Replicating Bird Song with Closed-Volume Pressure Control

How do birds sing? Much like you and I do, it turns out.

New research by Dr. Coen Elemans et al in the journal Nature Communications (“Universal mechanisms of sound production and control in birds and mammals,” published Nov. 27, 2015) finds that birds and mammals use the same physical mechanism of sound production – the myoelastic-aerodynamic (MEAD) mechanism. Crucial to this research was the ability to replicate tissue oscillation in the syrinx (the voice box, a bird’s correlate to our larynx) by finely controlling the pressures below and outside this organ. For this, Dr. Elemans chose Alicat’s dual-valve pressure controllers (PCD Series) on account of their fine accuracy and control within closed volumes.

Pressure control animates MEAD oscillations

A portion of Figure 1 from the team's article in Nature, showing MEAD oscillations. Click to see the full image.

A portion of Figure 1 from the team’s article in Nature, showing MEAD oscillations. Click to see the full image.

Animal vocal sounds are composed of discrete pulses of air generated by oscillations of vocal tissue that can occur at a frequency of several hundred times a second. The myoelastic-aerodynamic (MEAD) mechanism describes how these oscillations are sustained without the need for active muscle vibrations at the same frequency, which would surely tire out any animal. (The fastest known muscles cannot contract faster than 250 Hz.) In MEAD theory, air pressure builds below the closed vocal tissue until there is enough pressure to force them open. However, the tissue opens and closes asymmetrically, and the passing air is chopped up by its oscillations, making sound. The frequency of the oscillations determines the pitch of the sound, and the oscillation frequency is determined by the flow rate through the voice box (larynx for mammals, syrinx for birds) and the muscular tension of its tissue.

To determine whether MEAD functioned in birds as it had been demonstrated to operate in mammals, Dr. Elemans’ team had to replicate pressure-induced airflow through the syrinx. The team wanted to precisely control the pressure in the bronchial airways below the vocal tissue to test the presence of MEAD oscillations. Because the remainder of the bird’s respiratory system was effectively dead-ended, Alicat’s dual-valve pressure controller (PCD) was the perfect solution. The PCD’s two valves allow it to maintain the exact pressures required by adding or removing air from the closed system as needed.

Differential pressure controller for closed volumes (with remote display) by Alicat Scientific. Note the dual pressure sense ports on the front.

Differential pressure controller for closed volumes (with remote display) by Alicat Scientific. Note the dual pressure sense ports on the front.

The research team also needed to control a rather low amount of positive pressure (up to 3 kPa, or 0.4 psi, above atmosphere) to prevent damage to the biological structures. For this, Alicat specified use of a differential pressure sensor within the pressure controller. One of the two remote sense ports was plummed to the bronchial airways of the bird, and the other was left open to atmosphere. This meant that the bronchial pressure was always referenced to the exact local atmospheric pressure, no matter how slightly it changed throughout the experiment. Integrated analog and digital control signals made it easy for the team to subject the syrinx to bronchial pressure ramps.

Pressure control spaces and redundancy

A portion of Figure 6 from the team's article in Nature, showing redundant pressure control spaces. Click to see the full image.

A portion of Figure 6 from the team’s article in Nature, showing redundant pressure control spaces. Click to see the full image.

Beyond proving the operation of MEAD within birds, Dr. Elemens also wanted to determine whether bird vocalizations resulted from unique muscle commands or a redundant control space. To test this, the team varied the interclavicular air sac (ICAS) pressure that surrounds the syrinx, while subjecting the syrinx to bronchial pressure ramps and varying degrees of muscle stimuli. A second PCD was employed to vary the simulated ICAS pressure within the same 1-3 kPa(G) range as the bronchial volume. The team found that multiple combinations of pressures within the two pressure zones (as well as muscle stimuli) were able to yield the same fundamental frequency, a redundancy feature that is common to MEAD.

Significantly, Dr. Elemens’ team found that the aerodynamic engine that sustained the vocal tissue oscillations was not generated by “the mass inertance of air column in vocal tract, but by the tissue-wave-induced intraglottal pressure changes” (page 6 of their study). The alternation of low and high pressures necessary to maintain flow through the vocal tissue was not caused by the mass of air moving up the bird’s throat, thereby creating a region of low pressure behind it until the tissue opened again. Instead, the wave motion of the tissue edges generated the necessary pressure changes within the syrinx.

This study is a fantastic look into aerodynamics at work within animals. How can Alicat make your research proposal a reality? Contact one of our applications engineers by phone (888.290.6060), email (info@alicat.com) or chat to discuss your project.

 

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