18. Racks and resonances

Resonances play an important role in placing acoustic equipment in a room, not only when dealing with loudspeakers, but also on the devices that are used to drive them. Where they occur, they negatively influence a unit’s ability to perform according to spec and thereby change the sonic characteristics of our whole listening experience. Well-designed HiFi racks will help to keep both resonances and electric interferences at bay, however, not every good looking HiFi rack is acoustically ideal (very few are). It therefore makes sense to have some basic understanding of the subject.

A typical ‘old school’ HiFi system will probably consist of two or three sources, a pre-amp, and an amplifier. Each device is equipped with at least one power supply. Many high-quality power amplifiers will even have one large power supply per channel. In our current system, there is one power supply in the CD player, one on the DAC, one in the turntable, two in the pre-amp and two in the amplifier, bringing the total number to seven. Each power supply is humming at 50Hz. The low vibrations are affecting both the device itself and all adjacent devices. Other resonances result from city traffic, industrial activity, people walking nearby, household appliances, etc.

Although it may be argued that all HiFi units will benefit from resonance free placement, some are especially prone to instability, such as turntables and CD players. On turntables, any physical impulse coming from outside or within will be layered onto the audio signal and lead to inaccuracies in playback. On CD players the effect is not much different. Although some people argue that digital signals either work or don’t work, the effect of resonances can be heard in sonic brightness and harshness. If you have ever had an ageing CD player that would on occasion grab a CD slightly off center, you are already familiar with the effect, as before a CD skips, the music first becomes thin and strained.