so I'm no biologist, but I'm trying to get a grasp on some biological elements of the model for a resonant bubble with respect to the swim bladder. if a fish pushes against the bladder with muscles to make a noise, it looks like in some of the biology book pictures that the gas is moved around during vocalization. but the center frequency seems to be pretty constant in the files I look at. so if the fish isn't frequency modulating - but there is still some modulation present - shouldn't the radius factor (usually shown in centimeters) be a function of time, from when the vocalization begins to when it ends?
I believe the resonant bubble model fails because it does not consider the swimbladder wall. We have a paper, hopefully about to be accepted by the Proc Roy Soc London, that shows the bladder wall is a viscoelastic structure that causes rapid damping. Collagen and elastin occur in different layers (anisotropic structure) and there is about an 80% water content. Dry the swimbladder wall increases its modulus several hundred fold. People have tended to believe the resonant bubble model and then say rapid damping is caused by surrounding tissue. Hopefully we have shown that is not the case.
Further in swimbladders not associated with a bone or tendon, the fundamental frequency is determined by the muscle contraction rate. A toadfish contracts its bladder muscles 200 times a sec to produce a 200 Hz fundamental frequency and harmonics at multiples of that frequency. Note in some cases very small bladders are more effective at coupling high frequency sounds into water because of their smaller dimensions than are larger fish. This should require investigators to be careful about interpretations. Note that muscle contraction rate can decrease a small amount during a sound (muscle fatigue) and therefore produce a sound with a slightly lower fundamental frequency.
When the muscle stops contracting, the sound damps quickly, and each half cycle is slower than the one before. To me this indicates a forced rather than a resonant response.
I'd like to see that paper when it comes out, thanks for the response. would you say that if there is a forced response, there might be a modulation effect that affectd the way other fish perceive the information content in the call? I say info content because I don't want to get wrapped around pitch.
My old major professor Howard Winn did some playback experiments with boatwhisltes. Playing them back to a calling toadfish at a rapid rate would get the caller to call faster. He then tried tone burst playbacks and got the same results (Winn 1972). They would respond to pretty much any tone in their hearing range if the burst were not too long or too short. The fundamental frequency of the call changes with temperature and some seasonal factor (Fine 1978) that was never worked out. That and calling in shallow water can mean that the fundamental frequency can be filtered out by the environment so a fish could be responding to the fundamental frequency or even a higher harmonic (likely the second). So a long route to maybe not answer your question, they are not too specific in what they will recognize as a boatwhislte.
Ok, that's one I had not seen. I was going in with a bent based on some of Dr. Bass' work where he tested various modulations of the same tone on midshipmen, and basically the pure tone got 100% of the gravid females to come hither. the more modulation was introduced, the less interested the females got. at this point I'm wondering if a modulation effect isn't present that fades with maturity. I'll send you some stuff under separate cover.
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