How do I show a custom product attributes value in my woocommerce tab

I am trying to show a custom product attributes value in my woocommerce tab the attributes is named specification. Below is my code which outputs nothing. Ideas?

add_filter( 'woocommerce_product_tabs', 'woo_new_product_tab' );

function woo_new_product_tab( $tabs ) {

// Adds the new tab

$tabs['test_tab'] = array(
    'title'     => __( 'Features', 'woocommerce' ),
    'priority'  => 50,
    'callback'  => 'woo_new_product_tab_content'
);

return $tabs;

} function woo_new_product_tab_content() {

// The new tab content
$pa_value = get_post_meta($product->id, 'pa_specification', true);
    echo $pa_value;

}

via Chebli Mohamed