- Details
THE DESIGN PROTOCOL: PART 1, SPEAKER CONFIGURATION
Speaker configuration addresses serious aspects of the system under design : driver selection, cabinet size, special crossover properties, overall cost. One must carefully consider input information so that all these aspects be defined effectively.
-Isn't it enough to say that we wish all operation parameters of our speaker under design to reach maximum quality ?
No we can't say that, because it can not be done. We have already stated that speakers are closely related to their environment. A reference high-cost three-way 80-liter giant speaker can not operate in a teenager room ! Speakers are "operation-specific" devices.
| PART 1: |
- INPUT CONSTRAINTS: |
- MAJOR DELIVERABLES: |
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- special use ? |
-> speaker volume | |
|
- room size ? |
-> cabinet requirements | |
| - nominal impedance ? | -> number of acoustic branches (ways) | |
| - power handling capacity ? | -> number of woofers | |
| - cost level ? | -> woofer cost level | |
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- MINOR DELIVERABLES: | |
| -> special crossover tasks | ||
| STEP 1: |
SELECT SPEAKER CATEGORY - TARGET LISTENING SPACE |
-> SPEAKER VOLUME | -> ADDITIONAL PROPERTIES |
| special use | near-field monitor | small volume |
high quality drivers |
|
speaker resting on a shelf or flush mounted on a wall |
very small volume |
height no more than 30cm, special crossover tasks |
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| desktop speaker connected to a computer | very small volume |
low power drivers |
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| corner placement | whatever |
better be closed box design |
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| hometheater, center speaker | small volume |
2-way, 2 woofers, horizontally directive, D'Appolito configuration, closed box. |
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| hometheater, rear satellite speaker | small or medium volume |
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| typical domestic use |
small-sized room, <7m2 | very small volume | |
| medium-sized room,8-16m2 | small volume | ||
| average-sized room,17-29m2 | medium volume | ||
| large living room,>30m2 | large volume | |
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| STEP 2:SELECT SPEAKER VOLUME |
SELECT NOMINAL IMPEDANCE | -> WOOFER CONFIGURATION |
->2 OR 3-WAY) |
-> ADDITIONAL PROPERTIES |
| very small speaker volume, <12 lt | system at 8 Ohms | single 5.5 inch woofer |
2-way | |
| system at 4 Ohms | two 4-4.5 inch woofers | 2-way | increased vertical directivity | |
| small speaker volume, 12-20 lt | system at 8 Ohms | single 6.5 inch woofer | 2-way | |
| system at 4 Ohms | two 4.5-5 inch woofers | 2-way | increased vertical directivity | |
| average speaker volume, 20-35 lt | system at 8 Ohms | single 8 inch woofer | 2 or 3-way | |
| system at 4 Ohms | two (2) 6.5 inch woofers | 2 or 3 way | increased vertical directivity | |
| large speaker volume, >35 lt | system at 4 or 8 Ohms | single 10 or 12 inch woofer | 3 way | |
| system at 4 Ohms | two 8 or 10 inch woofers | 3 way | increased vertical directivity | |
| STEP 3:SELECT TOTAL POWER HANDLING CAPACITY | -> WOOFER COST LEVEL / SETUP / CABINET REQUIREMENTS | ->SPECIAL CROSSOVER TASKS |
| <30 Watt RMS per channel | low cost drivers |
no need for impedance compensation |
| <6.5'' single woofer or two 4.5-5'' woofers | ||
| enclosure wall thickness can be less than 15mm | ||
| 30-50 Watt RMS per channel | medium cost tweeter |
no need for impedance compensation |
| 6.5-8'' single medium-cost woofer or two 6.5'' low-cost woofers | ||
| enclosure wall thickness should be 15mm or more |
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| 50-100 Watt RMS per channel | quality drivers |
better be be impedance compensated |
| 6.5-8'' single woofer or two 6.5'' woofers |
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| enclosure wall thickness should be 19mm or more, internal bracing could be considered |
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| >100 Watt RMS per channel | quality drivers | better be impedance compensated |
| 10-12'' single woofer or two 8'' woofers | ||
| enclosure wall thickness should be 22mm, internal bracing mandatory |
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The majority of woofer, squawker and tweeter drivers have an SPL sensitivity of 90 dB (per 2.83 Vrms of input signal) approximately. This means that there are some tweeters with a 91-92dB SPL sensitivity value but most of tweeters have the common 90dB value.
10 or 12'' woofers may have a 91-92 dB sensitivity value. This value holds from 250-300Hz up to 1-2kHz. Below 250 Hz diffraction step shifts SPL levels down.
So in a 3-way project with one 10 or 12'' woofer we must choose a midrange and a tweeter driver with sensitivities similar to that of our woofer. If we have two large diameter woofers we will not be able to find a midrange and a tweeter driver with the resulting sensitivity value : 96-98 dB. In such a case we should look for woofers of very low sensitivity (84-86dB).
In single 8'' woofer projects the most common sensitivity is that of 90dB. This value holds from 400-500Hz up to 3-4kHz. Below 350 Hz diffraction step shifts SPL levels down. Depending on our preferences over crossover design targets we may choose our low-pass filter to give a smooth rolloff above 400-500 Hz. In this way the effective woofer sensitivity will be further reduced by 2-4 dB and all we have to do is to avoid choosing a 91-92 dB midrange or tweeter.
In two 8'' woofer projects (where the resulting unfilteres sensitivity level at 400-500Hz will be 95-96 dB) the low-pass filter rolloff trick will create a filtered woofer sensitivity of 91-92 dB which is quite manageable: a midrange and a tweeter driver of similar values can be easily selected in the market.
In single 6.5'' or less woofer projects the nominal woofer sensitivity value is furnished at a rather high (and narrow) frequency range. It is a rule of thumb to choose tweeters of equal or even slightly less sensitivity value. SPL matching is then assigned to the crossover filter responses.
In 2 X 6.5'' or less woofer projects we can safely use a tweeter of 90dB sensitivity and let again our crossover filters do the necessary SPL trimming of the 300-1000 Hz frequency range.