Yesterday was the day to install the PIAA slimline horn. I decided to break out my digital SPL meter and see what I could see about the sound level improvement this mod makes. I think that what I found is not what anyone would have expected.
I parked the bike out in the open driveway, to minimize reflected sound, and set up the SPL meter on a tripod at about the same height as the horns. I made before and after measurements of both horns from a distance of 20", directly in front of the bike.
For the individual horn measurements I positioned the microphone to the side, on a 45 degree angle, on the side of the active horn.
The power cord seen above is going to a battery charger, which I used to maintain a fairly constant 13.2VDC battery charge thru the tests. All measurements were made on the dB "A weighted" scale. A weighting approximates the frequency response of the human ear, and is therefore more representative of how you should perceive the horn loudness. Right and left refer to the sides of the bike when seated in a normal riding position.
Right side OE horn alone (high tone - 2799 Hz) - 108dBA
Left side OE horn alone (low tone - 341 Hz) - 101 dBA
PIAA Slimline Horn alone, installed in right side (mid tone - 508Hz) - 98 dBA
The reason for a difference between the left and right OE horns is likely to be the "A" weighting. Lower frequencies are scaled down as the human ear is theoretically less sensitive to them. But this does not explain why the PIAA Slimline horn measured lower SPL than the 341 Hz horn. This difference was also born out by the dual horn before and after SPL measurements.
Before - Dual OE Horns - 119 dBA
After - Left side OE Horn and PIAA Slimline on right side - 113 dBA
"OK", you're probably saying to yourself, "there must be something wrong with Fred-o's PIAA horn because when I put the horn on my bike it definitely sounded louder." Well, yes, and that is the really ironic thing. Even with these lower measured SPLs, the "After" configuration did seem like it sounded louder. I probably need to do some more experiments to figure out what is actually going on here (for one thing, repeat these tests using the flatter "C" weighting filter), but I have a theory.
I believe that we feel the horns are louder because the combined spectral pitch is lower. It sounds like it is a big car now and not a motorcycle. So even if it isn't actually louder we sense that it is.
Part of what makes horns attention getting is the sound modulation that occurs because the two horns are at different frequencies. The two different frequencies mean that the sound waves will arrive at your ear in varying phases relative to each other. At times, when they are in phase with each other the sound waves will be constructive (louder). Conversely when they are in opposing phase they will tend to momentarily nullify each other (destructive). The phase relationship will cycle at a rate equal to the difference between the two frequencies (508Hz - 341Hz = 167 Hz). In other words, the loudness of the combined horns will increase to ~ double, then decrease to a null 167 times per second. This modulation is the "warbling"/ pulsating effect that you get when you have two horns close in tone, that you don't get when you sound only one horn. It's the entire reason that they have a High tone and Low tone horn and not just two horns of the same tone.
In summary, I believe that these PIAA horns may sound "louder" more because of their better (lower) tone than the actual SPLs they emit. More testing needed to prove this.
One other interesting data point (which I need to explore further) is that I repeated the "After" horns measurement after starting-up the bike. The battery (actually alternator) voltage went up to 14.2VDC and the measured SPL of the pair of horns increased from 113 dBA to 120 dBA. Considering that dB's are a log scale, that is a very significant increase in sound pressure.
I parked the bike out in the open driveway, to minimize reflected sound, and set up the SPL meter on a tripod at about the same height as the horns. I made before and after measurements of both horns from a distance of 20", directly in front of the bike.
For the individual horn measurements I positioned the microphone to the side, on a 45 degree angle, on the side of the active horn.
The power cord seen above is going to a battery charger, which I used to maintain a fairly constant 13.2VDC battery charge thru the tests. All measurements were made on the dB "A weighted" scale. A weighting approximates the frequency response of the human ear, and is therefore more representative of how you should perceive the horn loudness. Right and left refer to the sides of the bike when seated in a normal riding position.
Right side OE horn alone (high tone - 2799 Hz) - 108dBA
Left side OE horn alone (low tone - 341 Hz) - 101 dBA
PIAA Slimline Horn alone, installed in right side (mid tone - 508Hz) - 98 dBA
The reason for a difference between the left and right OE horns is likely to be the "A" weighting. Lower frequencies are scaled down as the human ear is theoretically less sensitive to them. But this does not explain why the PIAA Slimline horn measured lower SPL than the 341 Hz horn. This difference was also born out by the dual horn before and after SPL measurements.
Before - Dual OE Horns - 119 dBA
After - Left side OE Horn and PIAA Slimline on right side - 113 dBA
"OK", you're probably saying to yourself, "there must be something wrong with Fred-o's PIAA horn because when I put the horn on my bike it definitely sounded louder." Well, yes, and that is the really ironic thing. Even with these lower measured SPLs, the "After" configuration did seem like it sounded louder. I probably need to do some more experiments to figure out what is actually going on here (for one thing, repeat these tests using the flatter "C" weighting filter), but I have a theory.
I believe that we feel the horns are louder because the combined spectral pitch is lower. It sounds like it is a big car now and not a motorcycle. So even if it isn't actually louder we sense that it is.
Part of what makes horns attention getting is the sound modulation that occurs because the two horns are at different frequencies. The two different frequencies mean that the sound waves will arrive at your ear in varying phases relative to each other. At times, when they are in phase with each other the sound waves will be constructive (louder). Conversely when they are in opposing phase they will tend to momentarily nullify each other (destructive). The phase relationship will cycle at a rate equal to the difference between the two frequencies (508Hz - 341Hz = 167 Hz). In other words, the loudness of the combined horns will increase to ~ double, then decrease to a null 167 times per second. This modulation is the "warbling"/ pulsating effect that you get when you have two horns close in tone, that you don't get when you sound only one horn. It's the entire reason that they have a High tone and Low tone horn and not just two horns of the same tone.
In summary, I believe that these PIAA horns may sound "louder" more because of their better (lower) tone than the actual SPLs they emit. More testing needed to prove this.
One other interesting data point (which I need to explore further) is that I repeated the "After" horns measurement after starting-up the bike. The battery (actually alternator) voltage went up to 14.2VDC and the measured SPL of the pair of horns increased from 113 dBA to 120 dBA.
Considering that dB's are a log scale, that is a very significant increase in sound pressure.
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