Trends in Technology: Loudspeakers
Sep 1, 2001 12:00 PM, By Allan Soifer
If you ask most jocks/announcers what type of loudspeakers they would like in the control room, you might hear a response like �lots of power so I can feel the music.� If you put this query to the station engineer, you’re just as likely to get a different response with specs on sensitivity and power handling. It’s more a question of personal tastes and desires, than the attributes of any given loudspeaker.
Since we are mostly concerned with monitoring the quality of our on-air signal, it stands to reason that a clear and reasonably loud sound field be maintained at the operator’s position. Given the less than perfect dispersion fields of many speaker units, and no standard placement in the control room, the clarity of the signal may be muddied by the acoustics of the room, reflections from various angular surfaces of the console and nearby equipment, and the quality of the amplifier driving the system.
Modern materials have improved sonic and electrical loudspeaker performance.
Many radio studios, and most recording studios, are moving to nearfield monitors. For a talk-show studio with multiple host and guest positions, the high-on-the-wall-looking-down position is better. For voice monitoring, positioning is not so critical as it is for qualitative listening to the mix of music, jingles, stingers, announcer voice-overs, etc. Cue speakers are another critical need in the studio. It is often advisable to place the cue speaker on the floor, behind the operator or in a position away from the main monitors so that a busy operator can identify and focus on different sounds during a lively show.
The usual desires of announcers range from loud to bleeding ears. Running speakers too loud can cause acoustic leakage to other studios and danger to hearing health as well. Scale down the type and style of booth speakers to something reasonable. Judicious attention to amplifier output rating and possible input signal limiting may be advisable for booth use. You can also reduce the overall level by moving the speakers as close to the operator as possible.
Not a plumber’s leak
Speaking of acoustic leakage, this has always been an engineer’s nightmare. Concrete blocks, bricks, lead sheeting and pads, rubber feet and absorbers are among the common materials used to minimize acoustic leakage. One of the more interesting absorptive mountings that I have seen had the speakers sitting in box-like enclosures filled with fine sand; somewhat like indoor planters. It looked strange but did the job well.
Self-powered nearfield monitors are gaining popularity in many production facilities.
Acoustic leakage can be attributed to a monitor level that is too loud in the adjacent studio. While solid architectural and acoustic designs can minimize airborne sound leakage, a direct physical connection can nullify any efforts and expense that have already been made. A mounting bracket or suspension scheme that does not rely on wall studs or common ceiling joists may be part of the solution.
Well-managed remotes need quality loudspeakers too. The sound quality of playback direct from air is critical in maintaining good client relations at the remote site. Self-amped speakers made for PA and music playback, mounted on heavy-duty tripods, are perfect for smart-looking remote operations. Since each speaker assembly carries its own amplifier, proper volume levels can be achieved.
Caring for loudspeakers
Speakers tend to be forgotten once they are installed and operational. Proper maintenance is important, however. Monthly dusting of the speaker cone using a soft vacuum brush applied gently is a good idea. The other threats to speaker longevity are over-powering and excessive continuous distortion. The first one is simple � do not install an amplifier with more power than the speaker can handle. Peak watts, the maximum amount of power that a speaker can safely dissipate for a fraction of time, are roughly double the RMS value.
When used near video monitors, nearfield monitors must be properly shielded to avoid magnetic video distortion.
Distortion damage can occur even when safe power levels are never exceeded. The speaker is an electro-magnetic device that wants its voice coil to move longitudinally over the magnet. When the amplifier doing its work is driven into distortion by too high an input, or some other defect, the percentage of square waves delivered to the speaker drastically increases, causing the voice coil to attempt to move laterally � sideways rather than in and out. Excessive distortion delivered to the speaker causes overheating of the coil, since it cannot self-ventilate as it would in normal longitudinal motion. The end result is a burned voice coil.
Basic electronics knowledge tells us that a thinner conductor is more likely to lose energy passing through that conductor as dissipated heat. We can deliver the most power from amp to speaker by using the right size of conductor. For the majority of systems under 100 watts, regular 16-guage lamp cord will do. Phasing is critical for proper operation. A set of out-of-phase loudspeakers makes it impossible to derive proper stereo imaging, and the apparent sound field will seem to wander around the room. It is vital that the �+� terminal on the amp be connected to the �+� terminal on the loudspeaker. A way to determine the correct phasing if a phase meter is not part of your instrument collection is to drive the set of speakers with a mono source. If the speakers are in phase (both cones moving outward at the same moment) and aimed properly, the sound field will appear to originate from a phantom center position.
Phase coherence between channels is important, but absolute phase is just as critical. The speaker pair in question may be in phase with each other, but the waveform may be reproduced in reverse. When a bass drum sound is produced, the speaker cone should move outward and push air. It should not move inward and pull air.
Some speakers provide digital audio inputs in addition to a self-contained power amplifier.
Generally, speakers are divided into powered and non-powered classes. Non-powered speakers require a power amplifier. Powered speakers have a self-contained amplifier within. With non-powered monitors, an amp failure can be corrected without removing the speaker. Likewise, a power amp can be placed in a more favorable position to allow for physical space limitations. Powered monitors can be much heavier than non-powered versions. Also, powered monitors require an electrical power source. For longer cable runs, it is better to run a line-level signal than a speaker-level signal. Finally, a powered monitor will also free some valuable rack space and remove a large heat generator from the confines of the rack.
Many models are also available in shielded models, which reduce the magnetic field outside the enclosure. This is important when a speaker is placed near a video monitor because the magnetic field can cause an undesired effect to the color display.
With the extremely wide and varied assortment of speaker systems, and the many discrete loudspeaker components and plans/specs for building one’s own enclosures, the broadcast engineer has the daunting task of choosing an appropriate sound reproducer, and possibly the right mounting system as well.
Allan Soifer is an audio recordist and production consultant in Eastern Canada.