Making a portable stereo ... should I port it?

[Doug[_3_]]

I'm making a stereo that is to be mounted on the back of my bike.
It's in a box that has internal dimensions of about 12" x 6" x 6" and
inside this you'll find a T-amp (it's rated at 100 watts/channel, but
I'm not going to power it anywhere near that high) and two 7 Ah lead
acid batteries. Input will come from an iPod that's outside the box.

It uses two Polk Audio db651 6.5" car speakers, one on either side.

I know it's not a good speaker design, but sound quality is one of the
lesser considerations (weight, size, battery life are also important)
-- and really, it's not that bad.

But I'm wondering if I should port it?

As I see it, there's a few advantages for porting --

-- better low frequency response

-- make it more efficient -- use less power for the same volume
(the power limited is limited, so this is a concern.)

-- if the batteries do vent some hydrogen when charged, it gives it
an easy place for it to escape. It also lets the amp cool a bit
easier, though it's so efficient and the power relatively low so
I don't think that's a big concern.

But the stuff on calculating port size is quite confusing. I imagine
I could calculate the frequency of the box (though the batteries and
amp inside the box are likely to confuse that calculation) and guess
at the Xmax values and go from there.

I could just guess, put in a port that's about as large as I have room
for, though if I understand it correctly, I want to tune the
diameter/length so that the wave coming out of the port is in phase
with the wave coming out of the front of the speaker -- so it *does*
matter. But wouldn't this calculation only be valid for a specific
frequency?

The calculators I find online talk about calculating things for
woofers and subwoofers. I guess the main speaker *is* a woofer. But
when the calculators as for a frequency ... is it the Helmholtz
resonator frequency for the cabinet I should use?

Maybe I just need to bite the bullet and find a book on speaker design
-- since it seems that that's exactly where I'm headed.

--
Doug McLaren,
--
Doug McLaren,
The dandelion swayed in the gentle breeze like an oscillating electric
fan set on medium.

[Dick Pierce[_2_]]

Doug wrote:
> I'm making a stereo that is to be mounted on the back of my bike.
> It's in a box that has internal dimensions of about 12" x 6" x 6" and
> inside this you'll find a T-amp (it's rated at 100 watts/channel, but
> I'm not going to power it anywhere near that high) and two 7 Ah lead
> acid batteries. Input will come from an iPod that's outside the box.
> ...
> As I see it, there's a few advantages for porting --
>
> -- better low frequency response

Only if the drivers and enclosure together are suitable for
a ported design.

> -- make it more efficient -- use less power for the same volume
> (the power limited is limited, so this is a concern.)

Porting an enclosure WILL NOT make the system any more
efficient. Not in the least. Ported systems CAN be more
efficient, not because they have ports, but because they
have drivers suited for the enclosure that result in
electromechanical parameters that mke the DRIVERS more
efficient to begin with. A more efficient driver designed
for a prted enclosure will result in a system with
exactly the same reference efficieny whether the port is
there or not.

> -- if the batteries do vent some hydrogen when charged, it gives it
> an easy place for it to escape. It also lets the amp cool a bit
> easier, though it's so efficient and the power relatively low so
> I don't think that's a big concern.
>
> But the stuff on calculating port size is quite confusing. I imagine
> I could calculate the frequency of the box (though the batteries and
> amp inside the box are likely to confuse that calculation) and guess
> at the Xmax values and go from there.

Wrong.

> I could just guess, put in a port that's about as large as I have room
> for, though if I understand it correctly, I want to tune the
> diameter/length so that the wave coming out of the port is in phase
> with the wave coming out of the front of the speaker -- so it *does*
> matter.

Wrong.

> But wouldn't this calculation only be valid for a specific
> frequency?
>
> The calculators I find online talk about calculating things for
> woofers and subwoofers. I guess the main speaker *is* a woofer. But
> when the calculators as for a frequency ... is it the Helmholtz
> resonator frequency for the cabinet I should use?
>
> Maybe I just need to bite the bullet and find a book on speaker design
> -- since it seems that that's exactly where I'm headed.

You simply cannot just take a pair of speakers, jam them
in a box, stick a port in it, and expect 1) for it to work
reasonably well, 2) for it to have "better low frequency
response and 3) more efficiency. It simply does not work
that way.

A speaker is a SYSTEM composed of drivers, enclosures and
amplifiers.

If I were hired to do such a job, the first thing I'd
do is sit down and try to come to an agreement about
what constitutes "efficient" and "good low frequency
response". The third factor is enclosure size, but you've
already constrained that, so the only two variables you
have left are efficiency and low frequency cutoff. Things
like XMax play no role at this stage: it's on;y relevant
in determining the MAXIMUM output.

And the conversation would go something like this: you
have presented me with a MAXIMUM enclosure size of 0.25
cubic feet, and maybe more like .15 cubic feet considering
the batteries. That's all of 5 liters. And you're going to
put TWO drivers in there, meaning the effective volume is
half that, 2.5 liters. And, at best, you're talking 5"
drivers. I'd stay with 4"

Now, the classic Thiele-Small efficiency/bandwidth/size
rule takes over and constrains reality. SP you have 2.5
liters per driver. The MOST your are likely to find in
the efficiency for a 4" driver is on the order of 89dB
SPL @1W/1m. That means, the absolute BEST you can expect
for a low-frequency cutoff is about 90 Hz. And that's
only with a driver PERFECTLY suited to the application.
Not just any 'ol driver. It would have to have T/S
parameters like and Fs of 90 Hz, Vas of 2.6 L,
Qt of around 0.4, and that would require a cone mass
of nearly 12 grams with magnet Bl product of 7 N/A.
That's a REAL tough driver to build.

And yes, then you'd stick two of them in the box, and
you'd stick two ports in, let's say you made them out
of 1" PVC, each about 2 7/8" long.

It's doable, but not the way you want to go.

Then again, just sticking what you have in the box
may well be good enough for a bike. That's for
you to decide.

--
+--------------------------------+
+ Dick Pierce |
+ Professional Audio Development |
+--------------------------------+

[Doug McLaren]

On 2011-08-12, Dick Pierce > wrote:

|> -- make it more efficient -- use less power for the same volume
|> (the power limited is limited, so this is a concern.)
|
| Porting an enclosure WILL NOT make the system any more
| efficient. Not in the least. Ported systems CAN be more efficient,
| not because they have ports, but because they have drivers suited
| for the enclosure that result in electromechanical parameters that
| mke the DRIVERS more efficient to begin with. ...

So I have even more to learn about this than I thought -- and I
thought I had a lot to learn before.

I'll go ahead and not add a port then.

| You simply cannot just take a pair of speakers, jam them
| in a box, stick a port in it, and expect 1) for it to work
| reasonably well, 2) for it to have "better low frequency
| response and 3) more efficiency. It simply does not work
| that way.

Sorry I used the wrong terminology. I was under the mistaken
impression that adding a port could help alleviate the limitations
imposed by the overly small case to some degree -- especially for
lower frequencies -- but at the cost of some sound quality.
(Obviously it's not so simple.)

And then I went looking for instructions on how speakers are typically
designed and got lost in all the calculators available and such.

| A speaker is a SYSTEM composed of drivers, enclosures and
| amplifiers.

Everything is a trade-off ... that much I understand. And I know that
keeping it small is a serious limitation, but going much bigger
becomes difficult.

| If I were hired to do such a job, the first thing I'd do is sit down
| and try to come to an agreement about what constitutes "efficient"
| and "good low frequency response".

My goal is five hours run time, with the volume being as loud as it'll
go and still achieve that goal with the energy available. At least I
assume that the battery capacity is the limitation -- it may not be.
The batteries should be able to provide 20 watts for 8 hours if the
rated capacity can be trusted, though in my experience it rarely can.

(It's hard to quantify just how "loud" I want it to be -- after all,
this thing is going to only be a few feet behind me.)

"Good low frequency response" was just me using the wrong terms. I
figured that the small case size would hurt low frequencies more than
higher frequencies, though I guess what I really meant was "sounds
good".

| Things like XMax play no role at this stage: it's on;y relevant in
| determining the MAXIMUM output.

It didn't seem terribly relevant to me either, but (some of) the
calculators were asking for it.

| Then again, just sticking what you have in the box may well be good
| enough for a bike. That's for you to decide.

It does work pretty well so far. I just figured I could make it work
a little better by going a little beyond "jamming some speakers in a
box".

--
Doug McLaren,

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[gregz]

Doug > wrote:
> I'm making a stereo that is to be mounted on the back of my bike.
> It's in a box that has internal dimensions of about 12" x 6" x 6" and
> inside this you'll find a T-amp (it's rated at 100 watts/channel, but
> I'm not going to power it anywhere near that high) and two 7 Ah lead
> acid batteries. Input will come from an iPod that's outside the box.
>
> It uses two Polk Audio db651 6.5" car speakers, one on either side.
>
> I know it's not a good speaker design, but sound quality is one of the
> lesser considerations (weight, size, battery life are also important)
> -- and really, it's not that bad.
>
> But I'm wondering if I should port it?
>
> As I see it, there's a few advantages for porting --
>
> -- better low frequency response
>
> -- make it more efficient -- use less power for the same volume
> (the power limited is limited, so this is a concern.)
>
> -- if the batteries do vent some hydrogen when charged, it gives it
> an easy place for it to escape. It also lets the amp cool a bit
> easier, though it's so efficient and the power relatively low so
> I don't think that's a big concern.
>
> But the stuff on calculating port size is quite confusing. I imagine
> I could calculate the frequency of the box (though the batteries and
> amp inside the box are likely to confuse that calculation) and guess
> at the Xmax values and go from there.
>
> I could just guess, put in a port that's about as large as I have room
> for, though if I understand it correctly, I want to tune the
> diameter/length so that the wave coming out of the port is in phase
> with the wave coming out of the front of the speaker -- so it *does*
> matter. But wouldn't this calculation only be valid for a specific
> frequency?
>
> The calculators I find online talk about calculating things for
> woofers and subwoofers. I guess the main speaker *is* a woofer. But
> when the calculators as for a frequency ... is it the Helmholtz
> resonator frequency for the cabinet I should use?
>
> Maybe I just need to bite the bullet and find a book on speaker design
> -- since it seems that that's exactly where I'm headed.
>
> --
> Doug McLaren,

I would suggest getting a woofer that actually has specs, and use only one,
or preferably isobaric, using two front to front, which work better in
small enclosures, each having a qts something like .6 sealed box, and use
separate tweeters.

But, you just will not get low bass outdoors, especially using a small
configuration, and I don't know if the above is worth the trouble.

Greg

[Dick Pierce[_2_]]

Doug McLaren wrote:
> On 2011-08-12, Dick Pierce > wrote:
>
> |> -- make it more efficient -- use less power for the same volume
> |> (the power limited is limited, so this is a concern.)
> |
> | Porting an enclosure WILL NOT make the system any more
> | efficient. Not in the least. Ported systems CAN be more efficient,
> | not because they have ports, but because they have drivers suited
> | for the enclosure that result in electromechanical parameters that
> | mke the DRIVERS more efficient to begin with. ...
>
> So I have even more to learn about this than I thought -- and I
> thought I had a lot to learn before.
>
> I'll go ahead and not add a port then.
>
> | You simply cannot just take a pair of speakers, jam them
> | in a box, stick a port in it, and expect 1) for it to work
> | reasonably well, 2) for it to have "better low frequency
> | response and 3) more efficiency. It simply does not work
> | that way.
>
> Sorry I used the wrong terminology. I was under the mistaken
> impression that adding a port could help alleviate the limitations
> imposed by the overly small case to some degree -- especially for
> lower frequencies -- but at the cost of some sound quality.
> (Obviously it's not so simple.)

It CAN, but you have to know a bit more about
the specific physical parameters of the components
of the systems to 1) determine if it''s possible,
2) figure out how to do it and 3) know when you've
done it.

> My goal is five hours run time, with the volume being as loud as it'll
> go and still achieve that goal with the energy available. At least I
> assume that the battery capacity is the limitation -- it may not be.
> The batteries should be able to provide 20 watts for 8 hours if the
> rated capacity can be trusted, though in my experience it rarely can.

Let's assume it can. Let's also assume your T-Amp has
an efficiency of, oh, 75%. If you assume the 20 watts
figure as your draw, then you can reasonably assume only
15 of those watts could possibly make it to the speakers.

And let's further assume the speakers are ideally suited
to the amp, in terms of havimg an effective impedance
that draws the most effective power from the amplifier.

Now, there is a notable lack of real and reliable technical
information on the drivers, but let's assume my original
guesstimate of a reference efficiency of about 89 dB is
TRVTH (tm) (and so it shall remain until B3TT3R TRVTH
comes along). That means that, at best, your system could
produce on the order of about 101 dB or so AT BEST 1 meter
on axis of the drivers. And the actual average sound level
will be substantially less than that, unless your only
musical interest is ultra-compressed pop stuff.

So, again, you have to come up with some acceptable
definition of "loud" that can be turned into some real
design goals. "Loud" doesn't make it.

And, lastly, making it as loud as possible for you also
makes it, likely, TOO loud for those near you who don't
share you particular musical and audio proclivity.

--
+--------------------------------+
+ Dick Pierce |
+ Professional Audio Development |
+--------------------------------+

[Trevor]

"Dick Pierce" > wrote in message
...
> And, lastly, making it as loud as possible for you also
> makes it, likely, TOO loud for those near you who don't
> share you particular musical and audio proclivity.

Presumably that's his point or he would simply use headphones/ear buds.

Trevor.

[Dick Pierce[_2_]]

Trevor wrote:
> "Dick Pierce" > wrote in message
> ...
>
>>And, lastly, making it as loud as possible for you also
>>makes it, likely, TOO loud for those near you who don't
>>share you particular musical and audio proclivity.
>
>
> Presumably that's his point or he would simply use headphones/ear buds.

Save that depending upon the jurisdication and the type
of vehicle, it may be illegal to wear any such applicances
while, alledgedly, controlling the vehicle.

Then again, in some jurisdictions, it may also be illegal
for the vehicle to generate excessive sound pressure levels
in a public area. Or, maybe, it should be.

--
+--------------------------------+
+ Dick Pierce |
+ Professional Audio Development |
+--------------------------------+

[Trevor]

"Dick Pierce" > wrote in message
...
>>>And, lastly, making it as loud as possible for you also
>>>makes it, likely, TOO loud for those near you who don't
>>>share you particular musical and audio proclivity.
>>
>> Presumably that's his point or he would simply use headphones/ear buds.
>
> Save that depending upon the jurisdication and the type
> of vehicle, it may be illegal to wear any such applicances
> while, alledgedly, controlling the vehicle.

Good point. Since it's compulsory to wear a helmet on a bike here (both
pedal and motor) wearing earbuds within your helmet is fairly common.
Whether it's a good idea on the road is another matter, but I doubt it's any
worse than having the same music SPL at your ears, just because it's coming
from speakers. If the level is high enough it will mask most external sounds
in any case. Something easily possible with most cars if you have both the
stereo and windows up.

> Then again, in some jurisdictions, it may also be illegal
> for the vehicle to generate excessive sound pressure levels
> in a public area. Or, maybe, it should be.

It is here, but it may be hard for him to reach that limit with anything he
can carry on a bike (as he mentioned) Possible on a motorbike however, and
with a car it's not at all difficult.

Trevor.