Wednesday, November 13, 2013

How Loud is it - Sound pressure requirements for movies and music



 
We see that dimensioning a sound reproduction system for at least 120dB output capacity is reasonable and not overkill if realistic sound is the goal.


So, how loud is it.

What kind of sound level exposure is to be expected, and what sound pressure level should be used for system requirements.

I did some measurements while playing music and movies, to show exactly how loud it is, at a reasonably loud level, and a little bit louder. This information can then be used to find requirements for loudspeakers, and also give a better understanding of why sound quality often is compromised.  

All measurements are done on a calibrated system. Using the -20dB RMS pink noise method and verified on frequency response measurements the calibration accuracy is within +-0.5dB, -20dB RMS equals 85dB from each main front speaker, 0dB peak level equals 105dB peak SPL.

First, some music.

Played at 0dB, we can see the RMS sound pressure level varies from 75dB to 95dB, while peak levels approach 113dB. Not perceived as very loud, most will find it this to be a pleasant volume, and the bass has some physical impact and realism.

At around time 02:30 level is increased to +8dB, and now it is quite loud, with RMS level above 100dB, and we can observe peak levels above 120dB.

Music playback at 0dB and +8dB, good recording



Another example, showing some of the problems with reduced sound quality in most newer recordings.

First a good old one, then a newer from 2009, same artist, all played at 0dB.

We see that the peak levels are the same, but the new recording is more compressed and has a louder RMS level, about 5dB higher. This higher RMS level means it is required to turn it down 5dB to maintain the same loudness, the problem then is of source that the peaks are also reduced by 5dB, and all the punch and impact and realism is gone.

This recording shows the same peak level as the first example, around 113dB.

Music - old and new, 0dB


The difference between the peak and RMS level is sometimes referred to as the "crest factor", a number that tells us something about how dynamic a piece of music is. Good recordings often have a crest factor of 20dB or more, while new popular music can have less than 10dB.

The music examples analyzed here tells us that loudspeakers must be dimensioned for at least 113dB if we want to play music at around 90-95dB sound pressure level, and if we want to play loud, more than 120dB can easily be required.

It is only peaks and transients of very short duration that reach those maximum levels, but the reproduction system must be dimensioned to be able to reproduce those peaks, or else the quality of the sound reproduction will be compromised. If the loudspeakers are not able to handle the peaks it will first start to sound more flat and the physical impact of drums and sound effects will be missing, and if you turn it up even further, it will start to sound bad due to distortion.

Note that the black RMS lines in the graphs is SPL-dB(Z), meaning full-bandwidth measurements, actual sound level exposure for comparison to noise level standards are much lower when C or A frequency weighting is used.

Now, a movie.

Master & Commander, DVD-version with the best sound, the cannon scene in the middle of the film. Here we see that the cannons peak out far above 120dB (124dB peak), but the RMS level is mostly at 80-95dB. This scene, played at 0dB as shown here, is not excessively loud, but the experience is realistic and powerful.

Movie at 0dB, Master & Commander action scene


The conclusion is that for realistic sound reproduction of music and movies it is required to have output capacity well above the average listening level, at least 20dB more for most good source material. The requirements given in the article How to set up a Home Theater Sound System, and numbers that can be found using the Audio Calculator are realistic and valid requirements, and dimensioning for less capacity will lead to compromised sound reproduction.

This also gives us some clues for reasons why many claim it sounds "too loud" playing a move at 0dB, because the speakers may not be properly dimensioned and will distort the sound, and distorted sound is perceived to be "too loud".

Other factors that affect perceived loudness is amount of early reflections and reverberation.
The room needs to have acoustic properties suitable for sound reproduction for higher sound levels to be experienced as pleasant and not too loud.


Friday, July 19, 2013

Audio Calculator for Speaker Requirements

I have now completed the first version of the Calculator for Speaker Requirements, for home theater and sound reproduction.

By entering data for room and speakers, the sound pressure level can be estimated to verify if the speaker configuration has sufficient output to meet the chosen required listening level.

A graphical presentation shows the current status, and provides point and click access to each speaker configuration section.
Speakers that do not meet requirements are indicated, it is easy to get an overiew of the current situation.

Speaker Calculator status image
 




The most difficult part was to decide how to calculate the reverberant sound field contribution - the sound that is added by the room itself.
The chosen method is approximately valid for relatively small rooms that are acoustically well damped, with reasonably normal height-length-width relationship.
For larger spaces or odd room shapes the model may be less accurate.

With this tool it is quite straight-forward to check whether a chosen speaker set will be usable, and also see how much amplifier power is needed.

This is very useful for subwoofer configuration - add or remove different subwoofers and see the results immediately.

It is easy to see that the most difficult requirement is the subwoofer.
Reference level with a nice house-curve lift at the lowest frequencies will require massive output capability, most likely several powerful subwoofers must be used.

Tooltip provides additional information for many of the different data items.

Master Volume Listening Level

The loudest listening level is selected, this forms the foundation for the rest of the requirements.

Level can be chosen from a set of predefined values.
There is a reason for not using just a number to be entered - the range of values now reflect the level of accuracy one can expect, there are also descriptions for the different levels, and steps of 3 - 6 - 10dB are appropriate, as these levels provide suitable steps in loudness to span the range from reference (quite loud) to a lower -20dB.

Now, these are meant to be the systems maxiumum design requirement - that does of course not mean that one always must listen at these volumes.
Even -20dB may be too loud sometimes, and for quiet movie or documentary both the dialogue and sound effects should be clear and easy to understand at, say, -40dB.
But you don't dimension the home theater for that.

Room data

Dimensions and acoustic data for the room is used to calculate the resulting sound.

Here the reverberation time T60 may cause some uncertainty, regarding what value should be entered.
If the theater has not been built yet, how can you measure it.
The answer is, you do not need to measure it, you determine the value instead, and thereafter fix the room with acoustic treatments to reach that value.

A T60 around 0.25 or lower is nice for the smallest rooms, 40-60m3, or 12-20m2.
Larger rooms have longer reverberation, 0.3 and up to 0.4 can still be usable for rooms up to 40-60m2.

Speakers and room

Distance from listening position to speakers determines how much attenuation there is in the direct sound.

Speaker directivity is the radiation pattern of the speaker:
 1: Omnidirectional 4π radiating. Subwoofer or normal speaker at low frequencies.
 2: 2π radiating. Recommended for wide dispersion surrounds (180°).
 3: Dipole, and normal radiating. Recommended for mains L,R with low directivity - direct radiating mid and tweeter. 
 4: 45° radiating. Recommended for mains L,R with higher directivity - horns, waveguides.

The critical distance, calculated based on room data and speaker directivity, is displayed.
This is the distance where the reverberant field and the direct radiated sound are equally loud.
Beyond this distance, the sound from reflections around in the room will be louder than the sound that comes directly from the speaker.

Subwoofer configuration

Defines how to calculate subwoofer sound pressure level.
When main and surround speakers are set to Small - routing the sub frequencies to the subwoofer, the required capacity for the sub must include the added signals.

House curve means a customized frequency response target with a boost at the very low end, starting typically at around 50-100hz, full level at 20Hz.
Clearly this will also add to the required spl form the subwoofers.

Since the main and surround channels are very unlikely to have full level low-frequency content, the level of contribution can be adjusted.

Loss means correction from 1m/2π specification to in-room at listening position.
The actual correction will vary according to room construction and size, and subwoofer location.
A range of typical values can be selected, and when set to "Use distance" the spl is calculated using reverberant field and room data.

Speaker Requirements

Sensitivity for the speakers must be specified, as measured at 1m with 2.83V.
Often specified as dB SPL/1m/1W, as 2.83V equals 1W into 8 ohm load.
Some speakers are 4 ohm, and in some cases sensitivity is given for 1W into 4 ohm, then add 3 dB and you have the sensitivity for 2.83V.

Actual SPL at listening position is calculated using the entered Actual Power.
The Power calculated is the required power to reach the required spl.

Subwoofer

Four different subwoofer types can be selected, in any reasonable number.
The configuration table allows for quick and easy overview of different subwoofer configurations.

For subwoofer specification, the maximum spl at 20Hz, measured at 1m distance 2π (groundplane) must be used.
Make sure the value you use is a real, measured value, and not some marketing number.
Most subwoofers will be a lot more powerful at higher frequencies, like 60Hz, than at the lowest 20Hz and below, so be aware of getting the right spl number, you want the one for 20Hz.
If the specification is for 2m groundplane, just add 6dB to that value to get the 1m/2π.

For existing and new installations

The calculator is useful both for new and existing installations.
Due to flexibility in what data may be entered, there will always be a way to get approximate information about performance.

Monday, July 1, 2013

More on Compact Subwoofer Technology

The Compact Subwoofer Technology is a design method for small horn subwoofers, presented in the article available on the Kvålsvoll Design web pages.
The article describes briefly what it is, and how it works, read the entire article there, it is also available for download as .pdf file.

Now I will discuss some of the more practical implications of the small horn
designs, and give some examples on what can be achieved, in real subwoofers.

Several subwoofers have been designed, and some of those have been built and
tested.

The very small 6-14 subwoofer was designed to be compact, at the expense of
low frequency output capacity.  Design specification, output capability: 30Hz,
100dB (1m/2pi).
The 6.14 subwoofer. A very compact 30Hz, 100dB design.

This means it is not a real, full bandwidth subwoofer, and can not play powerful
 bass in a larger or even medium sized room with the physical authority and feel that larger systems have.
However, the small size have some advantages, and the output is enough for low and mid volume tv or movie, and music.
With two of them in a small room they played loud enough to have a gentle physical impact on the cannons in the Master & Commander movie.
Being small and nice looking, it is easy to find a place for them, they will also often fit under a sofa.
Another advantage of small size is that they are easier to move around for measurements and testing.

Utilizing a small 6.5" driver, this design really pushes the limit for output from such a small woofer.

30Hz extension is not enough to get the solid weight of low frequency effects in movies, and nowadays there are lots of music that also benefit from extension down to below 20Hz.
The difference really has to be experienced.

The T138 (left) is a real subwoofer with true 20Hz capacity. As we can see, it comes with a price.


Now, compare the size of the 6-14 to the T138.  This is a real subwoofer.
Extension to well below 20Hz, design output 112dB/1m/2pi at 20Hz.

Small is a relative term.
Even though the compact subwoofer technology enables very good performance for the size, it still can not beat the physics of low frequency sound reproduction.
More output at lower frequencies will still require a larger box.

SPL requirements

How much output does one need, then.
Let us assume movie playback at reference level on a calibrated system, we can calculate the theoretical maximum sound pressure level from the subwoofers, when bass management routes all LF to the sub, including LFE track:

Source SPL ref (dB) N Sum N (dB)
L 105 1 105
R 105 1 105
C 105 1 105
SL 102 1 102
SR 102 1 102
RL 102 1 102
RR 102 1 102
LFE 115 1 115
Sum SPL (dB)

124.079116

We see that maximum sub output may actually be 124dB.
However, we can make some assumptions that lowers this number considerably.
First, it is unlikely that there is full level content at 20Hz and below.
And, if there are any other sounds playing at the same time in the front and surround channels, some of the headroom is used for that signal, and thus can not have low output at maximum.
For most of the time, we can assume it may not be necessary having more than 115dB capability.

But, since we would like to have some reserve headroom for a nice house-curve with some boost down at the lowest frequencies, that must be added.
Let us assume +6dB.
We still end up with 115dB + 6dB = 121dB.

To achieve this kind of output will require some powerful subwoofers.
We need the real thing.

4 decent subwoofers is no overkill..

Simulations of the S4 subwoofer, a nice, 62l net volume/97l external design, shows output capability at 108dB/20Hz/1m.
Around half the size of the T138 pictured above.
Now, assuming we loose 3dB on average in-room from the 1m output to th listening position, we see that 4 of these subwoofers can give us 117dB:


Source SPL ref (dB) N Sum N (dB) SPL diff LP1 (dB) SPL LP1 (dB)
S4 108 4 120.0412 -3 117.0412


0 0
0


0 0
0



0
0



0
0



0
0
Sum SPL (dB)

120.0412
117.0412

Also consider that the output increases some up in the mid-bass range.
This should be a good start for a nice subwoofer setup.

Advantage of small horn - the compact subwoofer technology

To achieve the same output with sealed box subwoofers would require more subwoofers, and a lot more power:

Source SPL ref (dB) N Sum N (dB) SPL diff LP1 (dB) SPL LP1 (dB)
Sealed 50l 100 10 120 -3 117


0 0
0


0 0
0



0
0



0
0



0
0
Sum SPL (dB)

120
117

We see that using a driver with similar parameters in sealed boxes requires 10 subwoofers, and that is not really practical.
(For sealed simulation: mms increased to 2x, mounted in 50l sealed enclosure, driven at same voltage level, excursion xmax = 16.8mm).

An alternative could be to use larger vented boxes, but bigger boxes are more difficult to place, likely it is easier to find space for four smaller boxes than one or two monster-cabinets.
There is another advantage to using several smaller subwoofers as well - when located spread around the room they tend to even out the frequency response.
But that is a different subject on its own, so we can get back to that later some time.



Friday, June 28, 2013

Audio, design, the making of things

Here I will present articles and notes about things I make, new technology, new designs.

The content will be related to audio design and technology, control systems and cybernetics, and design - the making of things. 

My Kvålsvoll Design company web page www.kvalsvoll.com is now updated, and will continue to receive new articles and designs.

I will release some audio related articles here as well, and you will be welcome to join in and comment on those.