6. Tarhu Upper-body Grill Design as High Frequency Filter
Peter Biffin September 2019
Tarhu design is unusual in that it encases the vibrating membrane inside the upper half of the body. This is quite necessary due to the fragility of the cone – however it does create potential problems in terms of the dispersal of the vibrations that the cone generates. In all other stringed instruments these factors do not come into the design process as the vibrating membrane forms one of the instrument’s outside surfaces and is therefore in direct contact with the air.
Depending on the design of the grill pattern in the upper half of the body, a variable percentage of the cone’s output will be blocked by the components of the grill. The low frequencies produced by the cone are more able to bend around obstacles created by the grill pattern, whereas the higher frequencies are much less able to bend and are therefore reflected back inside the body. If the upper body has relatively few openings in the grill pattern, then a significant reduction in high frequency output can be expected.
Depending on the quantity of high frequencies the cone produces, having the grill pattern act as a high frequency filter could be either positive or negative – it all depends on what type of sound is wanted.
A Simple Experiment
Before a grill design is finalised for use in ongoing tarhu making, it would be worth investigating what affect the grill pattern has on the sound, and allow the results of such an experiment to indicate what the grill design should be.
It is much easier to cut-out the grill design without a neck being attached to the body. However, it is still quite possible to do a nice job of the grill pattern when a neck is attached, and the information provided by this experiment is worth the extra effort.
Choose a design that will allow you to start with just a few holes then be able to gradually increase the number of holes until you have reached the greatest hole surface area that is possible while still keeping enough strength in the shell to resist impact damage.
As the experiment progresses listen to the sound often, focussing especially on the high frequency part of the sound.
Having done this experiment myself, here are my design decisions:
1. I try and have the greatest amount of open space that is possible while keeping sufficient strength in the body
2. When designing the part of the neck that goes over the body (around the bridge), I make the bridge-well as small as possible – in kamancheh tarhu/shah kaman, that is an internal diameter of 50 mm, outside diameter of 74 mm. This is so that the sound radiating from the apex of the cone suffers a minimum amount of reflection.
3. The grill design begins as close to the neck as is possible – again because of wanting the cone-apex sounds to be unobstructed
4. The grill design goes as close as possible to the cone ring at the base of the cone
5. I prefer to make bodies out of laminated timber so that, (among other things), I can have as much open space as possible while retaining body strength against impact damage.
6. I prefer to control high frequencies by other means – to me, controlling them with grill design is too cumbersome, and too difficult to adjust.
The holes cover the greatest amount of surface area possible. Even though the cone is clearly visible, I have never had a cone damaged.