#31 in a series of articles about the technology behind Bang & Olufsen loudspeakers

 

 

 

from www.recordere.dk when they visited Struer for the BeoLab 20 launch.

#27 in a series of articles about the technology behind Bang & Olufsen loudspeakers

Introduction

To begin with, please watch the following video.

One thing to notice is how they made Grover sound near and far. Two things change in his voice (yes, yes, I know. It’s not ACTUALLY Grover’s voice. It’s really Yoda’s). The first change is the level – but if you’re focus on only that you’ll notice that it doesn’t really change so much. Grover is a little louder when he’s near than when he’s far. However, there’s another change that’s more important – the level of the reverberation relative to the level of the “dry” voice (what recording engineers sometimes call the “wet/dry mix”). When Grover is near, the sound is quite “dry” – there’s very little reverberation. When Grover is far, you hear much more of the room (more likely actually a spring or a plate reverb unit, given that this was made in the 1970’s).

This is a trick that has been used by recording engineers for decades. You can simulate distance in a mix by adding reverb to the sound. For example, listen to the drums and horns in the studio version of Penguins by Lyle Lovett. Then listen to the live version of the same people playing the same tune. Of course, there are lots of things (other than reverb) that are different between these two recordings – but it’s a good start for a comparison. As another example, compare this recording to this recording. Of course, these are different recordings of different people singing different songs – but the thing to listen for is the wet/dry mix and the perception of distance in the mix. Another example is this recording compared to this recording.

So, why does this trick work? The answer lies inside your brain – so we’ll have to look there first.

Distance Perception in the Mix

If you’re in a room with your eyes closed, and someone in the room starts talking to you, you’ll be pretty good at estimating where they are in the room – both in terms of angular location (you can point at them) and distance. This is true, even if you’ve never been in the room before. Very generally speaking, what’s going on here is that your brain is automatically comparing:

• the two sounds coming into your two ears – the difference between these two signals tells you a lot about which direction the sound is coming from, AND
• the direct sound from the source to the reflected sound coming from the room. This comparison gives you lots of information about a sound source’s distance and the size and acoustical characteristics of the room itself.

If we do the same thing in an anechoic chamber (a room where there are no echoes, because the walls absorb all sound) you will still be good at estimating the angle to the sound source (because you still have two ears), but you will fail miserably at the distance estimation (because there are no reflections to help you figure this out).

If you want to try this in real life, go outside (away from any big walls), close your eyes, and try to focus on how far away the sound sources appear to be. You have to work a little to force yourself to ignore the fact that you know where they really are  – but when you do, you’ll find that things sound much closer than they are. This is because outdoors is relatively anechoic. If you go to the middle of a frozen lake that’s covered in fluffy snow, you’ll come as close as you’ll probably get to an anechoic environment in real life. (unless you do this as a hobby)

So, the moral of the story here is that, if you’re doing a recording and you want to make things sound far away, add reflections and reverberation – or at least make them louder and the direct sound quieter.

Distance Perception in the Listening Room

Let’s go back to that example of the studio recording of Lyle Lovett recording of Penguins. If you sit in your listening room and play that recording out of a pair of loudspeakers, how far away do the drums and horns sound relative to you? Now we’re not talking about whether one sounds further away than the other within the mix. I’m asking, “If you close your eyes and try to guess how far away the snare drum is from your listening position – what would you guess?”

For many people, the answer will be approximately as far away as the loudspeakers. So, if your loudspeakers are 3 m from the listening position, the horns (in that recording) will sound about 3 m away as well. However, this is not necessarily the case. Remember that the perception of distance is dependent on the relative levels of the direct and reflected sounds at your ears. So, if you listen to that recording in an anechoic chamber, the horns will sound closer than the loudspeakers (because there are no reflections to tell you how far away things are). The more reflective the room’s surfaces, the more the horns will sound further away (but probably no further than the loudspeakers, since the recording is quite dry).

This effect can also be the result of the width of the loudspeaker’s directivity. For example, a loudspeaker that emits a very narrow beam (like a laser, assuming that were possible) would not send any sound towards the walls – only towards the listening position. So, this would have the same effect as having no reflection (because there is no sound going towards the sidewalls to reflect). In other words, the wider the dispersion of the sound from the loudspeaker (in a reflective room) the greater the apparent distance to the sound (but no greater than the distance to the loudspeakers, assuming that the recording is “dry”).

Loudspeaker directivity

So, we’ve established that the apparent distance to a phantom image in a recording is, in part, and in some (perhaps most) cases, dependent on the loudspeaker’s directivity. So, let’s concentrate on that for a bit.

Let’s build a very simple loudspeaker. It’s a model that has been used to simulate the behaviour of a real loudspeaker, so I don’t feel too bad about over-simplifying too much here. We’ll build an infinite wall with a piston in it that moves in and out. For example:

Here, you can see the piston (in red) moving in and out of the wall (in grey) with the resulting sound waves (the expanding curves) moving outwards in the air (in white).

The problem with this video is that it’s a little too simple. We also have to consider how the sound radiation off the front of the piston will be different at different frequencies. Without getting into the physics of “why” (if you’re interested in that, you can look here or here or here for an explanation) a piston has a general behaviour with repeat to the radiation patten of the sound wave it generates. Generally, the higher the frequency, the narrower the “beam” of sound. At low frequencies, there is basically no beam – the sound is emitted in all directions equally. At high frequencies, the beam to be very narrow.

The question then is “how high a frequency is ‘high’?” The answer to that lies in the diameter of the piston (or the diameter of the loudspeaker driver, if we’re interested in real life). For example, take a look at Figure 1, below.

Figure 1 shows how loud a signal will be if you measure it at different directions relative to the face of a piston that is 10″ (25.4 cm) in diameter. Two frequencies are shown – 100 Hz (the blue curve) and 1.5 kHz (the green curve). Both curves have been normalised to be the same level (100 dB SPL – although the actual value really doesn’t matter) on axis (at 0°). As you can see in the plot, as you move off to the side (either to 90° or 270°) the blue curve stays at 100 dB SPL. So, no matter what your angle relative to on-axis to the woofer, 100 Hz will be the same level (assuming that you maintain your distance). However, look at the green curve in comparison. As you move off to the side, the 1.5 kHz tone drops by more than 20 dB. Remember that this also means that (if the loudspeaker is pointing at you and the sidewall is to the side of the loudspeaker) then 100 Hz and 1.5 kHz will both get to you at the same level. However, the reflection off the wall will have 20 dB more level at 100 Hz than at 1.5 kHz. This also means, generally, that there is more energy in the room at 100 Hz than there is at 1.5 kHz because, if you consider the entire radiation of the loudspeaker averaged over all directions at the same time the lower frequency is louder in more places.

This, in turn, means that, if all you have is a 10″ woofer and you play music, you’ll notice that the high frequency content sounds closer to you in the room than the low frequency content.

If the loudspeaker driver is smaller, the effect is the same, the only difference is that the effect happens at a higher frequency. For example, Figure 2, below shows the off-axis response for two frequencies emitted by a 1″ (2.54 cm) diameter piston (i.e. a tweeter).

Notice that the effect is identical, however, now, 1.5 kHz is the “low frequency region for the small piston, so it radiates in all directions equally (seen as the blue curve). The high frequency (now 15 kHz) becomes lower and lower in level as you move off to the side of the driver, going as low as -20 dB at 90°.

So, again, if you’re listening to music through that tweeter, you’ll notice that the frequency content at 1.5 kHz sounds further away from the listening position than the content at 15 kHz. Again, the higher the frequency, the closer the image.

Same information, shown differently

If you trust me, figures 1 and 2, above, show you that the sound radiating off the front of a loudspeaker driver gets narrower with increasing frequency. If you don’t trust me (and you shouldn’t – I’m very untrustworthy…) then you’ll be saying “but you only showed me the behaviour at two frequencies… what about the others?” Well, let’s plot the same basic info differently, so that we can see more data.

Figure 3, below, shows the same 10″ woofer, although now showing all frequencies from 20 Hz to 20 kHz, and all angles from -90° to +90°. However, now, instead of showing all levels (in dB) we’re only showing 3 values, at -1 dB, -3 dB, and -10 dB. ( These plots are a little tougher to read until you get used to them. However, if you’re used to looking at topographical maps, these are the same.)

Now you can see that, as you get higher in frequency, the angles where you are within 1 dB of the on-axis response gets narrower, starting at about 400 Hz. This means that a 10″ diameter piston (which we are pretending to be a woofer) is “omnidirectional” up to 400 Hz, and then gets increasingly more directional as you go up.

Figure 4 shows the same information for a 1″ diameter piston. Now you can see that the driver is omnidirectional up to about 4 kHz. (This is not a coincidence – the frequency is 10 times that of the woofer because the diameter is one tenth.)

Normally, however, you do not make a loudspeaker out of either a woofer or a tweeter – you put them together to cover the entire frequency range. So, let’s look at a plot of that behaviour. I’ve put together our two pistons using a 4th-order Linkwitz-Riley crossover at 1.5 kHz. I have also not included any weirdness caused by the separation of the drivers in space. This is theoretical world where the tweeter and the woofer are in the same place  – an impossible coaxial loudspeaker.

In Figure 5 you can see the effects of the woofer’s directivity starting to beam below the crossover, and then the tweeter takes over and spreads the radiation wide again before it also narrows.

So what?

Why should you care about understanding the plot in Figure 5? Well, remember that the narrower the radiation of a loudspeaker, the closer the sound will appear to be to you. This means that, for the imaginary loudspeaker shown in Figure 5, if you’re playing a recording without additional reverberation, the low frequency stuff will sound far away (the same distance as the loudspeakers), So will a narrow band between 3 kHz and 4 kHz (where the tweeter pulls the radiation wider). However, the materials in the band around 700 Hz – 2 kHz and in the band above 7 kHz will sound much closer to you.

Another way to express this is to show a graph of the resulting level of the reverberant energy in the listening room relative to the direct sound, an example of which is shown in Figure 6. (This is a plot copied from “Acoustics and Psychoacoustics” by David Howard and Jamie Angus).

This shows a slightly different loudspeaker with a crossover just under 3 kHz. This is easy to see in the plot, since it’s where the tweeter starts putting more sound into the room, thus increasing the amount of reverberant energy.

What does all of this mean? Well, if we simplify a little, it means that things like voices will pull apart in terms of apparent distance. Consonant sounds like “s” and “t” will appear to be closer than vowels like “ooh”.

So, whaddya gonna do about it?

All of this is why one of the really important concerns of the acoustical engineers at Bang & Olufsen is the directivity of the loudspeakers. In a previous posting, I mentioned this a little – but then it was with regards to identifying issues related to diffraction. In that case, directivity is more of a method of identifying a basic problem. In this posting, however, I’m talking about a fundamental goal in the acoustical design of the loudspeaker.

For example, take a look at Figures 7 and 8 and compare them to Figure 9. It’s important to note here that these three plots show the directivities of three different loudspeakers with respect to their on-axis response. The way this is done is to measure the on-axis magnitude response, and call that the reference. Then you measure the magnitude response at a different angle, and then calculate the difference between that and the reference. In essence, you’re pretending that the on-axis response is flat. This is not to be interpreted that the three loudspeakers shown here have the same on-axis response. They don’t. Each is normalised to its own on-axis response. So we’re only considering how the loudspeaker compares to itself.

Figure 7, above, shows the directivity behaviour of a commercially-available 3-way loudspeaker (not from Bang & Olufsen). You can see that the woofer is increasingly beaming (the directivity gets narrow)  up to the 3 – 5 kHz area. The midrange is beaming up above 10 kHz or so. So, a full band signal will sound distant in the low end, in the 6-7 kHz range and around 15 kHz. By comparison, signals at 2-4 kHz and 10-15 kHz will sound quite close.

Figure 8, above, shows the directivity behaviour of a 3-way loudspeaker we made as a rough prototype. This is just a woofer, midrange and tweeter, each in its own MDF box – nothing fancy – except that the tweeter box is not as wide as the midrange box which is narrower than the woofer box. You can see that the woofer is beaming (the directivity gets narrow)  just above 1 kHz – although it has a very weird wide directivity at around 650 Hz for some reason. The midrange is beaming up from 5kHz to 10 kHz, and then the tweeter gets wide. So, this loudspeaker will have the same problem as the commercial loudspeaker

As you can see, the loudspeaker with the directivity shown in Figure 9 (the BeoLab 5) is much more constant as you change frequency (in other words, the lines are more parallel). It’s not perfect, but it’s a lot better than the other two – assuming that constant directivity is your goal. You can also see that the level of the signal that is within 1 dB of the on-axis response is quite wide compared with the loudspeakers in Figures 7 and 8. The loudspeaker in Figure 7 not only beams in the high frequencies, but also has some strange “lobes” where things are louder off-axis than they are on-axis (the red lines).

When you read B&O’s marketing materials about the reason why we use Acoustic Lenses in our loudspeakers, the main message is that it’s designed to spread the sound – especially the high frequencies – wider than a normal tweeter, so that everyone on the sofa can hear the high hat. This is true. However, if you ask one of the acoustical engineers who worked on the project, they’ll tell you that the real reason is to maintain constant directivity as well as possible in order to ensure that the direct-to-reverberant ratio in your listening room does not vary with frequency. However, that’s a difficult concept to explain in 1 or 2 sentences, so you won’t hear it mentioned often. However, if you read this paper (which was published just after the release of the BeoLab 5), for example, you’ll see that it was part of the original thinking behind the engineers on the project.

 Addendum 1.

I’ve been thinking more about this since I wrote it. One thing that I realised that I should add was to draw a comparison to timbre. When you listen to music on your loudspeakers in your living room, in a best-case scenario, you hear the same timbral balance that the recording engineer and the mastering engineer heard when they worked on the recording. In theory, you should not hear more bass or less midrange or more treble than they heard. The directivity of the loudspeaker has a similar influence – but on the spatial performance of the loudspeakers instead of the timbral performance. You want a loudspeaker that doesn’t alter the relative apparent distances to sources in the mix – just like you don’t want the loudspeakers to alter the timbre by delivering too much high frequency content.

Addendum 2.

One more thing… I made the plot below to help simplify the connection between directivity and Grover. Hope this helps.

• #24 in a series of articles about the technology behind Bang & Olufsen loudspeakers

As you may already be aware, Bang & Olufsen makes headphones: the BeoPlay H6 over-ear headphones and the BeoPlay H3 earbuds.

If you read some reviews of the H6 you’ll find some reviewers like them very much and say things like  “…excellent clarity and weight, well-defined bass and a sense of openness and space unusual in closed-back headphones. The sound is rich, attractive and ever-so-easy to enjoy.” and “… by no means are these headphones designed only for those wanting a pounding bass-line and an exciting overall balance: as already mentioned the bass extension is impressive, but it’s matched with low-end definition and control that’s just as striking, while a smooth midband and airy, but sweet, treble complete the sonic picture.” (both quotes are from Gramophone Magazine’s April 2014 issue). However, some other reviewers say things like “My only objection to the H6s is their volume level is not quite as loud as I normally would expect.” (A review from an otherwise-satisfied on Amazon.com). And, of course, there are the people whose tastes have been influenced by the unfortunate trend of companies selling headphones with a significantly boosted low-frequency range, and who now believe that all headphones should behave like that. (I sometimes wonder if the same people believe that, if it doesn’t taste like a Big Mac, it’s not a good burger… I also wonder why they don’t know that it’s possible to turn up the bass on most playback devices… But I digress…)

For this week’s posting, I’ll just deal with the first “complaint” – how loud should a pair of headphones be able to play?

Part 1: Sensitivity

One of the characteristics of a pair of headphones, like a passive loudspeaker, is its sensitivity. This is basically a measurement of how efficient the headphones are at converting electrical energy into acoustical output (although you should be careful to not confuse “Sensitivity” with “Efficiency” – sensitivity is a measure of the sound pressure level or SPL output for the voltage at the input whereas efficiency is a measure of the SPL output for the power in milliwatts). The higher the sensitivity of the headphones, the louder they will play for the same input voltage.

So, if you have a pair of headphones that are not very sensitive, and you plug them into your smartphone playing a tune at full volume, it might be relatively quiet. By comparison, a pair of very sensitive headphones plugged into the same smartphone playing the same tune at the same volume might be painfully loud. For example,let’s look at the measured data for three not-very-randomly selected headphones at https://www.stereophile.com/content/innerfidelity-headphone-measurements.

Brand	Model	Vrms to produce 90 dB SPL	dBV to produce 90 dB SPL
Sennheiser	HD600	0.230	-12.77
Beoplay	H6	0.044	-27.13
Etymotic	ER4PT	0.03	-30.46

If we do a little math, this means that, for the same input voltage, the Etymotic’s will be 3.3 dB louder than the H6’s and the Sennheiser’s will be 14.4 dB quieter. This is a very big difference. (The Etymotic’s are 7.7 times louder than the Sennheisers!)

So, in other words, different headphones have different sensitivities. Some will be quieter than others – some will be louder.

Side note: If you want to compare different pairs of headphones for output level, you could either look them up at the stereophile.com site I mentioned above, or you could compare their data sheet specifications using the Sensitivity to Efficiency converter on this page.

The moral of this first part of the story is that, when someone says “these headphones are not very loud” – the question is “compared to what?”

Part 2: The Source

I guess it goes without saying, but if you want more out of your headphones, the easiest solution is to turn up the volume of your source. The question then is: how much output can your source deliver? This answer also varies greatly from product to product. For example, if I take four not-very-randomly selected measurements that I did myself, I can see the following maximum output levels for a 500 Hz,  0 dB FS sine tone at maximum volume sent to a 31 ohm load (a resistor pretending to be a pair of headphones):

Brand	Model	Vrms	dBV
Lenovo	ThinkPad T420	0.317	-9.98
Apple	iPhone 3Ds	0.89	-1.01
Apple	MacBook Pro	2.085	+6.38
Sony	CDP-D500	6.69	+16.51

In other words, the Sony is more than 26 dB (or 21 times) louder than the ThinkPad, if we’re just measuring voltage. This is a very big difference.

So, as you can see, turning the volume all the way up to 11 on different product results in very different output levels. This is even true if you compare iPod Nano’s of different generations, for example – no two products are the same.

The moral of the story here is: if your headphones aren’t loud enough, it might not be the headphones’ fault.

Part 3: The Details, French Law, and How to Cheat

So much for the obvious things – now we are going to get a little ugly.

Let’s begin the ugliness with a little re-hashing of a previous posting. As I talked about in this posting, your ears behave differently at different listening levels. More specifically, you don’t hear bass and treble as well when the signal is quiet. The louder it gets, the more flat your “frequency response”. This means that, when acoustical consultants are making measurements of quiet things, they usually have to make the microphone signal as “bad” as your hearing at low levels. For example, when you’re measuring air conditioning noise in an office space, you want to make your microphone less sensitive to low frequencies, otherwise you’ll get a reading of a high noise level when you can’t actually hear anything. In order to do this, we use something called an “weighting filter” which is an attempt to simulate your frequency response. There are many different weighting curves – but the one we’ll talk about in this posting is an “A-weighting” curve. This  is a filter that attenuates the low and high frequencies and has a small boost in the mid-band – just like you do at quiet listening levels. The magnitude response of that curve is shown below in Figure 1. At higher levels (like measuring the noise level at the end of a runway while a plane is taking off over your head), you might want to use a different weighting curve like a “C-weighting” filter – or none at all.

So, let’s say that you get enough money on Kickstarter to create the Fly-by-Night Headphone Company and you’re going to make a flagship pair of headphones that will sweep the world by storm. You do a little research and you start coming across something called “BS EN 50332-1” and “BS EN 50332-2“. Hmmmm… what are these? They’re international standards that define how to measure how loudly a pair of headphones plays. The procedure goes something like this:

1. get some pink noise
2. filter it to reduce the bass and the treble so that it has a spectrum that is more like music (the actual filter used for this is quite specific)
3. reduce its crest factor so your measurement doesn’t jump around so much (this basically just gets rid of the peaks in the signal)
4. do a quick check to make sure that, by limiting the crest factor, you haven’t changed the spectrum beyond the acceptable limits of the test procedure
5. play the signal through the headphones and measure the sound pressure level using a dummy head
6. apply an A-weighting to the measurement
7. calculate how loud it is (averaged over time, just to be sure)

So, now you know how loud your headphones can play using a standard measurement procedure. Then you find out that, according to another international standard called EN 60065 or EN 60950-1 there are maximum limits to what you’re permitted to legally sell… in France… for now… (Okay, okay, these are European standards, but Europe has more than one country in it, so I think that I can safely call them international…)

So, you make your headphones, you make them sound like you want them to sound (I’ll talk about the details of this in a future posting), and then you test them (or have them tested) to see if they’re legal in France. If not (in other words, if they’re too sensitive), then you’ll have to tweak the sensitivity accordingly.

Okay – that’s what you have to do – but let’s look at that procedure a little more carefully.

Step 1 was to get some pink noise. This is nothing special – you can get or make pink noise pretty easily.

Step 2 was to filter the noise so that its spectrum ostensibly better matches the average spectrum of all recorded and transmitted music and speech in the world. The details of this filter are in another international standard called IEC 60268-1.  The people who wrote this step mean well – there’s no point in testing your headphones with frequencies that are outside the range of anything you’ll ever hear in them. However, this means that there is probably some track somewhere that includes something that is not represented by the spectral characteristics of the test signal we’re using here. For example: Figure 2, below shows the spectral curve of the test signal that you are supposed to send to the headphones for the test.

Compare that to Figure 3, which shows an analysis of a popular Lady Gaga tune that I use as part of my collection of tunes to make a woofer unhappy. This is a commercially-available track that has not been modified in any way.

As you can see, there is more energy in the music example than there is in the test signal around the 30 – 60 Hz octave – particularly noticeable due to the relative “hole” in the response that ranges between about 70 and 700 Hz.

Of course, if we took LOTS of tunes and analysed them, and averaged their analyses, we’d find out that the IEC test signal shown in Figure 2 is actually not too bad. However, every tune is different from the average in some way.

So, the test signal used in the EN 50332 test is not going to push headphones as hard as some kinds of music (specifically, music that has a lot of bass content),

We’ll skip Step 3, Step 4, and Step 5.

Step 6 is a curiosity. We’re supposed to take the signal that we recorded coming out of the headphones and apply an A-weighting filter to it. Now, remember from above that an A-weighting filter reduces the low and high frequencies in an effort to simulate your bad hearing characteristics at quiet listening levels. However, what we’re measuring here is how loud the headphones can go. So, there is a bit of a contradiction between the detail of the procedure and what it’s being used for. However, to be fair, many people mis-use A-weighting filters when they’re making noise measurements. In fact, you see A-weighted measurements all the time – regardless of the overall level of the noise that’s being measured. One possible reason for this is that people want to be able to compare the results from the loud measurements to the results from their quiet ones – so they apply the same weighting to both – but that’s just a guess.

Let’s, just for a second, consider the impact of combining Steps 2 and 6. Each of the filters in both of these steps reduce the sensitivity of the test to the low and high frequency behaviour of the headphones. If we combine their effects into a single curve, it looks like the one in Figure 4, below.

At this point, you may be asking “so what?” Here’s what.

Let’s take two different pairs of headphones and pretend that we measured them using the procedure I described above. The first pair of headphones (we’ll call it “Headphone A”) has a completely flat frequency response +/- < 0.000001 dB from 20 Hz to 20 kHz. The second pair of headphones has a bass boost such that anything below about 120 Hz has a 20 dB gain applied to it (we’ll call that “Headphone B”). The two unweighted measurements of these two simulated headphones are shown in Figure 5.

After filtering these measurements with the weighting curves from Steps 2 and 6 (above), the way our measurement system “hears” these headphone responses is slightly different – as you can see in Figure 6, below.

So, what happens when we measure the sound pressure level of the pink noise through these headphones?

Well, if we did the measurements without applying the two weighing curves, but just using good ol’ pink noise and no messin’ around, we’d see that Headphone B plays 13.1 dB louder than Headphone A (because of the 20 dB bass boost). However, if we apply the filters from Steps 2 and 6, the measured difference drops to only 0.46 dB.

This is interesting, since the standard measurement “thinks” that a 20 dB boost in the entire low frequency region corresponds to only a 0.46 dB increase in overall level.

Figure 7 shows the relationship between the bass boost applied below 120 Hz and the increase in overall level as measured using the EN 50332 standard.

So, let’s go back to you, the CEO of the Fly-by-Night Headphone Company. You want to make your headphones louder, but you also need to obey the law in France. What’s a sneaky way to do this? Boost the bass! As you saw above, you can crank up the bass by 20 dB and the regulators will only see a 0.46 dB change in output level. You can totally get away with that one! Some people might complain that you have too much bass in your headphones, but hey – kids love bass. And plus, your competitors will get complaints about how quiet their headphones are compared to yours. All because people listening to children’s records at high listening levels hear much more bass than the EN 50332 measurement can!

Of course, one other way is to just ignore the law and make the headphones louder by increasing their sensitivity… but no one would do that because it’s illegal. In France.

Appendix 1: Listen to your Mother!

My mother always told me “Turn down that Walkman! You’re going to go deaf!” The question is “Was my mother right?” Of course, the answer is “yes” – if you listen to a loud enough sound for a long enough time, you will damage your hearing – and hearing damage, generally speaking, is permanent.  The louder the sound, the less time it takes to cause the damage. The question then is “how loud and how long?” The answer is different for everyone, however you can find some recommendations for what’s probably safe for you at sites that deal with occupational health and safety. For example, this site lists the Canadian recommendations for maximum exposure time limits to noise in the workplace. This site shows a graph for the US recommendations for the same thing – I’ve used the formula on that site to make the graph in Figure 8, below.

How do these noise levels compare with what comes out of my headphones? Well, let’s go back to the numbers I gave in Part 1 and Part 2. If we take the measured maximum output levels of the 4 devices listed in Part 2, and calculate what the output level in dB SPL would be through the measured sensitivities of the headphones listed in Part 1 (assuming that everything else was linear and nothing distorted or clipped or became unhappy – and ignoring the fact that the headphones do not have the same impedance as the one I used to do the measurements of the 4 devices… and assuming that the measurements of the headphones are unweighted on that website), then the maximum output level you can get from those devices are shown in Figure 9.

So, if you take the calculations shown in Figure 8 and compare them to the recommendations shown in Figure 7, then you might reach the conclusion that, if you set your volume to maximum (and your tune is full-band pink noise mastered to a constant level of  0 dB FS, and we do a small correction for the A-weighting based on the assumption that the 90 dB SPL headphone measurements listed above are unweighted ), then the maximum recommended time that you should listen to your music, according to the federal government in the USA is as shown in Figure 10.

So, if I only listen to full-bandwidth pink noise at 0 dB FS at maximum volume on my MacBook Pro over my BeoPlay H6’s, then the American government thinks that after 0.1486 of a second, I am damaging my hearing.

It seems that my mother was right.

Appendix 2: Why does France care about how loud my headphones are?

This is an interesting question, the answer to which makes sense to some people, and doesn’t make any sense at all to other people – with a likely correlation with your political beliefs and your allegiance to the Destruction of the Tea in Boston. The best answer I’ve read was discussed in this forum where one poster very astutely point out that, in France, the state pays for your medical care. So, France has a right to prevent the French from making themselves go deaf by listening to this week’s top hit on Spotify at (literally) deafening levels. If you live in a place where you have to pay for your own medical care, then you have the right to self-harm and induce your own hearing impairment in order to appreciate the subtle details buried within the latest hip-hop remix of Mr. Achy-Breaky Heart’s daughter “singing” Wrecking Ball while you’re riding on a bus. In France, you don’t.

Appendix 3: Additional Reading

ISVR Consulting’s page

Rodhe and Schwarz’s pdf manual for running the EN 50332 test on their equipment