front of — the speaker and that the calibrated microphone is connected to an accurate audio meter reading dB SPL .
Considering these quantifications :
• Normal loudspeakers have a sensitivity of 85 to 95 dB .
• Nightclub speakers have a sensitivity of 95 to 102 dB .
• Rock concert , stadium speakers have a sensitivity of 103 to 110 dB .
Although sensitivity rating provides a way of comparing the efficiency of speakers and enclosures , it doesn ’ t provide the complete picture . Directional characteristics and the physical ability of components to move large amounts of air also play into the equation . For example , cone speakers can be compared by their cone excursion , which is the maximum amount of travel distance of the voice coil around the magnet and is specified as the X max
. Though this is more commonly specified in car audio system subwoofers than in professional audio speakers , the concept provides an instructive visual image .
POWER , SPL , AND SENSITIVITY Bear in mind that there is a fixed relationship between power and SPL . Doubling the power , given the identical input source , results in a 3-dB increase in SPL . Therefore , a 100-watt speaker with a sensitivity of 95 dB will produce 98 dB when given 200 watts . Conversely , when sensitivity is considered , a speaker with a 3-dB increase in sensitivity will require half the power to achieve a given SPL . Seemingly small decreases in sensitivity imply substantial increase in wattage demands .
The assessment of power-handling capabilities , sensitivity , and desired SPL go hand in hand . As with other speaker assessments , specifications only provide useful information when considered altogether and from a quantifiable vantage point .
CORRECT POWER RATINGS FOR EACH CABINET The cabinet must be capable of handling the power supplied by the amplifier at the specified impedance . For example , a cabinet rated to handle 600 watts RMS at 8 ohms should be paired with an amplifier that can deliver 600 watts RMS to an 8-ohm load . A more powerful amplifier could cause damage to the speaker components . Conversely , though an amplifier with less power than the specified cabinet rating might not damage the components , it also might not produce optimal sound quality . Sometimes an underpowered amplifier works too hard to achieve the desired SPL , delivering a distorted waveform to the cabinet that is irritating to the listener .
The ideal combination delivers an accurate powered signal with minimal distortion characteristics to sufficiently rugged and accurate speaker components .
BRIDGING A STEREO AMPLIFIER The bridging connection turns a stereo power amplifier into a more powerful mono amplifier — it combines the left and right outputs from the power amplifier into a mono output with increased power .
The bridged connection is simple to accomplish . From the power amplifier , connect the positive post from output 1 to the positive post on the enclosure , then connect the positive post from output 2 to the negative post on the enclosure . See Figure 1 on the next page .
Bridging is common in large systems in which multiple components are powered by several amplifiers . A theoretical four-way stereo system might include two high-frequency horns , two mid-frequency cabinets , two enclosures for the low-mid frequencies , and a subwoofer for the left channel , along with an identical complement of speakers on the right . That adds up to seven cabinets per side , for a total of 14 .
In a large system , each cabinet will typically be fed from its own bridged power amplifier . Bridging is an effective way to provide more power to each enclosure while increasing the fidelity and flexibility of the system . Modern speaker components are efficient and capable of handling impressive amounts of power — power that is effectively delivered by bridging .
Be sure the power amplifier is designed to be bridged . Any amplifier is easily bridged by simple connection modification , but if the amp isn ’ t designed to be bridged , it could be damaged .
Some amplifiers provide a switch to select bridge mode with a diagram showing proper speaker connection . Amplifiers using banana connections typically position the outputs so that bridging is easily accomplished by connecting the plug across the left-right outputs instead of between the positive and negative posts on just one of the outputs .
When the amp is bridged , the resulting power output doubles into a load that also doubles . In other words , two 200-watt channels into 4 ohms bridge together to provide 400 watts into 8 ohms . At an identical load , bridging quadruples the amplifier ’ s potential ! Because two identical amplifiers work together in bridged mode , it ’ s easy to see how the power output might be doubled , but this change of impedance matching doubles the fun again . However , in the real world , the bridged power potential is typically about three times the single left or right channel rating — this is a result of inadequate power supplies and output devices .
The following specifications represent the power specifications of a couple professional power amplifiers offered by reputable manufacturers . See Figure 2 on the next page .
Bill Gibson This article is an excerpt from The Ultimate Live Sound Operator , 3 rd Edition by Bill Gibson .
Teacher at Berklee College of Music Online and The Conservatory of Recording Arts and Sciences ( CRAS ), content creator for LinkedIn Learning , and author of more than fifty books and videos about live sound and studio recording . Most recent book releases : Recording and Mixing Drum : Contrasting Techniques from Seven Lifetimes of Experience , The Ultimate Live Sound Operator ’ s Handbook , 3rd Edition , and The First 50 Recording Techniques You Should Know to Track Music .. Link to BillGibsonCourses . com
120 May 2023 Subscribe for Free ...