A couple of months ago, while I was borrowing the travelling Daedalus Sparrow, I set it up alongside my Hansen Minispinner Pro and my Electric Eel Wheel 6.0, and downloaded a decibel meter app to my tablet, and got them all set up for measuring the volume of each. I tried as hard as I could to make it fair, removed the brake bands and positioned the flyer hooks near the front of the flyer, and moved the tablet between each measurement so it was the same distance from each machine while measuring. Rather than taking notes as I went, I recorded the whole process. When I came to edit the video and make a pretty table of all my measurements, I realised they were pretty much meaningless. The decibel measurement didn’t strongly correlate with the actual noise levels.
As an example, here are two sections of the video. Both of these spinners measured 73-74 decibels at the speed they are going in the video, and the noise level of each sounds similar to how it did in person… the same dB measurement, totally different volume. I know that decibel meter apps aren’t hugely accurate, but they are good enough for these purposes.
What gives?
To explain, I need to give a little background, which is hard as the dB scale isn’t that easy to understand intuitively. It’s a logarithmic scale rather than linear, which makes sense given the enormous range of figures we are talking about. A sound at 120dB is one trillion (1 with 12 0’s after it!) times the intensity of the quietest sound a person (with perfect hearing) can hear. A linear scale that goes from 0 to more than one trillion is hard to work with, but converting this to a logarithmic scale makes the numbers more manageable, once you understand the relationship between them. The decibel scale is a logarithmic scale based on powers of 10. That means, a sound 10 decibels louder than another is 10 times the intensity. So a 30 decibel noise is 10 times the intensity of a 20dB one, and 100 times that of a 10dB one.
When we are looking at e-spinners, the noise will be in the range of 50-70 decibels or so, and while our instinct may be to think that 70 isn’t that much more than 50, with this scale, it’s actually 100 times more. But 100 times more what? As I said above: sound intensity. That is, the amount of energy taken to produce the sound. You would perhaps expect this to equate to volume, but it doesn’t, for every 10dB increase in intensity, you perceive a 2-fold increase in volume, so that e-spinner that runs at 70dB will sound 4 times louder than the one at 50dB.
This was my understanding until I did these tests, and it’s correct, but incomplete. Sound intensity is not the only thing that affects how we hear sounds, the frequency (ie. pitch) of the noise also matters. Googling to figure out if frequency makes a difference to percieved volume led me to learn about phons. Wikipedia tells us:
“The phon is a logarithmic unit of loudness level for tones and complex sounds. Loudness is measured in sone which is a linear unit. Human sensitivity to sound is variable across different frequencies; therefore, although two different tones may present an identical sound pressure to a human ear, they may be psychoacoustically perceived as differing in loudness. The purpose of the phon is to provide a logarithmic measurement (like decibels) for perceived sound magnitude, while the primary loudness standard methods result in a linear representation.â€
Tl;dr: 2 sounds measuring the same number of dB can have different volumes.
This handy graph shows you clearly… each of those red lines, at any point along the line, your ears will hear the sounds as being the same volume, due to the change in frequency, despite the fact that the sound intensity differs greatly.
The area of the chart we are interested in is mostly towards the left, your average flyer will rotate at 500-2000 rpm, which, divided by 60 gives 8-33Hz, which I think needs to be doubled to take into account the fact there are 2 flyer arms. The motor moves faster than the flyer, by a factor of 2-4 or so depending on the spinner in question, but doesn’t contribute much to the noise on modern e-spinners. There are also other noises involved, possible chatter of the bobbin, friction of the brake band on the bobbin, tension spring pulling against its mount, basically any part that touches something else and moves, will make a noise, some louder or more annoying than others. Some e-spinners have excellent noise dampening at certain frequencies, and poor dampening at others, so while they all get louder as they go faster, some don’t get as loud as others.
There are also other factors at play in how annoying or distracting we find a noise, clicking noises may be more irritating than humming noises, noises in the frequency range of what we are watching on TV may be harder to ignore than noises outside this range. Certain frequencies may cause the entire body of the espinner or the table it is on to resonate, creating more noise than you’d expect and more than at either a slightly faster or slower speed. Some people are just more sensitive to noise than others. I am one of those people, and most of my spinning is done in a room with little or no other background noise, which is why the volume of my e-spinners is such a big deal to me.
I’m not saying that decibels are a completely useless measurement for e-spinner volume. If you see that a certain model runs at 80 decibels, which is the same volume as a loud sewing machine, then this is going to be far louder than one at 50dB, frequency and type of noise notwithstanding. However if you have two machines that run at about the same sound intensity level, don’t assume that the one with lower dB is going to sound quieter.
I am now ready to write my comparison post, with more unscientific, but more accurate ‘this one is louder than that one’ descriptions, rather than a neat table of decibel measurements at different speeds. So watch this space!