SOFT FEET?

TECHNICAL APPRAISAL OF THE USE OF “SOFT FEET” IN PLACE OF SPIKES AND SINGLE POINT ISOLATION SUPPORTS

The following explains why the use of “soft feet” are superior to spikes and single point isolation supports when used under loudspeakers and other components.

It was written as background to explain the use of Sonic Design Audiophile Damping Feet, a Swedish product. It was drawn to the attention of Ringmat Developments by the Danish hi-fi company, Holfi, who found that the Sonic Design Damping Feet worked “miracles” when use in conjunction with Ringmat Feet and Statfeet.

Ringmat Developments believed that an even better product could be designed for use with Ringmat Feet in place of the Sonic Design Audiophile Damping Feet, and this gave rise to the development of Ringmat Domes.

Consequently, we believe the best support available for loudspeakers as well as other hi-fi and audio/visual components are the “soft feet” concept of Ringmat Domes used with Ringmat Feet and Statfeet.

At present, Ringmat Feet are designed to take weights up to 21kilos. Larger, extra heavy duty Ringmat Feet are presently under development to be used with the larger Ringmat Domes for heavier speakers and other equipment.

For a review by "Dave Ayers" of this concept please click here.

DAMPING FEET

The mission of loudspeaker damping feet is to make the loudspeaker cabinet motionless and to isolate it from the floor and other objects in the room. This has been hard to achieve but it turns out that it is possible to get substantial improvements compared to the standard solutions of today.

What difference will the Sonic Design feet give in practice? Can you really hear any improvements?

These feet will give a substantial decrease of the cabinet's movements. This is apparent when you put one finger softly against the top of the front baffle, as it vibrates much less when playing music. The transmission of vibrations from the cabinet into the floor will be reduced. The music will be cleaner, with a less coloured sound quality. The transmission of noise to the neighbours will be reduced.

TECHNICAL DESCRIPTION
Sonic Design Audiophile Damping Feet for loudspeakers
Every loudspeaker rests on a surface that can be compared to a spring, even though the surface may be quite hard. As a result, a resonance appears within or outside the loudspeaker's frequency range. Below this resonance frequency the loudspeaker is coupled to the floor, and above this resonance it will be more or less isolated from the floor.

The traditional approach has been to try to make the suspension so stiff that the resonance might be placed above the audible range. This is one of the basic ideas behind the use of "spikes". However, this approach does not work. The resonance between the cabinet and the floor will not be any higher than the upper bass range. Not even a concrete floor will do, as anyone who has drilled a hole in a concrete wall will be aware of after having experienced the resonances and sound transmission in this material.

The spokespersons for spikes also bring forward two more incompatible theories:

1. Spikes work as mechanical diodes, transmitting vibrations one way only.

2. Spikes rest on an infinitesimal area unable to transmit vibrations.

Regarding theory 1, Sir Isaac Newton has yet to be proven wrong. A force causes a counterforce. This is of course valid both ways.

Regarding theory 2, the tip of the spike may be very small, but not smaller than necessary to carry the weight of the speaker. In that way a force exists between the spike and the floor. This force will be modulated by vibrations and thus transmit them.

Consequently, to isolate the loudspeaker from the floor, the resonance frequency should be lower than the lower limit of the frequency range of the loudspeaker.

This leads to some very low frequencies, far below 20 Hz.

This concept can be compared with that used in turntable tonearms, where these ideas have now been accepted for a very long time. There nobody claims that the fundamental resonance of the tonearm/cartridge combination should be in the audible range to produce solid bass or musical sound.

In article 34452 of the rec.audio Usenet newsgroup a calculation of the loudspeaker cabinet movement magnitude was presented. An 8" woofer vibrating at 50 Hz with a 0.5 cm cone displacement in a 20 kg cabinet will move the cabinet 6x10^-6 m, (6 millionths of a metre). The cabinet in this example is assumed to be free floating in air, attached to nothing.

The conclusion that the resulting movement is small enough not to cause any trouble is surely correct. One could however get an implied conclusion that spikes could improve things ever so little. This is simply not so. A high Q resonance will multiply the movements until they become quite noticeable. The free floating approach should be the goal.

Until now it has been hard to design a foot made from rubber that is soft enough. Also, it has been impossible to prevent foam materials from gradually settling and eventually becoming hard. Also, sufficiently soft coil springs do not have enough horizontal stability without special mechanical arrangements.

Now Sonic Design can offer a foot made from a special engineering foam with open pores. The open pores make the material structure of the foam spring independent of any contained gas that would eventually escape. The material is created to be stable over long periods of time.

The Swedish audio society "Ljudtekniska Sällskapet" has taken measurements that show that a loudspeaker standing on a soft spring moves less from music than if put on hard feet. They also found that spikes contribute both measurable and audible distortion:

Sinus signal 125 Hz to the speaker (top),
floor movement below, using soft feet.
Floor signal magnified 2 x.

Sinus signal 125 Hz to the speaker (top),
floor movement below, using spikes.

Spikes can give an open but slightly hard and distorted sound, which some may say is more musical. But resonances and overtones in the floor are equivalent to harmonic distortion in the amplifier.

A soft overdamped support where the resonance frequency is a little too high seems to make the music sound dull.

The use of "Blue Tack" or damping feet with extremely high internal damping can be compared to a car with too hard shock absorbers, where much of the vibrations will pass through even if they are not amplified.