How to Measure an In Ear Monitor
Of great concern to many headphone and earphone users is the measured frequency response. Much emphasis is often put on the frequency response curve. With the recent work by Sean Olive et al. of Harman corporation (a good summary is available InnerFidelity), enthusiasts are much more aware of what can be considered ‘good sound’ from an objective standpoint. It can be instructive for the consumer to learn how measurements are made.
In-Ear Monitors (IEMs) are headphones designed to directly seal to the outer ear canal, and reside mostly or completely within the ear structure itself. They do not ride on or around your ear. In this way much of the variability often associated with headphone measurement is eliminated; there is no concern over proper width spacing, seal of an earpad, or the impact of the pinna (the outer part of the ear) on proper fitment of the IEM to the listener.
Of importance, however, is the seal of the IEM to the ear canal, or – in this case – the acoustic coupler. A coupler of a standard size is used to replicate the acoustic volume of the ear canal. The standard used for this coupler is IEC 60318-4, and it defines the physical parameters of the coupler. 60318-4 defines a standard internal diameter as well as a range of lengths (centered about 13.5 kHz resonance) and a controlled loss for the microphone capsule. Most microphones will be close enough to the specified capsule; thus the issue is to ensure the physical internal dimensions are appropriate.
Typical IEC 60318-4 couplers are between 7 and 8.5mm in internal diameter, and have an internal volume (net, exclusive of microphone and IEM displacement) between 1.7cc and 2.5cc. For the measurements shown here, we have standardized on a 7.5mm ID with a volume of 2cc. Smaller couplers tend to have higher resonant frequencies, while larger couplers have lower resonant frequencies. However, a 2cc coupler is a fairly good fit for approximately 85% of all adults.
The chamber has two connections: one side accepts our standard measurement microphone (a 1/4″ capsule with an 8mm OD), the other side is sized to accept an IEM with an attached earbud. The adapter, microphone, and IEM are placed within a sound chamber to reduce outside noise from corrupting the measurement.
For our measurements we utilize a CLIO test system. The IEM is driven with a test signal; we use a CHIRP (FM slide) signal to the IEM, with the level set to 1 mW (178 mV RMS for our 32 Ohm nominal IEMs). The resulting frequency response is captured by CLIO, and the measurement is displayed. We do not add any smoothing or processing, so the resulting frequency response is completely accurate, raw, and indicative of the true 1 mW efficiency of the IEM.
Several components are critical to a good measurement of an IEM:
- Properly sized coupler
- Calibrated microphone
- Test chamber
- Calibrated measurement system
It should be noted that an interested consumer/hobbyist could purchase a CLIO pocket (approximately $700), use one of several on-line suppliers of 3D printed parts to create the coupler (approximately $20), and use either a quiet closet in the home, or home-brew a test chamber (a large suitcase with two pillows inside, and the coupler/microphone/IEM stuffed between the two pillows will generally suffice).
Learning how a measurement is made can lead to a better understanding of what a measurement indicates. Because of the use of the coupler, frequency response is inherently rough in the higher frequencies; however the coupler is an accurate substitute for your own ear canal. Failing to use the coupler may result in a better visual measurement but one that is not representative of what you would actually hear.