Meinderts Hpd315 Project
User Manual: meinderts hpd315 project
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Meindert’s HPD315 project
History
Thirty years ago I bought a pair of HPD 315 speaker and build them into the Devon cabinet, one of the
enclosures in the construction guide of Tannoy. Ten years later I measured the TS-parameters and build,
using the original Small AES-paper, a 100 liter cabinet, a heavy triangular column 100 cm high. But, after a
while I changed to two B&W801 F’s, I sold a few years later because they didn’t sound good at all (too much
distortion in mid and high, and a thick bass). After I build the SEAS Odin MK3 (quite good), it was due to get
my Tannoy’s from the attic, repair the roll surround (see elsewhere) and build a new cabinet using the
modern measurement and simulation techniques the computer offers today.
The Plan
My personal design criteria are:
Active filtering: now easily done digitally with for example miniDSP
Small size, since my listening room is small (lowest fundamental 40Hz)
Simple, light but sturdy construction, to make it easily to build and to transport
The design procedure was as follows:
- Measurement of the frequency & phase response of the sound pressure
- Simulation of the speaker
- Designing of the speaker
- Tuning of the active filter
- Measurement of difference between passive and active filter
Measurement of the frequency & phase response of the sound pressure
I used the HobboyBox HBX V.6.5 measuring software with a iSEMcon EMM-8 calibrated microphone on my
DELL Studio laptop with good internal soundcard (24 bits, 48000kHz, THD<-70dB). The measurements were
done on a self made DIN 45575 standard baffle (165 x 135, speaker at 65 & 55 cm) on which I placed the
speaker in a 100 liter closed and moderate damped box.
Figure 1 Measuring a Tannoy HPD 315 on a DIN baffle with calibrated microphone using MLS & time windowing
I used the standard MLS-technique of the measurement software with a 1V noise signal and a 5 msec
measurement time window in order to make possible a semi-free field measurement under a low ceiling (2,6
meter) and a cramped space. The microphone was placed on axis at 1 meter measured from the baffle. The
results in a diagram in HBX V.6.5.0:
The woofer shows an interesting response: there is a dip between 1000 and 2000 Hz and has a steep peak at
2200 Hz. The response is only useful till 1000Hz. The waterfall spectrum shows like this:
Note the ringing of the woofer at a frequency around 2kHz. Even more interesting is the typical horn tweeter
response:
Figure 4 Amplitude (red) and phase (blue) of a HPD 315 Horn Tweeter on a DIN45575-baffle at 1 meter & 1V RMS
Figuur 2 Amplitude (red) and fase (blue) of a HPD 315 on a DIN45575-baffle at 1 meter & 1V RMS
Figure 3 A waterfall spectrum of a Tannoy HPD 315 Woofer on a DIN-baffle
With HBX you can export with it measurement points to be used as a input for the simulation on the BOXSIM
simulation program from Visaton. The data is set in steps of 150 Hz is:
Frequency Hz
Amplitude
Woofer dB
Absolute
Phase° woofer
Amplitude
Tweeter dB
Absolute
Phase°
Tweeter
150
84,0317
89,0833
56,5833
99,5905
300
85,0143
51,7021
69,6169
-46,0829
450
83,7141
44,821
79,8983
-83,0904
600
82,6962
39,1852
85,5849
-85,077
750
82,5677
35,2524
87,7585
-81,2497
900
82,591
35,5834
89,8359
-79,5615
1050
81,3322
36,9232
91,7805
-77,0611
1200
78,6538
29,4548
93,4815
-73,5094
1350
78,3203
12,2942
95,208
-68,4004
1500
81,8756
1,9884
96,7101
-60,5908
1650
84,2227
4,0812
97,2044
-52,2396
1800
85,837
10,9202
97,586
-45,0317
1950
86,1785
16,729
97,4913
-38,8633
2100
88,5742
25,4718
96,9107
-36,2472
2250
89,7168
43,0225
97,0453
-37,0036
2400
88,7109
61,9113
98,317
-35,972
2550
85,7669
72,6124
99,5719
-29,9004
2700
83,9098
73,9087
99,9861
-20,7375
Figuur 5 A waterfall spectrum of a Tannoy HPD 315 Tweeter on a DIN-baffle
Frequency Hz
Amplitude
Woofer dB
Absolute
Phase° woofer
Amplitude
Tweeter dB
Absolute
Phase°
Tweeter
2850
83,6772
74,3308
99,4905
-12,0265
3000
83,6293
78,2233
98,1774
-7,5593
3150
83,1959
83,8544
97,3375
-8,0581
3300
82,9866
91,5064
97,4184
-9,8615
3450
81,6866
100,1659
98,2589
-8,6886
3600
79,3604
103,5801
98,6546
-3,8065
3750
78,5478
103,5123
98,1696
0,941
3900
78,4081
107,4856
97,5518
2,9738
4050
76,9164
113,3141
97,3372
3,3799
4200
75,1397
116,3832
97,4868
4,1138
4350
73,497
117,4537
97,9334
6,9148
4500
71,7648
117,1228
97,9711
11,5734
4650
69,9131
114,4115
97,6858
16,2764
4800
68,6014
110,2524
97,384
21,0257
4950
67,2475
105,6449
96,671
25,4272
5100
65,8238
98,8228
95,8736
28,5836
5250
65,3872
91,4816
95,1496
31,2656
5400
65,0991
86,1708
94,1273
33,3324
5550
64,7069
81,8588
92,9533
33,6477
5700
64,57
78,8253
91,8748
32,1066
5850
64,1134
77,3318
91,0827
29,792
6000
63,2007
75,7445
90,2017
27,0211
6150
61,4966
70,496
89,1392
22,0637
6300
60,1679
58,7908
88,746
15,2792
6450
61,2898
47,8735
89,0452
9,817
6600
62,2416
42,9421
89,3748
6,8913
6750
63,1211
41,7587
89,5371
5,7363
6900
63,511
43,3633
89,2637
4,7462
7050
63,193
45,4201
88,8589
2,4195
7200
62,2987
45,1834
88,6412
-0,9815
7350
61,5247
41,7033
88,5732
-4,7768
7500
61,4996
37,7074
88,6789
-8,7625
7650
61,422
34,9812
89,245
-11,5652
7800
61,0398
31,5345
89,8831
-11,3363
7950
61,07
26,8961
89,6327
-9,9938
8100
61,7614
23,8659
88,6199
-12,4838
8250
62,4246
24,2376
88,3717
-19,2505
8400
62,1968
24,8381
89,201
-25,9045
8550
62,4358
24,4393
90,8146
-28,2798
Frequency Hz
Amplitude
Woofer dB
Absolute
Phase° woofer
Amplitude
Tweeter dB
Absolute
Phase°
Tweeter
8700
62,9202
25,725
92,2415
-24,9576
8850
63,4012
29,9761
92,5775
-19,2216
9000
63,1877
36,4102
92,414
-14,4329
9150
61,9637
41,3318
91,9796
-11,3131
9300
60,3804
41,4255
91,6343
-9,2865
9450
59,6126
38,0243
90,9515
-8,8574
9600
59,8219
35,9939
90,8422
-9,3225
9750
59,9772
37,2331
90,8318
-8,1994
9900
60,0041
40,9569
90,2467
-6,7537
10050
59,7117
46,907
89,4423
-6,8963
10200
58,6952
53,3921
88,3485
-9,995
10350
57,4516
59,3571
87,7367
-16,1047
10500
55,9586
65,746
87,5072
-24,4849
10650
53,9236
73,3452
88,9309
-30,9339
10800
50,3682
80,7612
90,6001
-30,5642
10950
42,3233
70,8506
91,4012
-25,3115
11100
35,5503
24,9625
91,2003
-19,3412
11250
44,7593
-6,8303
90,2428
-16,1428
11400
46,7577
-2,1598
89,4341
-16,2344
11550
46,3594
4,0792
88,9312
-17,4402
11700
44,6822
7,5581
88,0909
-20,2849
11850
41,8909
6,4703
87,7554
-25,2415
12000
36,3406
-11,5804
88,0941
-29,4492
12150
35,2147
-47,6217
88,6642
-30,5283
12300
42,3031
-65,4388
88,4854
-29,911
12450
45,562
-52,6158
87,5989
-31,8224
12600
43,5266
-34,0435
86,8543
-38,4268
12750
36,488
-33,0963
87,5667
-45,6444
12900
27,2124
-75,7796
88,6492
-48,7717
13050
35,9228
-113,798
89,1827
-49,1183
13200
36,2942
-121,506
89,106
-50,2542
13350
35,1647
-140,452
88,9721
-54,372
13500
37,4901
-164,525
89,504
-60,7211
13650
42,0076
-178,031
91,2849
-64,9382
13800
45,7887
-178,197
93,4239
-63,1184
13950
47,4365
-172,794
94,9317
-56,2123
14100
47,5004
-170,445
95,8609
-46,2272
14250
47,4554
-174,358
95,5687
-36,1687
14400
48,7308
-180,491
94,8037
-28,8348
Frequency Hz
Amplitude
Woofer dB
Absolute
Phase° woofer
Amplitude
Tweeter dB
Absolute
Phase°
Tweeter
14550
51,0721
-181,792
93,6115
-25,4296
14700
52,856
-176,412
92,9931
-25,0477
14850
53,1332
-169,545
92,3822
-26,9891
15000
53,1251
-164,175
93,5463
-26,3774
15150
52,4328
-160,746
93,4416
-22,0638
15300
51,4511
-161,182
93,3942
-18,2872
15450
50,7956
-165,469
93,5688
-12,5537
15600
51,486
-168,576
92,5868
-6,8813
15750
51,7379
-167,479
91,7708
-3,6601
15900
51,0207
-165,945
91,3651
0,6074
16050
49,7099
-167,005
89,6558
3,6587
16200
47,4755
-174,687
88,3303
2,5826
16350
46,0291
-191,345
87,8146
1,8458
16500
47,1259
-209,876
86,6559
2,0391
16650
49,5191
-222,521
84,9422
-1,1274
16800
52,5591
-226,231
83,881
-7,9472
16950
54,6811
-221,946
83,5684
-15,3615
17100
55,3162
-215,55
83,9219
-20,7377
17250
55,3876
-210,854
84,5737
-21,812
17400
55,361
-207,48
84,427
-19,6742
17550
55,1549
-204,758
83,4042
-17,9311
17700
54,6821
-203,543
80,6201
-25,5337
17850
55,0625
-200,794
81,6984
-37,2335
18000
54,3642
-195,629
83,594
-39,0459
18150
52,9153
-192,186
83,9358
-35,0158
18300
51,1073
-191,356
83,9898
-29,3323
18450
48,3348
-195,733
82,4636
-25,2588
18600
44,9677
-212,813
80,6198
-28,3458
18750
46,9287
-229,999
80,8457
-33,2493
18900
48,5063
-232,711
82,4939
-27,2686
19050
47,0648
-233,546
79,7701
-18,0867
19200
45,7229
-240,601
75,1173
-26,9577
19350
45,0465
-249,758
77,428
-35,4932
19500
43,7318
-263,022
75,558
-35,7624
19650
45,5384
-274,673
74,2924
-44,4212
19800
48,0363
-271,381
78,4803
-38,4685
19950
47,3961
-256,198
77,0377
-11,4954
Simulation of the speaker
In order to use the simulator properly, the simulation with Boxsim had to be calibrated first: the simulation
of the speaker build into the DIN-baffle had to give the same result as the measurement. The calibration
failed because of a few reasons (my informed guess): the actual passive filter I used was not the same as the
simplified filter I build into the simulator, and/or the acoustical centers of the speaker where not right (I did
not measure them). I experimented with different distances between the acoustical centers and I filter I
found on Hilberinks site, but I did not succeed in simulating the actual passive filtered speaker response.
So I took another route: I wanted to know the simulated effect of the enclosure on the response and than
choose the one with the smallest effect. Hence I loaded Boxsim with measuring points of an ideal speaker
woofer and tweeter with the bandwidth from 50 to 12000Hz. I filtered them actively at 1000 Hz and got the
best results with baffle of 80 x 50, the speaker placed 20 from the top and the baffle ends tapered.
Figure 6 Effect of the cabinet dimensions on a ideal loudspeaker response at the optimal box dimensions of about 80 x 50 cm
As you can see, the box dimensions and the active filter result into a lift between 500 and 1000 Hz, a dip
between 1000 and 2000Hz and a ripple beyond 2000Hz. The dip is especially nasty since the speaker itself
has an dip in this region, as you will see below. I will discuss this problem later on when I show the active
filter tuning. In the end I chose a cabinet of 63 x 41 x 32, for various reasons a explain in a minute, that gives
the following simulated response of figure if equipped with ideal speakers:
Figuur 7 Effect of the cabinet dimensions on a ideal loudspeaker response with baffle dimensions of 63 x 41 cm
Shaping the box
In choosing the exact dimensions I took in to account the following:
- The tweeter should be at ear height, that is 90 cm, so a small box on a standard or a big stand alone
box should work
- The cut off frequency of the speaker should be 40 Hz or higher, since the fundamental frequency of
my listening room is 40 Hz. Choosing a lower frequency is useless. If I want a speaker with ‘slam’ I
don’t need deeper basses, but a optimal timing between woofer and tweeter, so to produce shock
waves in my room.
The next two design criteria need some extra explanation:
A fast or a deep bass tuning?
Normally a Tannoy is build within a bass reflex box. This gives more bass in the frequency domain, but is
costs speed in the time domain, since a bass reflex box is in principle a box with a low frequency gong build
into it. A closed box design of 40 liter reacts fast (with Unified Box Model 408):
A optimal Butterworth bass reflex design of 100 liter reacts slower:
I still chose a bass reflex design of 70 liter with a cut off frequency of 40 Hz. If I wanted it to sound faster I
could simply fill the ports, giving the following approximated 70 liter closed box response:
Non-resonant internal dimensions
But what exact internal dimensions should the box have? There should be as little as possible standing waves
inside the box. So I calculated half the length of an standing wave of all the notes of western music. The
internal dimensions should be somewhere between these lengths:
Frequency
247
262
279
294
311
330
349
370
392
415
440
466
494
523
558
Half wave
length
0,695
0,656
0,615
0,584
0,552
0,521
0,491
0,464
0,438
0,413
0,390
0,368
0,347
0,328
0,307
Optimal
dimension
0,675
0,635
0,599
0,568
0,536
0,506
0,478
0,451
0,425
0,402
0,379
0,358
0,338
0,318
0,300
The internal dimensions chosen are 63,5 x 38 x 33,8 cm. At these dimensions also checked the higher order
resonances of the longer dimension at frequencies other than the western tuning, would not coincide with
the resonances in a shorter dimension. In the end the cabinet panels appeared remarkably silent.
Chipboard or MDF?
Normally you would take thick MDF to build a box. This would get very heavy, too heavy. A better choice is a
lighter but stronger cabinet using a internal board reinforcement, making the front baffle acoustically
practically dead. I choose 16 mm chipboard panels for a calculated weight of 18kg (with speaker) and one
internal board reinforce just below the speaker, damping of every bending waves coming from it and .
Chipboard? Yes, chipboard is better. I tested the vibration sound of both a chipboard and a MDF 12 mm 80 x
50 cm panel in an closed box with a small speaker inside(green line). The result, chipboard (red) won:
Tuning the active filter
After assembling the cabinet with the speaker and placing it in a measurement setting, I started tuning the
active filter via the USB link to a miniDSP active filter. Using a interfacing and communication software the
‘TWO WAY CROSSOVER PEQ’ can be tuned as required:
Figuur 8 Sound pressure 50 cm from 12 mm MDF (blue) and chipborard (red) panel
For this tuning some knowledge of filtering techniques is required, since filters interact on mysterious ways if
you haven’t got the knowledge. This time I use HOLMImpuls software to measure the results.
The problem
This response is not easily to be filtered. The woofer rolls of at 1000Hz, the cut off frequency set by Tannoy’s
passive filter, but after 1300 goes up and down again with a peak at 2200Hz, leaving us with a gap to fill with
.. sound from the tweeter. The tweeter has a typical horn response to be made flat. What to do?
The solutions
I adopted a principle in filtering: Ockham’s razor, or: less is more. Then, first the simplest part: the tweeter
rolls off at low frequency even without any filtering added. So I added only a real life capacitor of 30 uF
(2x15) only to protect the tweeter and giving it a 1st order high pass filter set at 500 Hz, the resonance filter
of the tweeter (it has resonance frequencies at about 2000, 8000 and 14500Hz ... ).
Second, I gave the woofer a second order low pass Bessel filter at 1000Hz. Bessel is a very moderate filter,
resembling a 2nd order Linkwitz Riley Filter (the LW rolls off faster, too fast here). More filter options are
possible, but I chose Bessel because of its moderate nature in the time domain. In the next step the tweeter
response is flattened out with a step filter: -9 dB flat from 5300Hz downwards (Q=3). The result:
In the high frequencies there are a few resonance peaks to dealt with later, but first the deep dip between
1000 and 2600Hz. There are a lot of options, two simple ones: lifting this regions with the parametric
EQualiser or fiddling with a forgotten parameter: time delay, either of the tweeter, or the woofer. The time
delay needed depends on the physical dimensions of the woofer/tweeter combination and the phase of
both woofer and tweeter driven with a certain filter in the cross over region, so trial and error is the easiest
way to optimize the results. Delaying the woofer (not the tweeter) between 0,2 – 0,3 msec (7 – 10 cm;
depending on the type of woofer filtering used) did the trick (blue line).
Last but not least: supressing the (resonance) peaks and checking the phase (dotted line):
An alternative solution with 3de order Butterworth filtering looks quite similar:
The response bellows up and down between 1 and 5 kHz. I couldn’t hear the difference between Bessel and
Butterworth filtering, yet. A could manage to get it flat with the parameticequaliser, but I choose not to: less
is more.
Is active filtering better?
I did a comparison of the original passive filter with a slithly different active filter tuning:
The top red line is the original filter, the top blue line the active filter: the active filtered is less uneven.
Below a indication of the distortion of the speaker: the active filtered speaker is a bit better: approximately -
46 dB (0,5%) THD between 2 and 5kHz, less than 1% above 800Hz. Not bad for a 30 year old speaker. The
results could get better if my measuring conditions would have been better (the distortion measurements
does not use time windowing, so panel vibrations from my room could influence the measurements).
What’s up next?
After this project I would like to try and check a few things:
- Simulate the speaker using LSPcad from Ijdata, like all big speaker builders do
- Measure the psycho-acoustic differences between different filter settings
- Test the psycho-acoustic effect of a bigger baffle (50 x 80) (need a bigger house first)
- using a automatic filter optimalisation routine I would like to tune the digital filter so to get a
Linkwitz-Riley response on both the woofer and tweeter measured sound pressure
Meindert Scholma, Amsterdam, 2011
pe1mmk
Digitally signed by pe1mmk
DN: cn=pe1mmk, o=hans
hilberink, ou=ucs,
email=pe1mmk@gmail.com,
c=BE
Date: 2011.08.07 00:59:23
+02'00'
