Denoising different types of noise.

<p>(a) White noise, (b) pink noise, and (c) brown noise.</p

Spectrogram representations of various bird species showing some of the typical appearances of sounds.

<p>(a) A fox sparrow (<i>Passerella iliaca</i>) song illustrating its syllables, phrases, and elements (S = syllable and E = element). (b)-(e) show representations of lines: (b) tui (<i>Prosthemadera novaeseelandiae</i>); (c) the <i>more-pork</i> sound of ruru (<i>Ninox novaeseelandiae</i>); (d) kakapo (<i>Strigops habroptilus</i>) <i>booming</i>; (e) brewer’s sparrow (<i>Spizella breweri</i>). (f)-(h) demonstrate blocks: (f) (long billed) marsh wren (<i>Cistothorus palustris</i>); (g) female North Island brown kiwi (<i>Apteryx mantelli</i>) call; (h) kakapo <i>chinging</i>. (i)-(j) show stacked harmonics: (i) male North Island brown kiwi whistles; (j) ruru <i>trill</i>. (k) oscillations: North Island saddleback (<i>Philesturnus rufusater</i>).</p

Wavelets and their relation to time-frequency resolution and wavelet packet decomposition.

<p>Time-frequency resolution in (a) STFT and (b) wavelets. Examples of mother wavelets: (c) <i>Haar</i>; (d) a subset of Daubechies wavelets; (e) Discrete Meyer wavelet. (f) Scaling and shifting the mother wavelet <i>Ψ</i>_{1,0(<i>t</i>)} gives two new wavelets <i>Ψ</i>_{2,0(<i>t</i>)} and <i>Ψ</i>_{2,1(<i>t</i>)}. (g) A level three wavelet packet decomposition tree (A- approximation and D- detail).</p

Box plot view of the results in (a) Table 3 and (b) Table 4.

<p>Box plot view of the results in (a) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146790#pone.0146790.t003" target="_blank">Table 3</a> and (b) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146790#pone.0146790.t004" target="_blank">Table 4</a>.</p

Box plot view of Table 5.

<p>(a) call series and (b) segmented calls.</p

Non-stationarity.

<p>(a) A non-stationary signal containing 20 Hz, 40 Hz and 80 Hz frequencies and (b) its power spectrum computed using the Discrete Fourier Transform.</p

Different mother wavelets produce different results.

<p>Same excerpt of a male kiwi whistle (a) original whistle and (b)—(e) denoised with different mother wavelets.</p

Examples of bird calls with various degrees of noise, the effect of band-pass filtering and power spectrum of white and pink noise.

<p>The top row of each sound figure displays the oscillogram and the second row the spectrogram. (a) A less noisy example of kakapo <i>chinging</i> with limited noise and (b) a noisy example of kakapo <i>chinging</i>. (c) An original male kiwi whistle and (d) its noise filtered (band-pass) signal. Noise is visible as a grey background in the spectrogram surrounding the sound depiction and most of the high-frequency variation in the oscillogram. Power spectrum of (e) white noise and (f) pink noise.</p

Birdsong Denoising Using Wavelets

<div><p>Automatic recording of birdsong is becoming the preferred way to monitor and quantify bird populations worldwide. Programmable recorders allow recordings to be obtained at all times of day and year for extended periods of time. Consequently, there is a critical need for robust automated birdsong recognition. One prominent obstacle to achieving this is low signal to noise ratio in unattended recordings. Field recordings are often very noisy: birdsong is only one component in a recording, which also includes noise from the environment (such as wind and rain), other animals (including insects), and human-related activities, as well as noise from the recorder itself. We describe a method of denoising using a combination of the wavelet packet decomposition and band-pass or low-pass filtering, and present experiments that demonstrate an order of magnitude improvement in noise reduction over natural noisy bird recordings.</p></div

Denoising overlapped songs.

<p>Male kiwi, female kiwi, and more-pork are overlapped in (a) and kakapo <i>chinging</i> overlapped with mottled petrels (<i>Pterodroma inexpectata</i>) in (b).</p