Table 6.

The frequency of four walls and bubbles for the first period.

Items	Bubble	$n=$ 15	$n=$ 10	$n=$ 6	$n=$ 3
$F_t$/Hz	238	242	250	123	136
$F_{n-i}$/$f_b$	1.000	1.016	1.050	0.516	0.571
$\beta$		$<$1	$<$1	$>$1	$>$1

Four cases are all performed when $\gamma =$ 1.68 and $R_m=$ 21 mm;

$f_n$ is the frequency of the composite wall vibration where $n$ is 15, 10, 6, and 3;

$f_b$ is the frequency of the bubble oscillations.