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Sound waves distorted in certain speaker enclosures?

  1. Aug 9, 2006 #1
    my theory may be sloppy and wrong - but here it goes

    when people start talking about speakers and thier enclosures there are generally two types used, ported and sealed. sealed being just that, and airtight box with a speaker in it. but a ported box is far from air tight instead they put a hole in a box so that when the speaker travels out it can suck air in and when the speaker travels in it pushes air out( HERE for a pic of a ported box).

    now here is my question. if the speaker pushes out a compression, then technically wouldn't a a rarfacation be pulled in at the same time, causing there to be canceling/distorting of a wave (at least in theory)? but i know for a fact it is no cancled these boxes are know to be louder, why?
     
  2. jcsd
  3. Aug 9, 2006 #2

    rcgldr

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    Louder but less accurate unless sufficient electronic feedback is used to compensate for the lack of dampening factor. This can be done in the speaker's electronics, or in the amplifier. The amplifiers that do this are expensive though.

    With a typical ported system, in the base range the effect is to "muddy" up the sound. A crisp thwack, becomes a lengthened whoooom. The worst sound is high level base from a car or van, because the vehicle's body will resonate with the lower frequencies.

    Most home theatre speakers use ports for the base and sub-woofers, and are underdamped, since for movies rumble is rumble and doesn't need to be musically accurate. It is still possible to find more musically oriented speakers, but the price is higher now because this has become a niche market. Some of these are still ported, but use electronic dampening (or rely on special amplifier setups) to prevent overshoot. Finding good acoustic suspension speakers is really tough these days.

    The most accurate speakers are true studio monitors, but the high powered ones are expensive. These are classified in terms of volume, from lowest to highest, they are called near field, middle field, far field. Also a lot of speakers claim to be studio monitors, but few truly are of studio monitor quality.

    Good commercial disco speakers are acoustic suspension, less efficient, but they're intended to be used with high powered amplifiers, sometimes 1400 watts or more per speaker at 8 ohms impedance or higher (18 inch woofers eat up a log of power). The sound is loud is enough that placement is tricky. Often these are suspended by chains from the roof in order to reduce resonating the floor or walls. They also use well padded enclosures. These are also next impossible to find. Cerwin Vega makes low quality home speakers, but their high end commercial disco speakers are very good (assuming they still make them).

    Personally I have a pair of Energy Veritas 2.8 speakers, which handle 400 to 500 watts per channel, impedance is about 4 ohms each. These are ported and many consider them good enough to use as reference monitors. However the base on my old acoustic suspension Saras, rated at 200 watts per channel, with impedance of 6.5 ohms, have a slightly tighter base.

    You nead a relativly "dead" room to keep from resonating the walls and floors when there is loud base. Also you don't want a lot of sound reflecting off of hard surfaces. I nixed my wifes idea of having a wooden floor in our family room for this reason.
     
    Last edited: Aug 9, 2006
  4. Aug 9, 2006 #3

    rcgldr

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    Below is a to the standard line of Cerwin Vega pro speakers. The disco speakers are combinations of these and/or custom made. The Vision series is a good example. They include a truss (suspended) setup in one of the pictures (in the discos I've seen chains were typical). The 3 way system and the 18 inch sub woofer pair recommend power of 1400 watts at 8 ohms, with peak power handling at 2800 watts.

    http://www.cerwinvega.com/products/newpro/index.html

    I'm not that familiar with which true studio monitors are really good, other than last time I checked these were running around $5,000 (US) or higher per speaker.
     
    Last edited: Aug 9, 2006
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