STONES SOUND STUDIO
SELECTING A SPEAKER FOR A DIY PROJECT
One of the most important parts of any speaker project is selection of the speaker driver.If you're building a kit or working from a set of plans, in most cases this has already been done for you. Still, even in the case of a kit, many different drivers can fit into the same panel cutout and you may wish to select alternatives. If you're building from scratch, driver selection is a required part of the design process.
Consider what is the primary purpose of your speakers first. Is your application for a home theater (HT) system, or for music? Most people will use theirs for both, so here are the basic guidelines you need to observe:speakers
Home-theatre multichannel requires at least 6 speakers (some 8 Speakers) two main (left and right front) speakers a center (front) channel speakers two rear channel speakers and a subwoofer. Digital Dolby can require still more speakers. For HT systems the recording engineers are in control of the sound field, so all 5 of the main and rear speakers should sound pretty much alike. Also, since there are so many of them, they should be as small and cost effective as possible. If you don't want to make all 5 alike, you should at least try to make the front 3 alike and the back 2 alike, both sets with a common "sound". The front 3 should also be magnetically shielded to avoid interference with the TV screen. All should have response down to around 80 Hz, where the subwoofer takes over . Since one of the things that contributes to the realism of HT is the bass, don't scrimp on the subwoofer!
DVD movie sound tracks and DVD audio discs offer multiple channels but ,most of your attention should be paid to the quality of sound coming from your front two speakers and the centre channel as they provide the location and depth of field for the music or video effects .The centre speaker is the one of the most vital links in multichannel digital surround systems providing the left to right movement of people effects objects and instruments steering you to the correct location in the sound field . If you wish to use a subwoofer, you will either need an electronic crossover in the signal path used in conjunction with a separate power amp , or the preferred easy option and less involved is to purchase a subwoofer amplifier which has the necessary bass filters and phase switching built in already and can be mounted straight into a subwoofer enclosure ready to go .
Shielded drivers The two front main speakers may be located far enough away from the TV screen to not necessarily require shielding but the center channel speaker will require shielding. Rear channel speakers in such a system should share the same sound as the front channel speakers usually using similar drivers with the same cone material and sensitivity
Choosing the number of drivers and their specific parameters sounds easy but just remember, the more drivers you use, the easier it is to mess things up and the more complex the hole design becomes . Two-way designs are the simplest and can sound amazingly good with high definition drivers . Three-way designs require more care to properly match all the speaker drivers in sound character (cone type) and SPL level . Four-way or higher-order designs get really complicated. Often, these will have crossovers in the middle of the critical musical range of 300-3,000 Hz, which makes driver matching much more critical and is very time consuming.
Choosing the type of enclosure for your driver using Thiel and Small parameters as a starting point Use a computer speaker design program like one of those listed on the contents page and apply these simple rules
A very basic general rule of thumb for enclosure types which will allow a you to quickly choose a driver or several drivers for your preferred type of enclosure and quick calculation of size
Ots < 0.5 use a vented enclosure
Ots > 0.5 use a sealed enclosure
Vas with values ( 5 to 60 ) litres will result in a small to medium enclosure size
Vas with values (80 to 250) litres will result in a large enclosure size
Vent as a general rule of thumb the vent size should be around 25% to 40% of the speakers piston area
Sealed enclosures do not have the bass extension generally as a similar sized vented enclosure but can be placed against or very near to walls in corners or in wall cabinets which can supply a boost in the bass level between 4 to 20db through room mode gain and compensate for the lack of bass. Sealed speakers do not suffer from vent velocity turbulence or wind noise of the vented enclosure but drivers usually require a larger Cms (compliance) a higher power rating and a lower Fs resonant frequency .The box or enclosure is usually lined of filled with acoustic damping material to reduce excessive cone movement and fine tune the system Q of the box ,remembering that the speaker acts like a spring that has to expand and compress air inside the chamber as the cone moves in and out with the music signal .Any poor glue joints on the cabinet will result in air leaks and thus poor and distorted sound so be liberal with the glue when building the cabinet .Sealed enclosures can be used for 4 to 6' drivers as a midrange cabinet where very low frequencies are not required and thus drivers with smaller cone excursions can be used . Drivers normally requiring a vented enclosure can be placed into a sealed box and used as a midrange for a 3 way speaker system where the low frequency roll of the cabinet driver combination can assist the demand on the crossover
Vented enclosures have their advantages over the sealed enclosure with reduced cone excursion at and around the resonant frequency of the System ( driver ,speaker vent and enclosure ) also providing an extended low-end response compared to a sealed enclosure of similar size .Increased power handling of the driver at low frequencies around the system resonance is improved and greater flex ability of system performance. Upgrading can be performed by only changing the vent length and diameter rather than changing the complete cabinet size as is required with a sealed enclosure. Small changes in the bass response can be achieved by varying the amount thickness and density of the acoustic material lining the inside wall or walls of the cabinet. Vented enclosures are tuned to get the best performance from the driver by the addition of a vent either in the form of a PVC tubular round pipe or rectangular vent made of the timber, as a general rule of thumb the vent size should be around 25% to 40% of the speakers piston area .Thiel and Small (two Australian Engineers from Sydney ) developed the world Speaker industry standard of acoustic speaker parameters when they derived a series of mathematical filter functions to calculate the enclosure and vent and size from a set of parameters measured from the speaker driver itself, these parameters are usually entered into a computer software program for speaker box design .These Thiel and Small (T&S) parameters are available from the manufactures or for higher accuracy SSS LEAP (T&S parameters ) see those provided in this book or on line.
Bandpass enclosures are simply a high pass enclosure (vented ) with an acoustic low pass filter (chamber) attached to the front of the vented enclosure which gives a flattened mountain top or bell shaped frequency response .The SPL (speaker pressure level) and efficiency is greater than a vented enclosure because of the reduced power bandwidth . Can be used as a subwoofers when no active filters or subwoofer amp is unavailable , Is used in the car and PA industry where high efficiency is on demand .Not normally recommended for home theatre or hi end sound ,cabinet construction can be complex
Subwoofer Enclosures are usually vented or sealed depending on the type of driver .Vented is the most common type used through the Home- theatre industry because of the size and efficiency obtainable from today's high powered high efficiency drivers. They can be vented any where on the cabinet and even with the vent facing the floor providing you allow at least 50mm spacing above the floor for the vent to work. At very low frequencies a developed in DVD thx movies the subwoofer driver must have response down to at least 30hz and down to 20hz for the larger 12 to 15" drivers . The inside walls of the cabinet need to be very well braced with timber allowing at least 25mm thick MDF (medium density fiber board ) front side and rear panels so as to reduce any baffle resonance to a minimum . The frequency response bandwidth is between 20 to 180 hz so a good long throw low fs high powered subwoofer driver is a must. A subwoofer is highly recommended for Home theatre speaker systems and the size will depend on the size of your speakers and room . I recommend Subwoofers 6" to 8" for small to medium rooms an 10" to 12" for large rooms 15" for extra large rooms
Choosing the right type of driver for your system
The type of driver will depend on the sound program material and type of system
Digital Home-theatre systems
Stereo music systems
PA music systems
Intercom conference system
Multi-room AV domestic house system
Computer DVD and music
Internet in room monitors
Digital Home-theatre systems speaker drivers need to be matched as near as possible in sound signature ie cone material type power rating and SPL sensitivity levels so as to obtain similar sound signatures from each of the surround channel speaker enclosures.
I suggest that the front speakers are probably the most important and if you are on a budget apply most of it to the fronts .
Digital HT and Music Speaker Bandwidth The speaker system needs to have a wide bandwidth to reproduce the demanding and dynamic sound tracks from a movies and music.
I recommend choosing drivers with wide bandwidths with a flat frequency response and
a smooth roll of at the upper and lower frequency extremes .Metal dome tweeters or metal cone bass-mid drivers are generally harder to apply when designing a speaker system with rivers usually require more complex crossovers due the inherent cone breakup modes
but can sound good when properly applied .
The speaker system for Home-theatre movie and music should have a frequency response from a minimum of 60hz to 20Khz preferably more in the top end to 40Khz if possible with an active subwoofer unit covering the lower octaves from20Hzto180Hz Remember that as technology improves so will the demand for wider bandwith systems so look always to the future when building a speaker sys tem.
The sound quality of the multi-channel movies or music sound tracks depend on what studio the DVD or Digital recording was recorded in eg (Sony WEA Artisan Entertainment CBS 20th Centry Fox ) and who did the sound mixing this puts a big variable into the sound quality which can vary from being very dynamic to dead flat undynamic unexciting this is why some DVDs sound better than others . If your speakers are properly designed
and are very detailed and dynamic you will be able to hear the radical differences in sound quality and realy make listening an enjoyable experience so do not scrimp on the cost of the speakers they are usually a long term investment