Measuring Resonance
It is interesting to explore acoustic resonance with a SPL meter. Such a meter is very useful, can be found at stores like Radio Shack, and cost as little as $30.00. You can also use a mic patched into your board, keeping an eye on the VU meter. Sound meters measure the strength of “sound pressure changes.” If the SPL meter reads 90 dB, that means the air pressure at the microphone is fluctuating strongly above and below ambient air pressure with a strength of 90 dB. Compare this to a 60 dB reading and notice that the fluctuations in pressure are much smaller and the sound is quieter.
By the way, dB, A is not a flat response curve. It is rolled off gradually below 1kHz as our own hearing response does. The dB, C scale is “flat” for most purposes. A mic, patched through without equalization will be close to dB, C levels, not dB, A levels. The dB, C or flat response weighting is best for room acoustic measurements and the mic should be an omni mic.
If the mic or SPL meter is moved from one end to the other end of a room that is in the fundamental mode of resonance, data points can be taken and plotted against position. High SPLs are detected at both ends of the room, and a low SPL in the middle. These are known in audio as “hot” and “cold” spots; the “hot spot” is where pressure changes strongly occur and the “cold spot” is a location where pressure only slightly changes.
Just because we don’t hear sound in the cold spot doesn’t mean the acoustic energy is gone. The sound may be “cancelled,” but the kinetic part of acoustic energy is in full presence. Although we can’t hear acoustic kinetic energy, a ribbon mic properly oriented can pick it up. Note that the same ribbon mic in a pressure zone will not register any sound. This is because ribbon mics pick up the air motion of sound while condenser mics pick up the air pressure of sound. For a ribbon mic to pick up the acoustic kinetic energy, it must be aligned per indicator to the direction of air motion. If rotated ninety degrees so the plane of the ribbon is aligned with the direction of the acoustic kinetic energy motion, the mic will not give a reading.
The frequency of the lowest room resonance (1,0,0) is easy to calculate from f100 = C/2L. Measure the length (L) of your room and use the equation to calculate the room’s fundamental resonant frequency. The graph of the equation is also useful to use. |
