Sound and spectrograms

Sound and spectrograms

This page deals with various technical questions about all types of sounds that bird can produce, their visual display and how to describe them. It’s also providing keys to analyze spectrograms.

  1. Sounds, songs and calls.
    1. What’s a sound ?
    2. Song or call ?
    3. Other bird sounds : drumming, wing sounds,  bill clapping…
  2. What’s a spectrogram ?
    1. Why spectrograms ?
    2. The three dimensions of a spectrogram
  3. Describing sounds
    1. Duration, pitch and loudness
    2. Fundamental frequency and harmonics
    3. Tone
    4. Sonic texture
    5. Spatial location

1. Sounds, songs and calls

1.1. What’s a sound ?

A sound is a wave, that is to say a mechanical vibration of air particles. The places where the particles are compressed are regions of high pressure. The places where the particles are pulled apart are regions of low pressure. Because your eardrum is very sensitive to small variations in air pressure, you can hear sounds.

Basically, producing a sound is hence just moving air particle is a specific way, that’s why you can hear the wind : the stronger (i.e most brutal) vibration of the particles, the louder the sound, The fastest the rhythm in pressure variation, the higher pitched the sound and so on…

1.2. Song or call ?

We commonly separate songs and calls . A song is often defined as a relatively structured vocalization used for reproduction purposes. Calls tend to be shorter, less built sounds used to communicate an anxiety or an individual’s location.

Each species and individual has a variety of songs and calls used in different contexts. In most cases the difference between a song and a call is straightforward once you’re familiar with the singing species. However, we are sometimes clueless to classify vocalizations as “songs” or “calls”. Therefore you can find words like “call-song”, “subsong”

Check this example for Marsh tit (Poecile palustris) :
Here is the typical song :

There the typical call :

And here a very surprising vocal production that I recorded in Brittany. It’s rather unique but Marsh tit are known to be very creative. This can be described as some kind of “call-song” :

Most birds you will hear singing are males, but recent articles show that female can actually sing too in most bird species.

1.3 Other bird sounds : drumming, wing sounds,  bill clapping…

In addition to the use of their vocal organs, many bird species produce other sounds, that can sometimes be diagnostic

Check out the following examples :

Wingflaps of a European nightjar (Caprimulgus europaeus) :

Drumming of a Eurasian three-toed woodpecker (Picoides tridactylus) :

Flight sound from the wings of a Common goldeneye (Bucephala clangula) :

Bill clappering of a White stork (Ciconia ciconia) :

 

2. What’s a spectrogram?

2.1 Why spectrograms ?

  • Humans are visual creature : for most of us, our visual memory is much better than our auditory memory.
  • Our ears are used to understand long low-pitched sequences of syllables, but not short high-pitched notes

Therefore, the idea to analyze unfamiliar sounds is to… convert them into something visual !

That’s what it is : a spectrogram is simply a two or three dimensional visualization of a sound.

2.2 The three dimensions of a spectrogram

Usually, there are two axes : time is displayed on the first one, while frequency is visible on the second one. There is also often a third dimension : loudness, that can be displayed through colours. The most intense colors are usually depicting the loudest sounds.

So the three dimensions are :

  • Duration, that is visible on the first axis of a spectrogram (abscissa). The unit of measure is the second (s). How many seconds does the sound last ? The longer the sound, the longer the spectrogram.
How to read duration on a spectrogram
  • The pitch, that is visible on the second axis of a spectrogram (ordinate). Actually, it is a frequency (the higher the frequency of sound waves, the higher-pitched the sound). In other words, high-pitched sounds occur when the air pressure on a given place undergoes rapid changes. Therefore, the unit of measure of the pitch is the Hertz (Hz), and for bird sounds it is usually comprised between 1 and 10 Kilohertz (kHz). The higher the point on the spectrogram, the higher-pitched the sound. Easy isn’t it ?
How to read pitch on a spectrogram
  • Loudness, often depicted with colors on spectrograms. This is why spectrograms are actually three-dimensional (third dimension = color). ). The unit of measure is the Decibel (Db). Usually, the more intense the colour, the louder the sound.

 

How to read loudness on a spectrogram

3.Describing sounds

There are six ways in which bird sound waves can be analyzed. They are:

  • duration
  • pitch
  • loudness
  • tone
  • sonic texture
  • spatial location

3.1 Duration, pitch and loudness

Luckily enough the first three components are immediately visible on the spectrogram, so just by reading numbers on a spectrogram you can already describe most of the sound !

Duration and pitch are very easy to describe, because you can directly read their value on the two main axes. Loudness is generally visible too by using colors or shades of gray. In most audio processing software you can get the value of the loudness by clicking on a given place of the spectrogram

Check this example of a Common crane’s (Grus grus) call opened with the RavenLite software :

By moving the cursor on a given part of the spectrogram, you can read the values at the bottom of the window.

We read the values of time ( duration of the sound = time at the end – time at the beginning), pitch (= frequency) and loudness (=power) just below the spectrogram.

 

3.2 Fundamental frequency and harmonics

Note that in this case (and quite often in bird sounds), there are several frequencies on the spectrogram. The lowest-pitched one is called the fundamental frequency (or first harmonic), and then you have the second harmonic and so on… So here we have four visible harmonics : the frequency of the first one is about 1kHz, the second about 2.3kHz, the third one 3.5kHz and the fourth one at about 4.7kHz (we guess there is also a very faint fifth one, about 5.9 khZ)

 

Fundamental frequency and harmonics

 

3.3 Sonic texture

Sonic texture describes the patterns occurring in bird songs. As we have seen in About bird songs, birds can produce several sounds at the same time. They also often repeat a single motive several times. This create a unique pattern of sound superposition or composition.

3.3.1 Isolated notes and sequences

The first thing to look at is whether the bird just produces an isolated sound, or whether it combines sounds to create a sequence.

Sequence and isolated sound

Most songs are sequences, while calls are often isolated sounds. But the delimitation is quite blurred by a huge variability across bird sounds.

It is often possible to count the number of a syllables in a sound, just as we do for the human language. Hence, isolated sounds can be monosyllabic, disyllabic or trisyllabic, and we can have disyllabic sequences like in this example :

Sequence of disyllabic sounds

Note that a trisyllabic “isolated” sound can often be considered as a sequence too.

3.3.2 Phrases, series, warbles and trills.

There are four basic patterns to describe sequences :

  • The first thing to look at is whether the notes are similar or not across the sequence. Usually this is quite obvious on spectrograms
  • The second step is to determine whether the notes can be easily separated or not (can you hear when the previous note ended and when the next one started, or is this difficult because the sequence is rather continuous ?). One other way to think of the problem is : would you be able to count how many notes there are when you listen at the sequence, or is it going to fast ?
The four kind of sound sequences.

 

3.3.3 Patterns in duration and pitch

Finally, it is often possible to distinguish some trends in a sequence : how does the sonic sequence evolves from its beginning to its end ? By focusing on speed and pitch we can figure out some patterns :

Sound patterns for speed and pitch

3.4 Tone

Tone is something more subjective and tricky. It depicts the shape of a sound on a spectrogram.

It can be described with several objective variables :

  • Modulation
  • Clarity
  • The variables you already know (duration, frequency…)

3.4.1 Modulation

Modulation quantifies the changes in pitch that occur in a single sound. Highly modulated sounds are sounds in which the frequencies varies a lot. Unmodulated sounds are often named “monotonous sounds”

Modulated and monotonous sounds.

 

The modulation is visible on a spectrogram as a sequence of upcurved and downcurved lines

Upcurves and downcurved spectrograms

 

3.4.2 Clarity

We say that a sound is clear when its shape on a spectrogram is sharp and well-defined. Otherwise, the sound is said to be noisy, or polyphonic if there are several overlapping sounds

 

Clear sound
Noisy sound
Polyphonic sound

 

 

3.4.3 Put it all together

With those objective criteria, you can describe tones. We use a lot of subjective single words such as “nasal”, “whistling”… that describe specific patterns in modulation,clarity,pitch and speed. See the following examples :

A buzzing sound is a sound with very fast modulation and a repetitive pattern.
A clicking sound is a very short, monosyllabic sound that covers a wide range of frequencies
A nasal sound is a noisy sound, with strong harmonics.
A whistling sound is a slightly downcurved, medium or high-pitched sound.

 

 

3.5 Spatial location

Finally, the spatial location refers to how a particular bird integrates its song in a whole sonic atmosphere (a soundscape). For example, the courtship flight (“roding”) of an Eurasian woodcock (Scolopax rusticola) can be characterized by a very special trajectory, with different calls (low-pitched grunts, and high-pitched squeaks) produced at different moments of the flight. In addition, males interact with each other.

 

Read the next page of “The quick guide to soundbirding” : Which recording device ?

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