Hookemeister wrote:I dunno Greg... If I were to put the sound in the room on a graph of frequency against time,
Nath, you're a legend... why didn't I think of using a graph to illustrate my point, could've saved myself a lot of writing.
OK here's Mark's supposition.
A room mode at 100Hz may initially exist, it may then combine with another mode slightly higher in pitch and decay at 125Hz. Starts at 100Hz, finishes at 125Hz = pitch change of mode.
Now here's a graphical representation of that with Frequency on the y axis and Time on the x axis
You can see that that the frequency of Mode 1 - 100Hz, does not change as a result of the introduction of Mode 2.
The amplitude of Mode 1 changes over time but
not it's frequency.
What we have here is a change of modes... individual modal frequencies remain unchanged.
The frequency of an individual mode does not change over time, only it's amplitude.
Hey Greg, I have randomly been pondering this...
I think Mark maybe correct. A room mode isn't sound. It is a phenomenon that happens to sound; that being a collection of resonances. Resonance, while effecting a waveform at a specific frequency, is an amplitude based phenomenon. When a 100hz waveform constructively interferes with itself, the amplitude changes...
What Mark has said is that a room mode can start at one frequency and drift to finish at another, if combined with a higher room mode overtime. It is like an EQ with some boosts applied. If you bring two boosted bands close enough together, they will alter the amplitude of the frequencies in the middle.
The key conclusion I have come to is... a room mode doesn't exist without preceived resonance. No sound equals no resonance equals no room modes. Say you introduce sound into a room and it results in resonance at 100 and 150hz initially, but as the sound decays 100 and 150hz disappears complete and the resonant peak moves to 125hz... The resonance (thus the room mode) has moved frequecies.
You do need a waterfall graph to accurately show it. The frequency of the center peak, the amplitude to represent the actual resonance of the room mode and time. Flicking through a bunch on the net, there is certainly cases of room modes starting at one frequency and finishing at another with a gradual transistion between frequencies.
