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.
Disclaimer: I work for a hearing aid transducer manufacturer.
The question is not about concurrency. It is not that a single speaker cannot reproduce low frequencies and high frequencies at the same time, but a single speaker will perform within a limited bandwidth all the time. A speaker can be designed to reproduce sound over a very broad frequency range, which is what we do for most hearing aid speakers. However, speakers can be specialized to perform well within limited bandwidths and ganged together for a total smooth, broadband response. A lot of engineering goes into the design of the individual speakers, as well as the electronic crossovers to correctly filter the signals to each speaker and the acoustic plumbing to make for smooth mixing back in the air, or the ear canal. The net result is that multi-way systems have a broader and smoother frequency response with lower distortion than a single speaker on its own.
Of course, like anything, it can be overdone. It is quite easy to appreciate the difference between a single speaker and a two-way speaker in an insert earphone. Even a three-way earphone would make an audible improvement for most users. More than that, though, would require the most discerning ear to appreciate.
Thank you for your comments. More information can be found on the excellent Knowles website (at Knowles.com)
Hello Marshall: This comment is off subject. I had never thought to ask before but since I have had less than good luck with digital aids, do you know if any business out there could put together custom analog aids? I at least know there are a number of hearing parts (microphones, speakers etc.) that are manufactured here in the U.S. by several corporations that act as suppliers for hearing producers but I have not been able to find out exact specifications of the various parts yet and if any company could make custom analog aids utilizing customer specified parts. I was thinking along the lines of attempting to get a BTE analog aid that had treble and bass adjusting pots plus volume control and low distortion parts.
Best regards – Bruce Bowen