The major theme when rating the quality of audio recordings is this:
How loud is the target bird sound compared to the background noise in the recording?
For bird sound recordings, the term “background noise” means all sounds other than the target bird species: wind, water, cars, planes, boats, insects, other bird species, talkative birding companions, barking dogs, roosters, loud music, and more. It’s a noisy world out there!
An excellent (5-star) sound recording has a very loud target species and virtually no background noise. A very poor (1-star) sound recording has so much background noise that it is difficult to even hear the target species. When you listen to a sound recording for quality rating purposes, pay close attention to how loud the target species is, and then also listen carefully to see how much background noise is audible. Good speakers or headphones are best for hearing all the details of a recording, but if you can only listen through laptop speakers, be sure to turn up the volume so that you can hear as much detail (both good and bad) as possible.
Here’s a quick guide to assigning audio quality scores:
5 Stars: very strong target sound with little or no background noise
4 Stars: strong target sound with limited background noise
3 Stars: good target sound with moderate background noise
2 Stars: weak target sound with significant background or handling noise
1 Star: very weak target sound that is barely audible due to high background or handling noise
The best way to assign an audio quality score is to listen to a recording while also viewing the associated Macaulay Library spectrogram. A spectrogram is a visual representation of a sound recording that shows different sound frequencies (y-axis) against time (x-axis). And, most importantly for quality rating, a spectrogram also shows the relative loudness of different sounds. Very loud sounds appear black in ML spectrograms, while soft sounds are light gray, and silence is white. Below are spectrograms of recordings ranging from 5 stars to 1 star:
Note the strong contrast in this spectrogram between the two loud (black) calls and the nearly silent (white) background. Click on the spectrogram to hear the full recording.
This spectrogram also shows a strong contrast between the target sound and the background, but there is some low-level background noise (the gray shading seen in much of the spectrogram), as well as an audible background species.
Note how the overall appearance of this spectrogram is much darker and grayer than the 5-star and 4-star spectrograms. This means that there is more background noise throughout the recording. And, in certain places (around 4 kHz and 8 kHz), there are prominent insect sounds.
The two target sounds in this spectrogram are still easily seen, but the darker gray background indicates that there is even more noise in this recording. In addition, there is a strong insect band visible between 6 and 7 kHz, and a rather loud, nearly continuous background bird species visible at 2 kHz.
In this spectrogram, there is very little contrast between the three descending target sounds (circled) and the noisy gray background.
∙ LISTEN TO IT ALL: Be sure to listen to more than just the first few seconds of a recording before assigning a quality grade. In many cases, the quality will improve later in the recording as a result of the recordist moving closer to the target bird.
∙ HANDLING NOISE: If a recording contains a significant amount of handling noise—noise made by the recordist by moving, hitting, or excessively gripping a microphone or smartphone—consider deducting one star from the score that the recording might otherwise receive. Below is an example of a recording with prominent handling noise:
∙ RECORDING LEVEL: Don’t be fooled by a spectrogram like this that is predominantly white:
This spectrogram might seem like it shows a high-quality recording because the background is entirely white and white backgrounds are an element of 4-star and 5-star recordings. However, high-quality recordings contain both a white background and strong, black target sounds. In this spectrogram, the target sounds are light gray and barely visible. This recording was made at an extremely low recording level, resulting in this washed-out white spectrogram and a target sound that is barely audible. If you see a spectrogram that looks like this and listen to a recording where it is difficult to hear the target sound, give the recording a 1-star or 2-star rating.
∙ EXTREME EDITING: If you see a recording like this that has large portions of audio deleted, give the recording a 1-star or 2-star recording depending on the severity of the editing. In this case, the editing is very extreme, with all sound above 2 kHz removed, so the recording should be given one star:
The Macaulay Library encourages recordists to perform only minimal editing on their recordings, using only a gentle (<250 Hz) filter when necessary. The above recording sounds very unnatural, because so much sound has been removed from it. In addition, valuable acoustic information about the target species has been lost due to the extreme editing. Compare the above recording with this other recording of the same species:
Note the rich harmonic content that is visible (and audible) above 2 kHz, which is where the heavily-edited recording is truncated.