Morphological differences affect speech transmission over bone conduction.

In bone conduction (BC), acoustic signals travel through an individual's bones and soft tissues rather than travelling through the air. While bone conduction hearing and communication are important in everyday life, nature, and technology, little is known about how individual differences affect the transmission of bone-conducted sound. Individuals differ in the sizes, shapes, and proportions of their craniofacial bones, leading to potentially different bone-conducted sound transmission effects in different individuals. Individual differences may influence the audibility and quality of bone-conducted sound, and this was studied using speech intelligibility as an assessment criterion for bone-conducted sound transmission. Thirty-two human participants were first subjected to a series of anthropometric craniofacial measurements. Eight morphologically diverse talkers were recorded with bone microphones placed at different skull locations, and 24 morphologically diverse listeners listened to these samples over bone conduction headphones. Modified Rhyme Test results suggest that skull morphology influences BC speech intelligibility and does so differently at different skull locations. Understanding morphological effects can improve bone conduction sound transmission models and may help to enhance BC technology for a diverse user population.

The effect of vocal and demographic traits on speech intelligibility over bone conduction.

Bone conduction (BC) communication systems provide benefits over air conduction systems but are not in widespread use, partly due to problems with speech intelligibility. Contributing factors like device location and background noise have been explored, but little attention has been paid to the role of individual user differences. Because BC signals travel through an individual's skull and facial tissues, demographic factors such as user age, sex, race, or regional origin may influence sound transmission. Vocal traits such as pitch, spectral tilt, jitter, and shimmer may also play a role. Along with microphone placement and background noise, these factors can affect BC speech intelligibility. Eight diverse talkers were recorded with bone microphones on two different skull locations and in different background noise conditions. Twenty-four diverse listeners listened to these samples over BC and completed Modified Rhyme Tests for speech intelligibility. Forehead bone recordings were more intelligible than condyle recordings. In condyle recordings, female talkers, talkers with high fundamental frequency, and talkers in background noise were understood better, as were communications between talkers and listeners of the same regional origin. Listeners' individual traits had no significant effects. Thoughtful application of this knowledge can help improve BC communication for diverse users.

Effects of Bone Vibrator Position on Auditory Spatial Perception Tasks.

OBJECTIVE: This study assessed listeners' ability to localize spatially differentiated virtual audio signals delivered by bone conduction (BC) vibrators and circumaural air conduction (AC) headphones. BACKGROUND: Although the skull offers little intracranial sound wave attenuation, previous studies have demonstrated listeners' ability to localize auditory signals delivered by a pair of BC vibrators coupled to the mandibular condyle bones. The current study extended this research to other BC vibrator locations on the skull. METHOD: Each participant listened to virtual audio signals originating from 16 different horizontal locations using circumaural headphones or BC vibrators placed in front of, above, or behind the listener's ears. The listener's task was to indicate the signal's perceived direction of origin. RESULTS: Localization accuracy with the BC front and BC top positions was comparable to that with the headphones, but responses for the BC back position were less accurate than both the headphones and BC front position. CONCLUSION: This study supports the conclusion of previous studies that listeners can localize virtual 3D signals equally well using AC and BC transducers. Based on these results, it is apparent that BC devices could be substituted for AC headphones with little to no localization performance degradation. APPLICATION: BC headphones can be used when spatial auditory information needs to be delivered without occluding the ears. Although vibrator placement in front of the ears appears optimal from the localization standpoint, the top or back position may be acceptable from an operational standpoint or if the BC system is integrated into headgear.

Sound localisation ability of soldiers wearing infantry ACH and PASGT helmets.

Helmets provide soldiers with ballistic and fragmentation protection but impair auditory spatial processing. Missed auditory information can be fatal for a soldier; therefore, helmet design requires compromise between protection and optimal acoustics. Twelve soldiers localised two sound signals presented from six azimuth angles and three levels of elevation presented at two intensity levels and with three background noises. Each participant completed the task while wearing no helmet and with two U.S. Army infantry helmets - the Personnel Armor System for Ground Troops (PASGT) helmet and the Advanced Combat Helmet (ACH). Results showed a significant effect of helmet type on the size of both azimuth and elevation error. The effects of level, background noise, azimuth and elevation were found to be significant. There was no effect of sound signal type. As hypothesised, localisation accuracy was greatest when soldiers did not wear helmet, followed by the ACH. Performance was worst with the PASGT helmet.

A free-field method to calibrate bone conduction transducers.

Bone conduction communication systems employ a variety of transducers with different physical and electroacoustic properties, and these transducers may be worn at various skull locations. Testing these systems thus requires a reliable means of transducer calibration that can be implemented across different devices, skull locations, and settings. Unfortunately, existing calibration standards do not meet these criteria. Audiometric bone conduction standards focus on only one device model and on limited skull locations. Furthermore, while mechanical couplers may be used for calibration, the general human validity of their results is suspect. To address the need for more flexible, human-centered calibration methods, the authors investigated a procedure for bone transducer calibration, analogous to free-field methods for calibrating air conduction headphones. Participants listened to1s third-octave noise bands (125-12,500 Hz) alternating between a bone transducer and a loudspeaker and adjusted the bone transducer to match the perceived loudness of the loudspeaker at each test frequency. Participants tested two transducer models and two skull locations. Intra- and inter-subject reliability was high, and the resulting data differed by transducer, by location, and from the mechanical coupler. The described procedure is flexible to transducer model and skull location, requires only basic equipment, and directly yields perceptual data.

The effect of bone conduction microphone placement on intensity and spectrum of transmitted speech items.

Speech signals can be converted into electrical audio signals using either conventional air conduction (AC) microphone or a contact bone conduction (BC) microphone. The goal of this study was to investigate the effects of the location of a BC microphone on the intensity and frequency spectrum of the recorded speech. Twelve locations, 11 on the talker's head and 1 on the collar bone, were investigated. The speech sounds were three vowels (/u/, /a/, /i/) and two consonants (/m/, /∫/). The sounds were produced by 12 talkers. Each sound was recorded simultaneously with two BC microphones and an AC microphone. Analyzed spectral data showed that the BC recordings made at the forehead of the talker were the most similar to the AC recordings, whereas the collar bone recordings were most different. Comparison of the spectral data with speech intelligibility data collected in another study revealed a strong negative relationship between BC speech intelligibility and the degree of deviation of the BC speech spectrum from the AC spectrum. In addition, the head locations that resulted in the highest speech intelligibility were associated with the lowest output signals among all tested locations. Implications of these findings for BC communication are discussed.

Speech intelligibility and passive, level-dependent earplugs.

OBJECTIVES: Noise-induced hearing loss is one of the most common occupational diseases. Military personnel are at especially high risk due to the broad range of military noise hazards and the frequency of exposure. Hearing protectors are vital for this particular workforce, yet they can impede the ability to understand necessary communication in the field. Level-dependent hearing protectors are designed to protect the auditory system from the hazards of impulse noise, while preserving the ability to hear speech and other important auditory signals. The aim of this study was to evaluate the effect of two different passive, level-dependent earplugs (Combat Arms Earplugs; Sonic II Ear valves) on speech understanding of normal-hearing listeners in the presence of low-level background noise. The Combat Arms Earplug, developed specifically for use by military personnel, represented devices that attenuate impulse noise using small orifices and the Sonic II ear valve represented devices using an internal diaphragm. DESIGN: This study used a repeated-measures experimental design. Four scrambled lists of each of the four Northwestern University No. 6 50-word lists were presented in random order at 65 dB SPL in the presence of quiet and two different types of background noise: multitalker and military vehicle noise; using three ear conditions: NP (open ear), CA (Combat Arms Earplugs), and SO (Sonic II earplugs); and three signal-to-noise ratios (SNRs): -10, 0, and +10 dB. Word recognition scores (WRSs) of 18 native English-speaking adults with normal hearing sensitivity were measured in all test conditions. The percentage of words correctly repeated was used to determine differences between the two different level-dependent devices, types of background noise, and SNRs. RESULTS: Results showed a statistically significant increase in WRS as SNR increased from -10 to +10 dB. A repeated-measures analysis of variance for ear condition × noise × SNR indicated a significant main effect for SNR but not for type of noise or ear condition. A slight but significant interaction was found for SNR and ear condition. CONCLUSIONS: SNR had great impact on the ability of listeners to understand speech in the presence of background noise; however, the type of noise and the type of level-dependent device used did not. The results of the study support the notion that individuals potentially subjected to high-level impulse noise should be able to use level-dependent earplugs in low-level continuous noise without compromising speech understanding. More specifically, the passive, level-dependent earplugs currently used by military personnel do not appear to be detrimental to speech communication for listeners with normal hearing when the speech is at an average conversational level and the listener is actively attending to the signal.

Effect of static force on bone conduction hearing thresholds and comfort.

OBJECTIVE: To assess the effect of the static force of a bone vibrator on the results of bone conduction (BC) threshold measurements and comfort. DESIGN: BC thresholds were determined for 40 participants using the standardized P-3333 headband and a leather adjustable headstrap with variable static forces (2.4, 3.4, 4.4, 5.4 N). Comfort ratings were examined using a five-point Likert scale. RESULTS: Mean BC thresholds were within ± 2 dB across all conditions; differences may be considered small enough to be clinically insignificant. Participants experienced significantly greater discomfort with the P-3333 versus the adjustable headstrap. The mean static force of the P-3333 varied considerably and was higher in situ than the calibration standard of 5.4 N. CONCLUSIONS: The results suggest that future revisions of relevant international and national standards should address the use of an adjustable headstrap and a static force less than 5.4 N.

The effects of reverberation on a listener's ability to recognize target sentences in the presence of up to three synchronized masking sentences.

OBJECTIVE: To determine the effects of room reverberation on target sentence recognition in the presence of 0-to-3 synchronous masking sentences. DESIGN: Target and masker sentences were presented through four loudspeakers (± 90° and ± 45° azimuth; 1m from the listener) in rooms having reverberation times (RT) of 0.2, 0.4, 0.6, and 1.1 s. STUDY SAMPLE: Four groups of 13 listeners each participated in the study (N = 52). RESULTS: In rooms with RTs of 0.2, 0.4, and 0.6 s, mean speech recognition scores (SRSs) were similar, with scores ranging from 96-100%, 90-95%, 75-80%, and 53-60%, when 0, 1, 2, and 3 competing sentences were present, respectively. However, in the room with a RT = 1.1 s, SRSs deteriorated significantly faster as the number of competing sentences increased; mean scores were 93%, 73%, 26%, and 10%, in the 0, 1, 2, 3, competing sentence condition, respectively. The majority of errors in SRSs (98%) resulted from listeners reporting words presented in masking sentences along with those in target sentences (mixing errors). CONCLUSIONS: Results indicate that reverberation has a similar influence on SRSs measured in multi-talker environments, when room reverberation is ≤ 0.6 s. However, SRSs are dramatically reduced in the room with a RT = 1.1 s, even when only one competing talker is present.

Effects of white noise on Callsign Acquisition Test and Modified Rhyme Test scores.

The Callsign Acquisition Test (CAT) is a speech intelligibility test developed by the US Army Research Laboratory. The test has been used to evaluate speech transmission through various communication systems but has not been yet sufficiently standardised and validated. The aim of this study was to compare CAT and Modified Rhyme Test (MRT) performance in the presence of white noise across a range of signal-to-noise ratios (SNRs). A group of 16 normal-hearing listeners participated in the study. The speech items were presented at 65 dB(A) in the background of white noise at SNRs of -18, -15, -12, -9 and -6 dB. The results showed a strong positive association (75.14%) between the two tests, but significant differences between the CAT and MRT absolute scores in the range of investigated SNRs. Based on the data, a function to predict CAT scores based on existing MRT scores and vice versa was formulated. STATEMENT OF RELEVANCE: This work compares performance data of a common speech intelligibility test (MRT) with a new test (CAT) in the presence of white noise. The results here can be used as a part of the standardisation procedures and provide insights to the predictive capabilities of the CAT to quantify speech intelligibility communication in high-noise military environments.

Speech intelligibility of the callsign acquisition test in a quiet environment.

This paper reports on preliminary experiments aimed at standardizing speech intelligibility of military Callsign Acquisition Test (CAT) using average power levels of callsign items measured by the Root Mean Square (RMS) and maximum power levels of callsign items (Peak). The results obtained indicate that at a minimum sound pressure level (SPL) of 10.57 dBHL, the CAT tests were more difficult than NU-6 (Northwestern University, Auditory Test No. 6) and CID-W22 (Central Institute for the Deaf, Test W-22). At the maximum SPL values, the CAT tests reveal more intelligibility than NU-6 and CID-W22. The CAT-Peak test attained 95% intelligibility as NU-6 at 27.5 dBHL, and with CID-W22, 92.4% intelligibility at 27 dBHL. The CAT-RMS achieved 90% intelligibility when compared with NU-6, and 87% intelligibility score when compared with CID-W22; all at 24 dBHL.

Task-related suppression of the brainstem frequency following response.

Recent evidence has shown top-down modulation of the brainstem frequency following response (FFR), generally in the form of signal enhancement from concurrent stimuli or from switching between attention-demanding task stimuli. However, it is also possible that the opposite may be true--the addition of a task, instead of a resting, passive state may suppress the FFR. Here we examined the influence of a subsequent task, and the relevance of the task modality, on signal clarity within the FFR. Participants performed visual and auditory discrimination tasks in the presence of an irrelevant background sound, as well as a baseline consisting of the same background stimuli in the absence of a task. FFR pitch strength and amplitude of the primary frequency response were assessed within non-task stimulus periods in order to examine influences due solely to general cognitive state, independent of stimulus-driven effects. Results show decreased signal clarity with the addition of a task, especially within the auditory modality. We additionally found consistent relationships between the extent of this suppressive effect and perceptual measures such as response time and proclivity towards one sensory modality. Together these results suggest that the current focus of attention can have a global, top-down effect on the quality of encoding early in the auditory pathway.

Impact of a bone conduction communication channel on multichannel communication system effectiveness.

OBJECTIVE: In this study, the impact of including a bone conduction transducer in a three-channel spatialized communication system was investigated. BACKGROUND: Several military and security forces situations require concurrent listening to three or more radio channels. In such radio systems, spatial separation between three concurrent radio channels can be achieved by delivering separate signals to the left and right earphone independently and both earphones simultaneously. This method appears to be effective; however, the use of bone conduction as one channel may provide both operational and performance benefits. METHOD: Three three-channel communication systems were used to collect speech intelligibility data from 18 listeners (System I, three loudspeakers; System 2, stereo headphones; System 3, stereo headphones and a bone conduction vibrator). Each channel presented signals perceived to originate from separate locations. Volunteers listened to three sets of competing sentences and identified a number, color, and object spoken in the target sentence. Each listener participated in three trials (one per system). Each trial consisted of 48 competing sentence sets. RESULTS: Systems 2 and 3 were more intelligible than System I. Systems 2 and 3 were overall equally intelligible; however, the intelligibility of all three channels was significantly more balanced in System 3. CONCLUSION: Replacing an air conduction transducer with a bone conduction transducer in a multichannel audio device can provide a more effective and balanced simultaneous monitoring auditory environment. APPLICATION: These results have important design and implementation implications for spatial auditory communication equipment.

The effects of simulated hearing loss on speech recognition and walking navigation.

OBJECTIVE: The objective was to assess whether a concurrent but independent navigation task exacerbates the effects of hearing loss on speech recognition and whether hearing loss degrades performance of the navigation task during the concurrent but independent listening task. BACKGROUND: Navigation performance and speech comprehension both decrease when a driver follows hard-to-hear concurrent verbal instructions. It remains unknown how much both tasks would be affected when performed concurrently, if tasks were independent. METHOD: Participants performed a listening task by responding to Callsign Acquisition Test (CAT) stimuli at three simulated hearing levels. For each hearing level, one trial was performed with the participant standing still and another trial was performed while navigating a path in a virtual environment using a handheld map. In one more trial, participants navigated a path with no CAT. The proportion of call signs correctly repeated and the total time required to walk the path were measured. RESULTS: CAT scores showed an expected negative effect of hearing loss. Concurrent navigation produced an even larger decrease in CAT score. Hearing loss caused a slight but not significant decrease in navigation task performance. CONCLUSION: A person with hearing loss may communicate less effectively while walking than predicted on the basis of hearing loss alone. The hearing loss, however, does not significantly decrease walking performance in a simple navigation task. APPLICATION: Obtained results may guide soldier performance modeling and requirements for communication systems used during physical activity when a soldier's hearing becomes compromised during dismounted combat operations.

The effect of bone conduction microphone locations on speech intelligibility and sound quality.

This paper presents the results of three studies of intelligibility and quality of speech recorded through a bone conduction microphone (BCM). All speech signals were captured and recorded using a Temco HG-17 BCM. Twelve locations on or close to the skull were selected for the BCM placement. In the first study, listeners evaluated the intelligibility and quality of the bone conducted speech signals presented through traditional earphones. Listeners in the second study evaluated the intelligibility and quality of signals presented through a loudspeaker. In the third study the signals were reproduced through a bone conduction headset; however, signal evaluation was limited to speech intelligibility only. In all three studies, the Forehead and Temple BCM locations yielded the highest intelligibility and quality rating scores. The Collarbone location produced the least intelligible and lowest quality signals across all tested BCM locations.

Speech intelligibility measured with shortened versions of Callsign Acquisition Test (CAT).

UNLABELLED: The Callsign Acquisition Test (CAT) is a new speech intelligibility test developed by the Human Research and Engineering Directorate of the U.S. Army Research Laboratory (ARL-HRED). CAT uses the phonetic alphabet and digit stimuli combined together to form 126 test items. OBJECTIVE: The purpose of this study was to assess the reliability of data collected with shorter versions of CAT. DESIGN: A total of 5 shorter versions of the original list (CAT-120, CAT-60, CAT-40, CAT-30, and CAT-24) were formed and evaluated using 19 participants. Each of the subsets of CAT was presented in pink noise at signal-to-noise ratios (SNRs) of -6dB and -9dB. RESULTS: Results showed that shortened CAT lists have the capability of providing the same predictive power as the full CAT with good test-retest reliability. CONCLUSIONS: Under the experimental conditions of this study, any of the shorter versions of the CAT can be utilized in place of the full version to reduce testing times with no effect on predictive power.

Bone conduction reception: head sensitivity mapping.

This study sought to identify skull locations that are highly sensitive to bone conduction (BC) auditory signal reception and could be used in the design of military radio communication headsets. In Experiment 1, pure tone signals were transmitted via BC to 11 skull locations of 14 volunteers seated in a quiet environment. In Experiment 2, the same signals were transmitted via BC to nine skull locations of 12 volunteers seated in an environment with 60 decibels of white background noise. Hearing threshold levels for each signal per location were measured. In the quiet condition, the condyle had the lowest mean threshold for all signals followed by the jaw angle, mastoid and vertex. In the white noise condition, the condyle also had the lowest mean threshold followed by the mastoid, vertex and temple. Overall results of both experiments were very similar and implicated the condyle as the most effective location.

Callsign Acquisition Test (CAT): speech intelligibility in noise.

OBJECTIVE: The study was designed to assess the effects of noise on the intelligibility of speech elements used in the Callsign Acquisition Test (CAT), developed by the U.S. Army Research Laboratory. The CAT consists of 126 test items, or callsigns, each of which is made up of a two-syllable word selected from the 18-item military alphabet (Alpha-Zulu) followed by a one-syllable number (all numbers from 1 to 8, excluding 7). DESIGN: The CAT items were mixed with one of three different types of background noises (pink noise, white noise, and multitalker babble) and presented to 18 listeners. Speech-to-noise ratio for all three noises and the overall level of pink noise were varied in two separate experiments to determine how these variables affected speech intelligibility of the CAT items pronounced by a male talker. CONCLUSIONS: Test results demonstrate speech-to-noise ratio has a significant effect on speech intelligibility of the CAT items under all conditions. Pink noise generated the lowest speech intelligibility scores followed by multitalker babble and then white noise. A change in the overall level of pink noise had only small effect on CAT intelligibility.

Spatial audio through a bone conduction interface.

Headphones are the standard presentation device for radio communication in the military. Although bone conduction devices possess several advantages over headphones for some military applications, they are generally considered inappropriate for inclusion in a multi-channel system. The current study tested the feasibility of a multi-channel bone conduction system by measuring the localizability of spatialized auditory stimuli presented through a pair of bone conduction vibrators. Listeners localized a Gaussian noise stimulus spatialized with individualized head-related transfer functions (HRTFs). The sounds were presented from eight virtual locations on the horizontal plane (0, +/-45, +/-90, +/-135, and 180 degrees ) through either stereo headphones or a stereo bone conduction system. Localization performance was found to be nearly identical for both audio systems, indicating that bone conduction systems can be effectively used for displaying spatial information.

Most Comfortable Listening Level and Speech Attenuation by Hearing Protectors.

Hearing protectors attenuate both the background noise and the useful sounds embedded in noise such as the sounds of speech and warning signals. An effective hearing protector is one that attenuates background noise while leaving sufficient energy of speech and warning signals to reach the ear of the worker. At present, however, there are no established criteria for assessing effective change in speech-to-noise ratio caused by hearing protection devices (HPDs). One such criterion could be a change in most comfortable (listening) level (MCL) for speech caused by the presence of HPDs. In this study the HPD-related shift in MCL for speech presented in quiet was measured and compared with two measures of noise attenuation: Noise Reduction Rating (NRR) and high-medium-low (H-M-L). The results indicate that the MCL shift may be a sensitive measure of speech attenuation by HPDs, which together with the appropriate H-M-L may describe technical properties of HPDs.