Have you ever watched a bombastic movie in a theater with a heavy-duty bass system and felt as battered as the film's hero afterward? Ever wondered how a kick drum sound could cause that 'chest punch' sensation? Did you know that at a low enough frequency, high-amplitude sound may even be able to breath for you?
In this article we look at how bass is felt rather than heard, and, after reviewing some of the research that has been done in this area, we investigate the points at which low frequencies go beyond sound and become a tactile sensation in an experiment of our own with willing volunteers.
We caution readers NOT to try these experiments at home.
Read: Bass: the Physical Sensation of Sound
Please be sure to share your tactile bass stories with us here.
Gene I think you are a frustrated physiologist.
I'm glad you used only short bursts of low frequency sound.
Much of what you found out relates to high frequency jet ventilation technology.
Of all the vital organs to take a hit from severe insults the lungs are top of the list. Severe sepsis, trauma, burns and a myriad of insults trigger the adult respiratory distress syndrome, known as ARDS. In addition respiratory failure is the biggest challenge in the care of premature infants.
The upshot if this is that in ICUs the world over, mechanical positive pressure ventilation is required. Mechanical ventilation becomes required when there is an imbalance of lung compliance and respiratory effort. All shortness of breath has at its core a decrease in lung compliance unless the is neuromuscular failure, in which case there is still imbalance between force and compliance.
Now in normal respiration there is negative intrathoracic pressure in inspiration, and positive pressure on expiration. The negative phase is important in maintaining cardiac filling. Once mechanical ventilation is established, then on inspiration intrathoracic pressure is positive.
Under severe condition the lungs get stiff and boggy, and the pressure required gets to the point where the pressure is damaging and barotrauma starts to occur which can make recovery impossible. In addition positive end expiratory pressure is required to prevent the alveolar sacks where oxygen exchange takes place form collapsing.
In severe cases all this leads to a check mate situation. Peak airway pressures over 36 cm of H2O, plateau pressures over 30 cm of H2O and mean pressures over 16 cm H2O are associated with increasing lung damage or baratrauma as it is known.
In order to overcome this high frequency jet ventilation has found a place. In severe cases extracorporeal oxygenation can be tried, known as ECMO.
Now the high frequency jet ventilation in audio terms is low frequency in audio terms.
Frequencies employed vary from 1 Hz to 30 Hz and rates as high as 50 Hz have been tried.
The idea is to provide gas exchange without lung movement.
This also has application in ENT procedures where a tube though the vocal cords would make the procedure impossible. The canula can be placed in the supraglottic region, but the surgeon must keep the airway open. It is also used in delicate lung surgery where the lungs need to be still.
So where does all this link up with what you found?
Now the loudest sound that can be reproduced without distortion in free air is 194 db. That is because the rise and fall of pressure is one atmosphere. That is 1033 cm of water. That is a high pressure, but not enough to cause lung and other injuries. Serious injuries occur at 3 psi or around 15,000 cm H2O. Pressures over 9 psi or 45000 cm of H20 are almost uniformly fatal. So I don't think you will blow out a lung with a sub!
You started your test at 95 db and increased to 101db and then 112 db. So 95 db is 1.125 pascals or 0.0115 cm of of H2O. 101 db would be 2.24 Pascals or 0.02 cm of H2O. 107 db would be 4.48 Pascals or 0.0457 cm of H2O.
The lung, aorta, Carotid bodies of the carotid arteries, are full of baroceptors and so that likely explains why the deep base sounds are felt. There would be some ingress and egress of air in and out of the lungs.
In order for low frequency sounds to cause significant ventilation you would have to be at the 155 db spl point which would result in pressures of 1000 pascal or 10 cm of H2O.
I would expect that to have significant physiological effects on the subjects depending on frequency.
From studies of jet ventilation frequencies above 3 Hz are associated with increasing trapped gas volumes in the lungs. As a consequence and in addition airway pressures rise. Optimal frequency is open to debate but frequencies of around 1.7 Hz seem most popular.
So subjecting volunteers to spl of 150 db and higher at frequencies over 20 to 30 Hz range could be very hazardous and result in significant trapped air volume in the lungs, with consequent rise in intrathoracic pressure and reduction in cardiac output.
So Gene, I think you were wise to limit the spl where you did.
Changing gears the optimum speaker loading for these tests has been debated and transmission line loading favored for these studies. I note John Wright was referenced in my researches at TDL back in the sixties, when he constructed TL loudspeakers able to produce significant spl down to 7 Hz.
Personally I'm convinced that TLs do produce the best quality bass, and do it without fuss.
I noted a number of investigators have used TLs and actual organ pipes. The TL after all is a specialized Gadeckt organ pipe and couples to the space with the same attributes.
What I have noticed here is that it is not just movies that are impressive. These lines are in fact even more dramatic than the Twin Cites top Atmos theater in the bass. The thing that distinguishes these is how you get the chest and body vibrations at very low volume. Even in very soft passages you feel your chest and internal organs vibrate even when an organist just touches a low pedal softly and gently.
When I was at the Temple Church for my niece, wedding, in one piece Roger Sayer held a soft sustained deep pedal for a period of time. Everybody felt it and many where surprised. My wife looked at me and acknowledged these speakers sound exactly the same and produce just this effect.
A couple of nights ago I was playing a Gothic CD of the Wannamaker Grand Court Organ. It was around 11 PM and I had the volume well notched back. Much to my surprise, my nine your old grandson came up and said his bed was shaking! He was sleeping in the floor below, but not directly under this studio, which is over the garage.
