Amplitude-modulated Noise Research Articles

Factors affecting masking release for speech in modulated noise for normal-hearing and hearing-impaired listeners

The Speech Reception Threshold for sentences in stationary noise and in several amplitude-modulated noises was measured for 8 normal-hearing listeners, 29 sensorineural hearing-impaired listeners, and 16 normal-hearing listeners with simulated hearing loss. This approach makes it possible to determine whether the reduced benefit from masker modulations, as often observed for hearing-impaired listeners, is due to a loss of signal audibility, or due to suprathreshold deficits, such as reduced spectral and temporal resolution, which were measured in four separate psychophysical tasks. Results show that the reduced masking release can only partly be accounted for by reduced audibility, and that, when considering suprathreshold deficits, the normal effects associated with a raised presentation level should be taken into account. In this perspective, reduced spectral resolution does not appear to qualify as an actual suprathreshold deficit, while reduced temporal resolution does. Temporal resolution and age are shown to be the main factors governing masking release for speech in modulated noise, accounting for more than half of the intersubject variance. Their influence appears to be related to the processing of mainly the higher stimulus frequencies. Results based on calculations of the Speech Intelligibility Index in modulated noise confirm these conclusions.

Properties of Correlated Neural Activity Clusters in Cat Auditory Cortex Resemble Those of Neural Assemblies

Spiking activity was recorded from cat auditory cortex using multi-electrode arrays. Cross-correlograms were calculated for spikes recorded on separate microelectrodes. The pair-wise cross-correlation matrix was constructed for the peak values of the correlograms. Hierarchical clustering was performed on the cross-correlation matrix for six stimulus conditions. These were silence, three multi-tone stimulus ensembles with different spectral densities, low-pass amplitude-modulated noise, and Poisson-distributed click trains that each lasted 15 min. The resulting neuron clusters reflect patches in cortex of up to several mm(2) in size that expand and contract in response to different stimuli. Cluster positions and size were very similar for spontaneous activity and multi-tone stimulus-evoked activity but differed between those conditions and the noise and click stimuli. Cluster size was significantly larger in posterior auditory field (PAF) compared with primary auditory cortex (AI), whereas the fraction of common spikes (within a 10-ms window) across all electrode activity participating in a cluster was significantly higher in AI compared with PAF. Clusters crossed area boundaries in <5% of the cases were simultaneous recording were made in AI and PAF. Clusters are therefore similar to but not synonymous with the traditional view of neural assemblies. Common-spike spectrotemporal receptive fields (STRFs) were obtained for common-spike activity and all-spike activity within a cluster. Common-spike STRFs had higher signal-to-noise ratio than all-spike STRFs and showed generally spectral and temporal sharpening. The coincident and noncoincident output of the clusters could potentially act in parallel and may serve different modes of stimulus coding.

Detection thresholds for amplitude modulation of noise with simultaneous modulating tone

In speech, the combination of amplitude-modulated (AM) noise and a low-frequency periodic source characterizes voiced fricative sounds. To investigate the modulation’s contribution to fricative auditory quality, AM white noise, with simultaneous sinusoidal component at the modulating frequency, provided stimuli for perceptual tests. Two AM detection-threshold experiments were conducted to establish the effect of varying the relative amplitude and phase of the tone. In the first experiment, tone and noise stimuli were separated within each trial by short pauses; in the second, the tone played continuously throughout the trial. AM detection thresholds when the tone was absent were similar to previously published thresholds for AM noise with no tone (averaging approximately −22 dB). The effect of the quieter tones (10–30 dB tone-to-noise ratio, TNR) was dependent on test condition. In the broken-tone experiment, a slight increment in threshold occurred for all relative phases. The constant-tone experiment revealed impaired detection when tone and noise modulation were in phase, and the converse when in antiphase (5-dB difference in threshold at 30-dB TNR). As TNR increased further (40–50-dB TNR), thresholds rose in all cases. Future work is needed to evaluate how this detectable AM percept influences quality and categorization of fricatives.

Behavior of the practically implemented filtered reference LMS algorithm in an active noise control system

A filtered-reference LMS algorithm is often used in practical active noise control systems. This algorithm is derived under the assumption of a stationary noise signal, and the order of the signal convolution is switched in the derivation process. However, the order of convolution cannot be changed in a real physical process. We examine the differences between these situations, i.e., theoretical formulation compared to practical control process, for various kinds of noise signals. Amplitude-modulated and low-pass noise signals are used as examples of disturbance signals. Results of the numerical simulations indicated that the practical implementation of the algorithm requires severer conditions for the convergence coefficient than those in the theoretical prediction. Analytical examination with the transformed coefficient update procedure also reveals the difference between them. To reduce this difference and achieve robust attenuation under practical conditions, a modified version of a filtered-reference LMS algorithm is introduced. Advantages of this modified version are verified through a series of simulations.

Speech masking release in listeners with flat hearing loss: Effects of masker fluctuation rate on identification scores and phonetic feature reception

Consonant identification was measured for a stationary and amplitude-modulated noise masker in four listeners with flat cochlear hearing loss, and four age-matched normal-hearing listeners. The masker modulation rate was systematically varied between 2 and 128 Hz. Masking release (MR), that is better identification performance in fluctuating, than in stationary noise, was highest in a masker fluctuating at 8–16 Hz in all normal-hearing listeners. In comparison, MR was only observed in two out of the four impaired listeners. In these listeners, MR was poorer than normal, and peaked at lower rates, that is 2 or 8 Hz. MR corresponded to increased reception of information for voicing, place, and manner between 2 and 64 Hz in all normal-hearing listeners. In impaired listeners, increased reception of information was mainly observed for manner, and mainly reduced for place, but these differences were not significant. For all phonetic features, MR was observed at lower masker fluctuation rates (≤32 Hz) than in normal-hearing listeners. This study therefore shows that cochlear damage affects MR, both quantitatively and qualitatively.SumarioSe midió la identificación de consonantes con un ruido enmascarador estacionario y de amplitud modulada en cuatro sujetos con pérdidas auditivas cocleares planas, y en cuatro sujetos normo-oyentes de edades similares. La tasa de modulación del enmascarador se varió sistemáticamente entre 2 y 128 Hz. La sustracción del efecto de enmascaramiento (MR), que tiene un mejor desempeño de identificación con ruido fluctuante que con ruido estacionario, fue mayor con un enmascarador que fluctuó entre 8 y 16 Hz, en todos los sujetos normales. Comparativamente, el MR sólo se observó en dos de los cuatro sujetos hipoacúsicos. En estos sujetos, el MR fue menor al normal y su pico se obtuvo a tasas bajas, esto es, de 8 a 12 Hz. El MR en sujetos normo-oyentes correspondió a un incremento en la recepción de la información en sonoridad, punto y modo de articulación entre 2 y 64 Hz. En sujetos hipoacúsicos, se observó un aumento en la recepción de información en modo, y aunque se vio reducida principalmente en cuanto al punto de articulación, estas diferencias no fueron significativas. Para todas las características fonéticas, el MR se observó a tasas más bajas de fluctuación (≤32 Hz) que en los sujetos normo-oyentes. Este estudio, por consiguiente, muestra que el daño coclear afecta el MR tanto cualitativa como cuantitativamente.

A perceptual learning investigation of the pitch elicited by amplitude-modulated noise

Noise that is amplitude modulated at rates ranging from 40 to 850 Hz can elicit a sensation of pitch. Here, the processing of this temporally based pitch was investigated using a perceptual-learning paradigm. Nine listeners were trained (1 hour per day for 6-8 days) to discriminate a standard rate of sinusoidal amplitude modulation (SAM) from a faster rate in a single condition (150 Hz SAM rate, 5 kHz low-pass carrier). All trained listeners improved significantly on that condition. These trained listeners subsequently showed no more improvement than nine untrained controls on pure-tone and rippled-noise discrimination with the same pitch, and on SAM-rate discrimination with a 30 Hz rate, although they did show some improvement with a 300 Hz rate. In addition, most trained, but not control, listeners were worse at detecting SAM at 150 Hz after, compared to before training. These results indicate that listeners can learn to improve their ability to discriminate SAM rate with multiple-hour training and that the mechanism that is modified by learning encodes (1) the pitch of SAM noise but not that of pure tones and rippled noise, (2) different SAM rates separately, and (3) differences in SAM rate more effectively than cues for SAM detection.

Détection de la modulation d'amplitude au niveau du colliculus inférieur : influence du signal porteur

Sound envelope considered as amplitude modulation contains relevant information about speech intelligibility. Hewitt and Meddis developed an auditory pathway model that codes amplitude modulation. It was extended to a multi channel one. Inferior colliculus response to amplitude-modulated white noise is compared with the response to amplitude-modulated tone in terms of firing rate and synchronisation index. The carrier signal does not affect single characteristic frequency unit. This result lets us propose a more complete structure to be explored.

Mechanisms underlying the auditory continuity illusion

This study investigates the auditory continuity illusion, combining psychophysics and magnetoencephalography. Stimuli consisted of amplitude-modulated (AM) noise interrupted by bursts of louder, unmodulated noise. Subjective judgments confirmed that the AM was perceived as continuous, a case of illusory continuity. Psychophysical measurements showed that the illusory modulation had little effect on the detection of a physical modulation, i.e., the illusory modulation produced no modulation masking. Duration discrimination thresholds for the AM noise segments, however, were elevated by the illusion. A whole-head magnetoencephalographic system was used to record brain activity when listeners attended passively to the stimuli. The AM noise produced a modulated magnetic activity, the auditory steady-state response. The illusory modulation did not produce such a response, instead, a possible neural correlate of the illusion was found in transient evoked responses. When the AM was interrupted by silence, oscillatory activity in the gamma-band range as well as slow evoked potentials were observed at each AM onset. In the case of the illusion, these neural responses were largely reduced. Both sets of results are inconsistent with a restoration of the modulation in the case of illusory continuity. Rather, they point to a role for onset-detection mechanisms in auditory scene analysis.

Effects of amplitude compression on first- and second-order modulation detection thresholds in cochlear implant listeners

The aim of this study was to examine the effects of instantaneous non-linear amplitude mapping on the detection of single-component and multicomponent temporal envelopes. To address this issue, first- and secondorder amplitude modulation detection thresholds were measured in four cochlear implant users with the intervention of the compression device of the implant processor. The compression device is set to produce either a strongly or a weakly logarithmic mapping of stimulus amplitude to electrical amplitude. ‘First-order’ modulation detection thresholds indicate the ability of listeners to detect sinusoidal amplitude modulation (SAM) applied to a white noise carrier; they are measured as a function of the rate of that modulation, fm. ‘Second-order’ modulation detection thresholds indicate the ability to detect sinusoidal modulation applied to the depth of a sinusoidally amplitude-modulated signal (here, a 16-Hz sinusoidally amplitude-modulated white noise); they are measured as a function of the rate of the modulation applied to the modulation depth (referred to as fm'). In each task, stimuli are transformed by the implant processor and are presented through one electrode at approximately the same level. The results show that, in cochlear implant listeners, both first- and second-order modulation detection thresholds measured at the lower rates (≤7 Hz) decrease slightly by about 3–6 dB when the stronger compression is used. No effect of compression is observed at higher rates. These results suggest that instantaneous logarithmic amplitude mapping has beneficial— but limited—effects on the detection of single-component and multicomponent temporal envelopes. These results are discussed in light of current models of temporal envelope processing.

Tonotopic organization in human auditory cortex revealed by progressions of frequency sensitivity.

Functional neuroimaging experiments have revealed an organization of frequency-dependent responses in human auditory cortex suggestive of multiple tonotopically organized areas. Numerous studies have sampled cortical responses to isolated narrow-band stimuli, revealing multiple locations in auditory cortex at which the position of response varies systematically with frequency content. Because appropriate anatomical or functional grouping of these distinct frequency-dependent responses is uncertain, the number and location of tonotopic mappings within human auditory cortex remains unclear. Further, sampling does not address whether the observed mappings exhibit continuity as a function of position. This functional magnetic resonance imaging study used frequency-swept stimuli to identify progressions of frequency sensitivity across the cortical surface. The center-frequency of narrow-band, amplitude-modulated noise was slowly swept between 125 and 8,000 Hz. The latency of response relative to sweep onset was determined for each cortical surface location. Because frequency varied systematically with time, response latency indicated the frequency to which a location was maximally sensitive. Areas of cortex exhibiting a progressive change in response latency with position were considered tonotopically organized. There exist two main findings. First, six progressions of frequency sensitivity (i.e., tonotopic mappings) were repeatably observed in the superior temporal plane. Second, the locations of the higher- and lower-frequency endpoints of these progressions were approximately congruent with regions reported to be most responsive to discrete higher- and lower-frequency stimuli. Based on these findings and previous anatomical work, we propose a correspondence between these progressions and anatomically defined cortical areas, suggesting that five areas in human auditory cortex exhibit at least six tonotopic organizations.

Influence of amplitude modulated noise on the recognition of communication signals in the grasshopper Chorthippus biguttulus.

The detection of acoustic communication signals in the presence of sinusoidally amplitude modulated noise was investigated in males of the grasshopper Chorthippus biguttulus. The auditory system of grasshoppers exhibits only poor spectral resolution. Hence, these animals are ideally suited to investigate noise tolerance in a system operating in the temporal domain. As a sensitive indicator for signal recognition the conspicuous phonotactic turning responses of males were recorded. The main result was that noise modulated at low frequencies (1.5-5 Hz) did not impair recognition compared to a unmodulated noise. With long stimuli even a moderate improvement of noise tolerance was observed, an effect that can probably be attributed to the existence of long troughs at low modulation frequencies during which the masking of the signal was reduced. Higher modulation frequencies (15-150 Hz), however, rendered detection and recognition increasingly difficult, due to a strong interference of the sound pulses of the masking noise with the syllable-pause structure of the species-specific signals. There are no indications for the operation of mechanisms analogous to comodulation masking release as found in vertebrates, nor for a spatial release from masking.

Amplitude modulation of underwater noise produced by seagoing vessels

Data obtained from the processing of records of underwater noise produced by seagoing vessels are used to refine the model of the noise amplitude modulation caused by the rotation of the propeller shaft and ship roll and pitch in a seaway. Specifically, the width of the frequency band of the modulation process, the value of the modulation coefficient, and the distribution of the magnitude and phase of the modulation coefficient over the carrier frequency spectrum are investigated.

Age-related alterations in the neural coding of envelope periodicities.

This research was guided by the working hypothesis that the aging auditory system progressively loses its ability to process rapid acoustic transients efficiently, and in elderly listeners, this results in difficulties in speech perception. Neural correlates of age-related deficits in temporal processing were investigated by recording from inferior colliculus (IC) neurons from young adult and old CBA mice. Single-unit responses were recorded to sinusoidally amplitude-modulated (SAM) noise carriers, presented at 65-80 dB SPL, having modulation frequencies (MFs) that ranged from 10 to 800 Hz. Because phasic-type temporal response patterns dominate responses to tone and noise in mammalian IC, we limited our analyses to only phasic units. Modulation transfer functions (MTF) for both rate (rMTF) and synchronization (sMTF) measures were used to derive respective best modulation frequencies (rBMF and sBMF). The main age-related finding was that there was an overall increase in response rate to SAM noise carriers and a decrease in the median upper cutoff frequency in units from old mice. At rBMF, the median spike count from units from old animals was 1.63 times greater, and at the sBMF, the median spike count was 2.29 times greater than the young adult sample. We explored whether the increase in driven activity was due to a change in the transient (first cycle response) or periodic (remaining response) component of the response to SAM noise. Median spike counts of the transient component decreased with increasing MF for both young adult and old units, with median counts consistently greater in the old sample as compared with young. Median spike counts for the periodic response remained relatively constant as a function of MF; however, there was a significantly greater (3 times) response for older units in a restricted range of MFs. The greater median spike counts found for the transient and periodic response was also evident when we analyzed the cycle-by-cycle response. The magnitude of the differences between the young adult and the old spike median responses was greatest at low MFs and then declined as MF increased. Finally, the young adult distribution of rBMFs extends to higher MFs than the old, with 36.0% of units having rBMFs >100 Hz as compared with only 12.5% of the old unit sample. We postulate that this age-related difference in rate coding of SAM noise carriers is consistent with a loss, or imbalance, of excitatory and inhibitory neural mechanisms known to shape encoding of envelope periodicities in the IC.

Measurement of auditory temporal processing using modified masking period patterns.

A common metric of auditory temporal processing is the difference in the threshold for a pure-tone signal masked by either unmodulated or amplitude-modulated noise. This technique may be viewed as a modification of the masking period pattern technique. Such measurements have been proposed as an efficient means of estimating auditory temporal resolution in a clinical setting, although in many cases threshold differences may reflect additional spectro-temporal processes. The primary purpose of the present experiment was to examine interactions among signal frequency and masker bandwidth and the effects of modulation frequency on modified masking period patterns. The results revealed unmodulated-modulated threshold differences that increased with increasing masker bandwidth and decreased with increasing modulation frequency. There was little effect of signal frequency for narrow-band noise maskers that were equal in absolute bandwidth across frequency. However, unmodulated-modulated threshold differences increased substantially with increasing signal frequency for bandwidths proportional to the signal frequency and for wideband maskers. Although the results are interpreted in terms of a combination of both within-channel and across-channel cues, the specific contributions of these cues in particular conditions are difficult to ascertain. Because modified masking period patterns depend strongly upon a number of specific stimulus parameters, and because it is difficult to determine with any precision the underlying perceptual processes, this technique is not recommended for use as a clinical measure of auditory temporal processing.

Signal detection in amplitude-modulated maskers. I. Behavioural auditory thresholds in a songbird.

Vertebrates have evolved mechanisms to exploit amplitude modulations in background noise for improving signal detection. However, the mechanisms underlying this masking release are not yet well understood. Here we present evidence for masking release observed in European starlings (Sturnus vulgaris, Aves) that were trained in a Go/NoGo paradigm to report the detection of a short tone (20 ms) in 100% sinusoidally amplitude-modulated noise maskers (400 ms duration). Maskers centred at the tone frequency were composed of one, three, or five spectrally adjacent noise bands each of auditory filter bandwidth. Envelopes of the masking noise bands were either in-phase (i.e. coherent) or successively phase shifted by 90 degrees (i.e. incoherent). A release from masking of up to 28 dB was observed for detection of signals presented in dips of the envelope of coherent maskers compared with those presented in peaks of coherent maskers and in incoherent maskers. For maskers limited to one auditory filter (i.e. limited to the analysis channel tuned to the test signal) this masking release was about 10 dB less than that observed for maskers allowing a comparison across three or five auditory filters. This indicates that both within-channel cues and across-channel cues are important for signal detection. These behavioural data provide the reference for the study of responses of auditory forebrain neurons in the same species reported in a companion paper [Nieder & Klump (2001) Eur. J. Neurosci., 13, 1033-1044].

Representation of Temporal Features of Complex Sounds by the Discharge Patterns of Neurons in the Owl's Inferior Colliculus

The spiking pattern evoked in cells of the owl's inferior colliculus by repeated presentation of the same broadband noise was found to be highly reproducible and synchronized with the temporal features of the noise stimulus. The pattern remained largely unchanged when the stimulus was presented from spatial loci that evoke similar average firing rates. To better understand this patterning, we computed the pre-event stimulus ensemble (PESE)-the average of the stimuli that preceded each spike. Computing the PESE by averaging the pressure waveforms produced a noisy, featureless trace, suggesting that the patterning was not synchronized to a particular waveform in the fine structure. By contrast, computing the PESE by averaging the stimulus envelope revealed an average envelope waveform, the "PESE envelope," typically having a peak preceded by a trough. Increasing the overall stimulus level produced PESE envelopes with higher amplitudes, suggesting a decrease in the jitter of the cell's response. The effect of carrier frequency on the PESE envelope was investigated by obtaining a cell's response to broadband noise and either estimating the PESE envelope for each spectral band or by computing a spectrogram of the stimulus prior to each spike. Either method yielded the cell's PESE spectrogram, a plot of the average amplitude of each carrier-frequency component at various pre-spike times. PESE spectrograms revealed surfaces with peaks and troughs at certain frequencies and pre-spike times. These features are collectively called the spectrotemporal receptive field (STRF). The shape of the STRF showed that in many cases, the carrier frequency can affect the PESE envelope. The modulation transfer function (MTF), which describes a cell's ability to respond to time-varying amplitudes, was estimated with sinusoidally amplitude-modulated (SAM) noises. Comparison of the PESE envelope with the MTF in the time and frequency domains showed that the two were closely matched, suggesting that a cell's response to SAM stimuli is largely predictable from its response to a noise-modulated carrier. The STRF is considered to be a model of the linear component of a system's response to dynamic stimuli. Using the STRF, we estimated the degree to which we could predict a cell's response to an arbitrary broadband noise by comparing the convolution of the STRF and the envelope of the noise with the cell's post-stimulus time histogram to the same noise. The STRF explained 18-46% of the variance of a cell's response to broadband noise.

Use of a simultaneous sentence perception test to enhance sensitivity to ease of listening.

The purpose of the study was to determine if a divided-attention, sentence-recall task was more sensitive to distortion of the speech signal than a conventional focused-attention task. The divided-attention task required listeners to repeat both of two sentences delivered simultaneously to the same ear. The focused-attention task required listeners to repeat a single sentence presented to one ear in quiet or in amplitude-modulated noise (0 dB signal-to-noise ratio). Distortion was introduced by peak clipping. Eighteen listeners with normal hearing were tested under three levels of peak clipping: 0 dB, 11 dB, and 29 dB (re: waveform peak). The effects of clipping were similar, on average, for simultaneous sentences and single sentences in noise. When data were separated by sentence length, however, the effects of clipping were found to be greater for the simultaneous-sentence task, but only for the short sentences (6 words or fewer). The simultaneous-sentence test, in its present form, is not more sensitive to the effects of clipping than is a single-sentence test in noise. Modification of the simultaneous-sentence test to include only short sentences, however, may provide greater test sensitivity than more conventional tests using single sentences in noise.

Voice-selective areas in human auditory cortex

The human voice contains in its acoustic structure a wealth of information on the speaker's identity and emotional state which we perceive with remarkable ease and accuracy. Although the perception of speaker-related features of voice plays a major role in human communication, little is known about its neural basis. Here we show, using functional magnetic resonance imaging in human volunteers, that voice-selective regions can be found bilaterally along the upper bank of the superior temporal sulcus (STS). These regions showed greater neuronal activity when subjects listened passively to vocal sounds, whether speech or non-speech, than to non-vocal environmental sounds. Central STS regions also displayed a high degree of selectivity by responding significantly more to vocal sounds than to matched control stimuli, including scrambled voices and amplitude-modulated noise. Moreover, their response to stimuli degraded by frequency filtering paralleled the subjects' behavioural performance in voice-perception tasks that used these stimuli. The voice-selective areas in the STS may represent the counterpart of the face-selective areas in human visual cortex; their existence sheds new light on the functional architecture of the human auditory cortex.

Quantifying the distortion products generated by amplitude-modulated noise.

When sinusoidal amplitude modulation (SAM) is applied to noise or tone carriers, the stimuli can generate audible distortion products in the region of the modulation frequency. As a result, when bandpass-filtered SAM noise is used to investigate temporal processing, a band of unmodulated noise is typically positioned at the modulation frequency to mask any distortion products. This study was designed to investigate the distortion products for bandpass noise carriers, and so reduce ambiguity about the form of this distortion and its role in perception. The distortion consists of two distortion-noise bands and a distortion tone at the modulation frequency. In the first two experiments, the level and phase of the distortion tone are measured using two different experimental paradigms. In the third experiment, modulation-frequency difference limens are measured for filtered SAM noise and it is shown that performance deteriorates markedly when the distortion tone is canceled. In a fourth experiment, masked threshold is measured at low frequencies for bands of high-frequency, unmodulated noise with the same levels and spectra as the SAM noises in the earlier experiments. The results confirm that unmodulated noise also produces quadratic distortion which may explain some aspects of earlier reports on remote masking.

The role of envelope modulation in spectrally unresolved iterated rippled noise

Iterated rippled noise (IRN) produces a pitch corresponding to the IRN delay. The pitch persists even when the sound is high-pass filtered at 12 times the reciprocal of the IRN delay, i.e., in the absence of resolved spectral peaks. Typically, when a sound produces a pitch in the absence of spectral cues, the pitch is explained in terms of periodic envelope modulation, for example, the pitch of a high-pass filtered cosine-phase harmonic complex, or the pitch of sinusoidally amplitude-modulated noise (SAMN). This study presents experiments designed to search for periodic modulation in IRN. The occurrence and significance of modulation is investigated in the envelope of the stimulus waveform as well as in the IRN envelope as represented after narrow-band filtering similar to that occurring in peripheral auditory filters. The results indicate that the envelope of band-pass filtered IRN reveals modulation but that the order of modulation (corresponding to the number of envelope maxima recurring every period) increases with increasing filter bandwidth. The occurrence of first-order modulation, like that of SAMN, is indirectly demonstrated for spectrally unresolved IRN in the lower unresolved frequency range between the 10th and 20th spectral peaks. The significance of recurring transients sometimes visible in the IRN waveform with respect to their contribution to the IRN pitch was assessed by replacing portions of the IRN period with random noise. The results of this experiment indicate that this ‘waveform modulation’ is not essential for the IRN pitch perception. The presence of temporal pitch in the absence of first-order modulation is demonstrated in two experiments involving the detection of phase delays and f 0 differences for spectrally separated, narrow bands of harmonic complexes and IRNs.