Is this a good buy for a novice speaker builder?

Discussion in 'Home Cinema Speakers' started by great_sushi, Jan 10, 2012.

  1. great_sushi

    great_sushi
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  2. PSM1

    PSM1
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    If looking to make your own speakers then post in the DIY section. Making your own speakers and/or designing the cabinet is a very difficult thing to do and get it right so you get a good sounding speaker. Make sure you do lots of reading first. If this is just to save money then I would suggest just buying a normal consumer ready made speaker instead. If making your own speakers is some deep seated passion that you want to do then lots of reading etc. is the way forward.
     
  3. BlueWizard

    BlueWizard
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    The best advice is to simply copy existing proven DIY designs the first few times. There are several places on the 'net where you can find plans for speakers. Check out DIYAudio.com forums -

    Multi-Way - diyAudio

    Here is another source of good designs -

    DIY Loudspeaker Projects Troels Gravesen

    You can just pick a crossover at random, it has to be compatible with your speakers, and you speaker have to be chosen to be compatible with each other.

    In general, you can believe the specs on tweeter, and you can believe the low response spec on the woofer, but NOT the high frequency spec on the woofer. You will see woofer with a stated 40hz t0 4000hz response, but if you look at the specs sheet and the graph of the frequency response, you will see that few of them are stable above 1khz, though a few might be functional up into the 2khz to 2.5khz range.

    Tweeters will have a rated low response of say 1.5khz, but it would be best to not use them below about 2k to 2.5khz. Tweeter tend to have low power ratings and the closer you are to the rated low end, the less power they can handle. To raising the low end of a tweeter can increase the power handling noticeably.

    I have used off the shelf 3-way crossovers in a speaker build, but because it was 3-way, (bass, midrange, treble) it was easy to find woofer that went high enough at tweeters that functioned in the working range. In my case, the crossovers were at a somewhat generic 800hz and 5500hz. That was right at the recommended low crossover of the midrange.

    In a two way system, you very much have to make sure your woofer and tweeters are both functional at the selected crossover point. That can be a stuggle as you really can't go by the specs, you need to see a response graph on both the woofer and the tweeter, though the full spec sheets are usually available from the manufacturer.

    Here is an example of a bass driver spec sheet -

    http://www.parts-express.com/pdf/295-305s.pdf

    Notice a claimed high end of 4khz, but from the supplied graph, things are starting to get ragged above 2khz.

    Steve/bluewizard
     
  4. BlueWizard

    BlueWizard
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    I will add that the kit of speaker you linked to are probably OK, but there are a few problems.

    First, they are 10" woofer, that means a cabinet about 12" wide.

    Second, the specification indicate the need for a cabinet between 70 and 80 liters, which is about 2.6 cubic feet. Think about the dimension of the cabinet required to equal 2.6 cubic feet. If that cabinet size doesn't bother you, then you should be OK.

    Work on those dimension of a while and see if you can come up with a workable cabinet. If we use standard architectural dimension, and make the front 12", then to maintain proportion, the depth would be 20", and the height would be 34". The problem is, that is 8160 cu.in. or about 4.7 cu.ft. About twice as big as you need.

    If we narrow the dimensions to the narrowest possible of about 11", and make the cabinet 16" deep, if that is possible, then make it 28" high, we have 4928 cu.in. The goal is between 2.47 cu.ft. (70 liters) and 2.82 (80 liters). The last set of dimension are 2.85 cu.ft. So, we have a close working approximation, but can you handle a cabinet that big, and can you actually fit the speakers into the available space.

    Third, the bass drivers are 4 ohms making the entire system 4 ohms. That's not so bad in and of itself, but it means you need a decent amp. A bargain basement discounts store amp is not going to cut it. Though most any reasonable amp will, but it also means you can never add a second set of speakers. If your OK with that, then I'm OK with that.

    Next, we have to determine the bass port, or the vent whole in the cabinet. You need to look up the specifications on the Peerless SKP254 10" bass units and determine the resonance frequency (fs). That will tell you what to make the cabinet resonate at.

    For example, if the bass drivers resonate at 30hz, then you can make the cabinet resonate in the 30hz to 40hz range. But no lower than the resonance frequency of the speaker.

    Likely you want to use flared ports, both flared on the inside and the outside.

    Here is a port/vent caculator that might help, though if you search the Internet you should find more -

    LinearTeam

    Click on the picture of the speaker box/port until it shows a flared port.

    Using the above dimensions, we have a port diameter of about 6.25 inches, not counting the flares of the port.

    However, we have to allow for missing volume in the cabinet. The speakers take up volume and the cabinet requires a separate sealed chamber for the mid-range. If we knock off a (best guess) 0.75 cu.ft. for that, then the port length extends to 9.9 inches plus an additional (best guess) 2" for the port flares, making the ports overall depth about 11.9 inches deep. Remember the cabinet is only 16 inches deep and you need some clearance at the back of the port. Likely though in this case, the remaining 4" might work OK.

    If we go to a smaller 3" in diameter port and use the same tuning frequency, and the same (best guess) internal volume, then the port is 4.5 inches in length plus an additional 2" for the flares.

    So, did you understand that, and do you understand were I'm getting my information from? Because, if you don't understand this, then I'm not sure you understand enough to build the speakers.

    Also, I'm not sure how workable the 11" face of the baffle is, you need internal braces to screw the front to and you need space to fasten the speakers down. It might work, but it will take some careful planning.

    There are many other considerations that I'm not mentioning, because they are too complicated to explain here.

    The Dimension aren't fixed but generally the ratio of the various sides should be in a ratio of 1.618 to each other, though there is some flexibility there. So, a 12" face = 12 x 1.618 = 19.41 inches should be the depth, and the height should 19.41 x 1.618 = 31.4 inches. And those are internal dimension.

    The internal volume of the cabinet, is the internal dimensions multiplied together minus the volume of internal structures like the drivers themselves and the volume of any internal chambers. That resulting volume you use for your port calculations.

    Lastly, the amount of insulation in the cabinets is the final tuning. Too little and the speaker tend to sound clutter because of all the sound waves bouncing around inside. Too much, and the speakers tend to sound flat. Also, the amount of insulation or padding in the cabinet effect the bass response graph.

    I tried to add more insulation to some factory made speakers I have, and I did get more of a bass peak, but it was followed by a much deeper dip in the bass response just above the peak. So, what I gained in one area, I lost in another. You job is to find the combination that best averages these gains and losses out.

    To really test and tune the speakers you will either need some bass tones or bass sweeps and an SLP meter to measure the output of the speaker at these low bass frequencies. Plot this out on graph paper to see how various amounts of insulation are affecting the bass response.

    I would suggest a bare minimum of the back and sides covered with 1" to 2" of poly batting from the fabric store.

    Forth, the crossovers, one has to wonder if these crossovers are specifically made for these speakers, or if they are just off-the-shelf crossovers? If they are off the shelf, then we really can't count on the crossovers frequencies being that accurate. If the are specifically made for these speaker, then they are probably OK.

    Still want to build your own speakers?

    What I have outlined here is only the bare essentials to building a speaker cabinets, there are many more complications, that are just too deep and complex to explain here. You'll have to research those on your own. Keep in mind, I have some speaker I built in 1980, since then I've built new cabinets and added tweters, later remodeled them to correct some mistakes, and am still thinking about ways I might improve them. A project like this is never done until you either die (let's hope not) or you sell the speakers.

    Once again, if you are still interested, have at it. The price of £120/pair seems very reasonable for the speakers and crossovers.

    Building speakers is a learning process, and you have to approach it as that. The more you learn, the more you realize you don't know. But that's just the way it is for everyone from the novice to the expert. Even the experts are stunned by how complicated it is and how much they don't know. Still, if you are determined, that shouldn't stop you. With every speaker you build, you learn more and become better.

    Just a few thoughts.

    Steve/bluewizard
     

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