Data from: Measurement artefacts lead to false positives in the study of birdsong in noise

1: Numerous studies over the past decade have reported correlations between elevated levels of anthropogenic noise and a rise in the minimum frequency of acoustic signals of animals living in noisy habitats. This pattern appears to be occurring globally, and higher pitched signals have been hypothesized to be adaptive changes that reduce masking by low-frequency traffic noise. However, the sound analysis methods most often used in these studies are prone to measurement errors that can result in false positives. In addition, the commonly used method of measuring frequencies visually from spectrograms might also lead to observer-expectancy biases that could exacerbate measurement errors.
2: We conducted an experiment to (a) quantify the size and type of errors that result from “eye-balling” frequency measurements with cursors placed manually on spectrograms of signals recorded in noise and no-noise conditions, and (b) to test whether observer expectations lead to significant errors in frequency measurements. We asked 54 volunteers, blind to the true intention of our study, to visually measure the minimum frequency of a variety of natural and synthesized bird sounds, recorded either in noise, or no-noise conditions. Test subjects were either informed or uninformed about the hypothesized results of the measurements.
3: Our results demonstrate that inappropriate methodology in acoustic analysis can yield false positives with effect sizes as large, or even larger, than those reported in published studies. In addition to these measurement artefacts, psychological observer biases also led to false positives – when observers expected signals to have higher minimum frequencies in noise, they measured significantly higher minimum frequencies than uninformed observers, who had not been primed with any expectation.
4: The use of improper analysis methods in bioacoustics can lead to the publication of spurious results. We discuss alternative methods that yield unbiased frequency measures and we caution that it is imperative for researchers to familiarize themselves both with the functions and limitations of their sound analysis programs.. In addition, observer expectancy biases are a potential source of error not only in the field of bioacoustics, but in any situation where measurements can be influenced by human subjectivity.

1: 近十年来，大量研究均报道了人为噪声（anthropogenic noise）水平升高与嘈杂生境中动物声学信号最低频率上升之间的相关性。这一现象似乎在全球范围内普遍存在，学界曾提出假设：高频信号是一种适应性变化，可降低低频交通噪声带来的听觉掩蔽效应。然而，此类研究中最常使用的声学分析方法极易产生测量误差，甚至可能导致假阳性结果。此外，这类研究中常用的手动将光标置于声谱图（spectrogram）上目视读取频率的方法，还可能引入观察者期望偏倚（observer expectancy biases），进一步加剧测量误差。
2: 本研究开展了一项实验，旨在(a)量化通过手动将光标置于声谱图上“目测”频率所产生的误差类型与大小，该操作针对噪声环境与无噪声环境下录制的信号；(b)检验观察者预期是否会对频率测量造成显著误差。我们邀请了54名对研究真实意图不知情的志愿者，让他们目视测量多种经噪声环境或无噪声环境录制的自然鸟鸣与合成鸟鸣声的最低频率。受试对象被分为两组：一组被告知该测量的假设性结果，另一组则未被提前告知相关预期。
3: 研究结果表明，声学分析中使用不当的方法可能产生假阳性结果，其效应量（effect sizes）甚至大于已发表研究中报道的数值。除上述测量伪差（measurement artefacts）外，心理层面的观察者期望偏倚同样会导致假阳性结果：当观察者预期噪声环境下的信号具有更高的最低频率时，他们测得的最低频率显著高于未受任何预期引导的不知情观察者。
4: 生物声学（bioacoustics）领域中，若采用不当的分析方法，可能会导致虚假研究结果被发表。本文讨论了可产生无偏频率测量值的替代分析方法，并提醒研究者务必熟悉其声学分析软件的功能与局限性。此外，观察者期望偏倚不仅是生物声学领域的潜在误差来源，在任何测量结果可能受人类主观性影响的研究场景中均存在风险。