequencies of recordings.

Traditional reel to reel tape machines could move tape at speeds of 3.75″, 7.5″, 15″, and 30 inches per second (IPS), with lower speeds equating to lower quality. Think of the IPS of tape like bit rate in the digital world. Low tape speeds generate more background noise which tends to alter the low end response, along with producing top end saturation that rolls off the higher frequencies

Think off these alterations in terms of high pass and low pass filtering. An IPS of 7.5 would fail to faithfully capture the sub frequencies (60hz and below roughly) and high end frequencies (12khz and above); that is why older recordings can sound very mid range focused, because the low end and high end is simply not there. As tape technology advanced to greater speeds, this issue became less and less apparent. 30IPS can faithfully capture frequencies across the spectrum of human hearing and beyond. However, the faster the tape moves, the more it shifts the frequency response in favour of the higher range of frequencies.

Notice the high end response of both 15 and 30IPS, and how it is quite flat. Also, note that tape alters the low mid frequencies, providing a slight bump to frequencies in the range of 60-200hz (varying on the IPS).

Now, what does this have to do with microphones? If you have ever felt inclined to look into the frequency responses of microphones (which you should), you would notice that essentially all well known and used condenser (and dynamics to an extent) have an increased top end response.

For the purposes of this discussion we will look into 3 condenser microphones from 3 different company’s, the AKG C414Xlii, the Neumann U87ai, and the Telefunken ElaM251E (251).

AKG C414Xlii

Neumann U87ai

Telefunken 251

Notice how each of the exemplar microphones have an increased top end frequency response. I have also not cherry picked the microphones that prove my hypothesis, if you would like to delve in further please look towards all the iterations of Neumann microphones, the AKG C12, the Sony C800G and modern day examples such as Lewitt.

I beleive that these microphones were designed around the flaws of tape. They provided increased top end response where tape speeds removed such frequencies. In the 1950’s when most of these legendary microphones were designed, slow tape speeds significantly altered the top end frequencies, so the design of these microphones aimed to counteract this shortcoming. But as tape advanced to greater speeds, it seems that the design philosophy of microphones did not. Now in the age of digital recordings, it seems that most recordings have a ‘clean’ and incredibly ‘bright’ characteristic, and this can be attributed to the way in which these microphones operate. Now, this could be an equalisation decision on behalf of the engineers, as you will find that most mix engineers tend to add high amounts of top end frequencies into their recordings to add clarity on top of already using microphones that accentuate the higher frequencies. This mixing convention of the past has carried over into the present day where we are not limited by tape. Digital seems to faithfully reproduce what it recieves whereas tape gave a musical quality to the recordings that were processed with it.

As with most things music, imperfections tend to be perfect. Tape for better or worse is in the past and as the digital world advances it is apparent that we retain an understanding of the why’s behind the design of past instruments of recording. If we know that digital recordings faithfully reproduce the frequency spectrum, then in order to counteract the harshness of excessive top end we should look towards the characteristics of tape. Tape machine emulations exist and are quite effective in addressing this, but from a recording perspective it may be of use to look towards microphones that fit the mould of the digital world, microphones that aim to provide a flat response across the entire frequency spectrum.