The Small Front Room
Thu, Feb 9 2012, 11:23PM
A similar topic appeared and almost immediately disappeared just recently and you may decide that this is as flimsy but here goes...
Listening in a small front room as I do I have some techniques, specific and general, to share. Firstly, the presence of a reflecting wall directly behind my head would cause secondary speaker sources to appear as though from a few metres behind me, confusing the stereo image with delayed information. To minimize this I place a stiff cardboard folder opened with the apex toward the back of my head ( sitting on a convenient radiator ) providing 45 degree reflection of the sound waves just after they pass my ears and preventing left ear information from reflecting to my right ear. The reflected waves bounce harmlessly off walls on either side returning so delayed and attenuated that they pose no further problem.
The amplifier is adjacent my listening seat so I have the speaker cable situation which I deal with using mains cable. This has the advantage that the live and neutral sheathed wires are equally spaced on each side of the hard copper earth. If you connect the signal to the outer two wires and the common to the earth then the electrical centre of the signal is coincident with the common which means there can be no inductance between the conductors. What happens to the cable capacitance I am not really sure.
In any case as I have a bi-amp system I use the common of one cable e.g. the tweeter as the common of the woofer and vice-versa hence between the signal and common of either connection there are several centimetres and as capacitance is inversely proportional to distance between conductors the capacitance is negligible. This enhances the benefit of bi-amping namely the non-use of a L-C crossover with associated phase-shifts.
The radiation polar diagrams of high frequency radiators (tweeters) show that directly ahead the intensity is a maximum and that on each side of the straight ahead the signal falls away.In other words it is naturally the case that the high frequency signal from the left tweeter is stronger at the left ear than at the right provided you do not point the speaker directly at your listening seat. It is worth experimenting with the degree of "toe-in". I have the speakers facing straight ahead but it all depends on the speakers and the amount of baffle (flat surface ) surrounding the actual sound source.
As I have built my own amps and speakers I have been able to use a 2 kHz crossover using a circuit from Wireless World which gives a phase matched crossover i.e. the signals to both speakers are exactly inphase with each other on both sides of the crossover frequency. This is another benefit of bi-amping. Traditionally as the 2kHz frequency is the one at which the ears are most sensitive to phase differences (directional hearing) and as L-C crossovers introduce phase shifts the crossover frequency has usually been set at 4kHz.
My title is cribbed from the short story by Nigel Balchin made into a B/W film in the 50's. This author seems to have gone completely off the radar and he has not been seen in Waterstones ('s?) for a long while in company with another good writer, John O'Hara, who also had a film (Butterfield 8) made from one of his short stories.
Sat, Feb 11 2012, 7:22PM
#2
RE: The Small Front Room
About the choice of a 2kHz crossover: it was initially experimental and I was expecting to have to change it but the results were so good that I have stuck with it. Minor points in favour were the reduced requirement on the woofer in the 2-4kHz region and the fact that you could obtain tweeters with a resonance below 1 kHz. The slightest degree of non-linearity in the speaker cone of the woofer will cause intermodulation (i.e spurious frequencies) so reducing the woofer signal by one octave would reduce this in the frequency range where it would be most audible. The distance between the two speaker centres is reduced at the crossover frequency to about one wavelength (17 cm) rather than over two for 4 kHz, the combination behaving more like one source than two separate ones.
... Traditionally as the 2kHz frequency is the one at which the ears are most sensitive to phase differences (directional hearing) and as L-C crossovers introduce phase shifts the crossover frequency has usually been set at 4kHz.
Not that I'm any sort of expert, but what made you choose 2kHz? Was it the driver specs?
I take your point about the phase-perfect active crossover, but a crossover is still a crossover. Isn't it always better, if possible, to keep it away from the "intelligence band"?
Robin