Loudspeaker Selection

Basic Loudspeaker Selection:
By Greg T. Vincent
Principal Consultant

To select loudspeakers, ask the following questions:

•	What is the intended use of the facility's audio system?
•	Speech
•	Stereo playback
•	Performances
•	Paging
•	What is the quality level desired?
•	What is the volume of the space?

As an example, let's look at an auditorium that is approximately 100 feet deep: For basic use of speech and some limited playback of lower quality, a central cluster is normally used. This cluster would consist of several cabinets to cover the area with the addition of a small bass bin to make it sound more natural and not too trebly. The speakers would be selected by many factor including:

1.) Coverage:
All speakers have a specific nominal area they can cover usually 90 or 60 degrees horizontal and 40 to 50 degrees vertical, of course, this is not perfect and at different frequencies, this coverage varies greatly. One has to study polar plots of speakers at sample frequencies to determine the coverage area for all frequencies) but in this case, lets just say this is a perfect world and this is a perfect speaker. Therefore, by laying out the coverage area on a floor plan one can determine what coverage area speakers to use.

2.) Efficiency and Power Handling:
You now have selected a speaker that has the dispersion to handle the area to be covered but can it throw sound that far? Another specification given is usually a dB rating of the speaker with1 watt of power measured at 1-meter say around 95dB. Now a designer must determine a desired dB level for the room which is based on all sorts of things (ambient sound levels, type of program, intended use, etc.) so one must calculate at what distance will that speaker roll off considerably and back up from there.

At this point, you will have calculated that to reach a desired dB level of say 100dB at say 90ft you will need 700 watts of power. However, the speaker you selected can only handle 400 watts of power. Therefore, you back track from there and discover that the speaker can deliver uniform levels at say 75ft and it drops off rapidly from that point. Now what do you do to cover the rest of the space?

Well you could look for a speaker with the same coverage area that might be more efficient to handle more power so it will throw sounds further. Then start your calculations all over again. Usually you are walking a tight line trying to balance between the clientÍs budget and bigger speakers or the addition of delay speakers to cover the rest of the area. Throw in Architectural space limitations and this adds more complexity to the selection process.

So now it is determined that bigger speakers are not feasible due to space and budget so you look for alternatives to reduce the size/cost of the main cluster and start selecting delay speakers. A delay speaker is where the speaker is located at some distance from the primary source speakers. In order for the sound from the primary speaker to arrive in time with the sound from the delay speaker the electronic signal being sent to the delay speaker is delayed in time to arrive at the same time as the primary thus eliminating the undesirable effect of an echo. By reducing the size/cost of the speakers, the coverage area shrinks to say now 65ft but by adding the delay speakers, you can now cover the entire area. Now the same calculations must be done for the selected delay speakers and to make sure that placement of the delay speakers overlaps the main cluster coverage area seamlessly.

Never design to the limits of a speaker system and its electronics, always leave enough headroom both acoustically and electronically for peaks and clarity Now you can upgrade this system by using higher quality speakers or more powerful speakers, add left and right speaker clusters to add stereo playback capabilities for typical performance venues.

This all is based on the intended use of the system, space, and budget.

Multiple ceiling speakers or otherwise know as a distributed sound system is used in many facilities for paging, background music, and speech lift systems. These types of systems have their own set of design rules but use a lot oft he same calculations (coverage, level, power etc.). Again you must know what the space is going to be used for to see if this approach is the best way to go or not. The whole idea to this type of system is to be able to have such uniform coverage and levels that the ceiling seems to act as one huge speaker.

If designed and implemented properly one should be able to walk about the entire space with your eyes shut and not be able to pinpoint a single speaker location with your ears. Of course, this is the perfect room and architectural restrictions sometimes have a great effect on this and in some severe case can render the system to be ineffective.

Some designs/systems to avoid or what not to do: These are rules for systems with Microphones and do not reflect movie theaters or home stereo systems or "surround" applications:

1.	Never put speakers on the sidewall facing each other. The headphone theory does not work here unless your head is as big as the room.
2.	Never put speakers on the back wall facing the presenter. Yes, there are speakers in your surround system at home and in a movie theater but again we are not talking about those spaces here and besides unless it is a special effect all voice usually emanates from the center dialogue channel anyways.
3.	Always locate the speakers ahead of the microphone/presenter and slightly above the audience area. This will greatly reduce the possibility of feedback and everyone will thank you.
4.	In program playback applications, always locate the speakers flanking the screen slightly above the listener's head. This is used for "localization." There is nothing worse that having to listen to speakers with the sound coming from someplace else than where you are looking (Screen) A known effect of this so known as "listener fatigue" after a while the brain gets tired of trying to associate the unnatural sound location with a different visual location.
5.	Avoid spacing speakers so far apart that it creates a "hole" in the middle. This is especially necessary for any stereo program systems to maintain the stereo image.
6.	Avoid placing speakers in hard corners. This is not a home stereo system and not a small room
7.	There is no substitution for size. The laws of Physics dictate this. A 4" speaker cannot move as much air/sound waves as a 18"speaker. For lower frequencies, an 18" speaker can reproduce the lower frequencies a lot better that a 4" speaker can.. There is only so much you can do with a 4" speaker no matter how many of them you use.
8.	Avoid at all costs any architectural restrictions or obstructions to any speaker. You can only predict what a speaker will do based on the manufacturer literature; this is based on a freestanding cabinet. All bets are off as soon as you throw restrictions or obstructions in front, top, or to the sides of a cabinet.
9.	Avoid "hiding the speaker" syndrome. This usually ends up being a disaster.
10.	Avoid any "Styrofoam, becomes part of the wall, disappears" (Some Architects actually believe this stuff) speaker. Face it here folks, what kind of sound quality do you think your going to get from a speaker made of Styrofoam and covered with gypsum mud and or wallpaper? Just how far do you think the sound will go and at what levels?
11.	Avoid any designs that have the word "Surround Sound" used anywhere in the spec unless you are actually building a movie or home theater.
12.	Avoid any designs that have a majority of home hi-fi speakers or equipment in the system.
13.	Unless your designing a Disco, rock club, Movie theater, Home theater or a high end or very large performing arts venue, or an artsy boardroom seldom is a sub-woofer necessary.

return to top