German Physiks Newsletter No. 49

July 2023 | Newsletter No. 49

Dealing with unwanted cabinet resonances

Welcome to the July 2023 German Physiks newsletter.

In this newsletter we would like to talk about some of the techniques we use to deal with cabinet resonances. Unless measures are taken to eliminate, or at least very significantly suppress cabinet resonances, these can seriously reduce a loudspeaker’s clarity. Loudspeaker manufacturers use a number of different approaches to this problem, including complex machined metal cabinets, cabinets made from exotic materials such as carbon fibre and lossy materials like MDF. We are traditionalists and our cabinets are mostly made from flat panels of MDF. This approach has the additional advantage of allowing us to offer veneer finishes, which are not easy to apply to curved or complex surfaces.

HRS-130 & Borderland Mk IV

German Physiks HRS-130 & Borderland Mk IV

On our HRS-130 and Borderland models we use an octagonal shaped cabinet. This is more expensive and more time consuming to make than a conventional square -section cabinet, but it results in panels that are 50% narrower and therefore much stiffer than the panels of an equivalent height and volume square-section cabinet.

In addition, two different densities of MDF are used and these are used on alternate panels of the cabinet. This reduces the energy transfer between adjacent panels.

HRS-130 & Borderland Mk IV cabinet cross-section showing different grades of MDF in alternate panels

Bracing inside the cabinet provides extra stiffness.

Hawaphon® vibration suppression material

Residual resonances are minimised by applying a special damping material called Hawaphon® to the inside of each panel. Hawaphon® is a polymer sheet containing a matrix of small cells filled with very fine steel shot. It was originally developed as an anti-surveillance measure for use in military and government buildings. It adds mass to the panel to reduce the resonant frequency and the ability of the shot in each cell to move against each other provides a very effective way of converting vibration energy into heat. Hawaphon® achieves a broadband attenuation of structure-borne sound of more than 50dB, which is a remarkably high figure.

Driver mounting method reduces resonances
The way the DDD driver and woofer are mounted on the cabinet provides a bonus as far as reducing cabinet vibration is concerned.

On a conventional loudspeaker the drivers are generally mounted flush onto one or more of the cabinet’s panels and the reaction forces produced by the drivers are in the same direction that the panels would naturally want to flex. On the HRS-130 and Borderland, the drivers are mounted so that the reaction forces they produce are perpendicular to the direction that the cabinets panels would most easily flex, so less vibrational energy is transferred into the cabinet. This is reduced further by the fact that the DDD driver’s moving mass is very small, less than 3 grams and so the reaction forces it produces are also very small compared to an equivalent pistonic driver.

PQS-302 upwards

PQS-302 with two DDD drivers

On the PQS-302 and upwards, where two or more DDD drivers and much larger bass systems are used, a different construction is employed.

The DDD drivers are housed in a column that is separate from the main bass cabinets. Its cylindrical shape makes standing waves on the column surface less likely than if a square cross-section was used.

As with the HRS-130 and Borderland, the reaction forces from the DDD drivers being perpendicular to the direction that the column surface would most easily flex, reduces the likelihood of resonances being excited.

Bass cabinet cross-sections

The PQS-302, PQS-402 and Loreley use two separate bass cabinets, as this allows for smaller and therefore stiffer panels than would be the case with a single larger cabinet. For the same reason the Emperor uses four separate bass cabinets. The bass cabinets are stacked on top of each other rather than being rigidly attached together, as this reduces the transfer of vibrational energy between the cabinets.

On the PQS-302, PQS-402 and Loreley bass cabinets, two different densities of MDF are used and different densities are used on alternate vertical panels of the cabinet. This reduces the energy transfer between adjacent panels.

On the Emperor bass cabinets this is taken further, by also using material of different thicknesses for each panel.

To reduce standing waves within the cabinet that might help to create resonances, there are no parallel vertical faces. The shape of the bass cabinets also enables them to act as diffusers for the rear-ward energy radiated from the DDD drivers. This allows these models to be positioned against the wall.

As with the HRS-130 and Borderland, Hawaphon® is applied to the inside of each panel to reduce any tendency to resonance.

Until next time

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If we can be of help with anything at all, please do not hesitate to get in touch with us here. You can find your national distributor here.

Holger Mueller | German Physiks