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RE: Re: building a house from scratch

Posted by KlausR. (A) on May 17, 2011 at 08:32:28

In Reply to: RE: Re: building a house from scratch posted by David Aiken on May 15, 2011 at 16:01:30:

>In a bigger room maintaining the same listening triangle, the speakers are going to be further from at least one set of walls, and the same goes for the listening position. The reinforcement walls provide at low frequencies is going to reduce, definitely an issue with 2 way standmounts with a 6.5" midwoofer like my Dynaudios. With the longer reflection paths, the level of the reflected sound is going to drop. With the greater air volume in the room, high frequency absorption by air is going to increase. Many walls and ceiling materials do absorb to some degree and the increase in wall and ceiling area is going to result in a proportionate increase in absorption. Increasing room size also increases floor size and that will further increase absorption if the room is carpeted or larger rugs are present.<

All this is quite true, that’s why I was cautious to write “anechoic sound pressure level”. If a system is capable of producing 100 dB at 3 m in the anechoic chamber, then it will produce that SPL at that distance in a large room. Using data for relative reflection levels provided by Devantier (AES paper 5638), with a source producing 80 dB the reflections produce additional 4.3 dB. An increase of 3 dB requires twice the amplifier power, so yes, with less support of early reflections some systems may get into trouble in large rooms, but not those mentioned above. The smallest model of Klein+Hummel produces 107.7 dB/1 m under semi-anechoic conditions, i.e. a chamber with only the floor reflection, in a reverberant room that is 105 dB at 2 m, 102 dB at 4m, 99 dB at 8 m, that’s still very loud.

Boundary reinforcement is not related to room size, it’s depending on distance to the boundary only, “above λ/2 the boundary has virtually no effect on radiated power (Allison, “The influence of room boundaries on loudspeaker power output”, JAES 1974, p.314). Maximum effect is at about 0.1 wavelength, at λ/4 the gain is zero. This means that when placed at 1 m from the boundaries, only the frequencies below about 86 Hz are experiencing gain, with maximum gain below about 34 Hz, the closer you get, the higher the limit of the frequency range that is affected. This might cause some problems when using large speakers in very small rooms because you might be obliged to place them close to the walls.

>I have a sneaking suspicion that desktop monitors are designed for use close to room surfaces and that they would show up the effects of reduction in such support a lot more quickly than your and my normal speakers would.<

If I’m not mistaken, genuine desktops are designed taking the desktop reflection into account, since when used as intended, i.e. neafield listening, this reflection is the only reflection arriving at the listening position, and being the only it may cause audible harm.

>A big issue your question about increasing room size while maintaining the same listening triangle raises is whether or not someone who moves to a bigger room will maintain the same listening triangle.<

That’s another question entirely, but you are probably right when saying that they won’t. If listening distance increases you need a system with more output, either more efficient speakers or more amp power. Bigger speakers alone won’t do the trick. However, the only way to know what the speaker is delivering is performance data, anechoic or semi-anechoic SPL, which to this date I have never ever seen from consumer speaker manufacturers.


>With a big new room and our usual belief that bigger is better (after all, both you and I are basically in agreement about making the room as big as you can), do you really expect that most—ie more than 50%—of us aren't going to think that bigger speakers won't also help with the sound we get?<

Most of us would, sure, but I think that a large room as such does not necessarily require more power. With the listening triangle unchanged, more power is required only when the existing system has not enough headroom to compensate for the decreased support of early reflections. More power is probably needed when you change the triangle and increase the listening distance. It looks as if there is no straightforward answer.

Klaus

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