carpet speaker

[Background to the creation of carpet speakers]

Carpet speakers are speaker systems with an extremely simple structure, consisting only of a rolled up carpet and a speaker unit placed on top.
By making the enclosure out of sound-absorbing material, it is possible to absorb the out-of-phase sound from the speaker back pressure without interfering with the pure sound that is inherent to the speaker unit, and since there is no box bending or resonance, the sound is clear.
I made it myself because it's easy to configure.

[Materials and Equipment]

・Speakers: MarkAudio CHR-70v3 (later changed to ALPAIR7v3)
・Amplifier SMSL SA-36A
・Carpet, Nitori, 80cm x 150cm, 571 yen.

[Design and Production]

There is a lot of information about carpet speakers when you search online. It takes only 30 minutes to roll up a carpet and place a speaker on it. However, just rolling up a carpet is not a good design. So...
First, I wrapped it together with the PU leather. Then I wrapped the ends around the back side as well.

The clasp fastens with a magnetic button, which secures it comfortably.

Next, make the stand.
It is machined using a precision drill bit and the void pipe is glued in.

The paint is our standard, India ink + matte urethane clear.
In order to emit low frequencies, rubber feet are attached to the bottom, raising it by about 15 mm.

The baffle plate was also made in the same way.
Finally, attach a weight to the speaker.

To make the weights, I bought lead ingots and melted them in a pot. I then poured the molten lead into an aluminum can with a long screw already set into it.

Once it has cooled, cut off the edge of the aluminum can and hammer it to shape it.

Secure the nut to the speaker with epoxy adhesive.
This allows you to remove or replace the weight at any time.

[Thoughts on the sound]

First, I rolled up the carpet, placed the speakers on it, and played the music. I was surprised at how such a simple setup produced a three-dimensional sound with a resolution I'd never experienced before. Compared to Krypton's HQM, which has a reputation for being ultra-resolution, the HQM sounds muffled. Once you experience the resolution of a carpet speaker, there's no going back.
The vocals and violin sounds are absolutely beautiful. On the other hand, the bass is a little lacking.

[Soft Enclosure Analysis]

Some people online say that carpet speakers produce the same sound as real speakers, but it's not that simple.
To begin with, producing pure speaker sound is an ideal that is extremely difficult to achieve.
Normally, there is always some extraneous sound due to reverberation inside the enclosure, resonance of the enclosure, and various other resonances. However, a soft enclosure can be easily constructed to almost completely eliminate these.
Speakers with strong reverberation are often sold as being like musical instruments.
However, that is just an excuse and it will always result in a muddy sound.

"Comparison with flat baffle/open rear type"

An ideal flat baffle would be an infinitely flat surface that completely separates correct and false sounds and reproduces the sound of the speaker itself.
However, in reality, the wide baffle surface resonates and, since the physical size is limited, the off-axis sounds mix with the correct sounds.
Carpet speakers do not completely absorb background noise, and their cylindrical shape allows bass to be extracted from underneath.
In addition, flat baffles and open-back speakers are difficult to install as they are heavily affected by reflected sound from the back side, but carpet speakers absorb background sound inside the tube, so they can be easily installed anywhere.

"Air column resonance"

Carpet speakers are shaped like air column resonators.
A good example of a resonator is a pipe organ, which has a number of different length pipes, and the length of the pipes determines the resonating sound.
It is not hard to imagine that if the cylinder were made of hard paper or PVC, the resonance would be extremely loud.
In the case of carpet speakers, the pressure is absorbed and air column resonance does not seem to occur.
This depends on the material; when tested on a yoga mat, the characteristic tube sound remains.

"The weight effect"

It is said that suppressing vibrations in the speaker unit is an effective way to make sound clearer.
We attached a weight to the magnet to increase its weight ratio to the diaphragm and thereby suppress the vibration of the speaker unit.
However, contrary to my expectations, I honestly didn't see any results.
In general, I think that weights have a much greater effect in pressing the speaker to the enclosure and suppressing the vibration of the enclosure itself than in isolating the unit itself.

"Electrical correction"

I tried correcting it with an electrical circuit to further enhance the bass.

However, in exchange for deep bass, resolution is reduced.
I tried adding a high-pass filter circuit and replacing resistors and capacitors, but the clear tone did not return.
It seems that even simple electrical circuits result in a decrease in sound quality.
This may be especially noticeable because the carpet speakers have a clear tone.
I think this is why speakers with built-in circuits, such as multi-way speakers, produce muddy sound.
Thus, the "Carpet Speaker Prototype No. 1" was completed.
With this simple configuration, it is possible to reproduce open, dynamic, realistic and three-dimensional sound.