Picking up where we left off last issue, there is another important psycho-acoustic effect to remember when working with IEMs. When IEMs are used in both ears, there is a “stereophonic” effect (stereo mix not required, just both ears used) known as binaural summation which yields a perceived 6dB increase in volume without any change in the level of either ear’s input volume. This means the left and right IEM are each outputting 90dB SPL, but when both IEMs are inserted, our brain sums them together and we hear an equivalent 96dB SPL, yet without the hearing damage associated with those extra 6dB SPL.
You can try this experiment yourself by turning on your MP3 player, setting a level, and putting in one earbud. When you add the second, you will notice a substantial jump in level. The practical upside of this is to always use both IEMs and not just one like so many performers I see on TV. It makes me cringe to think of how much louder they are blasting their IEMs to get the same volume. Even worse, if they are using floor wedges in an attempt to get the “best of both worlds,” they will be blasting their open ear too.
I learned another trick for wireless IEM users from Mike Prowda, monitor engineer for Nine Inch Nails and David Bowie. Prowda likes to use a compressor and limiter before the wireless transmission stage. Wireless systems have fairly narrow bandwidths in which each channel operates, so to best exploit what is available, it is important to aggressively compress and limit the signal before the wireless stage to keep it from overloading while at the same time not leaving any dynamic range unused – and therefore wasted.
This is similar to the approach radio stations take with their transmissions, using multi-band compressors and limiters to deal with different frequencies separately so that the overall energy level is controlled while not making the music sound overly squashed. At the time, Prowda was using Aphex Dominators, though there are similar units that can also handle this multi-band compression approach. For those of you without access to advanced tools such as this, try experimenting with whatever compression you do have before the wireless stage to see if you can find improvement. If you have any questions, please drop me a line.
Keith Gordon is a veteran audio engineer who helped develop a DSP-based hardware/software IEM system (inearsounddesign.com) in conjunction with Westone Laboratories. He can be reached at firstname.lastname@example.org.