Noise Caused by Vibration
What is “vibration isolation” and why should we care?
Noise comes from two sources: Airborne noise and structure borne noise. Airborne noise is produced by fluctuation in sound waves from a source to a receiver. Think about when you’re stopped at a stop light and the car next to you pulls up, with windows down, blasting “Fight the Power” by Public Enemy. The low base sounds create an extremely long sound wave that you can’t help but hear. Structure borne noise is like the scene in Jurassic Park when the camera is focused on a clear glass of water on the table. You then hear a dull thump in the distance and ripples appear in the water. The vibration the Tyrannosaurus Rex is making with his measured walking is causing the ground under the table to vibrate and that manifests itself into sound energy.
I’ve been learning about and selling vibration isolation products for 37 years. I’d like to share with you three very interesting calls I’ve made over my career.
The first visit I made early on in my career was to a phone call center. Oddly, the reason for the appointment was to help sound-proof the bosses’ office from the rest of the call center. If you can imagine a room with desks lined up within feet of each other, and each one occupied by a worker with a headset making phone calls all day, it was LOUD. They certainly needed much more absorption in the space (in addition to boosting the STC value of the bosses’ walls) as all the voices sounded like an AM radio with a lot of static. I did meet with one fellow who asked if I was the “sound guy”. I told him indeed I was, and he asked if he could show me something.
This poor chap was lucky, in that he had his own private office in the back, but it was a small office with no windows. To make matters worse, the AC condensing unit was hard mounted to the roof directly above his office. When the unit would kick on, the vibration of the fan prop and motor would transmit directly to the roof above his head and cause a dull roar – that would cycle on and off every day. He told me he had to take frequent breaks from his office and typically went home daily with headaches. His productivity and health were clearly being compromised by noise caused by the vibration of the unit overhead. We ended up putting the condensing unit on spring isolators and, “walla” – noise and headaches simply went away!
The second vibration issue I witnessed was similar, but with a twist. I was called down to a hospital in St. Louis where a nurse on the 3rd floor had been complaining about a noise in her office. I thought perhaps the same thing may be happening that I witnessed in the call center.
Turns out however, that the space above her office was just a storage space with no mechanical equipment. After some thought, I asked the facility manager if he had blueprints of the building – turns out he did. After a review of the mechanical and structural drawings, I discovered that there was a steel support beam that ran above the nurse’s office. This beam was at least 100’ long and ended in a mechanical room, above the nurse’s office at least 30 yards away. Turns out there was a pump that was hard mounted to the concrete floor on the 4th level that sat near the end of this steel beam. The vibration from the pump was shaking the concrete floor, which excited the steel beam and the small movement of the beam was manifesting itself as noise 30 yards away and one floor below in the nurse’s office. It was amazing to me that the vibration of this pump could travel down the length of a steel beam 100’ long. We ended up putting the pump in the mechanical room on a concrete inertia base with springs to absorb the shock of the vibration of the pump and again, “walla” – no more noise in the nurse’s office!
The last and perhaps most intriguing visit I made was just before the pandemic. I had received a call from a facilities manager in a high-rise condominium office in a very posh part of town. There was a woman on the 2nd floor of the building that was complaining about a noise source in a pipe chase in her condo. This was in a bump out behind the wall near her kitchen. She would hear a “knocking” sound every so often. It was not consistent, or ever present, which may have made it worse (sometimes you can get used to a passing train near your neighborhood if it happens all the time – but if it’s just once in a great while, it’s more disturbing).
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The facility manager thought there might be air in the pipes behind the wall so he called an HVAC contractor to flush the system. Sadly, this did not fix the issue. I had told them we offer a pipe wrap product that has a limp vinyl mass “blocking” layer, coupled with a 1” thick fiberglass layer (for sound “absorption”) that may help. They demoed the wall to expose the pipes and a colleague and I went to the site to wrap the pipe. While we were getting started, I noticed the knocking noise, but it didn’t appear to be coming from the pipe itself. It sounded like it was coming from the floor. I asked if we could see the space below this condo unit and found it was in the kitchen. After further review, we found that the hot water and chilled water piping for this unit was coming from the kitchen – through some bored holes in the concrete floor/ceiling assembly of the unit above. I also discovered that the contractor who installed this piping through the concrete floor has put wooden shims in the concrete opening to hold the pipe in place. What was happening was that when different temperatures of water were travelling through this pipe, it was expanding or contracting as it went from hot to cold. The expansion of the pipe was causing pressure on the shims holding the pipe in place and the “knocking” this woman was experiencing was actually the shims slightly moving against the expansion of the pipe. We removed the wood shims and installed a flexible polystyrene material around the pipe and guess what? You got it – “walla” – no more noise.
Vibration noise is real, it’s disturbing (just as much, if not more so, than airborne noise) - it affects health, productivity, concentration, learning and so much more. Sometimes you almost need to be a forensic isolation expert to see what is at play. That is why we exist.
- Tom