Bass Regeneration
Introduction
Bass regeneration is a special class of feedback, or regeneration, that exists in all live amplification systems. (See all L1 Wiki articles on feedback.)
Bass feedback is responsible for a lot of problems on stage and in the audience, and is a hard-to-confront phenomenon. Resonances of all kind, including those of musical instruments and those of large “boomy” rooms also contribute to bass regeneration. And the more system gain you need, either by having soft sources to amplify or by needing to play loud, the more unstable and unpleasant the regeneration becomes.
The net result of this is a sonic “doom” that makes musical presentations sound like they were being passed through a wet towel. Severe cases of bass regeneration will favor discrete frequencies at unfortunate musical notes and may make it very difficult to play bass instruments (like electric bass) due to certain notes sustaining unnaturally and out of control.
Independent of most actions to prevent it, bass tends to go everywhere. Thus, it tends to be picked up, more than any frequency range, by all open microphones and “re-broadcasted” through the systems that amplify those sounds. The more sources of sound and the more devices that pick up and amplify this sound there are, the worse bass regeneration gets. So, in a complicated triple amplification system, this can be a real runaway problem, especially with a lot of players and especially if the band wants to play loud (i.e. with lots of system gain).
The L1 approach uses a minimum of sources/pickups (one source only gets connected to one speaker, not many) but bass regeneration will still occur. There are many ways to minimize this and, like a good work of art, detailing will always result in excellent sound and minimum bass regeneration.
Setup Technique to Reduce Bass Regeneration
The main approach is to reduce the amount of bass that is re-introduced into the system and the following are specifics of how to do this:
- Use a highpass (low-cut) filter such as the presets. This reduces bass gain while not affecting the gain of the upper part of the audio range. For instance, if you are a singing bassist, use a highpass filter on your vocal microphone to keep your bass signal out of the regeneration loop. Use only the range you need and filter the rest.
- Keep open microphones away from bass boxes: If you are setting up an L1 system as recommended (on the backline) and are using a microphone pickup for, say, a small guitar amplifier, don’t place the guitar amp next to, say, the bass boxes that the bassist is using. If you do this, it will amplify this part of the bassist’s sound, but not in a predictable or deliberate manner. Also, don’t set the little guitar amp next to the L1 system that is being used for electric guitar. You will not get good results, due to self-feedback in the bass. Rather, set it offstage where the bass from the entire band is relatively quiet. In amplifying a kick drum, get the pickup microphone as close to the beater for a strong signal. This way, you can reduce system gain on the microphone. Also, get the kick drum as far from the drum’s bass box(es) as possible. This is a bit of a conflict because the drummer wants to hear and feel the kick drum, and the drummer’s system should be close enough to the actual drum kit so the sound appears to be coming from the drums. Using the method described next, for kick drum, will allow very high levels with minimum bass regeneration.
- Reduce resonances in bass or “kick” drums. Especially in strong amplified music, getting a solid kick drum sound usually involves placing the pickup microphone in the drum itself. The shell of the drum will have its own resonances and will thus tend to “ring” at these specific frequencies. If a second or back drum head is uses, especially one with a resonator hole in it, the resonances will change. A successful method is to reduce these resonances by filling the drum cavity with Dacron® wool, dampen the beater head with typical methods (felt strap, rubber pad, etc) and loosening the back head so that its resonance is low. The Dacron filling will also dampen its resonance. You can also use fiberglas, but you might not like its itchy nature and you may not appreciate glass fibers getting into your favorite microphone. Whatever method you use, you can acheive a very dampened and “tight” acoustical drumshell cavity that will not cause excessive system gain at specific frequencies. The microphone will essentially pick up the character and snap of the beater head. This really works well.
- Use KickGate for amplifying acoustic kick drum. KickGate both allows high gain on the kick but prevents other instruments from regenerating through the kick drum mic.
- Get the highest signal possible to the microphones, thus reducing system gain – Sing and play strongly. Don’t play too loud, like through a mic’d guitar amp, so that you interfere with and spoil the sound distributed by the Bose Personalized Amplification System™. You will have to achieve a good balance between too loud and too soft.
- Mute open microphones that are not in use- Using your remote control (L1 Classic), turn off your microphone if you don’t need to use it. This is just good practice. A noise gate will do this automatically (ToneMatch mixers have a Noise Gate feature), so try the ones we have provided in our presets.
- Don’t play so loud. We’re so used to this artifact of the triple system (playing too loud) that we don’t think about it much. Your audience will appreciate it and so will you when your ears don’t ring. A nice benefit of playing softer is that you need less system gain. This results in less bass regeneration.