Let me start with this: No we will not be making speakers out of concrete any time soon. First, because we have our hands full with our SoundSommeliers line of audiophile loudspeakers. What we have sounds great as is and is easier to make and thus highly affordable. Second, we would enslave ourselves to United Parcel Service.
But I am currently thinking about a bookshelf form factor for our line of speakers and since I am in "loudspeaker design mode", I ran into some really fun speakers made from concrete I would like to share here.
Why concrete? Well versatility for one. The casting process lets you make very cool designs. I also find them beautiful to look at. But what about the acoustics? The short answer: It depends. Anyone who has ever yodeled in a tunnel knows that concrete reflects sound very well, causing echoes. Now echoes are only a problem if they reach the ear and they have several ways of doing that. One through any opening in the enclosure. The other through the cone itself where sound radiating backward from the driver reflects and hits the membrane and hits the listeners ear with some delay causing a smeared stereo image. Lastly the vibration of a cabitnet itself propagetes sound outward. Concrete speakers address this last pount rather well, by being somewhat ridig.
The back loaded horns shown above were were created as a final year project Shmuel Linski at the Shenkar College of Engineering and Design in Tel Aviv. Linski went on to form his own company called Linski Design, so by all means go check them out. They solve the problem of sound being reflected back through the membrane by the conical shape that does not show any plans parallel to the driver. The back loaded horn approach puts intention to the sound emerging from the port. One might think about that whatever they want, but echo is certainly not going to be a bigger problem here than in other back loaded horn designs.
The following design, I have personal great acoustic hope for. Manufactured by the German Company Concrete Audio, they deal with the problem of sound being reflected back through the membrane in a similar way as our SoundSommelier line of speakers, by a curved wall that reflects the sound inward not forward. The rather wide cabinet leaves some concern of baffle step diffraction that may lead to a smeared stereo image, and some response anomalies. But for anyone who is not an imaging freak like we are, I am sure these sound entirely lovely.
They also look like the would play louder than our SoundSommeliers. Concrete has never hurt loudness. While the company rationalizes the use of concrete as a valid design choice to eliminate the vibration of the cabinet itself, I am sure that there must be a limit of what we will reasonably lift and pay for to optimize on this attribute.
That being said, I find the Concrete Audio N1 speakers outright gorgeous, they look like they might sound beautifully and they will probably stay put during a burglary.
Parenthesis open: Now on the subject of cabinet resonance, there is something to be said about a rigid cabinet. That´s why we use an extruded aluminum space frame type design for our SoundSommeliers. This also keeps them light (compared to concrete). Now, concrete will resonate as well, while given the higher modulus of elasticity, this happens at a higher frequency than wood, it is also less dampened and the effects can be dramatic.
The cause of the Tacoma bridge to collapse, by the way, was Helmholtz resonance of the wind blowing through the river valley. Parenthesis closed.
Next in line is the European Space Agencie´s LEAF. While heavy on the use of concrete, I suspect the Large European Acoustic Facility is not in it for the audiophile experience. The LEAF is capable of a +-2dB response from 25Hz to 10kHz and while this is certainly impressive, they are missing a Tweeter. Let´s send them one. The sound pressure from this front loaded horn would easily more than annoy the neighbors. It would kill them. Literally. Designed to test the damage to airframes and space structures under acoustic loads present during rocket launches, this system would probably have worked nicely to let Beethoven hear is own 9th symphony. Poor Ludvig was already completely deaf in 1814 or 10 years prior to its completion.
Next in line is something more palatable and usable in the average home. This is probably a mild case of "function follow form", the function in this case being "look cute and be made of concrete". A sealed box, I judge the bass to be clean, but not low, the directivity of the output stemming from the horned opening might cause uncessary problems and perhaps one might end up accidentally serving coffee in ones loudspeaker hungover after a night drinking with the boys. Personally I like the aesthetics of the design. This speaker is called the Jack and was produced as prototype only by a company called Anaesthetic Designs. They claim their concrete to be 10 times stronger than normal concrete. I am thinking that this might be more than needed for it to not blow up, but the same concrete also brings out fine detail in the cast. Hence probably the choice.
My last random example of concrete in speaker building comes from a company called Conscious Forms. The The Concrete Orbs are certainly a pleasure to look at and I would have probably started to save up to get a pair around the time I built my BLANDA Speakers from two IKEA salad bowls. The company describes the spherical shape as acoustically "sympathetic" and while avoiding sharp edges is helpful in loudspeaker design, I suspect these speakers to radiate a rather large amount of sound forward through the membrane again leading through a smeared image. I am also concerned by the acoustic obstruction through the undoubtedly pretty stainless steel grilles. I suspect that these things either don´t play low or loud or the grill rattles.
While we at KVART & BØLGE are not likely to get into concrete speaker design any time soon, the use of concrete in loudspeaker design can produce some fun and creative results. Combined with a solid acoustic approach, I am sure we are to see many great speakers using this material in the future. In my research for this blog post I have not found a single concrete quarter wave speaker and with that I must say I am tempted to make a trip to the home depot and get some Portland cement and rebar. I will keep you posted if I ever do.