If you ever order in-ear monitors, or IEMs, you are immediately confronted with a major decision: Do I choose a single-way (one loudspeaker) IEM like a hearing aid, or a 2-way system? What about a 4- or 5-way system? Current IEMs have up to five different outputs that can be configured to carry bass sounds (sometimes through more than one receiver), mid-frequency range, and high-frequency range.
Are more options, like French Fries, really better? To gain some insight, let’s look at the similarities and differences between speech and music.
Speech is “consecutive,” meaning that two things don’t really happen at the same time. We may hear or utter a consonant (C) sound, and a fraction of a section later, a vowel (V), and then another consonant or vowel a moment later. That is, you generally don’t get a speech sound happening at the very same time as another speech sound. We do have “coarticulation,” which occurs when some elements of one speech sound are adopted by an adjacent speech sound, but this is minor and does not really change the meaning of speech. Low-frequency vowels sounds simply do not occur at the same time as higher-frequency consonant sounds. They are uttered consecutively… CCVCVCCV…not simultaneously. Subsequently the receiver of a hearing aid or IEM has a relatively simple task: Do something and then, a moment later, do something else.
In contrast, music is “concurrent,” meaning that both low- and high-frequency elements of the music can happen at the same time. You just don’t have a bass note followed by a treble note; music is characterized by having both low bass notes and high treble notes played simultaneously.
This brings up the issue of whether a loudspeaker diaphragm in a receiver can vibrate as effectively as a slow-moving device (low frequency) as it can at the same time as a fast-moving device (high frequency). This is really the same as asking if a microphone can receive low- and high-frequency information at the same time.
It is true that in a recording environment many different microphones are used, but this is so that the recording engineer can independently manipulate different inputs- it has nothing to do with the nature of the microphone diaphragm.
I spent the last several weeks chatting with design engineers, microphone experts, and loudspeaker experts. A loudspeaker is really only a microphone in reverse that is configured to have as input a high-level electrical signal. I also pored through my old textbooks on electroacoustic transducer design. In my life before audiology I was as a mathematics nerd.
The conclusion is that I have absolutely no idea why a multi-receiver system would be any better than a single-receiver system.
I even chatted with various engineers from the IEM manufacturers and was given some information about isolation and differing levels. But, at the end of the day, I could not find out why a multi-receiver system was “preferable” to a single-receiver system in IEMs. I was even told that multi-receiver systems should be the norm for the hearing aid industry.
Whenever there is a mismatch between science and reality, I almost always conclude, perhaps a bit arrogantly, that the multi-receiver practice may be just a marketing ploy. After all, more receivers would typically mean a more expensive IEM. Perhaps the manufacturers of IEMs are just responding to the market- buyers of IEMs are demanding more and more receivers.
This blog is a bit unlike other blogs I have written…. I usually know everything! Alas, if someone out there has a different take on this than a “marketing ploy,” please put it in writing. This would also make an interesting AuD capstone essay for some unsuspecting graduate audiology student. Get a pair of IEMs, one with a single receiver and the other being a “top of the line” IEM with five receivers, and then perform a wide range of outcome measures to determine if there are indeed any measurable differences.
