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Headphone Testing (part 2)

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Graphic showing headphone testing part 2 article from Voice Coil Magazine

Headphone Testing Part 2

Author: Brian Fallon.  Reprinted from the April 2012 issue of Voice Coil.

In part 2 of this 2-part series, Brian Fallon discusses the challenges and practicalities of measuring headphones with built-in electronics such as digital and Bluetooth headphones. He covers connectivity, frequency limitations, test signals, measurement units and specialty measurements such as noise cancellation.

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Note: This article was written a few years back and there have been many advancements in the SoundCheck software and other accessories for measuring advanced headphones.

Read the first few paragraphs of Headphone Testing Part 2 Article

Introduction
“Headphone Testing (Part 1)” published in the December issue of Voice Coil, covered the basics of analog headphone testing: correction curves and fixturing, choosing hardware (ear simulators and couplers), the different requirements for testing in R&D vs. production, and the various essential measurements such as frequency response and distortion.

Analog headphones are relatively straightforward to test because there are only two electro-acoustic transducers to measure. Headphones with built-in electronics, such as digital headphones (including Bluetooth and USB), and noise-cancelling headphones are harder to test because the electronics and transducers need to be tested together as a complete system. In this article, test considerations for such headphone systems and the practicalities of testing them are discussed.

Testing Digital Headphones
Headphones with digital connectivity add complexity to audio testing. In addition to testing the acoustic transducers, the digital circuitry must also be considered. The fundamentals of analog headphone testing (the use of artificial ear simulators and couplers, the principals of repeatability vs. realism, and the tests that characterize the device) are the same, but the test signals must pass through the headphone electronics, which can greatly influence the test results. The principal distinction of digital headphones is that they contain a D/A converter and often DSP circuitry as well as a headphone amplifier. This means that unlike an analog headphone, where measurements are being conducted only on the electroacoustic elements, the measurements for digital headphones are being performed on the whole system which comprises everything from the digital signal to the acoustic output of the transducers. While it is certainly possible to isolate and test each of these components on its own, it is also very important to understand the intricacies of testing the complete device.

Managing Connectivity
The initial challenge of testing digital headphones is managing connectivity. The test system must be able to communicate directly with the device. A software-based system is ideal because it can communicate directly through the computer’s USB interface to the USB headphone, which will appear in the operating system along with other audio devices. Test signals can be sent digitally and the acoustic signals can be analyzed synchronously. If a hardware-based system is used, an extra program is usually required to connect the test system to the device under test.

Bluetooth headphones, however, require an additional interface for the computer to connect to the device under test. This may be a hardware Bluetooth communication box or a simple Bluetooth dongle, either built into the computer or externally connected by USB (see Figure 1). Bluetooth interfaces cause transmission delays in the audio chain. The test system must be able to account for these delays in order to take meaningful measurements. Some test systems can use an autodelay algorithm that looks at the system’s impulse response to calculate the delay and remove it from the measurement, if necessary.

Frequency Limitations
It is also important to be aware of frequency range limitations when designing tests for Bluetooth devices. Bluetooth devices typically operate at low-sampling rates of either 8 kHz (narrow band) or 16 kHz (wide band). These sampling rates limit the frequency range (because of the Nyquist frequency) to significantly narrower than analog headphones or even USB headphones. For example, a Bluetooth device with an 8-kHz sampling rate will only play audio up to slightly less than 4 kHz. Such limitations need to be considered when designing the test specifications. It can also be interesting to test the Bluetooth device beyond its cut-off frequency to see how well its anti-aliasing filter suppresses out-of-band signals.

Test Signals
Bluetooth presents a further challenge in that sine waves are not always transmitted accurately. When this occurs, alternative stimulus signals must be considered. Broadband noise is one possibility, but because of noise suppression circuits in some devices, this may not be a practical solution. A multitone signal, where several frequencies are played simultaneously, is another option. This produces a very fast frequency response measurement and is immune to the sudden dropouts that can occur in Bluetooth transmission. The downside of this test signal is that traditional harmonic distortion cannot be measured. Yet another possibility is the use of speech or music signals. These real-world audio signals transmit very well over Bluetooth, but their downsides are that they typically require long-term averaging and cannot be used for harmonic distortion measurements. If distortion measurement is required, non-coherent distortion may be measured using any test signal. This technique compares the input and output power spectrums to measure the non-coherent power and calculate the distortion plus noise (see Figure 2).

Measurement Units
Traditional analog headphones are tested with a stimulus level that is rated in terms of voltage or power. The sensitivity is also specified in these units such as dBSPL / mW. When testing headphones with USB or Bluetooth connectivity the stimulus is simply a digital signal whose level can be expressed in terms of digital full scale. The sensitivity is, therefore, expressed in dBSPL/FS. Manufacturers sometimes choose to relate these signal levels back to voltage, which can be done if the characteristics of the D/A circuit are known. In such cases, the gain of the built-in headphone amplifier chip must also be accounted for. Another method used for relating these digital units back to the analog domain is through the use of a codec (see Figure 3). A-law and M-law are two codecs widely-used in Bluetooth applications that can translate the digital units into “virtual volts.” These codecs are most commonly used in telecommunications, especially for headsets.

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Check out our main Headphone Testing page for up-to-date information on testing advanced headphones.

Headphone Testing (part 1) – The Basics

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Picture showing Headphone Testing Part 1 article from VoiceCoil

Headphone Testing Part 1

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.

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Read the first few paragraphs of Headphone Testing Part 1 Article

Introduction
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.

Considerations
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.

Headphone/Ear Seal
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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Check out our main Headphone Testing page.