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Can someone explain this acoustic phenomenon? #2751026
07/12/18 03:13 PM
07/12/18 03:13 PM
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Kitsap County, WA
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For context, the piano making the sound is a Yamaha CLP-685 which has a row of higher frequency speaker at the top of the cabinet and lower frequency speakers on the bottom. When sitting at the piano and adjusting for what seems to be a good volume for the person playing, we've discovered that anyone a few feet farther back hears the volume as significantly higher. This seems counter intuitive! We always tell my son to turn it down, but then you find that when sitting playing, the volume is just fine. What gives?



Currently learning: Beethoven "Easy" Sonata Op 49 No 2, JS Bach WTC Prelude No 2 in C minor
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Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751029
07/12/18 03:27 PM
07/12/18 03:27 PM
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You get "standing waves" in any closed room (actually it does not have to be closed), and this implies that there will be spots where the sound is particularly loud, and other places where it is particularly quiet. It may accidentally be that the place in front of the piano is a relatively quiet area, whereas a few feet behind it the sound is more powerful.

To make matters more complicated, it is frequency-dependent, so it is also a psycho-acoustic issue in that the sound level perceived by human ears may differ from that measured by technical equipment.

If you move the instrument, you will also move the "loud areas" and "quiet areas" around.

Last edited by QuasiUnaFantasia; 07/12/18 03:32 PM.

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Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751031
07/12/18 03:31 PM
07/12/18 03:31 PM
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Princeton, Texas
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Originally Posted by squidbot
...a row of higher frequency speaker at the top of the cabinet and lower frequency speakers on the bottom.


Can you be more specific about where the speakers are located and which direction(s) they are facing?

Last edited by Duke LeJeune; 07/12/18 03:33 PM.

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Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751038
07/12/18 03:42 PM
07/12/18 03:42 PM
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Standing waves are a low frequency phenomenon (it happens with long wavelengths). So if the areas of the room that seem louder seem to have a lot more low end in particular, that sounds like the right answer. If the high frequencies are louder, that's a different issue. High frequencies are more directional. If a high frequency speaker is a couple of feet over your head, it will not seem as loud to you as it will to someone further back and closer to ear level with the speaker.

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Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751039
07/12/18 03:44 PM
07/12/18 03:44 PM
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emenelton Offline
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Originally Posted by squidbot
For context, the piano making the sound is a Yamaha CLP-685 which has a row of higher frequency speaker at the top of the cabinet and lower frequency speakers on the bottom. When sitting at the piano and adjusting for what seems to be a good volume for the person playing, we've discovered that anyone a few feet farther back hears the volume as significantly higher. This seems counter intuitive! We always tell my son to turn it down, but then you find that when sitting playing, the volume is just fine. What gives?


It's possible if the sound is coming out of the back at the wall that the player might be in the 'doldrum' but the sound then develops a bit further into the room. Any time a sound is forced through a slot the sound has a tendency to 'throw' a distance before it develops fully.

If the speakers are facing the player but at his feet, the sound would simply be blowing past the player way off axis from his ears and then hitting the people in the room stronger.

Re: Can someone explain this acoustic phenomenon? [Re: emenelton] #2751044
07/12/18 03:57 PM
07/12/18 03:57 PM
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This here:
Originally Posted by emenelton
If the speakers are facing the player but at his feet, the sound would simply be blowing past the player way off axis from his ears and then hitting the people in the room stronger.
And it was probably intentionally designed that way. Player's ears aren't in line sight of the driver cones. Spectators get full blast.

Re: Can someone explain this acoustic phenomenon? [Re: Duke LeJeune] #2751058
07/12/18 04:44 PM
07/12/18 04:44 PM
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Kitsap County, WA
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Originally Posted by Duke LeJeune
Can you be more specific about where the speakers are located and which direction(s) they are facing?


Probably obvious, the green is the low frequency, the red is the high (it's a small strip of speakers coming from the top.)

[Linked Image]



Currently learning: Beethoven "Easy" Sonata Op 49 No 2, JS Bach WTC Prelude No 2 in C minor
Re: Can someone explain this acoustic phenomenon? [Re: anotherscott] #2751061
07/12/18 04:46 PM
07/12/18 04:46 PM
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Kitsap County, WA
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Originally Posted by anotherscott
Standing waves are a low frequency phenomenon (it happens with long wavelengths). So if the areas of the room that seem louder seem to have a lot more low end in particular, that sounds like the right answer. If the high frequencies are louder, that's a different issue. High frequencies are more directional. If a high frequency speaker is a couple of feet over your head, it will not seem as loud to you as it will to someone further back and closer to ear level with the speaker.


I'll have to pay a little more attention, but I think it's the high frequencies that are louder. And it's not otherwise positional, pretty much anywhere in the room is louder than sitting in the players seat.



Currently learning: Beethoven "Easy" Sonata Op 49 No 2, JS Bach WTC Prelude No 2 in C minor
Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751065
07/12/18 05:18 PM
07/12/18 05:18 PM
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Originally Posted by squidbot
Originally Posted by Duke LeJeune
Can you be more specific about where the speakers are located and which direction(s) they are facing?


Probably obvious, the green is the low frequency, the red is the high (it's a small strip of speakers coming from the top.)

[Linked Image]


Thank you.

I assume the high frequency speakers are facing you, same as the low frequency speakers?

Here's my guess: Your ears are very far off-axis of the low frequency speakers, and somewhat off-axis of the high frequency speakers, relative to someone who is standing behind you. Speakers are generally louder on-axis than they are off-axis, at least at the top ends of their ranges. So the midrange energy that comes from the low-frequency speakers will be louder in the audience area.

If that's a horizontal row of tweeters, or tweeters firing through a horizontal slot, then they probably approximate a line source up close. SPL changes more gradually with distance from a line source, so your being closer to the speakers doesn't increase the volume as much as you would expect. If your ears are also more off-axis than those of people out in the audience area, that could result in the highs being a little louder for them than for you.

Yamaha may have tailored the response of the speakers to be correct at your ear location when you are playing, which means that some frequency regions may be a bit over-emphasized out in the audience area. For instance if the top end of the low frequency speakers is emphasized to compensate for your being well off-axis when you are playing, that could make the speakers sound louder out in the audience area. And if the crossover between the low frequency speakers and high frequency speakers is fairly high up the spectrum, the upper-mids as well as much of what counts as "highs" could be coming from the low frequency speakers, and therefore those frequencies would easily be louder behind you than where your ears are when you are playing.

You might try moving your ears down until they are on-axis of the high frequency speakers while you play, and see if the highs then sound more like they do out the audience area.

If you really want to carry this experiment to the extreme. have someone else play while you move your ears down to the low-frequency speakers.


Last edited by Duke LeJeune; 07/12/18 05:24 PM.

Owner & designer, AudioKinesis
Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751526
07/14/18 04:46 PM
07/14/18 04:46 PM
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That’s very interesting thanks! I’ll have to try the “moving my head up and down” test. Would you please elaborate on what the speaker axis is and why it occurs?



Currently learning: Beethoven "Easy" Sonata Op 49 No 2, JS Bach WTC Prelude No 2 in C minor
Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751563
07/14/18 08:12 PM
07/14/18 08:12 PM
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Princeton, Texas
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Originally Posted by squidbot
Would you please elaborate on what the speaker axis is and why it occurs?

Imagine that the speaker cone (or dome) is a flashlight. The center of the flashlight's beam, where it is the brightest, would be “on axis”, and way off to the side would be “off axis”. Off to the side of the centerline a little bit, where there's still light but not quite as bright, would be “a little bit off-axis”.

On a speaker, what happens is, the radiation pattern tends to become more and more narrow as we go up in frequency. This phenomenon is called “beaming”, which brings to mind the flashlight analogy.

When the wavelengths are long compared to the diameter of the speaker cone or dome, the speaker has a very wide radiation pattern. When the wavelengths are short compared to the cone or dome diameter, the speaker's radiation pattern becomes much more narrow.

You may have noticed this effect out in front of a guitar cab: When you are on-axis, directly in front of where the speakers are facing, the highs are piercing. When you are fairly far off-axis, the highs are muffled. When you are off-axis a little bit, maybe fifteen or twenty degrees, you can still hear the highs but they aren't so piercing.

One of the reasons why most speaker systems include a large-diameter woofer and small-diameter tweeter is, the woofer would be beaming too severely at high frequencies (assuming the woofer could reproduce high frequencies to begin with). Let's look at an example:

Suppose we have a speaker system with a 6” woofer and a 1” dome tweeter. At low frequencies the wavelengths are many feet long, much greater than the woofer's diameter, so the low frequencies are omnidirectional – that is, they radiate equally in all directions. As we go up in frequency, the woofer's pattern progressively narrows.

Up at about 2.5 kHz, which is often the crossover region, a wavelength is about 5.4” long, which is approximately the woofer's diameter. Up here the woofer's radiation pattern has narrowed to about 90 degrees – it is “beaming”, though not too badly. So we cross over to the 1” dome tweeter, whose diameter is small relative to these wavelengths. The tweeter wants to have an omnidirectional pattern, but the speaker's front baffle acts as a 180 degree horn and prevents that. So the tweeter's radiation pattern is 180 degrees at the bottom end of its range, narrowing to about 90 degrees at 13 kHz, and narrowing even further above that.

If we had not crossed over to the tweeter, but let the woofer cover the whole spectrum (assuming it could), then its pattern would have been extremely narrow at high frequencies.

One consequence of the radiation pattern's change with frequency is, listening angle and listening distance both matter. The reason the angle matters is pretty obvious, but distance matters too, and here is why: The spectral balance of the reflections will be different from the spectral balance of the first-arrival sound, and the relative loudness of the two changes with distance (actually it is only the loudness of the direct sound that changes significantly with distance – the loudness of the reflected sound is usually close to uniform throughout the room).

So when you are only a few feet away from the speakers, the first-arrival sound delivers most of the energy to your ears. When you are much further away, the reflections deliver most of the energy to your ears. So in the “near field” the first-arrival sound dominates the perceived tonal balance, and in the “far field” the reflections dominate the perceived tonal balance. With most small speakers, their first-arrival (on-axis on nearly on-axis) sound is the most accurate, so they generally sound their best at short range, where the direct sound is loud enough to thoroughly dominate over the reflected sound. Hence the name “near-field monitors” - they are meant to be listened to in the “near field”, which is where the direct sound delivers more energy to your ears than the spectrally-skewed reflections do.

As we move further back from the speakers the direct sound falls off in volume, but the volume of the reflections stays pretty much the same (assuming an indoor venue). So in most venues, the audience is going to be in the “far field”, where the reflections dominate the perceived tonal balance. Therefore speaker systems optimized to sound good in the far field are used in sound reinforcement systems. The job of a PA system is not only to be loud enough, but also to still sound good out in the far field. Obviously some do this better than others.

That was probably a longer answer than you were looking for! I did not go into the actual acoustic mechanism that causes beaming, but doing so probably would have doubled the length of my reply, and would have made it a lot more boring.

Last edited by Duke LeJeune; 07/14/18 08:13 PM.

Owner & designer, AudioKinesis
Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751571
07/14/18 08:43 PM
07/14/18 08:43 PM
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Clear and interesting response Duke. Thank you for taking the time to educate us.

Feel free to "go into the actual acoustic mechanism that causes beaming" anytime. . .

Re: Can someone explain this acoustic phenomenon? [Re: newer player] #2751583
07/14/18 10:11 PM
07/14/18 10:11 PM
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Originally Posted by newer player
Feel free to "go into the actual acoustic mechanism that causes beaming" anytime. . .


Briefly, beaming has to do with there being different path lengths to different areas on the cone when you are off-axis. These different path lengths can cause partial cancellation, and therefore reduced SPL, at off-axis angles.

This illustration is going to be simplified a bit, but the principle described is correct.

Imagine you are exactly three feet in front of a speaker cone. The cone diameter is 6 inches.

Now you step three feet to one side, so you are now 45 degrees off-axis of the speaker cone. You take out a tape measure and measure the distance to the center of the cone, and it is 50.9 inches. You then measure the distance to the nearest edge of the cone, and it is 48.8 inches. That's a difference of 2.1 inches.

Now suppose we play a 3.2 kHz tone through the speaker. A 3.2 kHz tone has a wavelength of 4.2 inches, which is twice our 2.1-inch path length difference.

So at 3.2 kHz and 45 degrees off-axis, the sound coming from the center of the cone arrives ½ wavelength behind the sound coming from the nearest edge of the cone, due to the path length difference being ½ wavelength. So the output from the center of the cone arrives out-of-phase with the output from the nearest edge of the cone, and they actually cancel one another out!

In practice, there is enough sound from other areas of the cone whose path length difference is not exactly 1/2 wavelength that the net effect is only partial cancellation, but this example illustrates the mechanism that is in play. At any given listening angle, the output from every point on the cone is interacting with the output from every other point on the cone in this same manner. Only on-axis are all the path lengths the same.

If we make the off-axis angle greater, the net amount of cancellation increases. And vice-versa.

If we make the wavelengths shorter (higher frequencies), the net amount of cancellation increases. And vice-versa.

If we make the cone diameter larger, the net amount of cancellation increases. And vice-versa.

Unfortunately it's hard to find data from manufacturers that illustrates this. I think Selemium was the only company that published polar response plots of their woofers at different frequencies, until JBL bought them out and put and end to it. Here's a link to one of Selenium's older spec sheets, for a 15” woofer. Scroll down to the second page for the polar plots at different frequencies:

https://www.parts-express.com/pedocs/specs/264-393-selenium-15ws600-specifications-45430.pdf

Imo beaming and the resulting off-axis response variation is a very real phenomenon with imo very real and audible consequences, and is among the "dirty little secrets" that speaker manufacturers in general don't really want to raise awareness of.

Last edited by Duke LeJeune; 07/14/18 10:17 PM.

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Re: Can someone explain this acoustic phenomenon? [Re: squidbot] #2751872
07/16/18 10:21 AM
07/16/18 10:21 AM
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Thanks again! Very interesting!



Currently learning: Beethoven "Easy" Sonata Op 49 No 2, JS Bach WTC Prelude No 2 in C minor
Re: Can someone explain this acoustic phenomenon? [Re: anotherscott] #2751959
07/16/18 04:51 PM
07/16/18 04:51 PM
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Originally Posted by anotherscott
Standing waves are a low frequency phenomenon (it happens with long wavelengths). .


Standing waves can exist at any frequency. It's just that at low frequencies the wavelength is long and the distance between nulls and maxima is big enough for human beings to notice and be bothered by them.



Last edited by JohnSprung; 07/16/18 04:52 PM.

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