Fact or Crap? There is no replacement for displacement.

[Tom C]

The Heartmotion looks like a very interesting design approach. But I’m thinking it may be expensive. Equal performance in a smaller form factor usually is more expensive.

 

[Tom C]

This thread has some good explanations:

Bass production and membrane area

 

[Tom C]

SPL is part of the equation, but not the whole story, I don’t think. Comparing two systems here in my home, one is a multichannel home theater with 2-way speakers, having 5-in “woofer.” There’s a single 18” subwoofer in the room also. The other system is basically stereo, using 2-way speakers having a 10-in woofer. There are two 18” subwoofers in that room, one acting for the physical base in each of the two stereo channels.
The multichannel system will get pretty loud, but doesn’t have the same punch in the mid and upper bass frequencies that the stereo system has. If I turn off the upper cabinets in each system, what comes through the subwoofers in each room sounds pretty similar, not surprising. I hear the reverb from the strike on the tom (drum), but not so much the initial thwack. Turn the upper cabinets back on, and it’s the bold thwack that’s missing from the system with the 5-in woofers. I would like to address the situation by upgrading the multichannel system, but am trying to decide between several possibilities.
Something that caught my attention was a post by member @tomtoo, where he says that when people who play bass guitar want punch, they use a cabinet with 10” driver x 4. When they want boom, they use 18”. That makes sense to me, so if I want deep, suffusive bass filling the room, and that satisfying slam, I need both an 18” and a 10”. If this is so, then what is the best driver size to use for punch? Is 12” better than 10”? Is 15” even better, or is it only a compromise between 10” and 18”, without the full benefits of both?

 

[Kvalsvoll]

Yes, it can. If the room/listening distance is not too large, bass is reproduced by a decent bass-system, and the driver/speaker has high output capacity - it needs to handle both high current and quite long excursion.

This reply of mine does not make much sense, unless seen in context.

For a small acoustic radiator (size << wavelength), displacement follows max output capacity - for a given spl, there is a displacement. How the displacement is generated does not matter for low frequencies, you can have large area and small excursion, or small area and long excursion - same spl.

A small 5" can really not make it, even when used above 120hz only. 2 can barely hold up.

Calculating displacement is fairly easy - D=Sd*xmax.

Another problem with small drivers, is that they also have less excursion - a 5" can do 5mm, while a long-excursion 12" can do 10-20mm.

Displacement requirements take off as you go lower in frequency. For this small acoustic radiator (r<<wl), for a constant spl, the velocity doubles for each halving of frequency, and the displacement goes 4x for each f/2. So, you need 4 times displacement at 40hz compared to 80hz. So you need large displacement for low frequencies.

There is a way to go around this, and make a small driver reproduce low bass - horn loading. Then the driver will couple in a very different way, which increases its efficiency a lot. But the total size of the speaker will still be large - you can say, that you can choose between a closed box with a very large driver, or a box of at least equal size with horn loading and a small driver - same output, same size.

 

[Kvalsvoll]

SPL is part of the equation, but not the whole story, I don’t think.

Of course, because the sound intensity and directivity is directly related to the size of the radiator.

 

[Tom C]

Beaming is a little easier for me to conceptualize. For a given, fixed frequency, a larger driver will beam more than a smaller driver. Said another way, the higher the frequency, the more any given driver will beam. In this way, directivity differs with driver size.
Intensity seems like it should depend on power applied and efficiency, a corollary of sound pressure level. To the extent efficiency differs with driver size, I can see where intensity would also differ.
In the end, I’m a bit less concerned with exactly why, and am currently focused on simply understanding what I need to choose for the most effective and cost efficient way to get what I’m looking for.

 

[Kvalsvoll]

Intensity seems like it should depend on power applied and efficiency, a corollary of sound pressure level. To the extent efficiency differs with driver size, I can see where intensity would also differ.

Here I mean sound intensity, it is a defined physical metric, I = p x v, where I is the sound intensity, p is pressure and v is the velocity vector. Intensity is the power of the sound.

 

[G|force]

The best devastation of LF reproduction doesn't have any voicecoil Powersoft force solenoids 3 phase 208 is basic

 

[G|force]

You use wound voicecoil all your bass belong to us

 

[sarumbear]

Of course, because the sound intensity and directivity is directly related to the size of the radiator.

To clarify not the radiator area but the volume of the radiator displacement.

 

[abdo123]

Below 100Hz not really, above 100Hz however the direct sound will still have an influence on your perception and you will distinguish between the two speakers.

 

[sarumbear]

The topic comes up from time to time in discussions of speaker reviews. Rather than contribute to derailing of a review thread, thought I’d post here.
Can a 5-in mid/bass driver, properly implemented in a well designed system, eq’d and level matched, produce output that is audibly indistinguishable from the output of a system using, say, a 10” or 12” driver? Based on personal experience (a limited resource for sure), I would say no. The output of a larger driver will produce a superior sound quality in the mid and upper bass, and it will be easy to hear the difference compared to a smaller driver.

SPL is created by the displacement volume of the driver.

Vmax = (Diameter / 2) ^2 x Pi x Xmax

For every halving of driver diameter you need to increase Xmax four times in order to generate the same SPL. You can imagine the physical consequences of such an increase. No amount of clever design can surmount such a large barrier. A small speaker is most of the time will perform worse than a large driver.

 

[egellings]

Yep. Ya can't send a 6" driver out to do a man's job.

 

[phoenixdogfan]

Yes, for bass you need serious displacement. If you are talking sub bass then displacement and cabinet size is king. Think like a huge JTR sub. Big driver and big cabinet. Near field is so easy to do it is really almost buying any decent set of speakers.

I should bring up a point of safety here. Do not ask me how I know this OK, young and dumb. Do NOT ever think that subwoofer bass can't hurt your ears. It will and can hurt your ears at high levels. Protect you hearing at all times and don't put your head inside commercial sound giant bass bins and crank it up. You will regret it later in life. So, with safety out of the way, the old adage is buy as much and as many subs as you can until you hit the "just plain crazy" level.

My own personal test back in the day was the old incandescent lights bulbs, if you are breaking the filaments with your subwoofer set up, you have great bass!!! Or, maybe a bit too much? Either way, I liked it.

Old audiophiles are like cat ladies, only with subs.

 

[FrantzM]

SPL is created by the displacement volume of the driver.

Vmax = (Diameter / 2) ^2 x Pi x Xmax

For every halving of driver diameter you need to increase Xmax four times in order to generate the same SPL. You can imagine the physical consequences of such an increase. No amount of clever design can surmount such a large barrier. A small speaker is most of the time will perform worse than a large driver.

I would think this post rest the case.

 

[Kvalsvoll]

To clarify not the radiator area but the volume of the radiator displacement.

There is a difference. Same displacement from a large area creates a sound field with higher intensity compared to the same displacement with a smaller area.

At very low frequencies, the difference is no longer significant, because of 2 things - the knee-point on the radiators acoustic impedance is much higher in frequency (around where r == 1 wavelength), and the speaker couples to the boundaries of the room.

-------
Above the kr=1 point, the acoustic impedance is dominated by the real component, pressure and velocity are in-phase, creating higher sound intensity. Below the impedance becomes reactive, and pressure and velocity are 90 degrees out of phase -> less intensity for same pressure.

 

[Kvalsvoll]

SPL is created by the displacement volume of the driver.

Vmax = (Diameter / 2) ^2 x Pi x Xmax

For every halving of driver diameter you need to increase Xmax four times in order to generate the same SPL. You can imagine the physical consequences of such an increase. No amount of clever design can surmount such a large barrier. A small speaker is most of the time will perform worse than a large driver.

Exactly. Even if it was possible to make a driver with no excursion limit, it will soon move so far that the cone can no longer fit inside the cabinet. This will also lead to severe nonlinearity due to the compression of the air inside this cabinet.

Acoustic loading/horn loading of a small driver works very well to enable low bass from small displacement. But trying to make the horn very small also leads to problems with physics, the air velocity simply becomes too high, which creates very high losses and distortion.

 

[sarumbear]

There is a difference. Same displacement from a large area creates a sound field with higher intensity compared to the same displacement with a smaller area.

I am not sure I understand this. What is the definition of a high intensity sound field then just high SPL?

At very low frequencies, the difference is no longer significant, because of 2 things - the knee-point on the radiators acoustic impedance is much higher in frequency (around where r == 1 wavelength), and the speaker couples to the boundaries of the room.

-------
Above the kr=1 point, the acoustic impedance is dominated by the real component, pressure and velocity are in-phase, creating higher sound intensity. Below the impedance becomes reactive, and pressure and velocity are 90 degrees out of phase -> less intensity for same pressure.

You talk about kr but not define it. Is it k x r (radius)? If so what is k?

When we talk about low frequency (bass) we talk about frequencies around 40Hz where the wavelength is 8.5m. According to your description no driver on the market can couple to the room as even a 12" driver is 30 times smaller. Unless that k has a high value.

 

[xaviescacs]

the air velocity simply becomes too high, which creates very high losses and distortion.

Question. It's this due to "turbulence"? I mean, non-linearities of the fluid dynamics?

 

[Kvalsvoll]

I am not sure I understand this. What is the definition of a high intensity sound field then just high SPL?

You talk about kr but not define it. Is it k x r (radius)? If so what is k?

When we talk about low frequency (bass) we talk about frequencies around 40Hz where the wavelength is 8.5m. According to your description no driver on the market can couple to the room as even a 12" driver is 30 times smaller. Unless that k has a high value.

k=w/c0, where w=2*pi*f, c0=velocity of propagation of sound. The knee-point is more correctly referred to as critical frequency, when talking about acoustic transmitters;
w_critical=c0/R, where w_critical=2*pi*f_critical, c0=velocity of propagation.

This frequency is where the radiation shifts from real acoustic impedance loading to imaginary as frequency is reduced.

Then we see that all practical acoustic transmitters will behave similar for low frequencies, as they are all small compared to wavelength.

Intensity of the sound field is the power per area;
I=dP/dS,
Which can be written as:
I=p(pressure)*v(particle velocity)*cos(phi)(phase angle between pressure and particle velocity)

While the intensity is a measure of how much energy is transferred, it is the p(pressure) we hear.