about airflow

 

 

 

Sound production: Airflow

 

 

 

 

 

          

Bellows pressure
The airflow in a concertina is generated by expanding and contracting the bellows. The amount of air pressure generated is determined by the player. The more force the player applies to the bellows, the higher the air pressure. The size of the bellows also play a role in the amount of pressure that can be generated.

If the same amount of force (F) is applied by the player, smaller bellows will generate more air pressure than large bellows. Pressure is the force applied by the player, divided by the size of the bellows: P = F : S.  This formula illustrates that the pressure generated on a  concertina is much greater than on a full size accordion. That’s why we call concertinas high pressure and accordions low pressure free reed instruments.


airflow direction by expanding bellows

 

In a concertina the air flow is initiated by pushing down a key. The key is connected to a pad which opens the air hole of a reed chamber. When the bellows are expanded (pulled) it creates an airflow which passes through the selected air hole into the reed chamber. From thereon it will pass through the frame slot, which is obstructed by the reed, into the bellows. This obstruction creates a higher air pressure (P1) above the reed in the chamber than on the other side of the reed (P2). This pressure difference is needed to initiate and maintain a reed swing cycle (see 'About Sound generation').

airflow direction by closing bellows

 

When the bellows are closed (pushed), an overpressure is created in the bellows and under pressure in the reed chamber and the air flow moves in the opposite direction; from the bellows into the chamber and through the air hole. Now the reeds on the bellows side of the reed pan are activated.

The air pressure and air volume in the reed chambers is determined by the design of the concertina.  They vary in different parts of the instrument, depending on how much pressure and volume is needed.
For instance, high pitch reeds are small and need very little chamber volume to start the swing cycle, and require a very low airflow to coast.
Large reeds (low pitch) on the other hand need much more air volume to be readily available in the chamber to initiate the swing cycle. A chamber that is too small to provide the necessary air volume will results in a slow reed attack.  Because this problem is common in (vintage) concertinas, many players think that low notes are slow by definition.  However, when the air volume in the chamber is correct for the reed resistance, low pitch reeds can almost be as fast as high pitch reeds.
Other factors that affect the airflow in a concertina are the valves and pads/air holes.

 

Valves
Concertina valves traditionally are made out of leather such as hair-sheep, sheep or goat.  Valves close off the path to the reed in a chamber that is not activated. For instance, when the bellows are expanded, air flows in to the chamber from the outside and the reed in the camber is activated. The reed on the back side of the reed pan (‘push reed) is closed off with a valve.
The valve of the activated reed is blown open by the airflow.
The closing function is not that difficult, it just needs to be large enough to cover the vent opening and strong enough to resist the airflow and not be sucked inside the vent.

The opening function is a lot more complicated.  When a valve is placed in the airflow, it will always create an obstruction. Valve obstruction is caused by the resistance of the leather (stiffness) and mass. The obstruction affects several aspects:

  • Pitch, valve resistance can lower the pitch of a reed by several cents.

  •  Reed attack, the amount of resistance affects the initial airflow and therefore the time it takes for the reed to start sounding.

  • Reed coasting at low volume. High valve resistance increases the minimum airflow requirement for a reed to coast. Ideally, the valve resistance value should be lower than the airflow value needed for the reed’s coasting. This is one issue that is frequently found on instruments revalved by someone without the necessary knowledge. 

Valves also add variables that are not fully controllable:

  •   Over time the leather will become suppler, which reduces the amount of resistance. This affects the pitch of the reed.

  • When valves age, they loose strength and do not close fully. This is called valve leakage, and affects the reed attack.

     Because different reed sizes have different airflow values -large reeds have more airflow than small reeds-, valves need to be adjusted to match the airflow values. This is done by adjusting the thickness and resistance of the valves. 
With an Airflow velocity meter the airflow is measured before and after a valve is installed. The objective is to create a uniform airflow pattern. 

 

Pads and Air holes
The amount of air that can enter the reed chamber is determined by the size of the air hole and the air pressure.  It is obvious that larger reeds with larger chambers need large air holes to accommodate the higher airflow requirements. Small reeds (high pitch) only need small chambers and air holes. The scaling of the air hole sizes can easily be calculated.
Basic design vintage concertinas often have only one size air hole for every reed size in the instrument, which often results in uneven reed performance.

The pads that close off the air holes also have an effect on the airflow.  They need to open far enough to not interfere with the airflow. The limited space in the action cavity determines the thickness of the pads and the type of connection to the lever.

Setting up an action follows the following chain of decisions:
-The airflow value needed determines the required pad lift.
- the pad lift and available space determines the type of pad and lever connection.
- The total pad lift movement determines the key travel (how far the key needs to be
   depressed). The button guide pin often is a limiting factor, and may prevent the necessary
   button travel..
- The pad/hole overlap and the maximum air pressure on the pad determines the type of 
   pad leather and the type of springs needed and their pressure. This is the key pressure which
   is measured in grams. 

Traditional brass springs can not be calibrated accurately. Their adjustment usually in in 10 grams intervals. Another issue is that they tend to lose their tension over time.  High density steel springs are much more accurate. They can be adjusted in 1-2 gram intervals, and will keep their tension for many years.

The process of ‘setting up’ an action is called calibrating. Although essential for maximum performance, it is rare to find a correctly calibrated action in a concertina. It requires besides a lot of knowledge also specialized equipment to measure airflow, pressure, valve resistance, etc.. With all this information it is possible to make the correct parts (valves, pads, and springs) for an instrument.  

 

Evaluating Airflow
This is a simple way to evaluate one key element - the airflow accuracy- in your instrument without any measuring equipment:
push the button of a high and low note down at the same time without moving the bellows. Slowly move the bellows until you hear sound.  Ideally, both notes should start sounding at the same time. Variables in starting time (attack values) can be caused by the design of the instrument, valve resistance or incorrect action calibration.

 

Copyright © 2001-2018, Concertina Connection Inc8, All rights reserved.