Jacob":2ia1fl75 said:
I read somewhere of speakers being built in to the house using voids in the floor and other places as the horn. But powered by fairly cheap speakers, the sound quality being in the horn construction. Is this viable or just badly remembered?
It's been done. There's a really elegant example I've seen somewhere on the net of a pair of folded horns, built as brick labyrinths under the floor.
But bear in mind that it doesn't buy you everything. At bass frequencies, loudspeaker cones act as pistons. Just like in a car, if they move in free space, they don't do any work, as the air can move round the sides too easily. They also move too far in and out. The horn design is a sort of acoustic transformer, changing the big cone excursion (as much as an inch or more on a really big drive unit) into smaller pressure waves over a much wider area - the equivalent of what you hear naturally. This does work pretty well, and folded horns are used in many large PA systems, to couple the bass units to the air more efficiently. They often have very large cones too, and the labyrinth has another purpose, which also applies to mid-sized speakers in hi-fi designs: it couples the space inside, behind the bass unit to the outer air. In theory the delay from the labyrinth would be 1/2 wavelength of the sound you want to reproduce, so bigger = lower.
What about the organ? I've been in churches (Durham Cathedral particularly) during a virtuoso practice sessions and the sound was amazing, like nothing I'd ever heard anywhere else. Can you get that at home, would you need serried ranks of speakers like organ pipes, or just the right compact kit?
There are three problems:
1. the ability of the system to reproduce the 'sound'. The lowest rank of pipes in a properly indecent organ is usually 32ft. That's 10m, so the lowest 'note' is roughly 16.5Hz (assuming 1/2 wave). That's at roughly the bottom end of human hearing - below that we tend to perceive pressure waves as individual thumps rather than a 'sound'. And you can achieve lower notes still by 'beating' notes together. It's true those pipes usually don't speak on their own, but are added to the overall organ sound either in sympathy or dissonance (if it's Messiaen!).
Then you've got the building resonances. Apparently my dad got into trouble at school by sneaking into the chapel (with his brother!) and beating the lowest rank of pipes together in pairs until they found the building's resonances and nearly removed the window glass. It's impossible to divorce the cathedral organ as an instrument from the building it's situated in. I've been told that in certain places, certain combinations of stops and notes are banned, because of the risk of damage (but it may be an urban myth). But you can definitely create very low frequencies at quite significant amplitudes (loudness), vibrations you feel rather than hear.
How you make a hi-fi system to produce such sound is thoroughly non-trivial. Ignoring what plays the sound in the first place, the amplifier and loudspeaker systems become pretty complex and big!
2. what was originally recorded and mastered: Digital systems in common use usually are specified between 20Hz and 20kHz*. For a sound engineer, the problem with very low sounds is the amplitude (loudness) of them - you have to avoid overloading the recording system, and you ought to consider the playback system too.
Yes, you can turn down the wick a bit, but that makes everything quieter. When it's played back on a system that can't create the bass, it will be somewhat odd, and people would complain the CD (or Radio 3 broadcast) was too quiet on their ordinary systems. Sound 'balancing' IMHO is an art as well as a craft skill, but that doesn't stop excessive bass being a problem. So almost all recordings in the real world, will have some sort of filter applied, to 'roll off' or limit the bass end, so that it doesn't become excessive. Exactly how much and where it's applied all depends, but there will be something.
This is a particular problem with vinyl records. Because loudness = width of groove 'wiggle', and in stereo, channel difference = depth of groove, there are practical problems in cutting a disc with a lot of bass on it. Bass on just one channel is worst of all, as it has a lot of 'height'. You can only go so far before the cutter runs out of shellac and hits glass or aluminium underneath (and they do, and it's expensive). At the other extreme it could theoretically jump right off the surface! Similarly side-to-side, grooves can collide with each other. There are strategies to deal with this, nowadays involving computer-controlled cutters and a lot of number crunching, but the bottom line is that the nature of the system severely limits how you can handle bass frequencies. Mastering is the process of converting a recording into something that's actually cuttable
and playable too (it affects high frequencies differently, hence my comments about the 1812 overture).
3. neighbours.
So it's tough. In practice, you *might* get a digital system to produce something like what you experience in a cathedral with César Franck at full throttle, but you'd need big (really big) bass speakers and a reasonable space to listen in. It would be a practical impossibility with vinyl or most sorts of analogue tape.
Aside: this is actually my favourite from Hoffnung's "Symphony Orchestra":
Cheers,
E.
*OK there's 24/96 sampling but let's keep it simple!
