On August 15, 2023, Sennheiser presented a hands-on workshop in New York demonstrating advancements in the groundbreaking wireless technology called Wireless Multi-Channel Audio Systems, or WMAS. This new wireless tech has been under development for several years and promises to be a game-changer for large-scale events requiring large RF channel counts. (See Fig. 1) But before we dive into the capabilities of WMAS, let’s…
Rewind
Going back to 2010, the live event industry has been facing increasing challenges for availability of RF spectrum. The FCC has pushed us out of certain RF bands that we were able to use in the past and designated those bands for use by broadband mobile providers with deep pockets such as ATT, Verizon and T-Mobile. First, we lost the 700 MHz band (698 to 806 MHz), then we lost most of the 600 MHz band. Wireless mic users are still allowed to operate in VHF and UHF frequencies on TV channels 2-36 (which fall below 608 MHz), as well as small portions of the 600 MHz and 900 MHz bands. Portions of the 2.4 GHz and 5.6 GHz bands are also available for wireless audio systems, but these bands are unlicensed bands that can be subject to interference from Wi-Fi devices.
Unfortunately, the FCC doesn’t give a rat’s behind about the live event industry’s need for wireless bandwidth and is far more concerned with how much money they can bleed out of the aforementioned cell phone companies. I still feel that the issue will become of national importance as soon as some windbag politician’s wireless mic gets stepped on by an RF hit while in the middle of a speech.“How could this happen?” At that point, politicians will care, and perhaps pressure the FCC to show us more respect. Until that time, we’re dealing with less and less usable spectrum in which to operate our wireless gear, but at the same time, our productions are growing to include more and more wireless systems. Something has to give.
WMAS vs. Traditional Wireless
Sennheiser’s WMAS could be the solution needed for large-scale events requiring high RF channel counts such as festivals, major sports events, and theater. WMAS is a bi-directional, broadband wireless technology capable of combining wireless microphones, instruments, in-ear monitors and remote control into one broadband RF channel.
RF Pollution
A traditional narrow-band wireless system is limited to a bandwidth of 200 kHz and transmits with a power output of 50 mW (maximum permitted without a license), though some wireless systems can vary transmission power from around 10 mW up to 50 mW. As you increase the number of narrow-band systems operating in a local area, you also increase the “power on air” or total energy in the RF band (some might say total RF clutter). Twenty channels of wireless devices — not uncommon even on a small tour — produces one watt on air, enough activity to generate intermodulation or interference with nearby devices. The transmit power of a WMAS system is also 50 mW, but (wait for it…) the entire system operates at 50 mW, regardless of the number of audio channels being transmitted.
The Bad News
WMAS requires a wide band for transmission: 6 MHz in North America or 8 MHz in European countries, which is exactly the bandwidth of one TV channel in their respective locales. Unlike narrow-band wireless, the 50 mW transmission of a WMAS system is not concentrated within a 200 kHz band. Instead, it’s spread across a 6 MHz band, resulting in a power spectral density that is 30 times lower than that of a conventional wireless microphone, 40 times lower when used in an 8 MHz bandwidth (power spectral density is a measure of total signal power in a specified spectral bandwidth divided by that bandwidth, expressed in watts per Hertz).
A benefit of reduced power spectral density is that an RF channel can be “reused” more easily. “Reuse” means being able to operate a wireless unit on the same channel as your neighbor. Traditional wireless guidelines dictate that your neighbor should be roughly a quarter mile away from you to prevent interference when operating on the same RF channel. Using WMAS, however, two neighbors could be less than half that distance apart and still operate on the same channel without interference. In practical terms, this means that high-density RF environments such as Broadway could more easily secure channels on which to operate because neighbors would be able to reuse a channel.
One of the most impressive features of WMAS is that a WMAS belt pack is bidirectional and can operate as both a transmitter and receiver at the same time, for example transmitting an instrument signal while simultaneously receiving an IEM mix — which reduces the number of packs worn by a performer as well as the need to ensure that those packs are placed in a manner that prevents interference between them. Channel allocation is simplified because the instrument transmitter/IEM receiver operates on the same RF channel. This is in stark contrast to the current practice of allocating one vacant channel for wireless mics and instruments, while allocating a separate vacant channel for IEMs in order to reduce blocking of IEM receivers. And, WMAS enables multiple mics and body packs to be linked to one rack unit “base station” that could handle up to 32 I/O, so racks are reduced in size and weight. Surely, this must be voodoo. According to a Sennheiser press release, “Live audio engineers and frequency coordinators will benefit from additional control, less clutter and negligible RF fading, amongst many other advantages.”
DRA?
The bidirectional communication between a WMAS base station and body pack makes possible something called dynamic resource allocation (I’m calling it “DRA”). DRA enables an RF coordinator to assign spectral resources wherever they are needed most. One example would be allocating more spectral resources to a star performer who walks a large stage with a wireless mic and IEM system, while reducing the spectral resources dedicated to the IEMs for a keyboard player who remains in the same position throughout the show.
Practical Benefits
WMAS offers production pros a host of benefits. Rental houses would need to stock only one model of body pack instead of separate transmitters and receivers. Each performer would require only a single pack (with one battery), even if they’re using wireless instrument and IEMs, simplifying coordination and naming. Coax wiring is simplified due to a drastic reduction in the number of rack units, splitters, and combiners. And, an entire WMAS system consisting of a base station, belt packs and one or two antennas could be packed for travel in a relatively small rack or work box.
Trouble In Paradise
The fly in the ointment is that a WMAS channel requires availability of at least one unused TV channel in a locale. Most cities do have at least one unused channel. This, combined with its low transmission power, decreases the possibility for WMAS to interfere with transmissions from nearby TV stations and other wireless mic users, so it’s logical for regulatory agencies to clear WMAS for use. It has, in fact, been cleared for use in many European and Asian countries, but no one said that the FCC is logical — so we’re still in a holding pattern in the U.S. Let’s hope the FCC sees through the RF noise.
The author wishes to thank Sennheiser’s Joe Ciaudelli for his help in preparing this article.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
