The critical temperature of the listening room and why it should be controlled
One of our members comments that she can hears a marked change in the bass quality of a Harbeth speaker over recent months and has spent time and money 'upgrading' her amp in the fruitless and unsuccessful attempt to restore the quality she prefers. I asked her to confirm the temperature in her listening room. She checked: at the speaker cones the temperature was 30 degrees Centigrade in the late evening, and doubtless much higher at mid day. This was a shock to me. Clearly the issue is not with the amp, or CD, or cables.
Asked how she could tolerate such incredible temperatures indoors and in a windless apartment she said that the air-conditioning has been turned off because it made some acoustic and electrical noises which disturbed her hifi listening.
All loudspeakers from all manufacturers are designed at and with a target operational temperature in mind. By convention, this is 20 degrees C, which is universally deemed to be the ideal temperature for human comfort: neither too cold nor too hot: just right. So what? Well, 30 degrees C is a +50% rise on the target operating temperature (which would be, say 18-20 degrees = 20 degrees +/- 10%) and this has a dramatic effect on the molecules within the drive units - specifically in the plastic cones and the rubber surround. It does not effect the steel magnet parts.
Rubber is an extremely temperature sensitive material. You know that from playing with BlueTak or chewing gum or a squash ball. When it heats up it become much more elastic and that increase in elasticity even over the 20>30 degree range has a marked effect on the bass unit's rubber surround. Its throw increases, control, damping and Q all degrade. So all the Thiele-Small parameters that have been carefully calculated to give a nice smooth, tight bass at 20 degrees C are completely screwed by the temperature rise. So, when the temperature is elevated as high as 30 degrees, the bass will be softer, flabby and there will be a lot more of it.
Strange things happen to the molecules inside the cone too at elevated temperatures. For all typical bass/mid plastic cone materials (including RADIAL) if the temperature is substantially elevated above target, the acoustic damping characteristics of the cone change - the 'air' dries out around instruments, and the sparkle is lost as the clarity in the presence band diminishes.
I would add, that this temperature sensitivity makes it virtually impossible to design loudspeakers in the UK during the winter when the ambient temperature is around 5 degrees C. At these low temperatures, the bass unit's rubber surround tightens.
For me, critical design and measurement is only viable in the UK from April-June and September-October. Attached is a picture of me setting up to do some testing in the big open space of the Village Hall. When I made the reservation for the day I had to insist that the heating was turned on at 7.30 am so that by the time I arrived (11am) the entire hall was up to 19.5 degrees C: perfect. Conversely, last December, with an outside temperature of 2 degrees C, I was obliged to make some measurements on a prototype speaker to be shown at the Jan'06 CES. The only available large space was the local dojo hall, which they kindly loaned to me. Even though the heating had been on all day, the temperature only reached 10 degrees C and as expected, my acoustic measurement of the systems true bass output was completely useless. See how critical temperature is? Before you can even think about measuring (or designing) there as so many critically important but seemingly inconsequential little details that you have to get right first! Grrrrr!
Rubber and plastics have completely reversible temperature characteristics, so once the temperature is returned to 20 degrees C, all will be well again.
So taken together, the significantly elevated room temperature will .....
A. Give much more bass, less controlled and slower bass ...
B. Reduce clarity and detail in the presence band.
F.or critical listening, listen within a range of about 18-22 degrees and note that hifi demo rooms are normally set at this cool, shirt-sleeve temperature.
Alan A. Shaw
Harbeth Audio UK