Vocalisation and soundtrack: Interpretations and analyses

posted in: Vocalisation | 1

In an earlier post on the Oriental Magpie Robins’ (Copsychus saularis) vocalisation by Sun Chong Hong, Hui commented “Hi, I noticed that the vocalisation sounds are echoing one another. Is this a result of the second bird responding to the first bird’s call in a similar way or just an effect of the recording? Just curious.”

Chong Hong confirmed that there was no echo. In fact there were two calls by two different birds, as he explains below with the help of the soundtrack (below).

“Please see the attached waveforms which helps to visualise the soundtrack.

“…the audio recording was in stereo, meaning there were two tracks. The upper track represents the left channel, as what you would hear from the speaker placed on the left side. The lower track shows the right channel.

“The x-axis in the soundtracks represents the time scale in seconds: 0 to 60 seconds (represented as 1:00 = 1 min). From 1 min on, the scale shows 1:00 to 1:15 (1 min 15 sec). Thus for this Oriental Magpie Robin stereo sound track, the duration is about 1min 12 sec. The Y-axis indicates the amplitude (loudness) of the sound.

“In this particular example, bird 1’s vocalisation (represented by the large waveform, arrowed), when heard through a pair of speakers, would sound like it was coming from the middle in between the two speakers, because the left and right channels were of about the same loudness. As for bird 2 (smaller waveform, arrowed), the right channel sound could be seen to be louder than the left channel, and the sound would be heard as coming from the right side of bird 1.

“For the first few calls of bird 1 (represented by the bigger waveforms), no response or the so-called ‘echo’ could be heard (except the very small waveforms that followed the second, marked ‘x’ and the third calls, and if you refer to the earlier post and listen carefully with the volume turned up, you can actually detect the very faint call of a Spotted Dove.)

“It could be argued that this could be a case of ‘doctoring’ using the Audacity Digital Audio Editor, just like what Photoshop could do wonders to pictures. However, the convincing proof follows. The call of bird 1 @50 sec was preceded by the call of bird 2 (represented by the much smaller waveforms) in the background. It (bird 2) continued immediately after bird 1’s call @50 sec ended. This showed that there were 2 birds, one responding to the other.

“It is interesting to look at the waveform picture while listening to the calls in the video.

“By the way, @23 sec, the sound was made by bird 1. It was an interrupted or truncated song. Bird 1 was probably distracted midway through its vocalisation.

“Note on Audacity Digital Audio Editor: It can remove unwanted noise, generate silence between wanted sound, amplify soft sound, reduce loud sound, plus many other features.”

Sun Chong Hong
March 2011

  1. Sun Chong Hong

    Credit must be given to YC for his inclusion of arrows to the waveform picture and the tidying of some parts of the text to improve clarity.

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