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Practical Measurement of Bluetooth and Lightning Headphones

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Bluetooth Headphone Testing Article

Author: Daniel Knighten.  Reprinted from the July 2017 issue of Voice Coil.

In this article, Dan Knighten discusses Bluetooth headphone testing and Lightning headphone testing, specifically how to overcome the challenges of measuring headphones with wireless and digital interfaces such as Bluetooth, Lightning and USB-C to make the same measurements as on conventional wired headphones.

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Article Text

Practical Measurement of Bluetooth and Lightning Headphones

For decades, headphones have been passive devices with a direct, analog interface. Today, we are seeing a proliferation of headphones with wireless Bluetooth interfaces and various kinds of new and often proprietary digital interfaces. These new interfaces include Apple’s Lightning port and USB-C. In all cases these headphones present unique challenges to measurement because they cannot be directly connected to traditional test and measurement systems. In this article, we will explore how to overcome these interface challenges in order to make standard measurements on devices with nonstandard audio interfaces.

Closed Loop and Open Loop Testing
To begin, let’s define what we mean by open and closed loop testing. The test configuration for conventional headphone measurements, as seen in Figure 1, is what we term a “closed loop measurement.” This traditional type of audio measurement has been done for years with
all types of transducers (e.g., loudspeakers, headphones, microphones, etc.), and audio measurement systems can make these measurements without problems. The test signal passes from the audio interface through the speaker/headphone where it is converted to sound
pressure. Then, it goes through the microphone where it is converted back to voltage for analysis. The entire path from input to output is on the same interface, usually in the same domain (analog), and most critically, the analysis system’s input and output sample rate are
perfectly synchronous. The entire measurement from signal generation to capture of the device response simultaneously occurs with just a small amount of input to output delay added by the speed of sound.

In headphones with Bluetooth, Lightning, or other unconventional digital audio interfaces, this loop is broken. The input and output are on two different physical devices, which do not share a sample clock, and the signal goes through one or more analog to digital conversion stages. The delay from input to output of the device is likely quite long, compared to classic analog headphones. In fact, the delay might effectively be infinite. In the case of Lightning-connected headphones, there is currently no third-party solution available for injecting a test stimulus into a Lightning port. In Bluetooth systems, the connection is intrinsically non-synchronous. Bluetooth does not provide a synchronous sample clock across the wireless connection and instead relies on asynchronous sample rate conversion and various other techniques to maintain a glitch-free audio stream. This is what we mean by “open loop.” A closed loop system has a closed loop, synchronous signal chain.

An open loop system does not have a continuous or synchronous signal chain. However, SoundCheck makes it possible to measure all conventional parameters of a device, even when those devices are open loop devices, with a variety of tools including:
• Triggered acquisition—support for capturing measurements on playback devices
• File analysis—the ability to analyze signals captured by recording devices
• Resampling—conventional asynchronous sample rate conversion
• Frequency shift—the capability to align signals between non-synchronous systems

Let’s explore how these tools are applied in some typical test scenarios.

Bluetooth Testing
Figure 2 shows a typical Bluetooth setup. Since we are testing the Bluetooth headset using a Bluetooth interface, it is nominally a closed loop scenario. The audio signal comes out of the analog interface and is transmitted via the Bluetooth interface to the headset. It is then played
by the headset and picked up by the ear couplers where it is returned to the analog interface and computer for analysis. However, what makes this an open loop test is that Bluetooth does not transmit a sample clock and, therefore, the receiver and transmitter are asynchronous.

In this case, we will use frequency shift to align or synchronize the stimulus and response waveforms. Frequency shift uses a stationary reference tone to precisely find the difference in sample rate between two waveforms. Once the exact difference in sample rate is found, one waveform is then resampled with reference to the other. Frequency shift enables precise, conventional measurements to be made on Bluetooth devices despite their asynchronous nature.

When compared to a conventional test, only two changes need to be made. First, a short, stationary tone is pre-pended to the stimulus signal (see the Sidebar article). Typically 1 kHz for as little as 250 ms, this signal provides the frequency reference that the frequency shift step needs to align the stimulus and response signals in an asynchronous test scenario. Second, a
post-processing, frequency shift step is inserted into the test sequence between the acquisition and analysis step.

The rest of the sequence is identical to a conventional headphone test sequence and all normal parameters including frequency response, THD, polarity, rub and buzz, and so forth can be measured.

Lightning Headphone Testing
Any device that does not provide an analog or digital input and output is intrinsically an “open loop” device from a test perspective. Headphones that use the Apple Lightning port for connection are considered open loop because Apple does not provide Lightning audio
output adapters. The only device that can currently play audio into a Lightning headset is an iPhone. Measuring Lightning headphones requires an iPhone or similar Apple device to be used to store and play back the test signal (see Figure 3). This creates several open loop testing
challenges. To test a Lightning connected headphone, we will use three specific tools: triggered acquisition, resampling, and frequency shift.

Again, our test sequence will use a short 1 kHz tone, pre-pended to the normal test stimulus but this time it serves two purposes. First, it triggers a record-only acquisition, so that the test is automatically triggered when playback of the test signal begins. It is also used as the reference tone for frequency shift. Also, if our playback device, the iPhone, is using a different sample rate to the audio interface, we may need to use a resampling step. Finally, frequency shift will again be used to synchronize the stimulus and response waveforms. After the response waveform is captured via a triggered acquisition step and has been resampled and frequency corrected, calculation of the desired measurement parameters can proceed as with any conventional headphone.

Pre-written test sequences for both Bluetooth and Lightning headphones are available at no charge from Listen’s website, www.listeninc.com.

Lightning Headphone Microphone Measurements
Since most headphones now also include a microphone, it is worth mentioning how the microphone on a Lightning- connected headset is tested. The test sequence and method
for this is a little more complex, although ultimately it is really just the converse of testing the earphones. Figure 4 shows a typical test configuration. The preparation of the test signal and the use of resampling and frequency shift steps are identical to testing the earphones of a Lightning connected headset.

The difference is that instead of playing back the stimulus through the earphones and using a triggered, record-only acquisition, the stimulus is instead generated using a calibrated speaker or mouth simulator and recorded on an iPhone. The recorded signal is then transferred back
to the computer hosting SoundCheck and analyzed using a recall step to import the waveform into memory from storage on the iPhone.

Conclusions
Bluetooth and Lightning interfaces add an additional level of complexity to testing that is not there with their analog counterparts. However, because the SoundCheck test system is completely agnostic about where the stimulus and the response waveform are generated and
captured, these tests can be carried out with relatively simple modifications to existing test sequences. In fact, it pretty much comes down to a simple modification to the stimulus signal and some additional post-processing steps prior to analysis—all of which are easily automated.
This enables easy characterization and measurement of Bluetooth, Lightning, USB-C, and future devices with advanced digital audio interfaces.

 

Preparation of the Stimulus Signal (sidebar)
In SoundCheck’s frequency-shift algorithm, a Fast Fourier Transform (FFT) is used to extremely accurately calculate the centroid of a stationary tone. The result of this calculation can then be used to align or synchronize two signals even if they are sampled at different rates. A short, stationary signal is necessary for the frequency shift algorithm to lock on to. This is easily achieved by pre-pending a 1 kHz, 250 ms sine wave to the stimulus signal. Since SoundCheck enables the creation of compound stimuli, this short, single-tone burst can be followed with absolutely any test signal (e.g., a Farina log sweep, noise, speech, or other non-sinusoidal
waveforms).
The short sine wave also serves as the trigger tone for triggered record, as is necessary for testing Lightning headphones. The trigger tone clearly identifies the start of the signal. Care must be taken to set an appropriate trigger level. If it is too low, ambient noise can cause false
triggering; too high and it will never trigger. The trigger level should be set so that it is above the ambient noise and below DUT output level. The Multimeter virtual instrument is an ideal tool for finding the optimal trigger threshold.

A typical stimulus signal for open loop headphone test is shown in Figure 1. This compound stimulus works for both Bluetooth headphones where the sine wave is used for frequency alignment and for Lightning headphones where it additionally serves as a trigger and reference for frequency shift.

 

Additional Headphone Test Resources

More about Bluetooth headphone testing

Headphone Testing main page

 

Headphone Testing at Reviewed.com

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Headphone Testing article: A Visit to Reviewed.com

Author: Zarina Bhimani. Reprinted from the June 2016 issue of Voice Coil.

In this article we discuss headphone testing using SoundCheck at independent review site, Reviewed.com. Includes description of test lab, measurement methods and more.

Full Article in VoiceCoil Magazine (PDF)

 

Article text

A Visit to Reviewed.com

Reviewed.com, part of the USA Today network, carries out quantitative reviews on a wide range of products including appliances, headphones, cameras, televisions and more. Since the beginning, its reviews have been built on the principle of using standardized scientific testing procedures to examine the performance of products, and a proprietary scoring method to ensure a level playing field amongst all manufacturers. Recently, I met with senior scientist Julia MacDougall, toured the facility, and learned about its headphone test methods. I also received a demonstration of the company’s recently upgraded SoundCheck system.

The large brick building in Central Square, Cambridge, MA, is in a part of town renowned for its young startup culture and unconventional work environments, so it’s no surprise to see a ping pong table next to the large, glass-walled conference room. However, once you get beyond the main lobby, there is a labyrinth of test labs, each designed for testing a specific product. A room dedicated to camera testing features various test pictures on the walls, as well as 3-D models with many moving and rotating parts to evaluate the camera’s capture of movement. Another lab was filled with massive flat-screen televisions that were being tested for display performance, color measurement, luminance, contrast, and more. Perhaps the most impressive was the appliance lab, where staff get to do their own laundry while they work (in the interests of testing the washers) to scientifically evaluate the performance of the washing machines. Dishwashers, dryers, microwaves, ovens, and refrigerators are also tested there. Then, testers retreat to the floor above to write up product reviews for the company website.

 

The Audio Test Lab

The area that interested me the most was the smallest test area—the audio lab. Headphones are small and the test equipment is compact so a large room is unnecessary. Reviewed.com has been using Listen Inc.’s SoundCheck software since it first started looking for an objective way to test audio products back in 2007. Back then, SoundCheck was used for measuring mobile phones—smartphones were in their infancy and Reviewed.com was the first review website to measure the sound quality of a wide range of phones.

Reviewed.com’s audio test focus has changed over the years. Emphasis is placed on products customers want to know more about before buying, and the review focuses on the product most important to the customer. Since the smartphone market has matured and customer choice is driven by brand loyalty and the ecosystem over audio performance, Reviewed has moved on to testing other products. Headphones are one of the largest and fastest growing segments of the consumer electronics industry (in part, driven by the smartphone revolution), so now the audio test lab is primarily focused on headphones.

The audio test lab is a small, climate-controlled room on the ground floor of the building. It contains a computer with the SoundCheck system, a Brüel & Kjær Sound & Vibration Measurement A/S Head-and-Torso simulator (HATS), an amplifier to drive the headphones, speakers used for noise cancellation tests, and two SoundConnect microphone power supplies to power the ears of the HATS. The walls and the door are entirely covered in acoustical foam to acoustically isolate the room and minimize reflections. Other precautions have also been taken to ensure accurate testing. The HATS sits on a rigid rack mounted to the wall to prevent vibration. Initially, it was on a desk above the computer but the vibration from the computer fan interfered with the measurements. The Reviewed.com offices also have a more unusual noise issue to contend with. Due to its proximity to the local subway line, there is a low-frequency rumble every 5 minutes or so when a train goes by! Measurements are stopped or repeated if the train is not heard until too late.

 

Headphone Testing

Reviewed.com has been testing wired headphones for several years now, and has recently updated its setup to test the audio performance of wireless headphones for the same performance standards.

Figure 1 shows the test setup for a typical wired headphone. In-ear, over-the-ear, and on-the-ear headphones are tested using the same setup. The HATS is, as the name indicates, a device that replicates the acoustic behavior of a human head and body. The pinna is constructed to accurately replicate the average human ear, and behind the pinna is a very accurate measurement microphone representing the human eardrum. These microphones require 200 V polarization and are powered using SoundConnect microphone power supplies from Listen. When headphones are placed on the HATS, they must be placed as accurately as possible since small variations in fit can affect the test results. For this reason, four measurements are made on each headphone, and the best one is used for evaluation.

The SoundCheck software generates the test signal and receives and processes the recorded response. The test signal, a stepped sine sweep, is transmitted to the headphones via the audio interface headphone output of the amplifier. The sound is recorded by the microphones in the HATS and returned to the computer via the audio interface so it can be analyzed by the SoundCheck software.

Reviewed.com carries out six specific tests on every headphone: frequency response, distortion, tracking, leakage, isolation, and sound pressure level (SPL). The first three of these are the most important as they are the ones that have the greatest influence on the perceived sound quality.

Frequency response, distortion, and tracking are simultaneously measured using a stepped sine sweep from 20 Hz to 20 kHz. Figure 2 depicts a screenshot that shows the output from this test.

All measurements are then compared to standard curves to enable Reviewed.com to apply a numerical value to the data, which makes up the overall score. This enables unbiased and simple comparison between headphones, and these numerical values contribute to the device’s total score.

Audio Testing

Frequency Response – For measuring frequency response, recorded sound is compared to the original sound file to determine how the headphones have altered the sound. For consumer headphones, the frequency response is compared to the ISO 226:2003 equal loudness curve standard, which is the curve at which human ears hear notes at the same loudness. For studio headphones, limits are set at ±5 dB (SPL) against the response curve of the headphones. This means these headphones are not scored on the exact shape of the curve but rather how much the headphones deviate from these limits.

An objective measurement of distortion is obtained by measuring the total harmonic distortion (THD), a measurement of the distortion at every harmonic in addition to the fundamental. This is plotted and compared to a proprietary empirical data curve which represents Reviewed.com’s acceptable threshold, based on more than six years of headphone test results. Any measurements above this line have a negative effect on the score.

Tracking is a measurement of how the channels sound compared to each other. Ideally, the left and the right ears should sound the same. Both are simultaneously measured across a range of frequencies from 20 Hz to 20 kHz. Any difference of more than ±2 dB affects the score.

Other performance tests measure leakage, isolation, and maximum SPL. Leakage is an indication of how much sound escapes from the headphones. This is measured using a sound level meter placed precisely 6” from the ear. Initially, the ambient noise of the room is measured, pink noise is played back through the headphones at 90 dB, and the volume measured. A simple subtraction of the background noise from the measured noise offers a numeric leakage value that contributes to the overall score.

Isolation, the ability of the headphones to eliminate outside noise, is also measured using SoundCheck. The setup enables both passive isolation (i.e., isolation due to the mechanical structure of the headphone) and active isolation (i.e., the noise cancelling functionality) to be measured. Figure 3 shows the measurement when pink noise is played at 90 dB without the headphones on. Its is level measured across the frequency spectrum. The headphones are placed on the HATS, and the noise played again and measured so that the attenuation is calculated. In noise cancelling headphones, a third measurement is made with the active noise cancellation turned on. By subtracting active and passive noise cancellation curves from the unoccluded curve, the isolation can be numerically quantified. This is compared to the average values of hundreds of pairs of headphones to calculate their noise isolation score.  Last, SPL is measured. In this test, the volume of the stimulus signal is increased and the distortion analyzed until the peak THD reaches 3%, or the level of sound reaches 120 dB. There is no need to test higher than 120 dB as headphones are not intended to be played at that volume due to the risk of permanent hearing damage.

Wireless Headphone Testing

Recently, more and more headphones are being used in wireless mode, so Reviewed.com decided that it was important to test them under such conditions. (Until recently, wireless headphones were tested in wired mode.) The addition of a Bluetooth interface, the BQC-4148, to the test setup now enables wireless headphones to be tested exactly the same way as their wired counterparts (see Figure 4).

Instead of routing the test signal from the computer to the headphone via a headphone amplifier, it is routed via the BQC1448 Bluetooth interface. This small device connects to the computer via a USB, and is controlled via SoundCheck, where parameters such as the Bluetooth Protocol and transmitter power are set. It is paired with the headphones under test, and the signal is transmitted via the interface directly to the headphone. The recording and analysis side of the setup is exactly as before—the signal is transferred from the microphones within HATS to the computer via an audio interface and analyzed.

With this setup, wireless headphones are measured to exactly the same standards as wired headphones, so the score they receive is truly representative of the way they are commonly used. It is even possible to compare wired and wireless performances of the headphone by making the same measurement with both the conventional and Bluetooth setup.

In addition to the audio tests, the headphones are evaluated for comfort, control, and functionality. Specialty headphones (e.g., sports headphones) undergo additional testing, such as being worn on a long run to test for a secure fit in active conditions.

The Future

Mobile audio technology is evolving extremely fast. In the past 10 years, we have seen the smartphone revolution, the explosion of the headphone industry, the introduction of wireless headphones (in fact, wireless everything), the return of high-resolution audio, voice activated audio, and more. In addition, wireless homes are generating new audio opportunities (e.g., light bulbs that also function as wireless speakers). Reviewed.com will follow these trends, testing the hottest consumer products, particularly those where performance is a big differentiator.

The SoundCheck system is inherently flexible for testing any device, as it can support up to 64 channels of audio and there a range of interfaces enables Bluetooth, USB, MicroElectrical-Mechanical System (MEMS) microphones, and more to be tested. High-end audio interfaces permit accurate testing of high-resolution audio and the ability to custom-program virtually any test means the possibilities are endless.

This makes it a valuable tool for a constantly evolving product review site such as Reviewed.com, because the flexibility and forward compatibility of the SoundCheck system ensures that it will be capable of testing any audio device. This has already been demonstrated with the recent system upgrade to add the Bluetooth interface and update the software. As MacDougall, so succinctly explained, “We are very excited to use SoundCheck and the BQC4148 Bluetooth interface to broaden our headphone tests to include wireless headphones—it enables us to offer better, more relevant data to our readers.”

 

Curious to learn more ? Check out our main page about headphone testing.

Headphone Testing (part 2)

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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.

Full Article

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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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.

Full Article

 

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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