Editor’s Note: Attaining accurate frequency response data from headphones and in-ear monitors has long been a ‘missing link’ in a quality monitoring signal chain. This article details some current methodology aimed at standardizing headphone and earphone tests.
At its simplest, the basic method used for gathering performance data of loudspeakers is to place a measurement microphone at a standard distance from a loudspeaker in an acoustically appropriate space while the loudspeaker is driven by a test signal. The microphone acquires the acoustic output of the loudspeaker and passes it to an analyzer.
The process is similar for headphones and earphones/earbuds: drive an acoustic transducer, capture the result with a microphone, and analyze it. But unlike loudspeakers, headphones and earbuds are designed to be coupled to the human ear, which complicates the issue. The measurement microphone must be placed in a structure that acoustically models the human ear and the driver under test must be mounted in a way that represents actual use.
Types of headphones (Figure 1) include circumaural, which surround the ear; supra-aural, which rest on the pinnae; intra-concha, which rest on the surface of the auditory canal; and insert, which fit into the auditory canal. The resonance of the ear canal and the reflections from the body and the pinnae greatly affect the response at the eardrum (referenced as the Drum Reference Point, or DRP, which is where the measurement microphone diaphragm must be located). Earphones and headphones can be tested with a variety of acoustic test fixtures (ATF), chosen by headphone characteristics and applications.
The International Electrotechnical Commission (IEC) has specified an Ear Simulator to model the ear canal (called the IEC 60318-4 Ear Simulator), with a microphone at the DRP. An Ear Canal Extension and an Occluded Ear Canal Extension are specified accessories so that the Ear Simulator can be used in a number of fixtures and applications.
When fitted with an Ear Canal Extension and an artificial pinna, the Ear Simulator can be used to characterize headphones; with an Occluded Ear Canal Extension, it’s used for intra-concha and insert earphones (Figure 2).
Meanwhile, Figure 3 offers a diagram representing a loudspeaker response (where the loudspeaker is equalized to be flat) measured in free-field, and the same acoustic signal as measured at the DRP of the Ear Simulator in a HATS (we’ll discuss the Head and Torso Simulator manikin in a moment). The altered response due to the resonance and reflections is obvious.
For insert earphones, only an Ear Simulator with an Occluded Ear Canal Extension may be necessary (Figure 4), as external ear, head or torso effects are not relevant. For earphones and headsets that involve pinna, head or chest reflections, the Ear Simulator can be mounted in a larger fixture such as a HATS.
The Ear and Cheek fixture (Figure 5) has an Ear Simulator and artificial pinna mounted on a plane representing the cheek or side of the head, with an arm to set an adjustable pressure to the headphone-to-pinna coupling.
A HATS manikin models the reflections and acoustic shadow of the head, and the reflections and absorptions of the neck, shoulders and chest (Figure 6).
Ear Simulators and pinnae are mounted in each side of the head. For headset microphone testing, a mouth simulator transducer can be added. A HATS is particularly important for accurate microphone testing in headsets that incorporate a microphone.
Less expensive than a HATS but a more complete solution than some other options, a Headphone Test Fixture uses two Ear Simulators and artificial pinnae mounted in a large aluminum head (Figure 7).
Like a HATS, left and right headphones can be tested simultaneously. For production-floor testing, the pinnae can be replaced with conical aluminum fittings for fast and repeatable headphone seating.
Additionally, the large tubular head provides substantial acoustic isolation, required for measuring the effectiveness of ear-muff style hearing protectors and ANC (active noise canceling) headphones in reducing ambient noise.