The Roof Is All Wired
Aug 1, 2001 12:00 PM,
IN-CEILING SPEAKERS, LET’S FACE IT, ARE PRETTY COOL. DESIGNED TO FIT right into the ceiling or be flush-mounted in a wall, they blend into residential and business installations so you hardly notice they’re there, yet they fill the space with sound. Their low-profile design makes them popular with interior designers, architects and sound contractors alike for use in a range of venues such as homes, airports, malls and movie theater lobbies.
Most in-ceiling systems today are coaxial speakers with a cone size in the 4- to 8-inch range. Where the speaker is equipped with a crossover or transformer, it is usually secured to the woofer basket or to a printed circuit board, and mounted to a plastic frame. This plastic frame usually contains the necessary mounting hardware and an area for a metal grille, typically made of aluminum or steel with an acoustic grille pad attached to its inside surface. The acoustic grille pad hides the view of the speaker without affecting the acoustic performance of the system.
Perhaps the most common in-ceiling speaker is a full-range woofer design that uses a device known as a whizzer. This cone-shaped device is attached to the center of the woofer’s cone and helps extend the high frequency response of the speaker. This cost-effective solution is commonly found in large department stores where paging and background music is heard.
The co-motional in-ceiling speaker is a popular, cost-effective, full-range, woofer-and-tweeter design. This type of in-ceiling speaker uses a lightweight piezo tweeter bonded to the center of the woofer cone in place of the usual dust dome. The piezo tweeter actually moves with the woofer cone as the woofer and tweeter operate together in a co-motional manner, hence the name. Since the piezo tweeter appears to the input signal as a capacitor, it usually requires no crossover, and the design of the woofer is set to match its performance. The co-motional in-ceiling speaker exhibits good off-axis, high-frequency response characteristics.
To meet increased performance requirements, some in-ceiling speakers use a woofer and a separate tweeter with a crossover. These types of in-ceiling speakers (see Figure 1) are known as coaxial speakers, because both the tweeter and woofer voice coils are located on the same axis. This type of driver alignment helps provide very good off-axis characteristics since the distance of the listener from the woofer and tweeter, when measured along an arc, remains consistent. Even though the center location of the tweeter post can create some peaks and dips in the on- and off-axis frequency response, the coaxial speaker exhibits good high-frequency clarity and maintains good articulation. To further improve the articulation and off-axis characteristics, several manufacturers offer a swivel-tweeter feature. This feature allows the tweeter to be angled toward the listening area to improve the off-axis, high-frequency response.
In-ceiling loudspeakers are sometimes mounted in a wall; therefore, their maximum depth for residential use should not exceed 3.5 inches. For commercial use, the in-ceiling loudspeaker maximum depth restrictions are usually much higher, especially if the in-ceiling speaker will be used in a drop ceiling. Certain agency approvals will often require that a back-can be used with the in-ceiling loudspeaker. In the unusual event of a speaker somehow igniting, this back-can will keep the flames and toxic fumes from spreading. If a back can is used, the depth requirement can increase 10 inches or more depending on the size of the woofer used and the low-frequency performance requirements. The in-ceiling product will usually include a support bracket that can be used to reinforce the mounting.
DUAL-TWEETER IN-CEILING SPEAKERS
Now let’s get specific and look at another type of in-ceiling speaker that is growing in popularity: the stereo, dual-tweeter in-ceiling speaker (see Figure 2). The SDTIS was originally designed to provide a stereo sound in small rooms where only one speaker would fit. It actually includes two speakers in one. To produce stereo bass, the woofer uses a dual voice coil, a special voice coil wound with four layers of wire. Two sets of input terminals are provided on the SDTIS for connection to the left and right channels of the amplifier. The 4-layer voice coil operates in a single-magnet assembly and is attached to a single woofer cone. The signals from both the left and right channels of the amplifier are received by the dual voice coil and transmitted to the woofer cone to produce sound.
The most noticeable difference between the SDTIS and a mono in-ceiling speaker is that it uses two separate tweeters. Like the dual voice coils, one tweeter works with the left channel and one with the right channel of the amplifier. Usually each combination of woofer and tweeter in the SDTIS uses its own separate crossover to sum independently the sound from each of the woofer-and-tweeter combinations. The SDTIS has the advantage over the typical mono in-ceiling speaker of being able to play both channels of the amplifier simultaneously.
SDTIS WIRING CONFIGURATIONS
Since the SDTIS is actually two in-ceiling speakers combined into one, there is more than one option for wiring the speaker to the amplifier. Besides connecting in stereo, which is the most common method, you can connect in series or in parallel. Both of these wiring configurations make the SDTIS into a mono type of in-ceiling speaker, and each has performance advantages and disadvantages.
Connecting in Series
Connect one red (+) and one black (-) terminal from each of the two sets of terminals together. Then connect the amplifier to the unused red and black terminals remaining on each set of terminals. Compared to a stereo wiring configuration, connecting the SDTIS in series approximately doubles the speaker’s impedance rating. It also reduces the 2.83v/1m SPL rating of the speaker by approximately 3dB.
Connecting in Parallel
Wire together both of the red terminals, from each of the two sets of input terminals; then do the same with both of the black input terminals. The amplifier is then connected to only one set of red and one set of black input terminals. Compared to a stereo wiring configuration, connecting the SDTIS in parallel approximately cuts the impedance rating of the speaker in half. If each set of terminals for the SDTIS is rated at 8 ohms, then wiring the speaker in parallel will change the actual rating of the speaker to 4 ohms. Wiring the SDTIS in parallel also increases the stereo 2.83v/1m SPL rating of the speaker by approximately 3dB.
WHY USE SDTIS AS A MONO?
Why would you buy a stereo, dual-tweeter, in-ceiling speaker, designed for stereo wiring in small rooms, and wire it for mono in a series or parallel configuration? For the same reasons you would wire mono speakers in series or in parallel: increased power handling and sensitivity.
An SDTIS could be useful when you must provide sound for a large area and want to focus the high-frequency energy to a particular space. The dual-tweeter, in-ceiling speakers could be used in a series-parallel wiring configuration to keep the impedance level of the speaker arrangement above the minimum impedance speaker load rating of the amplifier.
Another reason, more specific to the SDTIS, is high-frequency pattern control from a mono source. If you don’t like the price or the sound quality of a horn-tweeter, in-ceiling, coaxial speaker, consider using SDTIS wired in series or parallel to achieve high-frequency directivity and performance effects.
The direction of the speaker’s tweeter array will determine the high-frequency horizontal and vertical dispersion patterns it produces. If a wide high-frequency coverage pattern is desired (see Figure 3), the SDTIS should be positioned with the tweeters on top of each other. For a narrow high-frequency coverage pattern, the SDTIS should have its tweeters aligned side-by-side.
When the SDTIS is wired in its usual stereo fashion, the high-frequency polar patterns will vary depending on the left and right channel signal content and phase relationships between the dual tweeters. For most types of signal information, the high-frequency polar patterns will approximate those of a single-tweeter coaxial and have a wide dispersion pattern. To maintain the best stereo effect from this dual in-ceiling speaker, the two tweeters should be aligned horizontally, one on the left and one on the right of the listening area.
SDTIS IN COMMERCIAL USES
Wiring in a series or parallel configuration pertains mainly to in-ceiling speakers designed for consumer use, with a 4- or 8-ohm rating for use with a conventional amplifier. If the in-ceiling loudspeaker is to be used in commercial applications, it will often be equipped with a 70/100-volt transformer. This type of installation is called a constant-voltage system.
The transformer’s job is to send a high-impedance load back to the driving amplifier and to isolate the amplifier from the low impedance of the speaker. This allows for several speakers to be connected to a special amplifier designed to drive a high impedance load and capable of producing its rated power output when set at a fixed voltage. The amplifier’s maximum output voltage will usually be set for 70 volts for U.S. installations and at 100 volts output for European installations.
The transformer supplied with the in-ceiling speaker is also known as a line-distribution transformer. The wattage setting of the transformer determines the amount of power drawn from the driving amplifier. If the amplifier is capable of producing 250 watts output, then the installer can connect 250 speakers with transformers set at 1-watt taps to the amplifier (line losses aside). However it’s a safer practice to use less than the calculated amount of in-ceiling speakers, so the output power available from the driving amplifier is not exceeded.
Several in-ceiling loudspeakers also provide a rotary switch so the wattage flowing into them can be easily and quickly adjusted. Since every doubling or halving of power is equivalent to a 3dB change in SPL, the switch settings are configured to approximate 3dB steps, up to the power rating of the in-ceiling loudspeaker.
Dean Rivera is the engineering manager for KSC Industries Incorporated located in San Diego, California. Dean wishes to thank Rich Eilenberg at KSC for his support with this article.
Low Tones from High Places: In-Ceiling Subwoofers
The typical 6.5-inch-diameter in-ceiling loudspeaker is capable of reproducing audio frequencies from 30 Hz to 20 kHz (measured in a 2š field using the -6dB point of the frequency response, referenced from the rated sensitivity figure for the speaker). Mounting a speaker flush to the ceiling helps eliminate edge reflections and provides very good loading to the woofer. This boundary-loading effect can increase the bass response of the in-ceiling speaker by as much as 6 dB.
So why is a subwoofer — and for that matter an in-ceiling subwoofer — necessary at all? For the same reason we add subwoofers to our home theater or home stereo systems: better, more powerful bass, or even the best bass possible. Although in-ceiling speakers do make bass, they are designed primarily to work with a tweeter and are designed with woofer parameters to have enough midrange energy to match up with the tweeter.
Our ears are less sensitive to bass energy; therefore, we require more bass level before we are satisfied. This puts more demand on the woofer and calls for a specially designed woofer that can handle the power demands and lower frequencies. This is where the subwoofer comes in. The design of the in-ceiling subwoofer provides for a flush fit. The acoustic coupling effect from the ceiling helps to maximize the in-ceiling subwoofer bass performance over conventional floor-standing subwoofers. Couple this with a driver design capable of handling the power requirements and additional woofer excursion necessary to produce powerful, floor-shaking — make that ceiling-shaking — bass, and you have an aesthetically pleasing bass performance enhancement for any sound system. There are several in-ceiling subwoofer products are on the market for home and commercial applications.