ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 104 (2018) 825 –829 DOI 10.3813/AAA.919232 Amplitude Modulation MayBeConfused with Infrasound Torsten Marquardt1),Carlos Jurado2) 1) UCL Ear Institute, University College London, 332 Gray’sInn Road, London, WC1X8EE, United Kingdom. [email protected] 2) Escuela de Ingeniería en Sonido yAcústica, Universidad de Las Américas, Avenue Granados and Colimes, Quito EC170125, Ecuador Summary Environmental infrasound is usually accompanied by low-frequency(LF)sounds. Considering that inner hair cell transduction equals half-waverectification, activity of low-frequencyauditory nervefibres may be indistin- guishable whether elicited by LF sound that is amplitude-modulated at an infrasonic rate, or LF sound that is superimposed onto infrasound that “biases” the basilar membrane position. We tested whether listeners are able to distinguish a63-Hz carrier tone, amplitude modulated at 8Hz, from a63-Hz pure tone that wasperceptu- ally loudness-modulated by an 8-Hz biasing tone. Using amaximum-likelihood procedure, 12 participants first adjusted the intensity of the 8-Hz tone so that the perceivedmodulation of the pure tone matched areference amplitude-modulated tone. Both stimuli types were then presented in random order,and participants had to iden- tify presentations which contained the infrasound tone. About half the participants performed close to chance. The best had 81% correct. Experiments with a125-Hz carrier tone gave similar results. Although performance may improve in a2-interval discrimination task, this would not be representative of real listening conditions. Results suggest that slowly amplitude-modulated LF sounds may underlie complaints about environmental infrasound, where measured infrasound levels are well belowsensation threshold. ©2018 The Author(s).Published by S. Hirzel Verlag · EAA. This is an open access article under the terms of the Creative Commons Attribution (CCBY4.0)license (https://creativecommons.org/licenses/by/4.0/). PACS no. 43.66.-x 1. Introduction stimulus types under consideration. Because the inner hair cell releases neuro transmitter only during basilar mem- Noise spectra assessed in response to complaints about en- brane (BM) movement towards scala vestibuli, signals be- vironmental infrasound most often reveal that the sound come effectively half-waverectified. In order to illustrate pressures of spectral components in the infrasound range this operation, the lower parts of the signals that are not (< 20 Hz)are well belowsensation threshold and there- coded by the AN are greyed out in the figure. Comparison fore should be inaudible to human listeners (e.g. [1]). of the remaining upper parts shows schematically that the Commonly,such finding closes the complaint case. The spiking probabilities of the AN fibres in response to those measured spectra, however, often cross the auditory sen- twostimuli are almost identical. sation threshold somewhere between 20 Hz and 100 Hz. Of course, this scenario is givenonly if the stimulus These supra-threshold low-frequencynoise components components are not spectrally resolved by the cochlea. might have pronounced envelope fluctuations with spectral This might easily be the case at its very apical end be- content well below20Hz, which might be easily mistaken cause the lowest auditory filters have relatively wide spec- as containing supra-threshold infrasound. tral tuning and more importantly,the lowest stimulus fre- Using simple sinusoidal stimuli, we tested under lab- quencythat has acharacteristic place on the human BM is oratory conditions whether listeners can actually distin- assumed to be as high as 50 Hz (see reviewing remarks in guish between stimuli mixes that contain true infrasound [2, pages 51–52]), if not even 80 Hz [3]. In other words, all and stimuli that are amplitude-modulated (AM) at an in- frequencycomponents lower than this share the very api- frasonic rate, i.e. do not actually contain frequencycom- cal end of the BM as their characteristic place (the place of ponents below20Hz. This might well not be the case if maximum vibration amplitude). The condition illustrated one considers the similarity in auditory nerve(AN)output in Figure 1istherefore not unlikely,and our test results that these twostimuli produce. Figure 1illustrates the two showthat listeners have indeed great difficulty in distin- guishing these twotypes of stimuli with carrier tones of Received9March 2018, 63 Hz and even 125 Hz. accepted 9September 2018. ©2018 The Author(s). Published by S. Hirzel Verlag · EAA. This is an open access article under the terms of the CC BY 4.0 license. 825 ACTA ACUSTICA UNITED WITH ACUSTICA Marquardt, Jurado: Amplitude modulation confusion Vol. 104 (2018) 2. Methods 2.1. Instrumentation The infrasound source used in this study is identical to that used by Kuehler et al. [4]: From ahermetically en- closed electrodynamic loudspeaker,an8-Hz biasing tone wastransmitted via an 8-m long polyethylene tube to the listener’sear.(It wasoriginally developed to be used in MEG and fMRI experiments.)For the last 40 cm, the 14 mm inner diameter of this tube wasreduced via acoupling into amore flexible tube with 2.5 mm inner diameter that took at its end an audiometric ear tip (ER3-14A, Etymotic Research)that washermetically fitted in the listener’sear canal. Before, in-between and after all psychometric mea- surements, the fitting wastested by measuring the SPL in situ with aminiature microphone (Knowles FG-23453) Figure 1. The twostimulus types considered in this study: abi- that wascoupled to the ear canal via asmall plastic tube ased tone (upper panel)and an AM tone (lower panel). The half- (20mmlength, 1mminner diameter)that penetrated the wave rectified versions of the signals (only the upper parts)are foam of the ear tip. It wascalibrated in a1.3-cm3 cavity almost indistinguishable. using aBrüel &Kjær 4153 microphone. Asecond such plastic tube penetrated the ear tip foam to deliverthe other sounds into the ear canal. These were produced by asmall biased were set to equal peak level. Levels corresponded insert earphone (ER4B, Etymotic Research)that wasdi- to either 40 phon, or 50 phon of the pure tone (ISO 226). rectly drivenbythe line output of the audio device (RME The phase of the amplitude modulation and that of the 8- Fireface UC). Using separate sound sources ensured that Hz biasing tone wasindependent and random in each trial. AM due to hardware non-linearities were <1%. In con- The duration of all stimuli was1200 ms, including 250 ms trast, the line output of the 8-Hz biasing tone waslow-pass cosine ramps at onset and offset. filtered (6 dB/octave,passive,fc=10 Hz)before power amplification (BEAK Type BAA120). 2.4. Procedures This filter,together with the acoustic low-pass filtering effect of the 8-m polyethylene tube and appropriate elec- The measurement of each subject, including breaks, took tric attenuation, made sure that at maximum electric out- up to three hours. It wasdone in the ear of their pref- put of the audio device, the sound pressure in the ear canal erence. Before the discrimination task could commence, could not exceed 105 phon [5, 6]. LRBT had to be determined for the eight AM reference stimuli. Initially,subjects made themselves familiar with 2.2. Subjects the infrasound biasing tone and its effect on the pure tone in asimple LRBT adjustment procedure (buttons “up” and Tenfemale and four male normal-hearing subjects (18to “down”), starting from lowlevels. Forall eight condi- 49 years), without self-reported hearing abnormality,were tions, theywere asked to set the biasing tone levelsothat recruited. Their auditory threshold for 8Hz, 63 Hz and the pure tone in the second interval wasperceivedwith 125 Hz wastested using the standard audiometric proce- equal modulation as the leading reference AM tone. These dure (but with 3-dB instead of 5-dB steps). None of the served as individual starting levels for the subsequent, subjects had thresholds above 10 dB HL [5, 6]. However, more accurate maximum-likelihood-tracking (MLT)pro- one male and one female subject were unable to perform cedure. Here, both kind of stimuli were presented in ran- the psychometric procedure so that only 12 subjects com- dom order in twointervals. In a2AFC task, the subjects pleted the study. were asked: “Which of the twointervals wasstronger modulated?” The tracks of the eight conditions, each end- 2.3. Stimulus conditions ing after sixteen trials, were interleavedinrandom order. Atotal of eight reference AM stimuli were used, four Foreach condition, the average LRBT of three repeats de- with acarrier frequencyof63Hzand four with 125 Hz. fined the individual’sLRBT,unless avalue deviated by We predicted that the infrasound detection might be eas- more than 4dBfrom anyother,inwhich case it wasex- ier at larger sound pressure, and modified therefore two cluded from the average. further parameters of the AM tone for which we expected The LRBT were then individually set for the final dis- the required biasing tone level(LRBT)tovary to produce crimination task, which wasa1-interval, yes-no proce- amatching perceivedloudness modulation of the biased dure with the question: “Did this stimulus contain infra- pure tone: the modulation depth of the AM tone (25% sound?” Foreach of the eight conditions, atotal of 50 tri- and 37.5%)and its sound pressure. All modulations were als were presented, where half consisted of the AM tone clearly audible. Since we expected asuppression in loud- and half of the corresponding biased tone (containing the ness due to biasing [7], the AM tones and pure tones to be infrasound at LRBT). This gave atotal of 400 trials, which 826 Marquardt, Jurado: Amplitude modulation confusion ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 104 (2018) were presented in random order.The variation in loudness and modulation depth across the eight stimulus conditions 116 modulation depth made the task less monotonous for the subjects. Before the 37.5% actual test, the subjects had atraining session with feed- 114 25% back, using ashorter series of 48 trials.