Bob Horn tells us a bit about his amp and mic technique in this clip from our recording episode. So what was Bob doing here, what are some other options, and how do they compare? Well, Bob used an SM57 and a Royer 101 ribbon on the Vox amp. He used his cell phone’s flashlight (check out this post for more good audio engineering apps) to check for the location of the center of the woofer and for the edge of the dust cover. He puts the ribbon mic in the center of the woofer because ribbon mics are usually darker sounding microphones, and the center of the speaker cone is generally the brightest. Then he put the SM57 where the dust cover meets the rest of the woofer.

So what factors were involved in this decision?

Well, the word “amp” is short for “amplifier”, which can be defined as anything that increases the power of a signal. There are electrical power, transistor, video, microwave, and magnetic amplifiers, among others.

In the case of audio, an instrument amplifier is increasing the amplitude of the audio signal. For example, an amplifier could take the very quiet input from an electric guitar, and increase the amplitude until it’s louder. We often think of amps as just speakers. But technically the amp is the component that translates this completely electronic, or otherwise very quiet, signal into an electronic signal that has enough power to drive the speaker. This is generally through a tube amplifier or a transistor amplifier. The speaker producing a louder sound is a result of the amplifier’s work, but the speaker itself is not the amplifier.

With that said, the amplifier then drives the speaker cone back and forth. This creates compressions and rarefactions in air density, which our ears then translate into audio. When you place a microphone at different distances and angles from this speaker cone, you will get a slightly different sound and frequency balance. To understand this, just think about the proximity effect, where a DJ or vocalist will sound deeper and fuller when they get right up close to the microphone. You can use this, and similar effects, to your advantage with mic placement on an amp.

Position Relative to the Speaker Cone

Generally, the edge of the speaker cone will have more bass, and will sound darker and heavier. The center of the speaker cone will have a brighter sound. This principle is probably why Bob placed his ribbon in the center of the cone. By placing the ribbon mic in the center of the cone, it will pick up a brighter sound than at the edge of the cone, and therefore mitigate the darkness of the mic pickup pattern.

Distance

Some of the facts about distance from the amp are fairly well known. The closer to the speaker cone, the less room noise you’ll get. This means that your sound will seem more direct and present the closer the mic is to the speaker cone. If your room acoustics are not ideal, you might want to take advantage of this effect and close mic your amp. Just keep in mind that you might get the proximity effect if you place the mic very close to the speaker. If you do, then you might find yourself adjusting the mic, or EQing the lows out a bit, depending on how it sounds.

For close mic placement, I was taught to start with the mic a couple of fingers width distance from the grill of the amp. Try starting at that distance, and then adjust the position of the mic depending on how you’d like to adjust the sound. As a general rule, closer will give you more bass, and farther will give you less.

People often distance mic their amps. It can add some interesting room tone and depth to your sound. Depending on the room, you could get some weird reflections doing this though, so just listen and make sure it sounds good during your setup. I always set up a close mic on the amp whenever I do a room mic, and I would definitely recommend doing so. Even if you think you will solely use the distance mic, it’s good to have a good alternative or backup. You can also rotate the angle of the mic slightly to put it on or off axis. On axis basically means that the mic is pointing directly at the speaker cone. To place a mic on axis, just place it so that the diaphragm of the microphone and the speaker cone are parallel. This positioning will make you more susceptible to the proximity effect, but can also make it sound brighter. So experiment with how it sounds to find the right fit for your track. When you angle the mic slightly, you’re placing it off axis. An off axis placement can be useful for smoothing out your sound. It can cut some of the harshness because some of the high end won’t hit the mic. This gives it a high frequency roll off effect.

Want to watch more of this episode? Bob Horn is a Grammy award winning engineer, and he supplies a wealth of information in the recording session episode that we had with him. You can check out the short version of the episode by here, or the complete and unedited version here.

Questions? Comments? As always, please share in the comments below!