Microphones: Podium

Many new, exciting mic-rophones are available today for podium applications. You don’t have to settle for that big ugly hand-held microphone mounted on a creaky gooseneck stand. First though, let’s look at how to properly use a microphone on a podium.

Figure 1

You probably only need one microphone on the podium unless the President is speaking. There is a simple reason why you always see three microphones on the President’s podium. If one fails, there is a backup. Even for the President, only one microphone is actually used for the sound system. In Figure 1, the top solid line shows the combined frequency response of two microphones if the talker is exactly the same distance between the two microphones. However, if the talker moves off center just a little, the sound will have to travel different distances to each microphone. That means the sound will arrive at the farthest microphone slightly later than the closest microphone. If you listened to each microphone separately, they would sound about the same. However, when the two equal signals combine together, there will be additions at certain frequencies and some cancellations at other frequencies. The result is unnatural sound. And worse yet, as the talker moves around, the sound quality will change because the distances to the microphones are changing. This causes the frequencies of cancellation to vary. The resultant frequency response curve is the gold line shown in Figure 1.

This same change in frequency response (called phase cancellation) occurs when both a podium microphone and a lavaliere (neck) microphone are on. There is a very large time difference between the sound arriving at the two microphones, and the combined result is very audible. Check it out yourselves. The sound is a very unnatural sound that cannot be made to sound right with the mixers’ tone controls. And the sound quality is constantly changing as the talker moves around.

This phase cancellation problem can even occur when one microphone is on a gooseneck above the podium. The sound from the talker travels from the mouth and strikes the microphone. Sound also travels down and bounces off the podium and back to the microphone. The path lengths are different, so there is time difference (phase difference). The resultant combination again produces a sound that is “unnatural.”

Fortunately, there are several things you can do to improve this situation. First, directional (cardioid) microphones can help. The reflected sound strikes the microphone on the microphone’s side where it is not as sensitive. The microphone picks up the direct front sound much stronger than the side (reflected) sound, so the problem is not as audible. Second, you could try putting carpet or some other absorbent material in the podium to absorb the sound. This may help stop the reflected sound from interfering. In fact, try this if you are having feedback problems. A reflection may be causing the feedback.

Figure 2

Third, don’t position the microphone too close to the surface of the podium. It should be at least 8 inches above the surface (Figure 2). If the microphone is only four inches above the podium, severe changes in the frequency response will occur.

Here are some other useful hints. If your podium is in the center of the platform, then put the microphone in the center of the podium. Now, as the talker’s head moves from side to side, the volume changes will lessen. If your podium is on one side of the platform, then you may want to place the microphone so it is in the center of the talker’s head rotation. For example, if your podium is on the left side of the platform (as you face the audience), then the microphone should be a little to the right of the podium center.

Figure 3

Moving away from a microphone by 4 times will result in a 12 dB drop in level no matter what distance you begin with.

Next, don’t be too close to the microphone. If you are speaking one inch from the microphone, and then move back another three inches, the change in distance will produce a large change in volume. That is not caused by the microphone. It is the law of physics call the inverse square law. A doubling of the distance will decrease the sound level by 6 dB. All microphones behave exactly the same. Moving from one inch to four inches from the microphone will cause the sound level to drop by 12 dB. Your ears will tell you that the level dropped by more than half (Figure 3).

If you are a foot from the microphone, the same three-inch change in distance will produce very little change in volume. You would have to move back three more feet to drop the level by 12 dB!

Finally, be sure all the microphones are off if they are not in use. This can be done by the switch on the microphone (if it has one) or by a sound operator. There are several reasons for this. First, the sound quality of the microphone which the talker is using will sound better. If unused microphones are on, they pick up the sound that is bouncing around the room. That reflected sound is mixed with the direct sound and amplified. This can cause the sound from the loudspeaker system to be muddy and unclear. Another reason they need to be off is so you will be able to get more gain before feedback (volume). In fact, every time you double the number of microphones that are on, the available gain before feedback drops by 3 dB. Try this experiment: Turn two microphones on and adjust each one until they each produce the same volume. Now slowly increase the master volume until you start to hear feedback ringing when you talk into the microphones. Now turn one microphone off. You should now be able to talk into the other microphone without any feedback ringing.

O.K., so what new microphone should you use for your podium? Good question! There are actually several choices. Please call us for our recommendation for your application.

Figure 4
Typical miniature gooseneck-mounted microphone.

Many schools can use the new miniature microphones. They are mounted on a thin gooseneck that actually doesn’t creak when you move it! This makes it easy to adjust for different height talkers. These microphones also have a very small profile, so they look good. And best of all, they sound excellent (Figure 4).

So don’t be content with that old ugly microphone any longer. It’s sound advice to look into a new podium microphone.

by Ron Huisinga © 1996 Internet Sound Institute (www.soundinstitute.com). This article is for personal use only. Any commercial reproduction is not permitted without permission. To obtain permission, contact ISI at hopi@soundinstitute.com

< Back