DESIGN CONSIDERATIONS FOR IMPROVING THE EFFECTIVENESS OF MULTITALKER SPEECH DISPLAYS

ABSTRACTAlthough many researchers have commented on the potentialof audio display technology to improve intelligibilityinmultitalkerspeech communication tasks, no consensus has been reached onhow to design an “optimal” multitalker speech display. This paperreviews a set of experiments that used a consistent procedure toevaluate the impact of six different parameters on overall intelligi-bility in multitalker speech displays: 1) the signal-to-noise ratio;2) the number of competing talkers; 3) the voice characteristicsof the talkers; 4) the relative levels of the talkers; 5) the apparentlocations of the talkers; and 6) the listener’s a priori informationabout the listening task. The results are discussed in terms of theirimplications in the design of more effective multitalker speech dis-plays.1. INTRODUCTIONMany critically important tasks require listeners to monitor and re-spond to speech messages originating from two or more competingtalkers. Air trafﬁc controllers, emergency service dispatchers, andmilitary commanders are just a few examples of personnel whoroutinely make life and death decisions on the basis of informa-tion they receive through multichannel communications systems.Although a number of researchers have commented on the sub-stantial beneﬁts that audio display technology can provide in thesemultitalker communications tasks, no consensus has been reachedon how to design an “optimal” multitalker speech display. In part,at least, this lack of consensus is a result of the extreme complexityof the multichannel listening problem— performance in such tasksdepends on a wide variety of factors, including: 1) the ambientnoise in the communications system; 2) the number of competingtalkers; 3) the voice characteristics of the talkers; 4) the relativelevels of the talkers; 5) the apparent locations of the talkers; and 6)the listener’s a priori knowledge about the listening task. A furthercomplicating issue is the variety of methodologies that have beenused to examine these factors: procedural variations often makeit difﬁcult to compare the results of different multitalker listeningexperiments. In this paper, we review a series of experiments thatused the same speech intelligiblity test (the Coordinate ResponseMeasure or CRM) to examine the impact of six different audio dis-play design parameters on overall intelligibility in a multichannelcommunications task. This allows a consistent comparison of therelative importance of each of these factors that can be used as aguide in the design of more effective multitalker speech displays.2. EXPERIMENTAL METHODOLOGY: THECOORDINATE RESPONSE MEASUREAll of the experiments described in this paper were conducted us-ing the Coordinate Response Measure (CRM). This speech intelli-gibility testwasoriginally developed to provide greater operationalvalidity for military communications tasks than standard speechintelligibility tests based on phonetically balanced words [1]. Inthe CRM task, a listener hears one or more simultaneous phrasesof the form “Ready, (Call Sign), go to (color) (number) now” withone of eight call signs (“Baron,” “Charlie,” “Ringo,” “Eagle,” “Ar-row,” “Hopper,” “Tiger,” and “Laker”), one of four colors (red,blue, green, white), and one of eight numbers (1-8). The listener’stask isto listen for the targetsentence containing theirpre-assignedcall sign (usually “Baron”) and respond by identifying the colorand number coordinates contained in that target phrase.Although the CRM was originally intended to measure speechintelligibilitywithanoise masker, itscall-sign-based structure makesit ideal for use in multitalker listening tasks. The embedded callsign is the only feature that distinguishes the target phrase fromthe masking phrases, so the listener is forced to listen to the con-tent of all the simultaneous phrases in order to determine whichphrase contains the most relevant information. In this regard, it issimilar to many command and control tasks where operators arerequired to monitor multiple simultaneous channels for importantinformation that may originate from any channel in the system.The experiments described in this paper were conducted usingthe corpus of CRM speech materials that has been made publicallyavailable in CD-ROM format by researchers at the Air Force Re-search Laboratory [2]. This CRM corpus contains all 256 possibleCRM phrases (8 call signs X 4 colors X 8 numbers) spoken byeight different talkers (four male, four female). The experimentsdescribed in the following sections were conducted using this cor-pus. In all cases, the stimulus consisted of a combination of atarget phrase, which was randomly selected from all of the phrasesin the corpus with the call sign “Baron,” and one or more mask-ing phrases, which were randomly selected from the phrases inthe corpus with call signs, colors, and numbers that differed fromthose used in the target phrase. These stimuli were presented overheadphones at a comfortable listening level (approximately 70 dBSPL), and the listener’s responses were collected either by usingthe computer mouse to select the appropriately colored numberfrom a matrix of colored numbers on the CRT or by pressing anappropriately marked key on a standard computer keyboard. Eachof the following sections discusses a different factor that inﬂuencesICAD02-1