The Flaming Weasels Web Project

Web Sites:

Amplifiers:

Speakers and Monitors:

 

The Problem:

 

You are the rock band the Flaming Weasels. Your last three sound engineers have quite on you and you now have to design your own sound system. Your microphones and other equipment are all OK, but your amplifiers, speakers, and monitors have all been mysteriously destroyed.

 

Problem #1 - Finding an Amplifier

DEFINITIONS:
  • Front of House Loudspeaker: The main speakers used in a performance. They are at the front of the stage and pointed at the audience.
  • Amplifier Power: The power produced by an amplifier is measured in watts.
  • Loudspeaker Capacity: Speakers must be connected to an amplifier that can deliver the right ammount of power. If you connect an amplifier that is too powerful, it may damage the loudspeakrs (and cause them to "blow out"). Connecting speakers to an amplifier that is too weak can also damage the speakers. In general, it is a good idea to match the loudspeaker capacity and the amplifier power within 20%.
  • Impedance: Impedance is a measure of the electrical resitance of the speaker and is measured in ohms.. An amplifier connected to a speaker with a lower impedance will deliver more power than the same amplifier connected to a speaker with a high impedance. Common impedances for stereo speakers are 2, 4 and 8 ohms. The specification for a professional amplifier will list how much power it will deliver at 2, 4 and 8 ohms.
  • Stereo Amplifiers: Most home stereo systems (and many professional systems as well) have two speakers, one left and one right. A stereo amplifier is essentially two amplifiers in one box, designed to drive two stereo speakers.
  • Bridging: Bridging is useful if you have a stereo amplifier but only one speaker. It allows you to connect both sides of the amplifier together to create a more powerf for the single speaker.

Your band mates have decided they want two JBL MR926 loudspeakers for the front of the house. Research these loudspeakers on JBL's web site and record their Power Capacity, Impedence, and Nominal Dispersion. Your first job is to find the proper amplifier for these speakers.

Your first idea is to use the Mackie 800 amplifier. Look at the specifications for the amplifier on Mackie's web site.

Next you decide on the Mackie 1400. Look at the specification for this amplifier.

Your drummer likes things loud and insists on the Mackie 2600 amplifier.

EXTRA CREDIT:

 

Problem #2 - Loudspeaker Dispersion

 

DEFINITIONS:
  • DISPERSION: When sound comes out of a speaker, it is in the shape of an irregular cone. The center of the cone is at the center of the speaker. If you are standing inside of this cone you will be able to hear the sound coming directly out of the speaker. If you are standing outside of this cone, the sound will have to bounce off of something first before you can hear it. Depending on how the speaker is designed, the cone can be very narrow or very wide. The shape of the cone is defined by two angles, one for the vertical dimension and one for the horizontal dimension (see illustrations above). The angles are known as the dispersion of the speaker. It is important to know the dispersion of a speaker so that you can determine how far away you need to place the speaker from the audience. If you place a speaker with a very narrow dispersion very close to the audience, the people sitting in the front row won't be "inside the cone" and the speakers will sound distorted (since all they will be hearing is reflected sound). In many speaker cabinets, there are actually two speakers. One is for the high frequency sounds and is called a tweeter or horn. One if for low frequency sounds and is called a woofer. The human ear is only able to tell the direction of high frequency sounds, so dispersion is measured from the center of the tweeter (as shown in the diagram above).

DIAGRAM:

It is now time to set up the speakers. You start with the right speaker and mount it on a tall pole. Using your trusty tape measure you determine that the height from the floor to the center of the speaker is exactly 9 feet. The speaker is mounted so that bottom of it is exactly parallel to the floor. Using the values for dispersion that you recorded above, detemine how far away from the front row the speaker must be so that people in the front row will be able to directly hear the sound coming out of the speaker (so that they are "in the cone").

 

Problem #3 - Monitors in Parallel

 

DEFINITIONS:
  • Monitor: A small speaker placed on the stage so a performer can hear himself or the other musicians.
  • Serial: When referring to electrical circuits, it means that current first flows through one device, than through a second device, then through a third device, and so on until it gets back to the source.
  • Parallel: Here, electrical current flows through all the devices at the same time.

 

PHYSICS:
  • When a speaker is connected to an amplifier, it is essentially a resistor in an electrical circuit.
  • When connecting speakers in series, the impedances are added together to form the total resistance.
  • When connecting speakers in parallel, the resistance is calculated using the following formula:

    1/RT = 1/R1 + 1/R2 + 1/R3 ...

    Where RT is the total impedance, R1 is the impedance of the first speaker, R2 is the impedance of the second speakers, R3 is the impedance of the third speaker, and so forth.

  • When one amplifier is connected to multiple speakers in parallel, the power is shared between all of the speaker in proportion to their impedance. For example, a 150 Watt amplifier connected to one 8 ohm speaker and one 4 ohm speaker would deliver 100 watts to the 8 ohm speaker and 50 watts to the 4 ohm speaker.

After four successful shows, your other band members are getting a bit big headed and everyone is demanding their own monitor. Currently you have two Community CSX28-S2 floor monitors, and you have dediced to purchase two additional Galaxy HotSpot monitors (one for the guitar player and one for the accordian player). You are not sure if your amplifier (a Mackie 1400) will be able to run all these monitors at once, since you are already using one channel of the amplifier to power a third main speaker you recently added to your setup.

First, go to the Community web site.

Then go to the web site for the Galaxy HotSpot monitor.

Solve the following:

Your drummer looks at this number and estimates that the amplifier will put out about 600 watts at this impedance.

Assume the drummer is correct, and that the amplifier will put out 600 watts.