The Transfer of Energy

When you place 2 similar-characteristic materials into firm contact with each other, say steel against steel, sound energy in one will readily pass through into the other. There is a low acoustic impedance at the junction. When you place 2 dissimilar-characteristic materials into firm contact with each other, say steel against rubber, sound energy in one will not readily pass into the other, it will bounce off the junction and be reflected back. There is a high acoustic impedance at the junction.

Take care not to confuse acoustic conduction with acoustic resonance. They are 2 completely different things. Resonance is a mechanical function. Take a steel rule and clamp it firmly at one end, flex and release the other end and it will spring back and forth at 3 or 4 Hz. Place a piece of plasticine on the end and the resonant frequency reduces. The characteristics of this resonance are set by the length of the rule, the springiness of the steel, the mass on the end of the rule and the air’s damping effect. But place the end of the steel rule firmly against something that is vibrating at 10 KHz say, and the vibration will be conducted from one end to the other at approximately 12,000mph (speed of sound in air at sea level, 768mph x 16 = 12,288mph), and with extremely low losses too. This function is set by the material’s molecular characteristics and the efficiency of the boundary at the source of the vibration.

Our hi-fi systems are constructed out of many different types of materials, some of them soft, and some of them very hard, and therefore they exhibit the different acoustic characteristics we have described. But for the purposes of us keeping the description fairly manageable we categorize structure into 3 groups, high, moderate and low acoustic transmission, and color them red, orange and yellow respectively on the diagrams. We also treat cables as high acoustic transmission items. Whilst the metals of the cables might not be quite as efficient as say the steel of the casework, your cables link directly the circuits and components in each box and thus are considered pretty efficient in this context.

In our materials diagrams, the circuits and circuit-boards in the CD player and amplifier are represented by the central red block, with the 2 short vertical lines representing physical connection to the outer case, normally by mounting pillars or screws. Also, the red block behind the domestic wiring is representative of the walls within which the wiring is laid (which can be a significant issue that we’ll discuss later).

Consider the vibration coming from transformers and motors, and the energy traveling all around the leads and look at the diagram carefully, and consider where the acoustic impedances are minimal, where the vibration is going to travel too. The energy in the circuits is going to stay in and circulate around the casework, and its going to travel up and down the cables, in other words, it can only stay in the red structure or travel along the pink cables. It can’t go anywhere else, such as through the player’s feet as most players feet are usually made of plastic with felt or rubber (a very big impedance mismatch). See diagram below.

Vertex AQ Systematic Approach – Acoustic Propagation
acoustic propagation – click to enlarge

Oh, and finally on this point, consider that the speakers put a large amount of energy into the air, and whilst the air’s contact with solid objects is not a particularly efficient union, very large objects, such as the walls of your room will pick up considerable energy and feed that straight back into the domestic mains wiring (especially at bass frequencies).