Electromagnetic Spectrum

I’ve been doing some research into RF based audio measurements and what immediately becomes apparent is that using RF based mics for anything is infinitely more complicated than using a cable. Even so there are times when RF based measurements are necessary due to time / distance / etc…

The term “radio frequency” is a man made term and it’s important to keep in mind that prior the the 1860s the only frequencies considered were visible waves.

WIKI – Electromagnetic spectrum

It’s helpful to look at the entire electromagnetic spectrum to understand where both audio frequencies and radio frequencies land. It’s also obvious that what we consider low and high frequencies in the audio world does not relate to the scale of electromagnetic frequencies.

0hz is equal to no frequency. 1hz is obviously a low frequency but is considered ELF (extremely low frequency). The same is true up to 30hz. 30hz to 300hz is considered SLF (super low frequency). 300hz to 3khz is considered ULF (ultra low frequency). 3khz to 30khz is considered VLF (very low frequency). 30khz to 300 khz is considered LF (low frequency). 300khz to 3mhz is considered MF (medium frequency). 3mhz to 30mhz is considered HF (high frequency). Yikes!!!

Obviously our scale and how we perceived frequencies is not in alignment the scale of the electromagnetic spectrum.

Relating to RF mics, we are familiar with the terms VHF and UHF but maybe not clear on what those ranges are in the context of the electromagnetic spectrum.

VHF = 30mhz to 300mhz
UHF = 300mhz to 3ghz (the range where most RF mics currently available are tuned to)

Moving up in the electromagnetic spectrum and skipping over microwaves which are next after radio waves, we eventually get into the visible radiation spectrum which covers a tiny sliver of the electromagnetic spectrum between 400 and 790 terahertz. Infrared (IR) landing between 300ghz and 400 terahertz and ultraviolet radiation (UV).

Staying on topic, for live acoustic measurements our interested in a the ELF to VLF region of the electromagnetic spectrum. One could even argue that what we’re most interested in when measuring live audio is the SLF (super low frequency) region starting at 30hz and going only partially upward into the VLF (very low frequency) range of around 15khz.

How to collect that data over the radio waves? I’ve spent the last few years looking into how to do that accurately and there are two things to share.

1. The list of tools is growing
2. The existing tools are relatively expensive

Compare a 100′ XLR cable to a Lectrosonics TM400 wireless measurement kit and the difference is startling.

100′ XLR = $25 to $75
TM400 kit = $2000

So why would someone spend $2000 on a wireless measurement rig when they can spend less than $100 on an XLR cable. One word. “TIME”! A typical procedure I’ve witnessed where a single mic gets moved around a venue to optimize each system / sub system might require hours to complete using an XLR cable. With a wireless measurement rig? Maybe 30 minutes to an hour. Maybe less.

As the available RF spectrum shrinks, manufacturers of pro audio wireless gear are forced to make changes to their products. The change that made RF measurement possible was a system that didn’t use a compander circuit which makes measuring audio with standard wireless gear inaccurate. Lectrosonics Digital Hybrid system was the first system I am aware of to allow for wireless measurements in the UHF range. If there were others first, please let me know. Next was Line 6’s 2.4ghz based X series which aren’t specifically designed for working with a measurement mic but have been adapted for that purpose by some. Most recently I learned that any of the 100% digital RF systems sold by Zaxcom in the UHF range are capable of accurately transmitting the desired data between transmitter and receiver. If the available RF spectrum continues to shrink, we may witness all pro audio wireless manufacturers moving in the 100% digital direction which may bring the cost of RF based measurement down. In the meantime, if you want an off the shelve solution that you can plug your existing measurement mic into, your options are limited.

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