Question about speaker sensitivity

[blubass]

Recently, I've been looking at a 6-string amp. The amp comes with some Chinese speakers, but they're rated at 1w/50cm (0.5m) instead of 1w/1m which is more commonly accepted.

I'm not apprehensive to using the speakers as I have no experience with them. If they're good and loud enough, so be it. However, it does make it hard to do comparisons against speakers using the more common 1w/1m rating.

Is there a way to convert the sensitivity (roughly) to 1w/1m to better make comparisons with other speakers such as Eminence or Celestion?

Here's the link to the exact speakers in this combo...

Invalid Link Removed

Thanks in advance!

 

[blubass]

After some research I found an article explaining db in relation to distance. Is it correct to assume about a +/- 6db change in sensitivity when increasing or decreasing the distance measured by one half?

 

[Kro]

Not sure how accurate this actually is, but for rule of thumb, ballpark type calculations, I've found this calculator to be helpful - includes distance as one variable among others.

Peak SPL Calculator

 

[Rick James]

Is it correct to assume about a +/- 6db change in sensitivity when increasing or decreasing the distance measured by one half?

Yes, it's called the Inverse Square Rule.

 

[blubass]

Yes, it's called the Inverse Square Rule.

Thank you!!!

Not sure how accurate this actually is, but for rule of thumb, ballpark type calculations, I've found this calculator to be helpful - includes distance as one variable among others.

Peak SPL Calculator

Bookmarked... that's cool, it should come in handy

 

[Rick James]

That calculator is off. It only shows 3dB gain close to a wall, it's actually 6dB. That's due to 1/4 Pi Space Loading. It also shows only 3dB per doubling of speaker count. That's also 6dB, due to Mutual Coupling. In practice in a normal room you probably won't get 6dB from either 1/4 space loading or mutual coupling, but it's almost always going to be more than 3dB.

 

[Kro]

It also shows only 3dB per doubling of speaker count. That's also 6dB, due to Mutual Coupling.

I'm no expert, but I thought the 6dB also required double the power: 3dB was gained purely from coupling (with the same power, now split between double the speakers), and 3dB from double power - which can either come from actually increasing power via another amp, or from the additional power that can come from the amp reacting to the impedance change.

Just stating this for the purpose of discussion. I fully expect to be misunderstanding.

 

[Kro]

It only shows 3dB gain close to a wall, it's actually 6dB. That's due to 1/4 Pi Space Loading.

Also, was just looking at this more, it seems like "away from walls" still means floor - so the floor would be the half space boundary, and already factors in a 3dB boost from that unless I'm misunderstanding. Therefore the gain from the additional boundary to get us to quarter space (wall) is another 3dB, right?

Edit: scratch that, it looks like it actually doesn't take into account the floor as a boundary, and goes right to what one might expect 1/4 being +3 and 1/8 being +6. Damnit. Bassists usually sit cabs on/near the floor!

 

[micguy]

For a point source, everything else held constant, doubling the distance does reduce the sensitivity by 6 dB.

Adding a wall reflection - inteh low frequency region, assiming a closed or ported box (and the port isn't blocked by the wall) - you get 6 dB from the reflection. However, that's only as long as the wall is significantly less than a quarter wavelength from the wall. At 100 Hz, for example, a wavelength of sound is 11 feet, so if the front baffle of the speaker is 11/4, or 2.75 feet from the wall, you get reinforcement well below that. As you close in on 100 Hz (your open G string's fundamental is 98 Hz), you get to a null - the wall being there cancels that frequency.

Same kind of thing happens with multiple drivers at low frequencies. Split the power between 2 drivers, each produces 3 dB less, but in the region where they're within a quarter wavelength, they add pressure, and the result is 3 dB more sensitivity for doubling of drivers at low frequencies. Above a quarter wavelength, you still get more sensitivity on axis, but off axis, things start to cancel - the thing "beams"

 

[Kro]

Adding a wall reflection - inteh low frequency region, assiming a closed or ported box (and the port isn't blocked by the wall) - you get 6 dB from the reflection.

That still takes into account the floor though, right? Or am I misunderstanding - I thought each boundary provided an additional 3dB. Floor+Wall=6dB. Floor+Wall+Wall (aka corner)=9db. The post I quoted above makes it seem like you get 6dB just from the wall. This could be semantics here, but it might make a difference to somebody with less understanding on the topic (arguably like me)!

Edit. God, please somebody set me straight. I've found sources on the web now, that seem to show it both ways. It's the internet, so they must both be right, but it's making my head hurt.

These sources below appear to contradict each other, and I'm not smart enough to make sense of it myself it appears.

Exhibit A

Exhibit B: Invalid Link Removed. My head hurts. Really sorry for the derail guys.

(for the tie breaker:

)

 

[micguy]

At low enough frequencies, it's 6 dB per boundary. In a 2 boundary scenario, you have the bounce off one wall (or floor - sound doesn't discriminate based on direction), and 2 bounces off two walls - they "go into the corner, bounce once off each wall, and then come back out". Those double bounces will have longer paths than the single bounces, so the frequency below which they work will be lower than the single bounce ones. For 3 boundaries, the same thing applies; you just have more bounces - now including 3 boundary bounces in addition to the 1 and 2 boundary ones, and the 3 boundary one is longer still. So, practically, you're not gonna get a full 18 dB "wall gain" from bass guitar cabinets up aganst a 3 wall boundary. You can get more gain from a smaller cabinet closer to the corner, but a smaller cabinet means you're starting out with less output to begin with.

Anyway, above the transition frequency, you get reinforcement that's up and down with frequency. By placing the speaker at different distances from the 3 walls, you can smooth things out, and in that case, what you'l get on average is 3dB per wall. So, both 3 dB and 6 dB are "right" - depending on what frequency range you're talking about. Might be why you have two sources that disagree.

My source: 4 years of Acoustical Physics in college plus 3.8 decades using this stuff to design pro audio products.

 

[Bill Whitehurst]

I am going to go slightly out on a limb and guess
that along with using an odd distance for their rating,
they didn't include much else that might help you compensate for it.
(Walls, floor, outdoors? Who knows?)

In other words, they want to be able to give you a rating without actually telling you anything.
My guess is that if you manage to translate to a normal 1W/1M number,
it won't be very impressive.

 

[Rick James]

These sources below appear to contradict each other, and I'm not smart enough to make sense of it myself it appears.
Exhibit A
View attachment 2915092
Exhibit B: Invalid Link Removed.
View attachment 2915123)

A is wrong. B is correct. There is a 'but', though. Free-space/whole space is at least two wavelengths from any boundaries. Even outdoors a true whole space measurement is impractical for full range speakers, because two wavelengths at 20 Hz is over 100 feet, so a whole space measurement would require hanging the speaker and mic 100 feet up in the air.
The usual method of quoting sensitivity is into half space, because it's easy to take that measurement. If the speaker is outdoors on the ground that's half space. If it's mounted in a wall in an anechoic chamber that's also half space. Quarter space and eighth space gains would be compared to those measurements, not to what you get in the middle of a room, because you could already be in eighth space in the middle of the room if the distances to the walls were short enough compared to the wavelengths. Plus you have to consider that the ceiling is a boundary too.
As for mutual coupling, when you add a second driver or speaker driven with the same voltage as the first the total volume of air displaced by the cones doubles. Doubling displacement gives a 6dB increase in SPL. The outputs of the two drivers or speakers is only fully mutually coupled if they're less than 1/4 wavelength apart, beyond that and the pathway differences from the two sources to both room boundaries and individual listeners will cause some frequencies to be less than 6dB. In extreme cases the two sources can cancel each other at some frequencies. But on the whole where low frequencies are concerned you'll get 6dB.

 

[winegamd]

This link might help: Invalid Link Removed

 

[charlie monroe]

This is great stuff, guys.

Great explanations in clear language.

 

[Downunderwonder]

Iirc standard sensitivity is in halfspace.

Doubling distance results in 6dB drop unless you achieve a line source which drops only a theoretical 3dB.

Then there's Vortex cabs but nobody has any maths for general consumption on that yet.

 

[HolmeBass]

That still takes into account the floor though, right? Or am I misunderstanding - I thought each boundary provided an additional 3dB. Floor+Wall=6dB. Floor+Wall+Wall (aka corner)=9db. The post I quoted above makes it seem like you get 6dB just from the wall. This could be semantics here, but it might make a difference to somebody with less understanding on the topic (arguably like me)!

Edit. God, please somebody set me straight. I've found sources on the web now, that seem to show it both ways. It's the internet, so they must both be right, but it's making my head hurt.

These sources below appear to contradict each other, and I'm not smart enough to make sense of it myself it appears.

Exhibit A
View attachment 2915092

Exhibit B: Invalid Link Removed. My head hurts. Really sorry for the derail guys.

(for the tie breaker:
View attachment 2915123)

The top chart/graphic you posted is what my understanding of this is. The bottom graphic appears to have inflated the expected dB gains for each scenario.

They obviously disagree with each other though, don't they?

Then again maybe what I had learned is wrong? It seems to have been very much ingrained that one additional barrier increases bass output by +3 dB, not +6, but @Rick James seems to know what he's saying and he's agreeing with +6 dB per boundary added.

 

[Rick James]

Mutual coupling and space loading are the same thing mathematically. SPL stands for Sound Pressure Level. Speakers really do pump air to create sound pressure. When you double the amount of air being pumped by doubling the cone displacement, either by adding a second cone driven at the same voltage or by doubling the voltage into one cone, you get 6dB. When you halve the space that the cone is pumping the air into you get twice the pressure for the same result, 6dB. When you see anyone talking about a 3dB increase that's almost always related to power; doubling power is a 3dB increase. Doubling power doesn't double the cone excursion for a doubling of pressure, so it doesn't give the same result as doubling the cone area with the same voltage or doubling the voltage with the same cone area. It all makes sense if you know Ohms Law, which tells us that doubling voltage into a given impedance doesn't double power for 3dB, it quadruples power for 6dB.

 

[AstroSonic]

So you are looking at an amp for 6-string bass (includes the low B string), and the amp (apparently a combo) uses 12-inch speakers with relatively small magnets, 70w thermal power handling, a 92 db/w/m sensitivity, Qts of 0.87, no Xmax spec and Fs of 115Hz. There is really no chance of success... :-( If the amp and cab are decent, it could be worth replacing the stock drivers, but why not buy what is going to work to begin with?

 

[HaphAsSard]

Recently, I've been looking at a 6-string amp.

So you are looking at an amp for 6-string bass (includes the low B string), and the amp (apparently a combo) uses 12-inch speakers with relatively small magnets, 70w thermal power handling, a 92 db/w/m sensitivity, Qts of 0.87, no Xmax spec and Fs of 115Hz. There is really no chance of success... :-( If the amp and cab are decent, it could be worth replacing the stock drivers, but why not buy what is going to work to begin with?

I'm no expert on guitar drivers any more that I am of bass or PA ones (read: I don't know diddly), but sensitivity seems a little underwhelming on this one.