B&W spaciousness from Bose-like reflection?

[Duke]

The tweeter without baffle will radiate quite an amount of sound (though not much in the top octave) to the back of the the tweeter by the phenomenon called in physics "diffraction" (like low frequency sound propagates also to the rear of the speaker having a front-mounted woofer).

What you're describing is caused by the wavelengths being long relative to the baffle's width and therefore wrapping around the edges of the baffle; it's not actually "diffraction" [edit: actually it is diffraction; I was incorrect], but yes that definitely happens.

The wrap-around energy decreases as we go up in frequency and the wavelengths become short relative to the baffle width, whether we're talking about woofers in boxes or B&W tweeters in their little eyeball thingy. Because of this, the wrap-around energy has a significantly different spectral balance than the direct sound.

Is the spaciousness of these loudspeakers partly from the reflection of upper mid and lower high from the wall behind the loudspeaker?

The spaciousness you mention may be largely due to the LACK of diffraction of the tweeter's output, which is because the surfaces around the tweeter are like the surface of a sphere, and are therefore non-diffractive. [edit: In this context I'm using the term "diffraction" to refer to the re-radiation of sound at an abrupt physical discontinuity, like a sharp cabinet edge, but maybe the correct term for that is "reflection"].

I would not expect the spectrally-incorrect wrap-around energy from the tweeter to make a large positive contribution to spaciousness because it is so dissimilar in spectral content to the first-arrival sound from the tweeter, but I could be wrong.

Some Burmester (a German high end brand) speaker models have a tweeter mounted at the back-side of the enclosure.

The original Revel Salon had a rear-firing tweeter. Numerous Von Schweikert models have had rear-firing tweeters. At least one big Wilson has a rear-firing tweeter. Several large Sonus Faber speakers have rear-firing midranges and tweeters. The (imo magnificent) Snell Type A had a rear-firing tweeter. I use 'em too.

 

[skyfly]

"it's not actually "diffraction""

Duke, it is diffraction. Ask your physics professor. Or ask Harman acoustic engineers.



Did you incorrectly memorize that only this is diffraction?

The reason we use enclosure is "diffraction." The sound at the back of the cone driver propagates to the front toward the listener (this phenomenon is called diffraction). So there will be destructive interference with the wave from the front side of the cone driver. Of course, the size of the cone driver and the wavelength matters.

We are on the Audio Science Review web site.

 

[Duke]

"it's not actually "diffraction""

Duke, it is diffraction. Ask your physics professor. Or ask Harman acoustic engineers.



Did you incorrectly memorize that only this is diffraction?

The reason we use enclosure is "diffraction." The sound at the back of the cone driver propagates to the front toward the listener (this phenomenon is called diffraction). So there will be destructive interference with the wave from the front side of the cone driver. Of course, the size of the cone driver and the wavelength matters.

We are on the Audio Science Review web site.

You are correct; I've amended my post.

"Diffraction" is when a sound wave moves around an object like the edge of an enclosure. I suppose the correct term for the re-radiation of sound that happens at the sharp edge of an enclosure would be "reflection", though I've seen the terms "diffraction" and "edge diffraction" used in that context.

Anyway that sharp-edge-of-the-cabinet reflection is what I was thinking of.

 

[skyfly]

Removing baffle is not the only way to combat the bad effect of baffle (showing up as uneven frequency response curves). Manufacturers other than Bowers & Wilkins use careful designing of wave guide, smoothly curving the baffle, etc.

The choice of removing baffle might be related to having more (though still far less than Bose 901) reflected sound energy in the listening room than competitors (Genelec, Revel, JBL, PMC, ATC, Focal, Dynaudio, etc.). See my markings on the ASR results, Revel vs B&W, on my post on this forum. B&W designers probably know this.

Bowers & Wilkins' marketing team might have decided to strictly hide that design intention, because of

[1] the B&W's monitor speaker heritage (most monitor speaker manufactures do not try to have more reflected sound energy) and

[2] the need to avoid being affected by the anti-Bose emotion among some hi-fi consumers.

 

[skyfly]

The choice of 1st order HP filter for tweeter on 702 S2 might also be an evidence of design intention to have more reflected sound energy than Genelec, Revel, JBL, etc.

With higher order filter, B&W might have achieved smoother on and off axis frequency response curves. However, if having more reflected sound energy is a design goal, we can understand why they used low order HP filter for no-baffle tweeter on 702 S2.

 

[skyfly]

I tend to not listen while behind the speaker.
You should read up on interpreting spinorama, I think you have jumped to several conclusions regarding so-called 'spaciousness'.
Have you ever heard a Bose 901? They really have nothing in common with the sound of one of these beaming B&Ws.

I heard Bose 901 many times. I posted a couple of threads about 901 on ASR.

I posted this in a previous post in this thread. The angles beyond 90 degrees are rear. You see the spectrum in speaker design in terms of the amount of high frequency sound toward the rear: Revel < Bowers & Wilkins << Bose 901. You do not need to use the emotional term "nothing in common," since scientific observation/analysis on speakers is not binary 1 or 0.

It is widely known that speakers making more reflected sound in the listening room tend to sound more spacious (of course, the reflected sound is one of the many factors, and the tendency is not like anything with correlation coefficient exactly 1). For example, the coaxial Tannoy sounding less spacious than speakers with dome tweeters on flat baffle, and those speakers sounding less spacious than 360 degree radiating MBL. I guess you heard the assertion about such a tendency many times since long time ago, long before seeing it in this thread.

 

[wwenze]

I have measured the B&W 804S:

B&W 804S quasi-anechoic measurements

Equipment used: REW v5.30.5 (mac) Calibrated UMIK-2 Playback: Asus Essence ST PCI soundcard (using ASIO in Foobar 2000, 24/48). Marantz SR-7013 (analog 7.1ch in (front), pure direct to avoid the slow/poor DAC filter). I started by measuring at tweeter axis (50cm): To make sure the distance...

www.audiosciencereview.com

The on-axis response and directivity looks weird, but the dip is almost gone at listening position in my room, and comes back when the measurement is gated, so it must be filled in by room reflections.

I measured at 0-170 degrees, but didn't include >90 deg since I don't know how reliable those results are. Anyway, here they are:

View attachment 372550


edit: 702 S2 has a similar on-axis dip, and off-axis peak:

Bowers & Wilkins 702 S2 loudspeaker Measurements

Sidebar 3: Measurements

www.stereophile.com

683 S2 tweeter has the same kind of weird peak at 6kHz

Bowers & Wilkins 683 S2 loudspeaker Measurements

Sidebar 3: Measurements I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Bowers & Wilkins 683 S2's frequency response in the farfield; and an Earthworks QTC-40, with its small, ¼"-diameter capsule, for the nearfield responses.

www.stereophile.com

 

[MattHooper]

Assuming that we all know that we must not judge a speaker’s sound by youtube, I post these for fun. I am not related to the two loudspeaker brands or the (seemingly - I don’t know that language - ) hi-fi retailer who made the video.

Bowers & Wilkins 702 S3

JBL S3900

Dear gawd. I can’t believe audiophiles are still playing that song.

 

[wwenze]

I heard Bose 901 many times. I posted a couple of threads about 901 on ASR.

I posted this in a previous post in this thread. The angles beyond 90 degrees are rear. You see the spectrum in speaker design in terms of the amount of high frequency sound toward the rear: Revel < Bowers & Wilkins << Bose 901. You do not need to use the emotional term "nothing in common," since scientific observation/analysis on speakers is not binary 1 or 0.
View attachment 372580

It is widely known that speakers making more reflected sound in the listening room tend to sound more spacious (of course, the reflected sound is one of the many factors, and the tendency is not like anything with correlation coefficient exactly 1). For example, the coaxial Tannoy sounding less spacious than speakers with dome tweeters on flat baffle, and those speakers sounding less spacious than 360 degree radiating MBL. I guess you heard the assertion about such a tendency many times since long time ago, long before seeing it in this thread.

Right... science... and what is science without numbers...

Typical tweeter size is 1-inch, typical woofer size is 5-inch to 6-inch. The typical speaker cabinet is just wide enough to house the woofer, just like B&W's tweeter enclosure.

Typical "omnidirectional behavior" starts at below around 200Hz to 250Hz. The entire graph becomes red from top to bottom. Then it gradually becomes directional because "diffraction" (sic with the inverted commas). If we want to scale that, we would expect around 1kHz to 1.5kHz for the tweeter.

Yet observations show:

1) The graph does not turn red from top to bottom at 1.5kHz. Heck, in that 805 S2 graph, there is a sharp drop in directivity behavior at 1kHz.

2) "Diffraction" reduces as frequency increases. The gradual directivity change be seen for woofers (Altho I will not claim that the entire behavior is due to "diffraction" alone. I mean... waveguides yea?). However in the 805 S2 tweeter data, there is no clear difference between the directivity at 1kHz and 8kHz.

If anything, I admit the sound's existence but it violates the diffraction "maths" too much to be called diffraction. I guess it could be more of the tweeter enclosure vibrating and radiating sideways.

That's why I said at the start: Find me a tweeter that shoots past 90 degree first. You can't calculate baffle effects if the wavefront doesn't even hit the baffle.

 

[skyfly]

Right... science... and what is science without numbers...

Typical tweeter size is 1-inch, typical woofer size is 5-inch to 6-inch. The typical speaker cabinet is just wide enough to house the woofer, just like B&W's tweeter enclosure.

Typical "omnidirectional behavior" starts at below around 200Hz to 250Hz. The entire graph becomes red from top to bottom. Then it gradually becomes directional because "diffraction" (sic with the inverted commas). If we want to scale that, we would expect around 1kHz to 1.5kHz for the tweeter.

Yet observations show:

1) The graph does not turn red from top to bottom at 1.5kHz. Heck, in that 805 S2 graph, there is a sharp drop in directivity behavior at 1kHz.

2) "Diffraction" reduces as frequency increases. The gradual directivity change be seen for woofers (Altho I will not claim that the entire behavior is due to "diffraction" alone. I mean... waveguides yea?). However in the 805 S2 tweeter data, there is no clear difference between the directivity at 1kHz and 8kHz.

If anything, I admit the sound's existence but it violates the diffraction "maths" too much to be called diffraction. I guess it could be more of the tweeter enclosure vibrating and radiating sideways.

That's why I said at the start: Find me a tweeter that shoots past 90 degree first. You can't calculate baffle effects if the wavefront doesn't even hit the baffle.

In science, there is no 1 vs 0 binary distinction between omni behavior vs beaming. The 1 vs 0 binary thinking and brand emotion such as anti-Bose feeling are what we need to avoid for productive conversation..

As for "2)," as you wrote in your post, it is not determined by the frequency alone. You should consider both the wavelength and the size of the objetct (e.g. baffle) or the hole (slit).

"Find me a tweeter that shoots past 90 degree first." There is no 1 vs 0 binary distinction between firing past 90 degree vs not firing past 90 degree.

 

[kemmler3D]

I have measured the B&W 804S:

B&W 804S quasi-anechoic measurements

Equipment used: REW v5.30.5 (mac) Calibrated UMIK-2 Playback: Asus Essence ST PCI soundcard (using ASIO in Foobar 2000, 24/48). Marantz SR-7013 (analog 7.1ch in (front), pure direct to avoid the slow/poor DAC filter). I started by measuring at tweeter axis (50cm): To make sure the distance...

www.audiosciencereview.com

The on-axis response and directivity looks weird, but the dip is almost gone at listening position in my room, and comes back when the measurement is gated, so it must be filled in by room reflections.

I measured at 0-170 degrees, but didn't include >90 deg since I don't know how reliable those results are. Anyway, here they are:

View attachment 372550


edit: 702 S2 has a similar on-axis dip, and off-axis peak:

Bowers & Wilkins 702 S2 loudspeaker Measurements

Sidebar 3: Measurements

www.stereophile.com

Your post was my first thought when I saw this one. IMO the only way this design doesn't look incompetent or crazy is if you assume they're going for some kind of reflected / spatial effect. There is plenty of precedent for that kind of thing, what is surprising is that the B&W speakers LOOK like they should be mostly front-firing, but then you see the spins and it seems they're trying to have it both ways.

 

[MAB]

I'm not sure if I can answer the OP question. Perhaps refine it? Here are some measurements that I hope can cast light on the sound field behind a speaker, and the effect of a baffle vs. no baffle.

I have an old ADS tweeter, with ~5cm minimal baffle. I think B&W has reduced the baffle more than this example, but it is OK. The ADS was a competent tweeter, I haven't used these in almost 40 years and am eager to see the measurements.

I made a quick and dirty baffle out of a board. It will introduce a host of issues.

I also have a box that will supposedly simulate a regular cabinet.

Also, Harry Olsen already did this study almost a 75 years ago. But not so focused on off-axis, and not on tweeters radiating from minimum baffle.

E-library page - AES

AES E-Library ← Back to search

aes2.org

If I consider the tweeter with a small but finite baffle, I expect to see an interference pattern in any rear-radiated sound. The frequency, the angle, and the distance to the microphone will modulate the pattern. Consider the ADS tweeter alone:

We expect a bright spot (peak) in the rear-radiated response at 180 degrees for an obstruction! This bright spot is seen in optics, and acoustic waves. And we expect the nulls and peaks in the interference pattern to be periodic in angle due to the superposition of waves of half-wavelength difference. The shape isn't regular like a sphere or a point-source, so this isn't really a simple textbook example.

Here are several off-axis measurements of ADS tweeter with no baffle, taken at 1 meter distance, angles 120 to 180 degrees:

The 180 degree off-axis measurement (heavy purple trace) has no peaks and nulls across frequency, this is the consequence of the 'bright spot' 180 degrees off axis as mentioned earlier. This also suggests I got the tweeter and mic lined up pretty well, I only see a wiggle at 21kHz so. I didn't take enough angular measurements to see the entire progression of peaks and valleys, but hopefully this makes the main point: we expect a peak in the 180 degree rear-radiated sound field, with nulls and peaks across angle and frequency. At 10kHz the rear-radiated signal is 20dB down, and 50dB down by 20kHz. Tweeters beam, especially radiating into free space, so this isn't too surprising.

The 180 degree does indeed have higher output at 180 degrees. The difference between no-baffle and the two baffle configurations at 180 degrees is an additional 10 dB or more of attenuation. Not that the no-baffle tweeter has much output at 180 degrees, but the tweeter with baffles produces even less.

Also, I did a crap job of making a good baffle:

I am pretty sure the jagged artifacts in the response of the no-baffle are the tweeter protruding out of the baffle. It clears up to a certain extent in the box since I adjusted the tweeter to be slightly more flush to the surface of the plywood. I used minimal smoothing, so you see all of the warts.

Going back to the no-baffle example, where is the energy in the 180-degree off axis coming from? In fact, the 180 degree measurement shows the SPL is higher than all of the other off-axis measurements (105 to 165 degrees).

So, yes, in a narrow band right behind a speaker you will find a tweeter has more energy. Go a few degrees off of that 180 degree axis and the energy is actually lower than the 180 degree response! Tweeters with no baffle don't just tend to beam in the forward direction, but also backwards!!! I have a hard time thinking this backwards beaming is strongly audible, perhaps if you had a highly reflective surface right behind the speaker with treated surface just off the 180-degree axis. Would need to reflect back at the main listening position. And would need to be significant relative to all of the another reflections in the room. And if audible, would be very MLP and speaker position dependent. As I said before, integrating over the entire sound field of direct and reflected energy, this is really small, and very room and setup-dependent. While this type of B&W are really fussy in setup and room, I am pretty sure it's not because of this.

Here is the Spin of the No Baffle compared to Full Baffle:

I used no smoothing. As stated earlier, the tweeter isn't so flush and well integrated in the baffle, but hopefully this makes the point. Yes, over a narrow angle you get 180 degree rear-beaming from the tweeter without the baffle, but less rearward energy to all other angles, and lots of peaks and valleys due to the superposition of all of those angles' radiation.

The baffle reduces that rear-beaming right on the 180 degree axis, and reduces the periodic peaks and nulls in the rear sound field. It also changes off-axis behavior of the direct sound field by reducing the directivity. Many find this extremely useful and audible since it is tens of dB more significant than the rear-radiated field.

I'm not saying the 180 degree off-axis beaming doesn't happen, it does as clearly seen in the measurements. I think what is going on in front of the B&W speaker of this type is what is dominating the sound. I hope this also puts in context the comment about Spin interpretation, you do need to integrate over the entire sound field, unless you have a reflector set up to beam that narrow band of energy right at the MLP, and even than its still tiny... Also, my comment about the Bose, which sends 90% of the sound backwards, the tweeter with no baffle is much less than 1% in that narrow rear-firing band.

I got through that without mentioning diffraction (Basil Fawlty would be proud). All this can be deduced without having to mention that though!

 

[wwenze]

Something fun happening at 5kHz with the baffle.

You might have discovered B&W's secret sauce.