On all « classically vented » drums, the first mode is hard to spot because of its heavy damping – numerical simulation showed for a 14×6,5″ drum a first mode at 50Hz – in this case both membrane are in phase opposition, compressing the volume of air which results in important flow and therefore energy loss in the vent. This mode is not heard nor seen on acoustical measurement probably because of its very damping and very small radiation surface (even though the heads are moving, most of the velocity is in the vent)
This second mode corresponds to the lowest tone perceived on a drum, and explains why deeper drums sound lower in tone : a common misconception is that it is linked to lower « cavity mode » of a larger volume – rather, in this case and IF the drum is vented (suppressing all pressure rise within the shell), the volume of air is mass loading the drum (see Shell depth , Venting & admittance) and the depth of the shell ends up having more importance for this mode than the tension of the heads. The venting suppresses some of the air stiffness but not all of it : the larger the opened surface, the lower the mode.
This is an attempt to explain observed peaks on the acoustical measurements, disappearing as soon as the vent section was modified – this partial is more related to a « Helmhotz » like resonator as the position of the vent in the shell (altitude) and width showed some clear effect on this behaviour. However this did not show up in the numerical simulation – For sure, the larger opened area leads to lower velocity and the profile of the vent cleans up the airflow : probably less damping of this mode. For sure the larger area and spatial distribution causes better radiation coefficient from the vent.
This is indeed observed on the acoustical measurement for repercussion snares only, and is unaffected by the snare effect.
On REPERCUSSION snares, the second mode is both lowered in frequency, and heard. In that case because of both the larger opened surface and the evenly distributed vent with respect to this membrane mode. see The snare was killing batter head sound ! for further explanations on snare effect : in essence the top head is now better decoupled from the snare head and able to vibrate longer. Soundwise this makes drums with increased presence and projection – remember we are talking about wavelength of about 2 meters, so the feeling for the audience around the drum is really different.
Also this eases lower mechanical impedance on the batter head saving your wrists (see Shell depth , Venting & admittance)
Intuitively when opening a shell, one could think that the effect would be mostly on the high frequencies, as there would be less shell wall to block those. This is true but the major effect is first found on the lowest components of the sound as a consequence of modifying the coupling. There is a subtle balance between opening up and letting more high frequencies escaping, and enabling too much of the lowest partials to take over. Repercussion snares sounds louder and with more presence because of the enhanced low end, better sound diffusion, and effects of the port acting as a magnifying glass for very high frequencies components – It acts as a « loudness » filter on your stereo.