Author: Steve Temme. Reprinted from the December 2011 issue of Voice Coil.
In part 1 of this 2-part series, Steve Temme discusses the basics of headphone testing. He covers the similarities and differences between loudspeaker and headphone testing. He discusses coupler correction curves and ear seal, and how these are important for accurate measurements. There are details on various types of headphones and how that affects the test equipment and configuration. Finally, he discusses the types of acoustic tests that are commonly implemented, and the standards that apply to headphone measurement.
Read the first few paragraphs of Headphone Testing Part 1 Article
As more and more loudspeaker engineers find themselves employed in the fast-growing headphone market, either through company diversification or changing jobs, it is important that the unique challenges of testing headphone packages are fully understood. Many of the characteristics that make for a good in-room listening experience with a loudspeaker—good frequency response, low distortion, no Rub & Buzz or loose particles, etc.—also apply to headphones, and many of the principles of loudspeaker test apply. However, there are some major differences and additional issues that need to be taken into account. These include couplers and associated correction curves, acoustic seal, fixturing, and additional tests such as L/R tracking. In this article, we discuss the issues that are common to testing all types of headphones. In Part 2 (a future article) we will address the specific needs of special cases of headphones such as Bluetooth and USB headphone testing, noise-cancelling headphones, and Max SPL measurements to prevent hearing loss.
Similarities and Differences
First, let us look at the similarities in testing loudspeakers and headphones. The set-up essentially consists of an electroacoustic measurement system, some kind of ear simulator containing a reference microphone, and the device under test. A test signal is sent to the transducer (headphone), which in turn is measured by a reference microphone in a coupler. The basic measurements made on headphones are very similar to those made on loudspeakers. These include frequency response, phase (polarity), distortion (THD and Rub & Buzz), and impedance.
In both cases, the test signal is usually a swept sine wave, and the level can vary. Some set the drive level to achieve a certain sound pressure level at a given frequency; others choose the level that equates to 1 mW of power. Certain products may necessitate testing the frequency response at one level and performing a second, higher level test for distortion. Now, let us look at the differences. The primary difference in the test set up between a loudspeaker and a headphone measurement is in the way in which the transducer interacts with the microphone. Whereas loudspeakers are tested in open air, a headphone or earphone must be presented with an acoustic load that simulates the human ear. It is common to compare the left and right-channel frequency response. Large differences at certain frequencies can be very audible in a stereo device, even though the individual responses may be within specification. Sometimes, electrical characteristics such as crosstalk may also be measured.
Before beginning to test headphones, there are two major considerations that need to be taken into account—correction curves, and the acoustic seal. These both have an effect of the frequency response. The latter also affects the repeatability of measurements.
Coupler Correction Curves
Loudspeaker engineers are familiar with the ideal frequency response for a loudspeaker measured in the free field being a flat line (see Figure 1a). For headphones, however, this is not the case. Headphone measurements are taken at what is known as the Drum Reference Point (DRP)—a point representing the human eardrum. Figure 2 shows where this is on a Head & Torso Simulator (HATS). If you were to measure the same loudspeaker that produced the flat free-field response curve in Figure 1a at the Drum Reference Point, the frequency response would look like Figure 1b. In other words, for a headphone to sound like a loudspeaker with a flat frequency response, it must produce a frequency response curve like Figure 1b.
This frequency response curve is a correction curve, or transfer function that represents the effects of the head, torso, pinna, ear canal and ear simulator. To further complicate matters, different correction curves are applied according to whether your measurements are made in the free field (anechoic room) or diffuse field (reverberation room) (see Figure 3). For the most part, like loudspeaker measurements, the free field is used. Typically, when making measurements, the subtraction of the correction curve from the actual measurement can be carried out in your test software, so that your output frequency response is shown as the familiar straight line.
Another issue that needs to be addressed when testing headphone is the acoustic seal, or leakage. Realistic headphone measurements (using a HATS or similar) have a certain degree of leakage as the headphone does not fit tightly to the pinna. This has an effect on the frequency response, with a demonstrable loss at low frequencies (see Figure 4). Although realistic, it affects the repeatability of measurement. In the R&D lab, this is compensated by repeating the measurement multiple times, removing and repositioning the headphone between each measurement and averaging; on the production line different couplers and fixtures are used to offer a more repeatable seal—these are discussed in more detail below.
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