Callas Acoustics

Room acoustics appears to be a field of tension regarding living space, aesthetics, interior and housemates. But the listening room certainly dictates 50% of the total reproduction regardless of the (financial) status of the HiFi gear. Room acoustics is a complex matter and has many parameters. There is usually talk of damping, but damping is something other than absorption. High pile and broadloom carpet is -as example- an ultimate absorber and most measurements show a rough descend beyond the 2500 Hertz. This is a kind of black hole for the higher frequency spectrum.

Absorption always means loss in the higher frequency range at the expense of clarity.

As with everything else it's all about balance, the trick is to maintain clarity at an acceptable reverberation (RT60) time.

The aim is to optimise by means of a lot of diffusion and just enough absorption.

Acoustical solutions are possible in the form of paintings, photographs, artwork etc. For example a living room with various artworks on the wall can be treated acoustically by the same. Acoustical spray ceilings, hidden tubetraps in front walls, there are many clever possibilities. However, reverberation (RT60) control is the first -and important- step towards a neutral space.

What is striking is that there is often focus on a short reverberation time, and in most cases an over focus. The result seems to be achieved: short reverberation as a measurable fact, job done. But how does it sound? Is there still life?

Below are three frequency plots (RT60) of our listening room. The first one shows a huge challenge. The graph is even limited to 2.5 sec for a mere visibility reason.

Acoustics measurement 2006


The second plot as measured with the Sprayplan ceiling by the company Warmteplan. Above the roughly 400 Hertz very nice within the ideal (green) lines. We could not imagine it having such a large and broadband effect. And the spray ceiling is also a very aesthetically pleasing solution. Incidentally, experience has shown that an absorbent ceiling and a neutral floor (in this case quartz particle epoxy) are preferable, which is at odds with the most common situations. The thickness of the SprayPlan ceiling s 15 mm. Please note that the area from roughly 80Hz to around 400Hz is a very essential area of voice and keynote.

Where the Midrange begins and/or the Bass stops remains debatable, but we may agree that the human voice should preferably be kept homogenous, as a whole. Consider then that the Male Bass reaches about 100Hz and the Female Soprano reaches about 1050Hz. A common error with larger three-way systems is that the woofer continues working above 80Hz precisely because a too small midrange unit is used. On topic.

The higher frequencies are easier to influence, but laws of nature cannot be circumvented:

The lower the frequency, the larger the object will have to become to absorb it.


The third plot with additional diffusers and basstraps*. It takes quite a lot to get the graphic line down in the lower frequencies. We could add some more bassstraps, but the effect is already substantial so we choose not to use any more. The placement and therefore the interaction with the speakers also have an effect on the reproduction. Meanwhile (Q3 2016) the plan has been made to install 12x Stockholm Optifusor units, mainly at the location of the first reflection LH/RH.

It looks as if the curve is still not quite right, but this concerns 10 to 15 Milliseconds, and although not perfect, it is marginal indeed. More important: the space still retains plenty of clarity and life. However the primary reaction in the listening chair is always the same: Calmness.

* 2 tubetraps of Ø50 x 125 cm. 2 tubetraps ofØ40 x 125 cm.


The Stockholm diffusers were installed at the end of 2016 and with particularly good results. Dimensions per side 180 high 240 wide. It has been estimated that almost two more months have been spent on the modular frames and painting. This painting is a gain concerning the non-absorption. The subjective experience with large diffusers left and right is like putting on headphones. The speakers are instantly harder to locate.


A so-called room in a room has been created here. The Basstraps and Stockholm Optifusors can be used as mobile castors and are stored in a separate storage. In this way a dedicated listening room is possible in about 5 minutes time.

Acoustics measurement 2012

In six years, things can change and evolve. In the meantime Toine has measured the current situation again after a major renovation, the passage at the back wall to a corridor and office has been completely closed. The wall of 21 square meters now consists of natural stone, which also acts as a very mild diffuser.

Selective Ears

The graph above shows curves at a certain sound level (SPL). The curves show the selective sensitivity of the human hearing. The reference point is set at 1000 Hertz. The least audible sound pressure (hearing threshold) is expressed on baseline p0. Around 3000 Hertz is the loudest frequency for the human hearing.

The curve indicates how much more dB is needed to subjectively experience the same sound pressure (SPL), we see that below 500 Hertz the curve is quite slanted, and from 5 Khz to about 10 Khz there is the same effect. This phenomenon explains to a large extent the subjective experience that speakers with a broad bass hump around 100 Hertz are experienced as 'musical' and with 'body' in the listening room at home. The counter effect is that speakers which measure almost linear in a dead room are more often experienced as thin sounding in the actual home situation.

The measurement also provides insight into the why of the Loudness Compensation. This is used to compensate the hearing curve at lower SPL's in particular.

The above frequency measurement graph is of the Capriccio Admonitor + Basso Submonitor loudspeaker. This curve shows an ideal Loudness Compensation. The dip is exactly at 3000 Hz and the hump is around 100 Hz. The theoretician will look at this design very differently (the ruler flat) than the acoustician. The actual listener at home will hear that this speaker sounds very good... The Capriccio is a design by Joseph Szall, not exactly a novice in this field.

Canalis Speaker setup

Article Speakerplacement by Allen Perkins

The Cardas method

Mit Physik den klang verbessern; Rainer Muller TU Braunschweig

A Deeper Note on the Subwoofer Subject

Would anyone buy these loudspeakers at a retail price of 116500 euro ?

Why then use one Subwoofer ?

The XY problem

What is it?

The XY problem is asking about your attempted solution rather than your actual problem. This leads to enormous amounts of wasted time and energy, both on the part of people asking for help, and on the part of those providing help.

- User wants to do X.
- User doesn't know how to do X, but thinks they can fumble their way to a solution if they can just manage to do Y.
- User doesn't know how to do Y either.
- User asks for help with Y.
- Others try to help user with Y, but are confused because Y seems like a strange problem to want to solve.
- After much wasted time, it becomes clear that the user wants help with X, and that Y wasn't a suitable solution for X.

The problem occurs when people get stuck on what they believe is the solution and are unable to step back and explain the issue in full.