In-ear monitors are personalized monitors that many musicians are now using. They look like hearing aids, smell like hearing aids, and perhaps, even taste like hearing aids. But they are not hearing aids- they have a pre-amp, some have an “in situ” microphone, most are built into custom shells, and they have a receiver…. well, maybe they are hearing aids after all!
They are used to replace the wedge floor monitors or the “side wash” monitors that many musicians use to hear their own music up on stage. Three advantages of in-ear monitors are that they (1) allow the musician to hear their music and selected music of those around them without having to acoustically fight with each other; (2) allow freedom of movement while up on stage without subjecting themselves to changing acoustic environments; and (3) the preferred sound level that reaches their ears, with counseling, can be up to 6 dB less intense than that which would be achieved with wedge monitors.
But unlike hearing aids, it is routine to have more than one receiver, each carrying its own frequency band of music. Why is this? Perhaps only marketing? After all, more is usually better than less, …, I think.
Let’s look at some differences between speech and music. Other than the obvious difference that music is more intense than speech and it has different crest factors, the big difference is that speech is linear and music is not.
When I say that speech is “linear” I mean that in a very strict sense. Vowels are followed by consonants in a strict temporal manner. One may have a low frequency vowel, and then a fraction of a second later, a high frequency consonant. It is impossible to have both low frequency sounds and high frequency sounds occurring at the same point in time. A hearing aid can transduce a low frequency vowel and then after the nucleus of the vowel is completed, a high frequency obstruent consonant sound follows. The important element is that the next sound “follows”- they do not overlap in time. Hearing aid receivers therefore have a relatively easy task.
In this same sense, music is not linear. One can have (and almost necessarily need to have) low frequency sounds occurring at the same time as a mid or high frequency sound. Can a receiver that is used for amplified speech be optimal for music? Can one receiver diaphragm oscillate simultaneously for low frequencies and for higher frequencies?
I think that the short answer is yes- especially if the transducer has a flat response. Flat transducers have minimal transient distortion and, within the limits of our perceptive abilities, should be sufficient to listen to, and play music.
The long answer is that I think so, but am not sure. If there are significant levels in different frequency regions then a single receiver may have difficulty transducing a very intense non-speech like, high frequency sound while also being within the operating range of an intense low frequency sound. Receivers by their very design are manufactured to operate near resonance in order to obtain the desired outputs and this has some inherent problems.
Virtually all in-ear monitors that are found in the market place have many receivers- up to five. Are five receivers better than one? I have my doubts about the need for five receivers but like the issue of multi-band compression, two or three may be better than one, at least in some environments. To date however, there are no definitive studies that more is necessarily better.
There are some well-defined situations where two may be better than one such as with drummers. The bass drum sound can be transduced through one receiver while the more treble sounds through the other.
But I am not sure. We cannot assume that more is better. As is the case in many aspects of life, less may be more,…, or not. Until the appropriate research is performed, and perhaps funded by the manufacturers where more means more money, one cannot assume that more receivers (found in more expensive in-ear monitors) is necessarily better.