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Amplifier Power Ratings & Speaker Sensitivity

We often hear comments about amplifier X does not have sufficient power to drive speaker Y or speaker Z requires minimum 200W to come alive. I am trying to figure out if there is a correlation between amplifier power ratings and speaker sensitivity in the prevention of over-driving of speakers when listening at high volume levels since stressing an amplifier beyond its intended output will cause the amp to clip that may subsequently cause damage to the coils of the speaker.

From what I have read, below is my understanding but please correct me if any of my interpretations are wrong.

1) For a manufacturer's stated sensitivity of say 86dB/W/m, 1W of power is required to produce 86dB SPL at a distance of 1m away from the drivers.

2) In the doubling of power, the SPL will increase by 3dB. For example :-
1W @ 86dB
2W @ 89dB
4W @ 92dB
8W @ 95dB
16W @ 98dB

3) For every distance added, the SPL will decrease by 6dB. For example :-
86dB @ 1m
80dB @ 2m
74dB @ 3m
68dB @ 4m
62dB @ 5m

4) Close placement to rear wall will boost 3dB in the bass

5) Close placement to a corner will boost 6dB in the bass

To determine whether an amplifier is capable of driving the speakers at a given loudness and distance without causing damage to either the speakers and/or the amp, can we use the power rating of the amp and the sensitivity of speakers as a basis in controlling the level of loudness we are getting from the system? Let say a 50W amp is used to drive a speaker with 86dB sensitivity and the listening distance is approximately 4 meters away. Can we work backwards from the peaks we are getting in the SPLs from the listening position? With the assumption of 90dB as the absolute peak SPL when listening at insane volume levels at 4 meters away(-18dB){at least in my case although I am aware some folks listen past 100dB}, the SPL at 1m away is 72dB.

At 4 meters listening distance,
1W @ 72dB
2W @ 75dB
4W @ 78dB
8W @ 81dB
16W @ 85dB
32W @ 88dB
64W @ 91dB

64W of power is required to produce 91dB SPL. If using lesser powered amplifiers of 60W and below, the amp will have potential to go into clipping. If listening at 88dB SPLs(peaks) and lower there would not be any concern on speaker or amplfier damage.

I do know that room conditions, amplifier gain, power supply and some other factors will play a part as well but the above should at least give a basic idea by providing a rough estimate on how loud should one be listening so as to prevent any damage to the components?

I understand that people with high-powered amps will not need to worry so much when cranking up the volume but it can be useful for some other folks who have flea-powered amps(especially tube amps), particularly those who like to listen loud.

Any advice or feedback would be most appreciated.
ryder
ryder OP
1,690 posts
 

09-26-09: Shadorne
At 4 meters this requires 88 + 12 = 100 db SPL from the speakers. From the amp this requires 32 watts. So you should be easily OK to reach these peak level without any problems of any sort from your amp or speakers.
Shadorne, there has been some changes. The peak SPL I am getting when listening at insane volume levels has increased to 96dB. My listening distance is now 3.5m(front plane of speakers to the ear). I have run a quick calculation. At 3.5m listening distance there would be an additional 10.5dB. The speaker would be producing 96 + 10.5 = 106.5dB at 3.5 meters away.

I have rechecked the specifications of my integrated. The power output is rated at 82W into 8 ohms and 127W into 4 ohms. My question is do we always take the figure going into 8 ohms although the impedance on the speaker will always be fluctuating from probably 4 to 8 ohms? An 82W amp(8 ohms) is only capable of 104.84dB, and the peak SPL of 106.5dB I am getting has slightly exceeded the maximum output of the amp. I reckon I am getting a slight distortion in the music listening at these levels.

Power output of amplifier at increasing SPL
1W @ 86dB
2W @ 89dB
4W @ 92dB
8W @ 95dB
16W @ 98dB
32W @ 101dB
64W @ 104dB
82W @ 104.84dB (extrapolated)
128W @ 107dB

I guess my amp may be working at borderline at a peak SPL of 106.5dB. This is only on the amplifier. I am not sure whether the speakers can handle this peak SPL or not. I need to check with the designer before I overstress the woofers.

Thanks in advance.
ryder OP
1,690 posts
 
The designer for Harbeth, Alan Shaw cannot factually answer the question of the maximum SPL the speakers can handle and has advocated good listening habits in respecting our ears. Harbeth speakers were said to have been designed to sound warm and natural at moderate volume levels.

As such, I have rearranged my listening position to be 3 meters away from the speakers. The reduced listening distance has brought down the peak SPL to about 93dB. With this arrangement the additional 9dB from speakers would produce 93 + 9 = 102dB at 3 meters away.

I'm covered now on both the amplifier and speaker end(I think the 8" driver of the SHL5 must be able to handle up to 102dB since the max SPL of the ATC SCM7 with 5" driver is up to 103dB).

No more issues.
shadorne's avatar
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shadorne
7,477 posts
 
I think the 8" driver of the SHL5 must be able to handle up to 102dB since the max SPL

I would be pretty sure it can handle that SPL at 1 meter. My guess is that the max speaker output will be around 105 db SPL at 1 meter (with still some headroom for transients at certain frequencies).

If 96 db spl at the listening position sounds insanely loud then you probably are getting distortion at that level.
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