How is this done?
Speaker mfrs tells to the customer that ”this” speaker is good for a room size of 10-15 m2 and amp power of 20-50 watts, and ”that” speaker is good for rooms of 20-30 m2 and 100-300 watts, how do they come up with these figures?
In my recent setup i have come to the conclusion that i need a lot less then i ever had thought, this in entirely because of the room used and how the speakers must be positioned
Speaker mfrs tells to the customer that ”this” speaker is good for a room size of 10-15 m2 and amp power of 20-50 watts, and ”that” speaker is good for rooms of 20-30 m2 and 100-300 watts, how do they come up with these figures?
In my recent setup i have come to the conclusion that i need a lot less then i ever had thought, this in entirely because of the room used and how the speakers must be positioned
It's often said here that the smaller the room, the bigger a speaker has to be.
There are, of course different aspects to consider, each with different solutions.
There are, of course different aspects to consider, each with different solutions.
that can not be true, unless you sit precisely in the middle of the room
yes, some might want to listen at very high spls, and some has neighbors to consider, some wants excessive bass and some do not, but in general, how is the calculation done?
They make assumptions, including:
- Desired SPL
- Listening distance
Everything else can be derived from those - start at the sensitivity of the speaker, and work from there.
Chris
i do not get it, how do they assume the room gain for a speaker at 2.5 meters away at 100db then?
SPL drops predictably: 6dB per doubling of distance. Room gain is due to whole-room pressurisation, and occurs at low frequencies - that's a different matter.
Let's take a 24sq.m room, and say it's 6m x 4m.
We want to hit 105dB peaks from each speaker at the listening position, and our speakers are 88dB@1w.
We're going to work two examples here. One with the speakers firing across the room, and the other example will be along the length.
At 3m listening distance (sofa to back wall, speakers almost touching the opposite wall), we'll need 250w/ch.
At 5m listening distance, we'll need something like 600w.
Peak SPL Calculator
The two factors at play are these:
-6dB per doubling of distance
+3dB per doubling of power
Once you make sensible assumptions about room dimensions (ie, a 24sq.m room is unlikely to be 1m wide and 24m long), it's pretty straightforward.
Chris
Let's take a 24sq.m room, and say it's 6m x 4m.
We want to hit 105dB peaks from each speaker at the listening position, and our speakers are 88dB@1w.
We're going to work two examples here. One with the speakers firing across the room, and the other example will be along the length.
At 3m listening distance (sofa to back wall, speakers almost touching the opposite wall), we'll need 250w/ch.
At 5m listening distance, we'll need something like 600w.
Peak SPL Calculator
The two factors at play are these:
-6dB per doubling of distance
+3dB per doubling of power
Once you make sensible assumptions about room dimensions (ie, a 24sq.m room is unlikely to be 1m wide and 24m long), it's pretty straightforward.
Chris
My best bet is they don't.
A lot of this stuff is marketing hype, said because it sounds good or sells more expensive products.
Consider the enormous variability of room shapes, sizes, decor, purpose and listening positions. There is no way on Earth they could scientifically account for all that to say... "You need a 15 inch woofer in a 7 cubic foot box, with a 4 inch midrange and a 1 inch horn loaded tweeter, with an amplfier 150 watts per channel"... It just isn't possible to be that specific.
What they can say is ... "We have a 15 inch woofer in a 7 cubic foot box with a 4 inch midrange and a 1 inch horn loaded tweeter and an amplifier of 150 watts per channel, that we want to sell to you."
On the topic of amplifier power... there is some science there. When a manufacturer specs a box at 10 - 100 watts, what he generally means is that 10 watt amplifiers are likely to clip before you get a decent spl going which could damage the tweeters and amplifiers beyond 100 watts could overheat and damage the voice coil in the woofer.
Beyond that it's all about common sense: small room, small speakers; medium room, medium speakers, etc.
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A 10w amp won't hurt the tweeter of a speaker rated for 100w. The additional harmonics from driving to square waves contain trivially small power levels.
The problem is actually the 10w amp being driven so far into clipping that it sounds bad, hence the minimum recommended power rating.
Chris
thanks, that makes sense, but there is no mention on speaker size? i thought there might existed some iec standard, some rec'd woofer size/room size ratio
🙂
🙂 i think you are correct
yes, but that rule of thumbs do not apply to my latest room, which is larger then before but requires smaller woofers or less low freq output, so i wondered how mfr's specifies there speakers
These days speakers are not designed and tested in real listening rooms. They are, instead, the product computer simulations and anechoic chambers. By themselves they look, sound and behave brilliantly... putting them into a room is a total crap shoot.
Another change that has gone by almost without notice is that the backing wall against which speakers were once tested has vanished from the anechoic rooms as well. Where they once designed speakers to be placed as furniture, near walls and other furniture, now they are designed to be out in the middle of the room, more or less on their own.
Essentially... You get what the room gives you.
Given a required SPL the laws of physics dictate the size of the drivers. For a typical listening distance of 4m and undistorted 105 dB peak levels at the listening location (about the sound level in a cinema) a good 10" woofer might be OK but the rule of thumb in the 70s when I started was a 12" which is more likely to be 2 x 8" today.
This excludes the lowest frequencies covered by subwoofers which are a bit different. Peak levels tend to be significantly closer to average given low frequency musical information tends to be more sustained than transient. The room will have a growing influence as the sound wavelengths approach those of the room. Distributed subwoofers (the only practical way to obtain high quality sound at low frequencies in the home) will absorb a significant amount of sound requiring more cone area to achieve a desired SPL. 4 x 10-12" subwoofers is likely to be a reasonable ballpark but the room shape and size will have more of an influence than with the mains.