After thirty years of walking on putting surfaces all around the world my feet can tell a firm green. I wear very soft soled Muck Boots that put my feet in close contact with the putting surface so as to assess the feel. A firm putting surface is truly a thing of beauty but what makes it so is that hollow sound.
I often use a stimpmeter to get a sense of ball roll distance on surfaces. For ease of measurement I simply flip the stimpmeter, roughly three feet in length to determine distance. It also serves the function of assessing putting green firmness. Sort of a poor mans USGA Tru-firm-sorry Matt.
That hollow sound is obviously from the air filled pores resonating from the surface vibration. In practical terms it adds a level of challenge to the game that our American golfers so desperately need. What separates the World golfer from an American golfer is the former is comfortable playing a ground game while the latter is accustomed to lobbing or stinging line drives into soft, spongy surfaces. Surfaces we all know are covered in annual bluegrass simply waiting to die from anthracnose!
The dry weather across the country has provided an excellent opportunity to offer firm putting surfaces. Obviously moisture is a key factor yet many are seeking sand specifications that result in extremely firm surfaces. In both cases golf superintendents walk that fine line where a little too firm or a little too dry and the green will fail.
In that case all you'll hear is the sound of silence.