Variable-rate frequency sweeps and their application to the measurement of otoacoustic emissions

Abstract

Swept tones allow the efficient measurement of otoacoustic emissions (OAEs) with fine frequency resolution. Although previous studies have explored the influence of different sweep parameters on the measured OAE, none have directly considered their effects on the measurement noise floor. The present study demonstrates that parameters such as sweep type (e.g., linear or logarithmic), sweep rate, and analysis bandwidth affect the measurement noise and can be manipulated to control the noise floor in individual subjects. Although responses to discrete-tone stimuli can be averaged until the uncertainty of the measurement meets a specified criterion at each frequency, linear or logarithmic sweeps offer no such flexibility. However, measurements of the power spectral density of the ambient noise can be used to construct variable-rate sweeps that yield a prescribed (e.g., constant) noise floor across frequency; in effect, they implement a form of frequency-dependent averaging. The use of noise-compensating frequency sweeps is illustrated by the measurement of distortion-product OAEs at low frequencies, where the ear-canal noise is known to vary significantly.

Muscles in and around the ear as the source of “physiological noise” during auditory selective attention: A review and novel synthesis

“…Before looking at each of these in more detail, it first helps to be clear that the sounds these muscles make has considerable power. Figure 1 shows the spectrum of sound in the ear canal as reported by Christensen and colleagues (Christensen et al, 2019), and it is notable that the sound power at about 50 Hz is some 40-50 dB higher than the power of otoacoustic emissions a little beyond 1 kHz. The low-frequency sound increases at a rate of 18 dB per octave, meaning that the power at 10-20 Hz, where most muscle sound peaks, is particularly strong.…”

“…Electronic noise is tens of decibels below the physiological noise. Reproduced from Christensen et al (2019) with permission of the Acoustical Society of America Figure 1 shows work done in 2019, but it is remarkable to read a report from 70 years earlier (Gordon & Holbourn, 1948) of the results of placing a microphone in the ear canal. Gordon and Holbourn studied a range of muscles-jaw, eyelid, and next to the ear-but the clearest results came from what they thought was the auricularis superior muscle when the pinna was voluntarily contracted (the ear was 'wiggled').…”

Extended low-frequency phase of the distortion-product otoacoustic emission in human newborns

“…Approximately half 1 of the duration was spent in the lowest-frequency octave through 0.5 kHz, half of the remaining duration in the octave through 1 kHz, and so on. Averaging in bands a fraction of an octave wide then reduces the noise at low frequencies more than it does at high frequencies [see further details in Christensen et al (2019) ]. To further lower the noise at low frequencies, the upper boundary of the frequency range changed with increasing repetitions.…”

Swept Along: Measuring Otoacoustic Emissions Using Continuously Varying Stimuli

“…Nothing requires that sweep trajectories be monotonic or traversed at a uniform rate [ 10 ]. For example, and can easily be varied in more complex and interesting ways, such as those shown in Fig.…”

“…Reducing the sweep rate at low frequencies can also be a useful tool for countering the frequency dependence of contaminating noise sources. Since biological and environmental noise is generally more troublesome at lower frequencies, adjusting the sweep rate to increase the “dwell” or effective measurement time at these frequencies—or, more generally, as a function of the expected noise floor—can help to ensure a more uniform SNR across frequency [ 10 ].…”