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Author Topic:   Sound Levels
macfam posted 04-29-2004 04:35 PM ET (US)   Profile for macfam   Send Email to macfam  
A friend is about to place an order for a new Nantucket with 135-HP Mercury Optimax engine. One point that was a big influence on his decision was the [sound levels] of the 135 Opti vs the 150 Opti.
With identical blocks, bore and stroke, why is the 150-HP engine so much louder in performance reports at cruising speed than the 135-HP engine? The readings are 89 vs 101 dB SPL. Being a logrithmic function, that is quite a bit louder.
There have been other reports on this 150-HO Opti from various boating magazines, all with basically saying the same thing. Are the reports whacky, or is the 150 Opti really noisy?
Jimh had just mentioned on the latest Cetacea page 78 how quiet the twin 150 Opti's were.
(Jimh and I are about the same age, if he's anything like me, hearing is no longer as acute as it once was.....along with some other things.....just kidding Jim)
Can anyone share some light?
Plotman posted 04-29-2004 08:17 PM ET (US)     Profile for Plotman  Send Email to Plotman     
How close does they guy doing the measuing hold the instument? Did they use the same instrument? Was it calibrated properly? Were they mounted on the same boat for the tests?

Any of these variables. plus a whole lot more, can impact the numbers they came up with. I spent last weekend on a fiends Dauntless 18, and I can say that his opti 150 was very quiet - the wind noise was much more of an issue than the engine noise in terms of how loud you had to talk when standing at the console.

There was nothing whatsoever about that engine that made me feel it was loud - in fact I commented on how quiet the engine was compared to my Merc EFI of the same size.

Another thing to think about - does it make sense to compare wot vs. wot? I would posit that he should compare wot with the 135 to the rpm necessary on the 150 to match that speed.

I would definately get the bigger engine - there is only $500 difference in price between them.

David

rtk posted 04-29-2004 08:36 PM ET (US)     Profile for rtk  Send Email to rtk     
It looks like the motors are the same size engine block according to the brochure I have. In order to get more horsepower out of the same block with most motors you need to get more air and fuel through the motor to produce the extra horsepower. To provide for the higher flow the exhaust is typically modified also. If they opened up the exaust on the Optimax 150 horsepower to get more air and fuel through that may explain a louder motor.

Are you comparing decible readings for each motor at the same RPM level? The "cruising speed" is different for a boat depending on which motor is run. If you want to compare apples to apples see if you can look at the reports and compare the decible levels at similar RPM values.

Rich


Dr T posted 04-29-2004 08:37 PM ET (US)     Profile for Dr T  Send Email to Dr T     
It could be that the sound level was tested at WOT. The 150 is pumping a bit more air through the system so that could account for the higher test number.

However, the 150 should be able to achieve the same performance level as the 135 at a lower throttle setting and thus be as quiet, or even quieter, than the 135 at cruise.

tds

jimh posted 04-30-2004 12:15 AM ET (US)     Profile for jimh  Send Email to jimh     
In the Boston Whaler measurements, the levels are taken at the helm, and according to the guys in Engineering, the transom design affects the levels quite a bit. If the boat is one of the larger boats with a closed transom and Euro-style bracket, the sound levels tend to be lower than they'd be on an open boat like a Montauk where the engine is closer and there is not much obstructing the sound path.

If the levels are taken on the same boat, then this influence should not matter.

On the ride with the twin 150-HP Optimax engines we had two people sitting in front of the motors and that may have attenuated the sound at the helm.

The frequency content of the sound can also affect the perception. The human ear is more sensitive at some frequencies than others, and this may affect the perceived loudness, too.

Regarding sound perception, it is difficult for a listener to recall a sound for more than a few seconds. When making comparisons it is important that the two sounds be presented in rapid succession, otherwise the listener will have little recall of the previous sound. Loudness perception is also affected by exposure, and after a period of listening to loud sounds the perceived loudness decreases.

macfam posted 04-30-2004 07:14 AM ET (US)     Profile for macfam  Send Email to macfam     
These are measurement taken by Boston Whaler on the same boat. A 190 Nantucket.
The rpm referred to was apples-to-apples, 4000-4500 average cruising speed.
Most outboards are pretty noisy "wide-open", but posing this question, we thought it was odd that the 150hp was that much noisier. 89 decibels compared to 101 at an average cruising speed.
I still don't understand why with he same block, bore and stroke, and at that rpm why the 150hp would be any louder, in fact, I expected that it would be less "stressed" and be actually quieter.
With all things considered, and the overall performance specs shown on the Whaler site, the 135 Opti probably is the better option for him.
cape_rover posted 04-30-2004 01:14 PM ET (US)     Profile for cape_rover  Send Email to cape_rover     
On the 17' montauk, BW shows the 90hp 4 stroke about 15db higher than the 90 hp optimax at WOT. I question the data....

[/url]http://bostonwhaler.com/Rec/pdfs/170MontaukEng.pdf[url]

Bigshot posted 04-30-2004 02:50 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I question it too. 101DB is louder than any outboard I have read about. At 120 You can go deaf.
LHG posted 04-30-2004 06:26 PM ET (US)     Profile for LHG    
If you don't trust BW's data on two same brand Mercury engines, why even bother buying a Whaler? Is it possible the place is this corrupt.

The 90 HP Mercury IS a quiet engine, more so than the relatively noisy 4-stroke. But the performance of the two engines is approximately the same. As Nick says, getting a high tech fuel injection system over traditional carburators could tilt the balance to Optimax. There is also a 26# weight saving. But don't expect either of these fine engines to be putting out more than 90HP. For that, you'll have to buy the Classic 2-stroke.

Moe posted 05-01-2004 12:46 AM ET (US)     Profile for Moe  Send Email to Moe     
I used to take my digital sound pressure level meter to Harley dyno shoot-outs so we could give an award to the guy with the loudest exhaust running WOT to redline on the dyno.

CRS has got the best of me, but IIRC, 15db was about the difference between a bike with near-stock mufflers and a bike with open pipes.

At any rate, its a helluva difference that you aren't going to see between unmodified 135 and 150 HP outboards.

--
Moe

jimh posted 05-01-2004 02:55 PM ET (US)     Profile for jimh  Send Email to jimh     
Sound Levels or Sound Pressure Level (SPL) is measured on a scale calibrated in decibels, a standard unit of sound level measurement. The decibel is one-tenth of a Bel, a unit of sound named after Alexander Graham Bell. The size of the unit was too large for most sound comparisons, so a smaller unit, the decibel was created.

The decibel reflects sound pressure changes in the same way that these pressure changes are perceived by the human ear. One decibel of change is about the smallest increment in sound pressure level that is recognizable by the human ear. There is an old joke among sound engineers that one decibel is the amount of volume change that occurs when you ask your teenaged kid to turn down the music. In other words, the music is turned down only enough to be barely perceptible, or one decibel.

The fact that sound pressure level changes in an exponential fashion in order to be perceived as volume increases (or that the decibel is a logarithmic expression of sound pressure changes) is not particularly important. The human ear perceives sound volume changes, and we don't really care how much additional pressure it takes to produce the change. We just hear it.

The ear is an amazing sensory organ and it can accommodate levels of sound that vary over an enormous range of sound pressures, on an order of a million-to-one changes in pressure.

A measurement that indicates two sources differ by 15 dB in level is representing a change in loudness that is quite significant. It is not important how much pressure increase was required to produce this perceived change, but it is significant that 15 dB represents a rather easily perceived and large change in sound level.

Generally when measurements are made of Sound Pressure Levels, some type of weighting is applied. The intent of the weighting is to make the measurement reflect more closely the actual response of the human ear to the different frequencies in the sound being measure. The ear is more responsive to certain ranges of sounds, and less responsive to others.

The lower the frequency of the sound, the less responsive the ear is to the sound. And, compounding this effect, the lower the level of the sound, the more the response of the ear varies. At very high sound levels the response of the ear becomes less variable with frequency, but at moderate to low levels of sound, the ear is not as sensitive to lower frequency sounds (i.e., sounds below about 400-Hz).

Four-stroke engines tend to produce sounds whose low frequencies are about half the frequency of a two-stroke engine running at the same speed. This is an artifact of the greater number of power cycles or combustion strokes in the two-cycle engine.

It could be that from a perceptual point of view, the sound from a four-stroke engine appears to be lower in level than the sound from a two-stroke engine, even though when measured on an un-weighted SPL Meter they would be the same level.

This effect could account for some of the perceptual differences between a two-stroke and a 4-stroke engine of similar horsepower, operating at similar engine speeds.

In the case of the measurements made on the 170 Montauk which compare the 90-HP two-stroke and four-stroke engines, a measured difference of 15-dB could be possible, although it does seem to be quite a large difference. Even if the difference were actually that large, the perceived sound of the four-stroke engine may not appear to be as loud as the measurement would indicate.

In this particular case, I, too, was surprised by this measurement when it first appeared. I exchanged some email on this topic with people in the Engineering Department at the factory, and I believe they are going to check this measure for accuracy.

Also, I do not recall if the measurements state any weighting. Often a notation of "dB-A" is given to indicate measurement weighting according to the "A-scale" which tries to impart some of the human ear characteristics to the measurement data.

For an excellent tutorial on sound measurement, see:

http://www.phys.unsw.edu.au/~jw/dB.html

jimh posted 05-01-2004 03:06 PM ET (US)     Profile for jimh  Send Email to jimh     
To go on a bit further into the topic of sound measurement (or really any measurement), it is often not trivial to get accurate measures of test data, even in the most controlled conditions. In the case of sound measurements, there may be some effects that are not controlled for in the measurement technique. For example, there may be some standing (sound) waves which could create variations in the sound pressure levels as a function of location. In general, SPL measurements probably need to be averaged over several locations to try to cancel any anomalies from the measurement position.

Also, it is possible that the device used to take the measurements could have had some error. It is not clear if there is any calibration standard or reference. In sound measurement in particular, is it difficult to generate known reference levels of sound for any type of field calibration. One must rely on the instrument for accuracy and stability. We don't really know of the instrument accuracy or calibration that was used. Given that these measurements are taken aboard a boat while underway, there certainly is room for some variation. Also, it is likely that the measurements, although perhaps made with the same instrument, were not made at the same time. This is another possible variation.

In most SPL meters there is a range adjustment or input attenuator, and this could be another source for error.

I do find the notion that the measurement being in favor of the two-stroke over the four-stroke has somehow implied an error. Were it the other way around the reaction would have been one of complete head nodding!

macfam posted 05-01-2004 08:55 PM ET (US)     Profile for macfam  Send Email to macfam     
jimh,
As always...above and beyond the call of duty.
Thanks for your thorough explanation on saound levels.
Probably too many variables to get accurate measurements consistantly.
prm1177 posted 05-02-2004 01:58 PM ET (US)     Profile for prm1177  Send Email to prm1177     
Jim doesn't mention it, but a difference of 10 dB SPL (sound pressure level)is close to double the perceived loudness is sound pressure. Thus, a source measured at 85 dB SPL is twice as loud as one measured at 75 dB SPL, but 1/2 as loud as one measured at 95 dB SPL.

As a frame of reference, normal conversation in a quiet room is typically 75-80 dB SPL, while the loudest sound from a single channel in a movie theater is 105 dB SPL(the subwoofers can reach 115 dB SPL, but as Jim mentions, low frequency wavelengths are less perceptible than higher ones.

One final note. Doubling the sources will add +3 dB SPL to the total measured noise level. If one engine idling produces 78 dB SPL; two will produce a combined SPL of 81 dB.

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