Tag Archive for: headphones

Headphone Test Sequence

This headphone test sequence measures a stereo headphone. Both left and right earphones are measured simultaneously using a standard 1/12th Octave stepped-sine sweep from 20 Hz to 20 kHz.

The analysis is then performed using Listen’s HarmonicTrak™ algorithm that measures harmonic distortion and fundamental frequency response simultaneously. Then the diffuse-field and free-field corrected Fundamentals are calculated. The diffuse-field correction curve compensates for the overall frequency response from the diffuse-field (sound in every direction) to the eardrum and includes the effects of the head, torso, pinna, ear-canal and ear simulator. The free-field correction curve compensates for the overall frequency response from the free-field (sound at 0 degree incidence to the nose of the Head and Torso Simulator – HATS) to the eardrum.

Further post-processing of the signal compares left and right earphone responses to show the difference curve (magnitude and phase are available). The average sensitivity from 100 to 10 kHz for both left and right earphone is calculated and the total harmonic distortion displayed.

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End of Line Distortion Measurements

Steve Temme discusses the importance of detecting manufacturing-induced defects such as Rub & Buzz and Loose Particles during end-of-line testing, and explains the various algorithms that are used. He compares conventional and perceptual metrics for the measurement of Rub & Buzz, including Listen’s new enhanced Perceptual Rub & Buzz algorithm, and discusses why it can be beneficial to use both conventional and perceptual measurements in tandem.

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Use the Harman Target Curve to Predict Headphone Preference

Screenshot from SoundCheck showing the Harman Target Curve being used to predict listener preference ratings of headphones

The Harman Target Curve is used to predict listener preference of headphones

In their landmark papers “A Statistical Model that Predicts Listeners’ Preference Ratings of In Ear Headphones – Parts 1&2” presented at the AES 143rd in October 2017, Sean E. Olive, Todd Welti, and Omid Khonsaripour of Harman International introduced a model that predicted listener preference ratings of headphones based on the Harman Target Curve. We have created a SoundCheck sequence that measures any headphones and predicts the listener preference rating based on comparing its measured results to the Harman Target curve using this model. Listen acknowledges and thanks the authors of the Harman paper for providing the spreadsheet which is used as the basis for the excel template used in this sequence.

This sequence, inspired by the Harman research referenced above, applies the Harman target curve for in-ear, on-ear and over-ear headphones to a measurement made in SoundCheck to yield the predicted user preference for the device under test. The measurements are made in SoundCheck and then saved to an Excel template which performs the necessary calculations to produce a Predicted Preference score using a scale of 0 to 100. The spreadsheet calculates an Error curve which is derived from subtracting the target curve from an average of the headphone left/right response. The standard deviation, slope and average of the Error curve are calculated and used to calculate the predicted preference score. The sequence also provides the option to recall data rather than making a measurement, which saves time for engineers who already have large quantities of saved data, and enables historical comparison with obsolete products.

Check out our short video where we demonstrate this sequence in action

Get the test sequence for predicting headphone preference based on Harman Target Curves

Learn more about headphone testing using SoundCheck

References
A Statistical Model that Predicts Listeners’ Preference Ratings of In-Ear Headphones: Part 1—Listening Test Results and Acoustic Measurements
A Statistical Model that Predicts Listeners’ Preference Ratings of In-Ear Headphones: Part 2—Development and Validation of the Model
A Statistical Model that Predicts Listeners’ Preference Ratings of Around-Ear and On-Ear Headphones

 

Practical Measurement of Bluetooth and Lightning Headphones

Picture of Bluetooth Headphone Testing Article reprint

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

 

SoundCheck ONE for headphone testing

seq_SC-ONE_headphones_display_and_memory_listWe are excited to announce a new low-cost headphone test package – SoundCheck ONE for headphone testing (part # 1007). This includes SoundCheck ONE, a basic version of the SoundCheck software, along with AudioConnectTM, an audio interface with built-in headphone amplifier. Only a headphone coupler or test fixture and associated power supply is additionally needed to make up a complete headphone test system. The SoundCheck ONE headphone test sequence carries out all basic headphone measurements such as  frequency response, sensitivity, THD, Rub & Buzz, Perceptual Rub & Buzz, loose particles, polarity and channel balance. It can be easily customized and saved for specific products by turning individual measurements on and off, and by adjusting settings within each sequence step such as stimulus range and level, tolerance limits, graphical displays, and data saving. It is an excellent low-cost entry level system, ideal for those on a tight budget.

Please note that although the SoundCheck ONE Headphone test sequence cannot have steps added/removed or the layout modified, customers may upgrade to a SoundCheck package that supports this at any time in the future, should testing needs expand to require additional functionality. Listen can also offer the GRAS 45CC headphone test fixture along with a SoundConnect 2 power supply for a complete production line test system.

 

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Production line headphone test setup with SoundCheck ONE and the GRAS 45CC

For more information, please contact your Listen sales engineer: sales@listeninc.com

Headphone Testing with SoundCheck ONE

This SoundCheck ONE template sequence contains all the essential steps for basic headphone measurements using SoundCheck ONE and AudioConnectTM. The sequence can be easily customized and saved for specific products by turning individual measurements on and off, and by adjusting settings within each sequence step such as stimulus range and level, tolerance limits, graphical displays, and data saving.
Please note that sequences in SoundCheck ONE cannot have steps added/removed or the layout modified – the full version of SoundCheck is required for this capability.

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Headphone Testing at Reviewed.com

graphic showing Headphone Testing article about a visit to Reviewed.com

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.

The Correlation Between Distortion Audibility and Listener Preference in Headphones

Picture of paper on listener preference & distortion audibility in headphones

Listener Preference & Distortion Audibility in Headphones

The correlation between listener preference and distortion audibility is investigated in this AES paper from Steve Temme, Dr. Sean Olive et al. Five popular headphones with varying degrees of distortion were selected and equalized to the same frequency response. Trained listeners compared them subjectively using music as the test signal, and the distortion of each headphone was measured objectively using SoundCheck. The correlation between subjective listener preference and objective distortion measurement is evaluated and discussed.

Authors: Steve Temme, Sean E. Olive*, Steve Tatarunis, Todd Welti*, and Elisabeth McMullin*            *Harman International
Presented at the 137th AES Conference, Los Angeles 2014

Full Paper

 

 

Listener Preference & Distortion Paper Abstract & Introduction

Abstract
It is well-known that the frequency response of loudspeakers and headphones has a dramatic impact on sound quality and listener preference, but what role does distortion have on perceived sound quality? To answer this question, five popular headphones with varying degrees of distortion were selected and equalized to the same frequency response. Trained listeners compared them subjectively using music as the test signal, and the distortion of each headphone was measured objectively using a well-known commercial audio test system. The correlation between subjective listener preference and objective distortion measurement is discussed.

Introduction
There has been much research published on how a loudspeaker’s linear performance, e.g. frequency, time and directional responses, affects perceived sound quality. However, there is little research published on how non-linear distortion affects perceived sound quality. In recent years, the increasing availability and affordability of high quality headphones and personal digital music
players e.g. MP3 players, has brought high quality music playback to the masses. The transducer performance is critical to listener enjoyment and Dr. Olive and others have presented research on what they believe the target frequency response of the headphone should be for optimum sound quality [1]. The attempt of this research is to determine what level and what kind of distortion is audible and how it affects the perceived sound quality.

Five different pairs of good quality over-the-ear headphones with varying levels of distortion were objectively measured and subjectively rated for their perceived sound quality. First, each headphone was equalized to the same target frequency response. Several different kinds of distortion metrics including harmonic, intermodulation, and non-coherent distortion, were measured for each headphone. A listening test was then conducted where the five headphones were rated by eight trained listeners based on preference and distortion using four short musical excerpts. The program material was selected for wide dynamic and frequency ranges to excite mechanisms in the headphone transducers that would cause distortion.

The different headphones were presented virtually to listeners via binaural recordings of the headphones reproduced through a calibrated low-distortion reference headphone, Stax SR-009. This virtual headphone test method minimized headphone leakage effects, and removed the influence of non-auditory biases (brand, price, visual appearance, comfort, etc.) from listeners’ judgment of sound quality. In this paper, correlations between subjective and objective ratings of distortion are examined (as was done previously [2]) in an attempt to develop an objective metric for measuring distortion audibility in headphones and other loudspeakers. This could possibly be extended to other types of audio devices such as amplifiers.

 

More about Headphone Testing using SoundCheck

A visit to Reviewed.com

Headphone Test using SoundCheckAbout 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, their 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, and received a tour of the facility and some insight into their headphone test methods, as well as a demonstration of their recently upgraded SoundCheck system.

The large brick building in Central Square, Cambridge, is in a part of town renowned for its young start up 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 it 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 3d 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 laundry while they work (in the interests of testing the washers), as well as working their way through many loads of white towels and stain strips that are marked with red wine, chocolate, sweat and more to scientifically evaluate the performance of the washing machines. Dishwashers, dryers, microwaves and ovens are also tested here, and a dedicated temperature and humidity controlled room contains many refrigerators filled with ‘dummy food’, the temperature of which is continuously monitored. The floor above the test labs is where their testers retreat to write up product reviews for their website, away from the whirr of tumble driers, swishing of dishwashers and stepped sine waves from the audio test lab.

 

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 also compact, so a large room is unnecessary. Reviewed.com has been using Listen’s SoundCheck software since they first started looking for an objective way to test audio products back in 2007. Back then SoundCheck was being used for measuring mobile phones – smartphones were in their infancy, the next ‘hot product’, and Reviewed.com was the first review website to measure sound quality of a wide range of phones.

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Measurement of Harmonic Distortion Audibility Using A Simplified Psychoacoustic Model

A perceptual method is proposed for measuring harmonic distortion audibility. This method is similar to the CLEAR (Cepstral Loudness Enhanced Algorithm for Rub & buzz) algorithm previously proposed by the authors as a means of detecting audible Rub & Buzz which is an extreme type of distortion[1,2]. Both methods are based on the Perceptual Evaluation of Audio Quality (PEAQ) standard[3]. In the present work, in order to estimate the audibility of regular harmonic distortion, additional psychoacoustic variables are added to the CLEAR algorithm. These variables are then combined using an artificial neural network approach to derive a metric that is indicative of the overall audible harmonic distortion. Experimental results on headphones are presented to justify the accuracy of the model.

Authors: Steve Temme, Pascal Brunet and Parastoo Qarabaqi
Presented at the 133rd AES Convention, San Francisco, 2012

Full Paper