Friday, February 5, 2010

Evaluating the Sound Quality of Ipod Music Stations: Part 1

For many consumers, an iPod Music Docking Station may be the primary audio device through which they experience most of their recorded music and infotainment. These ubiquitous devices offer a convenient, low cost, portable and easy-to-use solution for enjoying an Ipod through loudspeakers -- but what about their sound quality? What sonic compromises are made in order to achieve this level of convenience and portability? Do certain models or brands of Ipod Music Stations offer better sound than others, and if so, how can consumers identify which ones they are? These are legitimate questions that consumers should be asking when purchasing an Ipod Music Station. Unfortunately, the answers are not readily found.

Choosing an Ipod Music Station based on sonic performance quality is a daunting task for consumers. There are dozens of models to choose from that vary in price from $80 to as high as $3000 for a model designed by Ferrari. Competent in-store demonstrations and reviews of these products are difficult to find, and the technical specifications on the packaging provide no clear indication of how good they sound. For traditional loudspeakers, it is already possible to quantify their sound quality, but the audio industry continues to withhold this information from consumers. Without meaningful performance specifications in place, consumers cannot make sound purchase decisions, nor can manufacturers be easily held accountable for delivering products that sound “ not good enough.”

This article describes a listening test method used at Harman International for evaluating the sound quality of Harman and competitors’ Ipod Music Stations. The goal is to provide subjective ratings of Ipod Music Stations that are accurate, reliable and scientifically valid. From this data, a set of technical performance specifications can be developed that quantify how good the products sound.

Designing Listening Tests For Ipod Music Stations

Fortunately, there already exists a large body of scientific knowledge on how to design accurate, reliable and valid listening tests on loudspeakers. A key ingredient is careful control of listening test nuisance variables: these are psychological, electro-acoustical and experimental factors not directly related to the product(s) under test but nonetheless influence and bias the results (click on the figure below). Some of the more significant nuisance variable controls that should be in place but often are ignored by audio manufacturers and reviewers are:

  • Double-blind conditions (this removes the effects of sighted biases related to brand, price,etc)
  • Trained listeners with normal hearing (trained listeners are up to 20 times more discriminating and reliable than untrained listeners, yet their overall sound quality preferences are similar to those of untrained listeners)
  • Quiet listening room with acoustics that are representative of average homes (important for hearing low level sounds and the quality of the loudspeaker's off-axis radiated sounds)
  • Loudness matching between products (the perception of timbre, spatial and dynamic attributes are level dependent)
  • Selection of well-recorded music selections that are revealing of sound quality differences
  • Multiple comparisons among products which are more discriminating and reliable compared to single stimulus presentations

These important nuisance variable controls are essential for obtaining accurate, reliable and valid sound quality ratings of Ipod Music Stations.

Including the Acoustical Effects of the Wall and Desktop in the Listening Test

If audio products are not tested under similar conditions for which they were designed and intended to be used, the ecological validity (as well as the external validity) of the test may be compromised: in other words, the test results will be of little value or relevance to how the product is typically used in the real world.

Most Ipod Music Stations are intended to be placed on a desktop surface or bookshelf located near a wall, which will cause acoustical reinforcement and cancellation at certain audio frequencies. Below 500 Hz, there will be a gradual increase in sound pressure level that unless compensated for in the design of the product can make vocals and bass instruments sound tubby and boomy. Diffraction effects or reflections from the desktop/bookshelf may also produce audible effects that should be included in the listening test. For these reasons, listening tests on Ipod Music Stations are best done on a desktop/wall boundary.

A Video On How We Evaluate the Sound Quality of Ipod Docking Stations

The video shown at the top of the page illustrates how Ipod Music Stations are currently evaluated in the Harman International Reference Listening Room. The acoustical properties and features of the room have been described in detail in a previous posting.

In the video you see a trained listener comparing three different Ipod Music Stations situated on our automated in-wall speaker mover configured with a removable shelf and desktop. An acoustically transparent, visually opaque screen is placed between the listener and the products under test, so that the test is double-blind (note: the term double-blind implies that neither the listener nor the experimenter know the identities of the products currently selected since the computer controls and randomly assigns the letters A/B/C to the products in each trial.)

The listener can switch between the different products at will and enter their responses via a wireless PDA equipped a custom listening test software (LTS) client application. Sound quality ratings are given on a number of different pre-defined scales that include preference, spectral balance, distortion, auditory image size.This is repeated twice using four different programs.

The PDA client communicates with the LTS server application that performs the following functions:

  • A test wizard that defines of all experimental design and setup parameters (perceptual scales, presentation of stimuli, program, randomization of test objects, playback level,etc), which are then stored in a database
  • automation and administration of the listening test and its hardware (e.g. speaker mover, media player, DSP, audio switcher)
  • collection, storage and statistical analysis of listening test data
  • real-time monitoring of listener’s performance and ratings during the test

LTS makes conducting listening tests an efficient and repeatable process by minimizing human interaction and errors in the listening test setup, storage, and analysis of the results.


This article has described a listening test method used for evaluating Ipod Music Stations with the goal to provide accurate, reliable and valid sound quality ratings. In Part 2, I will show some results from a recent listening test conducted on different Ipod Music Stations, followed by some different acoustical measurements of the products in Part 3. By studying the relationship between well-controlled scientific listening tests and comprehensive acoustical measurements of Ipod Music Stations, a meaningful technical specification based on sound quality can be found.