Both the inches and the Watts are suggestive specifications of the following:
- Inches - The approximate diameter of the bass driver cone (if there is more than one driver, like in multi-way design). It is an approximation since some designs spell 8'' for what is really 7.8''. This spec vaguely correlates to the lowest frequency the speaker can produce.
- Watts - The power rating of a speaker/amp is a measure the safety power threshold of the device. Roughly correlates to the maximum output level it can produce.
The reason these are suggestive, is that they don't really tell us the important figures - they just suggest what these may be.
The Important Figures
In the spec sheets of a speaker you may find what you are really looking for, although these specifications can be manipulated by manufacturers to their advantage. This is either by means of the system used for measurements, or by the way information is presented. The specifications are:
The frequency response shows the range of frequencies the speaker can produced. This figure is always a measure that involves some deviation from a mean within a particular window of error (whether the error is presented or not). Often we say that a speaker has "flat" frequency response within a specific range, where "flat" means a certain error is allowed (±3dB, for instance).
For example, a speaker may show a spec like this:
This means that across the quoted frequency range the level of all frequencies are never 3dB more or less a chosen level (alternatively all frequencies are within a 6dB level window).
The speaker will still produce frequencies below (and above) the quoted range, but these will gradually decrease in level and will be lower (very rarely higher) by 3dB from frequencies in the quoted range.
The way manufacturers cheat is that a speaker with:
response, is given as:
The specification is still valid, but 30Hz may be produced at a level 10 dB below, say 80Hz.
The error margin is typically:
- ±3dB - High end and respectable manufacturers.
- ±5dB - Acceptable error margin.
- ±10dB - The manufacturer is likely to be cheating. Beefing their spec with a higher error margin.
- No error margin - The manufacturer is likely to be cheating.
Maximum Sound Pressure Level (Max SPL)
Given in dBSPL, typically at 1 meter away from the speaker, this is an indicator of how loud the speaker can get.
For studio speakers, 100-120 dBSPL is normal. PA speakers are often louder than this.
Again, manufacturers can cheat here. For instance a peak measurement will be higher than an RMS one; it is common practice to show Max SPL in peak, but RMS correlates to our loudness perception more than peak. Also, manufacturers may perform tests using a half-space setup (speaker close to one wall), which will increase the figure by approximately 3dB; so unless the full test specifications are mentioned, it's hard to tell the exact performance or to compare it.
What is crucial to mention is that all loudspeakers are essentially a calculated compromise between various factors like:
- Output Power
- Spectral Linearity
- Efficiency (how much input power translates into output power, with the rest converted into heat - either electronically or acoustically).
- Accuracy - For instance, do bass frequencies at the output linger much or little longer than the bass frequencies at the input.
In simple terms, this is a measurement of how much work the speaker can perform - the more work, the more air mass it can displace and thus the louder the output will be.
But there are many other factors that will affect this. The class of the amplifier design affects the efficiency of the amplifier, thus with some designs you can get higher output levels with less input power.
Cone Size (Diameter)
To reproduce a particular frequency, all that a cone needs to do is oscillate at that frequency, regardless of its size.
Reproducing loud low frequency sounds requires large displacements of air mass. There are two principals ways to achieve this:
- High Excursion - high amount of cone displacement (from its resting position).
- Bigger Cone Size - bigger cones will move more air.
There are many tradeoffs between the two. A small cone with high excursion will require less power to move, but it will also exhibit high distortion level and low efficiency (due to reduced acoustic impedance) compared to a bigger cone with lower excursion.
It is important to notice that the task is to produce "Loud Low Frequencies" - so the cone size also relates to output levels. That's why iMax theatres employ momentous subs, whereas the dubbing theatres in which films are mixed employ smaller subs.
But this is an over-simplified explanation. Practically, each manufacturer tweaks these two variables together with many others, like magnet size, cone depth and the cabinet (enclosure) design itself. See this link for more.
But you can say that bigger cones allow a more efficient low-frequency reproduction.